Coastal Lagoon Processes

COASTAL LAGOON PROCESSES

FURTHER TITLES IN THIS SERIES Volumes 1-7, 11, 15, 16, 18, 19, 21, 23, 29 and 32 are out of print. 8 E. LlSlTZlN 47 M.R. LANDRY and B.M. HICKEY (Editors) SEA-LEVEL CHANGES COASTAL OCENOGRAPHY OF WASHINGTON 9 R.H. PARKER AND OREGON THE STUDY OF BENTHIC COMMUNITIES 48 S.R. MASSEL 10 J.C.J. NIHOUL (Editor) HYDRODYNAMICS OF COASTAL ZONES MODELLING OF MARINE-SYSTEMS 49 V.C. LAKHAN and A.S. TRENHAILE (Editors) 12 E.J. FERGUSON WOOD and R.E. JOHANNES APPLICATIONS IN COASTAL MODELING TROPICAL MARINE POLLUTION 50 J.C.J. NIHOUL and B.M. JAMART (Editors) 13 E. STEEMANN NIELSEN MESOSCALE SYNOPTIC COHERENT STRUCTURES MARINE PHOTOSYNTHESIS IN GEOPHYSICAL TURBULENCE 14 N.G.JERLOV 51 G.P. GLASBY (Editor) MARINE OPTICS ANTARCTIC SECTOR OF THE PACIFIC 17 R.A. GEYER (Editor) 52 P.W. GLYNN (Editor) SUBMERSIBLES AND THEIR USE IN GLOBAL ECOLOGICAL CONSEQUENCES OF THE OCEANOGRAPHY AND OCEAN ENGINEERING 1982-83 EL NINO-SOUTHERN OSCILLATION 20 P.H. LEBLOND and L.A. MYSAK 53 J. DERA (Editor) WAVES IN THE OCEAN MARINE PHYSICS 22 P. DEHLINGER 54 K.TAKANO(Editor) MARINE GRAVITY OCEANOGRAPHY OF ASIAN MARGINAL SEAS 24 F.T. BANNER, M.B. COLLINS and K.S. 55 TAN WEIYAN MASSIE (Editors) SHALLOW WATER HYDRODYNAMICS THE NORTH-WEST EUROPEAN SHELF SEAS: THE 56 R.CHARLIER and J. JUSTUS SEA BED AND THE SEA IN MOTION OCEAN ENERGIES, ENVIRONMENTAL, ECONOMIC 25 J.C.J. NIHOUL (Editor) AND TECHNOLOGICAL ASPECTS OF ALTERNATIVE MARINE FORECASTING POWER SOURCES 26 H.G. RAMMING and 2. KOWALIK 57 P.C. CHU and J.C. GASCARD (Editors) NUMERICAL MODELLING MARINE DEEP CONVECTION AND DEEP WATER HYDRODYNAMICS FORMATION IN THE OCEANS 27 R.A. GEYER (Editor) -58- .P ..A.. P. .I R A 7 7 n l I MARINE ENVIRONMENTAL POLLUTION WORLD AT1 E 28 J.C.J. NIHOUL (Editor) MARINE TURBULENCE DE 30 A. VOlPlO (Editor) CHtMlCAL A S P t C I S THE BALTIC SEA 31 E.K. DUURSMA and R. DAWSON (Editors) MARINE ORGANIC CHEMISTRY 33 KHEKINIAN PETROLOGY OF THE OCEAN FLOOR 34 J.C.J. NIHOUL (Editor) HYDRODYNAMICS OF SEMI-ENCLOSED SEAS 35 B. JOHNS (Editor) PHYSICAL OCEANOGRAPHY OF COASTAL AND SHELF SEAS 36 J.C.J. NIHOUL (Editor) HYDRODYNAMICS OF THE EQUATORIAL OCEAN 37 W. LANGERAAR SURVEYING AND CHARTING OF THE SEAS 38 J.C.J. NIHOUL (Editor) REMOTE SENSING OF SHELF-SEA HYDRODYNAMICS 39 TKHIYE (Editor) OCEAN HYDRODYNAMICS OF THE JAPAN AND EAST CHINA SEAS 40 J.C.J. NIHOUL (Editor) COUPLED OCEAN-ATMOSPHERE MODELS 41 H. KUNZENDORF (Editor) MARINE MINERAL EXPLORATION 42 J.C.J NIHOUL (Editor) MARINE INTERFACES ECOHYDRODYNAMICS 43 P. LASSERRE and J.M. MARTIN (Editors) BIOGEOCHEMICAL PROCESSES AT THE LANDSEA BOUNDARY 44 I.P. MARTINI (Editor) CANADIAN INLAND SEAS 45 J.C.J. NIHOUL (Editor) THREE-DIMINSIONAL MODELS OF MARINE AND ESTUARIN DYNAMICS 46 J.C.J. NIHOUL (Editor) SMALL-SCALE TURBULENCE AND MIXING IN THE OCEAN ~

Elsevier Oceanography Series, 60

COASTAL LAGOON PROCESSES Edited by

Bjorn Kjerfve University of South Carolina, Belle W. Baruch Institute for Marine Biology and CoastaI Research, Columbia, S.C. 29208, U.S.A.

ELSEVl E R

Amsterdam

- London - New York - Tokyo

1994

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211,1000 AE Amsterdam, The Netherlands

L i b r a r y o f Congress Cata o g i n g - i n - P u b l l c a

i o n Data

C o a s t a l lagoon processes / e d l t e d by B j o r n K j e r f v e p. cm. -- ( E l s e v i e r o c e a ography s e r i e s : 60 ) I n c l u d e s b i b l i o g r a p h i c a l r e f e r e n c e s and I n d e x . ISBN 0-444-88258-8 (acid-free) 3. Lagoons. 1 . Lagoon e c o l o g y , 2. B r a c k i s h w a t e r e c o l o g y . I. K j e r f v e , B j o r n . 1944XI. S e r i e s . QH541.5.L27C63 1993 94- 1202 574.5'2636--dc20

.

CIP ISBN: 0 444 88258 8 (8

1994 Elsevier Science B.V. All rights reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publishers, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521,1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U S A . All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the materials herein. This book is printed on acid-free paper. Printed in The Netherlands.

This book is dedicated to my mother Valb rg and father h e

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vii

Prologue Coastal lagoons constitute a common coastal environment around the world. Whereas estuaries have been the focus of many texts and reviews, coastal lagoons have been neglected in comparison, sometimes lumped into the category of bar-built estuaries, and sometimes not dealt with at all. The standard definition of estuaries refers to coastal systems that are measurably diluted by seawater and in free connection with the open sea. This often does not fit coastal lagoons. Coastal lagoons are systems which can span the range of salinities from hypersaline to completely fresh, and which are often intermittently closed off from the adjacent sea. Definitions of coastal lagoons vary greatly (Fig. 1) and are addressed by several of the authors in this volume. Coastal lagoons indeed deserve a closer look. There exist only a few published works that attempt to provide a multidisciplinary coverage of salient aspects of coastal lagoons. The excellent early synthesis of coastal lagoons by Ayala Castaiiares and Phleger (1969) resulted from a joint UNAM/UNESCO symposium at Universidad Nacional A u t h o m a de Mexico in 1967. It remains a valuable resource reference. A follow-up UNESCO/IABO seminar in Beaufort, North Carolina, USA, in 1978, and a UNESCO (COMAR)/IABO/SCOR symposium in Bordeaux, France, in 1981, resulted in two valuable summaries, UNESCO (1981) and Laserre and Postma (1982), respectively. Together with an introductory text by Barnes (19801, these volumes appear to be the extent of dedicated multidisciplinary syntheses of coastal lagoons published in English. The current volume, Coastal Lagoon Processes, is an attempt to focus again on coastal lagoons and bring together the expertise and opinions on coastal lagoons in a single synthesis volume, serving to update the information and analyses in the older publications. B.C.

BY JOHNNY HART I

Fig. 1.The definition of coastal lagoon vanes widely, and it is about time for the scientific community to agree on an authoritative definition. Such a definition as well as a distinction between lagoons and estuaries will hopefully emerge in this volume. (Used by permission of Johnny Hart and Creators Syndicate, Inch

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This project started in fun in August 1988, during a n excellent Friday afternoon lunch, over smoked kassler and dark beer, in a crowded downtown Niter& (Brazil) restaurant, Bar Luiz. Bastiaan Knoppers, Luiz Drude de Lacerda, and I were discussing priorities for research and publications on the coastal lagoons along the Fluminense coast east of Rio de Janeiro. Thus was born the idea of an edited synthesis volume on coastal lagoons, recognizing that a lagoon does not necessarily function in the same manner as does a n estuary. I am indebted to Bastiaan and Drude for providing me with the idea to go ahead with this book project on coastal lagoons, and also grateful to them for many other stimulating discussions and interactions during subsequent research activities in Brazil. I would also like to express my gratitude for support from Diretoria de Desenvolvimento Cientifico e Tecnologico (CNPq), the National Geographic Society, and the National Science Foundation, allowing me the opportunity to carry out research on coastal lagoons in Brazil, and thus indirectly providing the fuel for this volume. Likewise, I am thankful for the kind and enthusiastic support and help provided me by Departamento de Geoquimica, Universidade Federal Fluminense (UFF) in Niter&, as well as individual faculty, staff, and students at that institution. I t would not have been possible to complete this edited book without the diligent assistance of many scientists who kindly gave of their time to review the many manuscripts submitted for inclusion in this volume. Each chapter was sent to four reviewers. I made a decision about inclusion of a manuscript after receiving at least two concurring, positive reviews. I would like to express my sincere thanks to each one of the reviewers listed below, who kindly critiqued one or more of the manuscripts. The reviewers were: Dr. Dennis M. Allen, Baruch Institute Coastal Marine Laboratory, University of South Carolina; Prof. Dr. Milton Asmus, Departmento de Oceanografia, Fundaqiio Universidade do Rio Grande, Brazil; Dr. H. John Bavor, Jr., Water Research Laboratory, Hawkesbury Agricultural College; Dr. Robert J. Byrne, College of William and Mary, School of Marine Science, Virginia Institute of Marine Science; Prof. Armando A. de la Cruz, Department of Biological Sciences, Mississippi State University; Dr. Ned Cyr, Office of International Affairs, NOAA, Rockville, MD; Dr. Richard F. Dame, Marine Science Department, Coastal Carolina College/University of South Carolina; Dr. R.A. Davis, Jr., Department of Geology, University of South Florida; Dr. John Mark Dean, Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina; Professor Colin D. Field, City Polytechnic of Hong Kong; Ms. Amy C. Frankenburg, Department of Geological Sciences, University of South Carolina; Dr. C.L.J. Frid, F.S.C. Research Centre, U.K.; Dr. S. Garcia, Marine Resources Service, F A 0 Fisheries Division; Dr. L.R. Gardner, Department of Geological Sciences, University of South Carolina; Dr. David Jay, Department of Oceanography, University of Washington; Dr. J. Kapetsky, Inland Water

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Resources and Aquaculture Service, FA0 Fisheries Division; Dr. David Krantz, College of Marine Studies, University of Delaware; Dr. J. Van de Kreeke, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami; Prof. Dr. Luiz Drude de Lacerda, Departamento de Geoquimica, Universidade Federal Fluminense, Brazil; Dr. William T. Lawrence, Center for Energy and Environmental Research, University of Puerto Rico; Dr. Liyuan Liang, Oak Ridge National Laboratory; Dr. Nelson Marshall, Coral Reef and Estuarine Studies, St. Michaels, MD; Dr. Robert H. Meade, U.S. Geological Survey, Boulder, CO; Dr. Hank N. McKellar, Jr., College of Health, University of South Carolina; Dr. C.B. Miller, School of Oceanography, Oregon State University; Dr. Stephen P. Murray, Department of Oceanography and Coastal Sciences, Louisiana State University; Dr. Charles B. Officer, Department of Earth Sciences, University of New Hampshire; Mr. Roger Palm, School of Fisheries, University of Washington; Dr. C.H. Peterson, Institute of Marine Sciences, University of North Carolina at Chapel Hill; Dr. Jay Pinckney, Department of Biology, University of South Carolina; Dr. J.P. Quignard, Universitk des Sciences et Techniques du Languedoc, Canada; Dr. Mitchell A. Roffer, Roffer Ocean Fishing Forecasting Service, Inc., Miami, FL; Dr. Paul M. Scholz, SCAMPI, Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina; Dr. Richard M. Showman, Department of Biological Sciences, University of South Carolina; Dr. Fred Sklar, Baruch Institute Coastal Marine Laboratory, University of South Carolina; Dr. Samuel C. Snedaker, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami; Dr. Ned P. Smith, Harbor Branch Oceanographic Institution, Fort Pierce, FL; Dr. Stephen V. Smith, School of Ocean and Earth Science and Technology, Department of Oceanography, University of Hawaii; Dr. Flora Chu Wang, Department of Oceanography and Coastal Sciences, Louisiana State University; Dr. Charles A. Wilson, Department of Oceanography and Coastal Sciences, Louisiana State University; Dr. W.J. Wolff, Research Institute for Nature Management, Netherlands Institute for Sea Research. In addition, I would like to acknowledge the help of Stephen C. Stonehill of the University of South Carolina. He handled most manuscript formatting, spell checking, letter writing, mailings, and other important chores related to the book. Without his help, the book would no doubt be several more years in production. However, for those errors that still no doubt occur, only I am to be blamed. References Ayala Castafiares, A., & F.B. Phleger (eds.). 1969. Coastal lagoons: a symposium. Universidad Nacional Autdnoma de MBxico. 686 pp. Barnes, R.S.K. 1980. Coastal lagoons. Cambridge University Press. 106 pp.

X

Laserre, P., & H. Postma (eds.). 1982. Coastal lagoons. Oceanologica A d a . Volume Special. 462 pp. UNESCO. 1981. Coastal lagoon research, present and future. UNESCO Technical Papers in Marine Science 33.348 pp.

Bjorn Kjerfve Columbia, SC, USA

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Contributing Authors Marlin J. Atkinson Hawaii Institute of Marine Biology, University of Hawaii, Honolulu, Hawaii 96822, USA

R.S.K.Barnes Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK E.C.F. Bird Department of Geography, University of Melbourne, Parkville, Vic. 3052, Australia John D. Boon I11 Virginia Institute of Marine Science and College of William and Mary, Gloucester Point, VA 23062, USA Michael A. Borowitzka School of Biological & Environmental Sciences, Murdoch University, Murdoch, W.A. 6150, Australia Saul Alvarez Borrego Centro de Investigaciones Cientifica y Education Superior de Ensenada, B.C., Av. Espinosa 843, Ap. 2732, Ensenada, Baja, Mexico Per Bruun 34 Baynard Cove Road, Hilton'Head Island, SC 29928, USA Ana Laura Lara Dominguez Programa de Ecologia, Pesquerias y Oceanografia del Golfo de Mexico (EPOMEX), Universidad Autonoma de Campeche, Apartado Postal 520, Campeche 24000, Campeche, Mexico

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Josh Maria Landim Dominguez Instituto de Geociencias, PPPG, Universidade Federal da Bahia, Rua Caetano Moura 123, Federaqiio, CEP40.210, Salvador, Bahia, Brazil Clifford Hearn Department of Geography and Oceanography, University College, The University of New South Wales, Australian Defence Force Academy, Campbell, ACT 2600, Australia Bjorn yjerfve Marine Science Program, Department of Geological Sciences, and Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, SC 29208, USA Bastiaan Knoppers Departamento de Geoquimica, Universidade Federal Fluminense, CEP-24.210 Niterbi, R J , Brazil Luiz Drude de Lacerda Departamento de Geoquimica, Universidade Federal Fluminense, CEP-24.210 Niterbi, FLJ,Brazil Rod Lukatelich BP Refinery Kwinana, Kwinana, W.A. 6168, Australia Donald J. Macintosh Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK Louis Martin Centre ORSTOM, 70-74 Route d'Aulnay, F-93140 Bondy, France

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AJ. McComb School of Geological and Environmental Science, Murdoch University, Murdoch, W.A. 6150, Australia

Maynard M. Nichols Virginia Institute of Marine Science and College of William and Mary, Gloucester Point, VA 23062, USA Daniel Pauly International Center for Living Aquatic Resources Management (ICLARM), P.O. Box 1501, Makati, Manila, The Philippines Ant8nio L. Philomena Fundaqiio Universidade de Rio Grande, Av. Italia, Cx. P. 474, CEP-96.200 Rio Grande, RS, Brazil

H. Postma Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB den Burg, Texel, The Netherlands Ned P. Smith Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway, Fort Pierce, FL 34946, USA) Stephen V. Smith Hawaii Institute of Marine Biology, University of Hawaii, Honolulu, Hawaii 96822, USA Malcolm L. Spaulding Department of Ocean Engineering, University of Rhode Island, Kingston, RI 02881, USA Alejandro Yaiiez-Arancibia Programa de Ecologia, Pesquerias y Oceanografia del Golfo de Mexico (EPOMEX), Universidad Authoma de Campeche, Apartado Postal 520, Campeche 24000, Campeche, Mexico

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Contents

Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bjorn Kjerfie Contributing authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 1. COASTAL LAGOON PROCESSES . . . . . . . . . . . . . . . . Bjorn Kjerfie Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coastal lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A multi-disciplinary synthesis . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 2. PHYSICAL SETTING AND GEOMORPHOLOGY OF COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E .C.F . Bird Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological background . . . . . . . . . . . . . . . . . . . . . . . . . . . . Climatic and hydrological factors . . . . . . . . . . . . . . . . . . . . . . Evolution of coastal barriers . . . . . . . . . . . . . . . . . . . . . . . . . Lagoon entrances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sedimentation in lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . . Swamp land encroachment . . . . . . . . . . . . . . . . . . . . . . . . . . Rounding and segmentation . . . . . . . . . . . . . . . . . . . . . . . . . Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 3. GEOLOGICAL HISTORY OF COASTAL LAGOONS . . . . . . Louis Martin and Jose' Maria Landim Dominguez Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The origin and evolution of coastal lagoons . . . . . . . . . . . . . . . . . Sea-level history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shoreface dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tidal range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological historyofcoastallagoons: Brazilianexamples . . . . . . . . . Lagoons formed on a low-relief coastal plain during sea-level rise . . . . . The Jequitinhonha strandplain . . . . . . . . . . . . . . . . . . . . . . . The Doce strandplain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lagoons formed on a low-relief coastal plain during sea-level drop . . . . Lagoons formed on a high-relief coastal plain during sea-level rise . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 1 3 5 7

9 9 13 15 19 25 29 30 32 34 36 41 41 42 42 47

48 49 51 53 57 60 63 65 66

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CHAPTER 4 WATER. SALT. AND HEAT BALANCES OF COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ned P . Smith Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Salt balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 5 . MODELING OF CIRCULATION AND DISPERSION IN COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . Malcolm L. Spaulding Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simplified circulation and pollutant transport models for inlet-lagoon systems hydrodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . Simplified pollutant transport models . . . . . . . . . . . . . . . . . . . . Two-dimensional vertically averaged models of inlet-lagoon systems hydrodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pollutant transport models . . . . . . . . . . . . . . . . . . . . . . . . . . Model selection process . . . . . . . . . . . . . . . . . . . . . . . . . . . . Model application to assess management strategies . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 6. MASS BALANCE OF NUTRIENT FLUXES IN COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stephen V. Smith and Marlin J . Atkinson Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Computational model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caveats to the application of this model . . . . . . . . . . . . . . . . . . Considerations in site selection . . . . . . . . . . . . . . . . . . . . . . . Alternativemodelsfor net nonconservativefluxanalysis . . . . . . . . Nutrient dynamics in coastal systems . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 70 71 85 91 98 98 103 103 104 114 116 118 119 . 121 125 127 133

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134 135 138 142 147 149 153

CHAPTER 7. SEDIMENT TRANSPORT PROCESSES IN COASTAL 157 LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maynard M. Nichols and John D . Boon 111 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Historical development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 General concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 Sources of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Sinks for sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Sediment dynamics in tidally-dominated (‘1eaky’)lagoons . . . . . . . . . 172 Basin morphology as a source of time-flow asymmetry . . . . . . . . . . 182 Textural patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

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Particle modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . Response to sea level change . . . . . . . . . . . . . . . . . . . . . . . . . Lagoon equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Global distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Storm effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generalizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 8. BIOGEOCHEMISTRY OF HEAVY METALS IN COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Luiz Drude de Lacerda Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sources of heavy metal input to coastal lagoons . . . . . . . . . . . . . . Fate of heavy metals in coastal lagoons . . . . . . . . . . . . . . . . . . . Heavy metal cycling in coastal lagoons . . . . . . . . . . . . . . . . . . . Fate of heavy metals in the water column . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

186 187 189 191 193 196 205 207 208 209 221 221 222 225 231 234 237

CHAPTER 9. AQUATIC PRIMARY PRODUCTION IN COASTAL 243 LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bastiaan Knoppers Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 The measurement of primary production in coastal lagoons . . . . . . . . 244 Physical setting and the primary producers . . . . . . . . . . . . . . . . . 247 256 Nutrient sources and sinks . . . . . . . . . . . . . . . . . . . . . . . . . . Nutrient supply by atmosphere and groundwater . . . . . . . . . . . . . 257 Riverine input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Autochthonous supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Nitrogen fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 261 Nutrient loss by denitrification . . . . . . . . . . . . . . . . . . . . . . . Nutrient loss by accumulation in sediments . . . . . . . . . . . . . . . . 261 New versus regenerated production . . . . . . . . . . . . . . . . . . . . . 263 Autotrophic versus heterotrophic metabolism . . . . . . . . . . . . . . . . 264 Case studies on primary production . . . . . . . . . . . . . . . . . . . . . 264 Phytoplankto based lagoons . . . . . . . . . . . . . . . . . . . . . . . . . 265 Macrophyte based lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Macroalgal based lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . 271 Arid choked to leaky lagoons: algal mat based systems . . . . . . . . . . . 273 Global lagoon primary production . . . . . . . . . . . . . . . . . . . . . . 274 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 CHAPTER 10. SECONDARY PRODUCTIVITY IN COASTAL LAGOONS . . 287 Saul Aluarez Borrego Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Zooplankton productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

xviii

Nekton productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benthic secondary productivity . . . . . . . . . . . . . . . . . . . . . . . The pelagic-benthic coupling . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

293 297 303 304 306

CHAPTER 11. MACROFAUNAL COMMUNITY STRUCTURE AND LIFE HISTORIES IN COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . 311 R.S.K. Barnes Nature of lagoonal organisms and communities . . . . . . . . . . . . . . 311 Processes structuring the macrofaunal communities . . . . . . . . . . . . 314 ‘Supply-side ecology’: problems of recruitment and colonization . . . . . . 316 Agents of mortality and their effects . . . . . . . . . . . . . . . . . . . . . 317 Competition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 Environmental adversity . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Life histories of lagoonal animals . . . . . . . . . . . . . . . . . . . . . . 342 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 CHAPTER 12. COASTAL LAGOONS AS FISH HABITATS . . . . . . . . . Alejandro Yaiiez.Arancibia. Ana Laura Lara Dominguez and Daniel Pauly Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Which fishes use coastal lagoons? . . . . . . . . . . . . . . . . . . . . . . Which features of lagoon-estuarine habitats do fishes actually use? . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 363

.

363 364 370 373

CHAPTER 13. FISHERIES IN COASTAL LAGOONS . . . . . . . . . . . . . 377 Daniel Pauly and Alejandro Yaiiez-Arancibia Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 Fisheries catches from coastal lagoons . . . . . . . . . . . . . . . . . . . 378 Fish population dynamics and its four factors . . . . . . . . . . . . . . . 379 Management of coastal lagoon fisheries . . . . . . . . . . . . . . . . . . . 391 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 CHAPTER 14. AQUACULTURE IN COASTAL LAGOONS . . . . . . . . . . 401 Donald J . Macintosh Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 The lagoon as a n environment for aquaculture . . . . . . . . . . . . . . . 403 Aquaculture practices in lagoons: some examples . . . . . . . . . . . . . 410 Aquaculture in pens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 Shrimp culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 Mollusc culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Seaweeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 Socio-economic issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436

XiX

CHAPTER 15. APPLICATION OF BIOTECHNOLOGY TO COASTAL LAGOONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Michael A. Borowitzka Aquaculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The production of high value chemicals . . . . . . . . . . . . . . . . . . . Environmental management . . . . . . . . . . . . . . . . . . . . . . . . . Waste treatment and environmental remediation . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 16. COASTAL LAGOON ECOSYSTEM MODELING . . . . . . Clifford Hearn. Rod Lukatelich and A.J . McComb Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coastal lagoons of southwest Australia . . . . . . . . . . . . . . . . . . Modelling the hydrodynamics of coastal lagoons . . . . . . . . . . . . . The addition of chemical and biological processes . . . . . . . . . . . . . Lagoon ecosystem models . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 19. FUTURE OF RESEARCH IN COASTAL LAGOONS . . . H. Postma Methodologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Problems in lagoon research . . . . . . . . . . . . . . . . . . . . . . . . .

443 449 458 458 459 460

. 471 471

. 472

.

477

. 489 493 502

CHAPTER 17. ENGINEERING PROJECTS IN COASTAL LAGOONS . . . . Per Bruun Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water table management of coastal lagoons . . . . . . . . . . . . . . . . A case study: Venice Lagoon. Italy . . . . . . . . . . . . . . . . . . . . . . Dredging in lagoons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acasestudy: dredgingandreclaiminginSouthCarolina . . . . . . . . . Problems associated with rising relative sea level . . . . . . . . . . . . . Water quality problems in coastal lagoons . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER 18. MONEY AND ENERGY ANALYSIS FOR COASTAL LAGOON RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AntGnio L. Philomena Integration of economy and ecology . . . . . . . . . . . . . . . . . . . . . General view of coastal lagoons . . . . . . . . . . . . . . . . . . . . . . . Money and energy flows: Lagoa dos Patos. Brazil . . . . . . . . . . . . . Coastal lagoon reserves . . . . . . . . . . . . . . . . . . . . . . . . . . . Implications and future directions . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

443

507 507 508 516 519 524 526 530 532 532

535 535

536

. 539 545 548 548

. . 553 553 555

Morphology and stability . . . . . . . . . . . . . . . . . . . . . . . . . . . Ecosystems under stress . . . . . . . . . . . . . . . . . . . . . . . . . . . Transport and pathways of materials . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

556 557 559 561 563

Coastal Lagoon Processes edited by B. yjerfve (Elsevier Oceanography Series, 60) Q 1994 Elsevier Science Publishers B.V. All rights reserved

1

Chapter 1

Coastal Lagoons Bjorn Kjerfve Belle W. Baruch Institute for Marine Biology and Coastal Research, Marine Science Program, and Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA, and a visitor to Departamento de Geoquimica, Universidade Federal Fluminense, CEP-24.210 Niterdi, RJ Brazil

Coastal lagoons are inland water bodies, found on all continents, usually oriented parallel to the coast, separated from the ocean by a barrier, connected to the ocean by one or more restricted inlets which remain open a t least intermittently, and have water depths which seldom exceed a few meters. A lagoon may or may not be subject to tidal mixing, and salinity can vary from that of a coastal fresh-water lake to a hypersaline lagoon, depending on the hydrologic balance. Lagoons formed as a result of rising sea level mostly during the Holocene and the building of coastal barriers by marine processes. They are often highly productive and ideal systems for aquaculture projects but are, at the same time, highly stressed by anthropogenic inputs and human activities.

Introduction Pritchard (1952) offered a definition of estuaries, based on the geomorphological structure of coastal systems. He divided estuaries into (a) coastal plaiddrowned river valley estuaries, (b) fjord estuaries, (c) bar-built estuaries, and later also (d) tectonically formed estuaries. Although many other criteria, such as water balance, ecological characteristics, circulation-mixing, and marine-fluvial processes, have been used to classify coastal systems, the geomorphological approach remains easy and very attractive. However, Pritchard’s (1952) early definition, where a n estuary is a semienclosed coastal body of water having a free connection with the open sea and containing a measurable quantity of sea salt, was very perceptive but is not generally being used. Rather, the later modified definition by Cameron and Pritchard (1963),where an estuary is a semi-enclosed body of water having a free connection with the open sea and within which the sea-water is

2

Coastal Lagoons

measurably diluted with fresh water deriving from land drainage, is most commonly adopted but is in reality a much too restrictive definition, since many types of inland coastal marine-connected systems are excluded. I would like to propose a modified division and definition of inland coastal ocean-connected waters based on their geomorphological structure, and recognizing that each type have many physical and functional processes in common. In this division, a n estuary is consistent with the Cameron & Pritchard (1963)definition, which then represents one type of inland coastal ocean-connected system. I propose that inland coastal ocean-connected waters can advantageously be organized into six categories: (1)Estuary: an inland river valley or section of the coastal plain, drowned as the sea invaded the lower course of a river during the Holocene sea-level rise, containing sea water measurably diluted by land drainage, affected by tides, and usually shallower than 20 m. This definition is consistent with the definition by Cameron and Pritchard (1963) and Pritchard (1967). It is only this type of system that should be referred to as a n estuary, which in fact is and has always been the practice in most of Latin America. The Chesapeake Bay and the Charleston Harbor-Cooper River systems in the USA are good examples of estuaries. ( 2 )Coastal Lagoon: a n inland water body, usually oriented parallel to the coast, separated from the ocean by a barrier, connected to the ocean by one or more restricted inlets, and having depths which seldom exceed a couple of meters. A lagoon may or may not be subject to tidal mixing, and salinity can vary from that of a coastal fresh-water lake to a hypersaline lagoon, depending on the hydrologic balance. Lagoons formed as a result of rising sea level during the Holocene or Pleistocene and the building of coastal barriers by marine processes. Laguna de Terminos, Mexico, Lagoa dos Patos, Brazil, the Coorong, Australia, are examples of coastal lagoons. (3) Fjord: a glacially scoured inland marine area with sea water measurably diluted by land drainage in the surface layer, consisting of high salinity waters in deep basins, affected by tides, and usually measuring several hundred meters in depth. Oslofjorden in Norway is a n example of a fjord. (4) Bay: a coastal indentation, usually the result of faulting or other tectonic or regional geologic processes, strongly affected by tides, and exhibiting salinities ranging from oceanic to brackish, depending on the amount of land drainage relative to oceanic exchange. Another common name for this type of a system is a gulf. San Francisco Bay, USA, Tokyo Bay, Japan, Baia de Guanabara, Brazil, are examples of bays. ( 5 ) Tidal River: a n inland river valley, drowned as the sea invaded the lower river course during the Holocene sea level rise, containing only fresh water, but subject to tidal sea-level variations and sometimes reversing tidal currents in the downstream section. As energy propagates more readily than salt is transported upstream, most estuaries have a n associated tidal river. The tidal river begins at the upstream limit of measurable ocean

B. Kjerfie

3

salinity and ends at the upstream limit of measurable ocean tide effects. Several hundred kilometers of the lower Amazon River, Brazil, is a n example of a tidal river. (6) Strait: an inland marine waterway, connecting two oceans or seas. Characteristics of sea straits with respect to circulation, salinity distribution, tidal processes, and water depth vary widely between straits. Strait of Magellan, Chile, is an example of a strait. Coastal Lagoons Phleger (1969) defined coastal lagoons as shallow inland marine waters, usually oriented parallel to the coast, separated from the ocean by a barrier, and connected to the ocean by one ore more restricted inlets. To this should be added that the ocean entrance(s) can at times be closed off by sediment deposition as a result of wave action and littoral drift. A workable definition of a coastal lagoon is thus a shallow coastal water body separated from the ocean by a barrier, connected at least intermittently to the ocean by one or more restricted inlets, and usually oriented shore-parallel. Coastal lagoons occupy 13% of coastal areas worldwide, and are often impacted by both natural and anthropogenic influences (Mee, 1978; Sikora and Kjerfve, 1985). Depending on local climatic conditions, lagoons exhibit salinities which range from completely fresh to hypersaline (Moore and Slinn, 1984; Kjerfve, 1986; Kjerfve and Magill, 1989; Merino et al., 1990;Knoppers et al., 1991). Barrier coasts are usually backed by lagoons and measure 32,068 of the world’s combined continental coastline with the contribution estimated at 17.6%for North America, 12.2%for South America, 5.3%for Europe, 17.9% for Africa, 13.8%for Asia, and 11.4%for Australia (Cromwell, 1971; Barnes, 1980). The size of coastal lagoons varies substantially with surface areas ranging up to 10,200 km2 as in the case of Lagoa dos Patos in Brazil. The water depth is typically 1-3 m, and almost always less than 5 m with the exception of inlet channels and isolated relict holes or channels. In general, coastal lagoons trap inorganic sediment and organic matter, and thus serve as material sinks or material filters. They often exhibit very high primary and secondary production rates and are valuable for fisheries and aquaculture, and sometimes for salt extraction. On a geologic time scale, they are short-lived landscape features, with an existence intrinsically linked to their filtering efficiency and the rate of relative sea-level change in response to global climatic change, local tectonic activity, and anthropogenic activities (e.g. river damming; water pumping, water diversion, and water use; land-use practices). Coastal lagoons experience forcing from river input, wind stress, tides, precipitation to evaporation balance, and surface heat balance, and respond

4

Coastal Lagoons

Fig. 1. Coastal lagoons can conveniently be sub-divided into choked, restricted, and leaky systems based on the degree of water exchange with the adjacent coastal ocean (after Kjerfve, 1986).

differently t o these forcing functions. Water and salt balances, lagoon water quality, and eutrophication depend critically on lagoon circulation, salt and material dispersion, water exchange through the ocean canal(s), and turnover, residence, or flushing times. The understanding of physical, chemical, geological, and ecological dynamics of lagoons is important for planning and implementation of coastal management strategies in coastal lagoons. Kjefie (1986) sub-divided coastal lagoons into three geomorphic types according to water exchange with the coastal ocean (Fig. 1).The rate and magnitude of oceanic exchange reflects both the dominant forcing function(s) and the time-scale of hydrologic variability. The divisions represent three points along a spectrum of lagoon variability. These points are: Choked Lugoons: Choked lagoons usually consist of a series of connected elliptical cells, connected by a single long narrow entrance channel, along coasts with high wave energy and significant littoral drift. Although lagoons experience tides that co-oscillate with tides in the coastal ocean, the entrance channel serves as a dynamic filter which largely eliminates tidal currents and water-level fluctuations inside the lagoon (Kjerfve, 1986; Kjefie et al., 1990; Kjerfve and Knoppers, 1991). Tidal oscillations in choked lagoons are often reduced to 5 % or less as compared t o the adjacent coastal tide. Choked coastal lagoons are characterized by long flushing times, dominant wind forcing, and intermittent stratification events due to

B. Kjerfve

5

intense solar radiation or runoff events. In arid or semi-arid regions of the world, choked coastal lagoons often become permanently or temporarily hypersaline (Copeland, 1967; Moore and Slinn, 1984). Choked lagoons are mostly oriented shore-parallel but are sometimes also found associated with river deltas and then occasionally oriented shore-normal. Examples of choked coastal lagoons include Lagoa dos Patos and Lagoa de Araruama, Brazil, Lake St. Lucia, South Africa, the Coorong, Australia, and Lake Songkla, Thailand. Restricted Lagoons: Restricted lagoons consist of a large and wide water body, usually oriented shore-parallel, and exhibit two or more entrance channels or inlets. As a result, restricted coastal lagoons have a well-defined tidal circulation, are influenced by winds, are mostly vertically well mixed, and exhibit salinities from brackish water to oceanic salinities. Flushing times are usually considerably shorter than for choked coastal lagoons. Examples of restricted coastal lagoons include Laguna de Terminos, MBxico, and Lake Pontchartrain, USA. Leak.. Lagoons: Leaky lagoons are elongated shore-parallel water bodies with many ocean entrance channels along coasts where tidal currents are sufficiently strong to overcome the tendencies by wave action and littoral drift to close the channel entrances. Leaky lagoons occupy the opposite end of the spectrum from choked lagoons. Leaky lagoons are characterized by numerous wide tidal passes, unimpaired water exchange with the ocean on wave, tidal, and longer time scales, strong tidal currents, and salinities close to that of the coastal ocean. Examples of leaky lagoons are Mississippi Sound, USA, and Wadden Zee, The Netherlands-Denmark. A Multi-Disciplinary Synthesis

This volume is an attempt to recognize the unique nature of coastal lagoons and describe them from a variety of points of view and demonstrate how they are similar to and differ from estuaries, fjords, bays, tidal rivers, and straits. Although the published literature on coastal lagoons does not yet match that on estuaries, many publications dealing with lagoons confuse them with estuaries. E.C.F. Bird describes the physical setting and geomorphology of coastal lagoons, how they formed, and how they vary in relation to geological, hydrological, climatic and ecological factors. L. Martin and J.M. Landim Dominguez focus their chapter on the geological history of coastal lagoons and geological processes controlling the origin and evolution of coastal lagoons and the history of the barrier island enclosing lagoons. They describe the three main factors that control the origin and maintenance of sandy barriers and thus determine the ultimate fate of coastal lagoons: sea-level history, shoreface dynamics, and tidal range.

6

Coastal Lagoons

N.P. Smith presents a balance of water, salt and heat for coastal lagoons and the importance of geomorphological factors which influence these balances and physical processes. He describes the wind, density-driven, and tidal circulation features in coastal lagoons, and discusses the salt balance due to advection and diffusion and heat balance due to advection, diffusion, and net radiation. M.L. Spaulding reviews strategies to model the circulation and pollutant transport in coastal lagoons based on simplified correlation analysis, analytic solutions to the ordinary linearized differential equations, and a numerical model that solves the governing equations for multi-inlet, multi-basin systems. He also demonstrates model application for management decision-making. S.V. Smith and M.J. Atkinson present a net mass balance approach to the analysis of coastal systems, including coastal lagoons, and a measure of how the system is biogeochemically coupled with adjacent systems. They demonstrate a method for evaluating nonconservative fluxes in coastal lagoons. M.M. Nichols and J.D. Boon I11 review sediment transport processes in coastal lagoons and how these processes act to modify, retain and accumulate sediment. They examine lagoon transport systems as a series of processes that distribute fine-grained sediment between sources and sinks. L.D. Lacerda describes how coastal lagoons are temporary sinks for trace metals and how accumulation is controlled by sedimentation rates and metal loads. B. Knoppers describes aquatic primary production in coastal lagoons and the mechanisms that regulate primary production. He estimates global coastal lagoon primary production at 10" kg C y r l , which is similar to the contribution by upwelling areas. S. Alvarez Borrego describes the secondary productivity in coastal lagoons and the fact that no single measurement can be used to characterize the secondary productivity of a community. However, it seems that the zoobenthos are the most productive animals in coastal lagoons, especially the suspension feeders. R.S.K. Barnes discusses the macrofaunal community structure and life history of coastal lagoons and the processes likely to structure the benthic animal communities, including patterns of recruitment and colonization; predation; disturbance; adulflarval interactions; interference and exploitative competition; and environmental adversity. A. Yafiez Arancibia, A.L. Lara Dominguez, and D. Pauley discuss fish species using coastal lagoons as habitats during at least some part of their life cycles with emphasis on latitudinal differences. D. Pauley and A. Yaiiez Arancibia describe lagoon fisheries in terms of recruitment, growth, natural mortality, and fishing mortality, and examine forms of lagoon fisheries harvesumanagement. D.J. MacIntosh discusses coastal lagoons as a location for aquaculture, and concludes that lagoon fisheries production can be enhanced significantly through aquaculture, provided appropriate culture sites and species are selected. He also examines the relationship between aquaculture and lagoon capture fisheries and states that Southeast Asia is responsible for 85% of the world's aquaculture production.

B. Kjerfve

7

M.A. Borowitzka describes existing and potential applications of biotechnology in coastal lagoons, including nutrition, disease management, and growth promotion in aquaculture; the culture of specific organisms to produce high value chemicals; environmental management; and waste treatment and environmental remediation. C. Hearn, R. Lukatelich, and A.J. McComb explore the use of mathematical modelling as a tool for understanding the functioning of lagoons, and address a range of models of process and ecosystem function. P. Bruun draws on his expertise in discussing a variety of engineering type projects in coastal lagoons including management of water tables, dredging of canals and basins, establishment of harbors and fish farms. A.L. Philomena explores the symbiosis between economy and ecology and how this alters the assessment of the value of coastal lagoon resources. He uses “emergy” as a measure of quantity and quality of energy and proposes this as a tool to establish ecological economic development. Finally, H. Postma concludes that every individual coastal lagoon has its own characteristics that require a separate study and at the same time make generalizations of results to other systems more difficult. The need for studies is in part generated by the great economic importance attached to coastal areas, including coastal lagoons, and the mostly negative effects of human interference. This volume is an attempt to synthesize a number of aspects of coastal lagoons and to demonstrate and argue that coastal lagoons represent a unique coastal environment, separate and different from estuaries, fjords, bays, tidal rivers, and sea straits, and thus require separate attention. References Barnes, R.S.K. 1980. Coastal Lagoons. Cambridge University Press. Cambridge, UK. 106 PP. Cameron, W.M. and Pritchard, D.W. 1963. Estuaries. pp. 306-324. In: The Sea, 2. (M.N. Hill, ed.). John Wiley & Sons, pp. 306-324. Copeland, B.J. 1967. Environmental characteristics of hypersaline lagoons. Publications ofthe Institute for Marine Science (University of Texas) 12: 207-218. Cromwell, J.E. 1971. Barrier coast distribution: a world-wide survey. p. 50. Abstract. In: Second National Coastal Shallow Water research Conference. Baton Rouge, LA. Kjerfve, B. 1986. Comparative oceanography of coastal lagoons. pp. 63-81. In: Estuarine Variability. (Wolfe D.A., ed.). Academic Press, New York. Kjerfve, B. and Magill, K.E. 1989. Geographic and hydrographic characteristics of shallow coastal lagoons. Marine Geology 88: 187-199. Kjerfve, B., Knoppers, B., Moreira, P. and Turcq, B. 1990. Hydrological regimes in Lagoa de Guarapina, a shallow Brazilian coastal lagoon. Acta Limnologica Brasiliensia 111: 931-949. Kjerfve, B. and Knoppers, B. 1991. Tidal choking in a coastal lagoon. pp. 169-179. In: Tidal Hydrodynamics (B. Parker, ed.). John Wiley & Sons, New York.

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Coastal Lagoons

hoppers, B, yjerfve, B. and Carmouze, J.P. 1991.Trophic state and water turn-over time in six choked coastal lagoons in Brazil. Biogeochernistry 14:149-166. Mee, L.D. 1978. Coastal lagoons. pp. 441-490. In: Chemical Oceanography. Second edition. Vol. 7.(J. Riley and 0. Skirrow, eds.). Academic Press. New York. More, N.H. and Slinn, D.J. 1984.The physical hydrology of a lagoon system on the Pacific coast of Mexico. Estuarine, Coastal and Shelf Science 19:413-426. Phleger, F.B. 1969.Some general features of coastal lagoons. pp. 5-26. In: Lagunas Costeras, un Simposio. (A. Ayala-Castaiieres, ed.). Universidad Nacional AuMnoma de M6xic0, MBxico, DF. Pritchard, D.W. 1952.Estuarine hydrography. pp. 243-280.In: Advances in Geophysics 1. (H.E. Landsberg, ed.). Academic Press, New York. Pritchard, D.W. 1967.What is an estuary: physical standpoint. In: Estuaries. pp. 3-5. (G.H. Lauff, ed.). American Association for the Advancement of Science. Publication 83.Washington, DC. Sikora, W.B. and Kjefie, B. 1985.Factors influencingthe salinity of Lake Pontchartrain, Louisiana, a shallow coastal lagoon: analysis of a long-term data set. Estuaries 8 (2A): 170-180.

Coastal Lagoon Processes edited by B. yjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

9

Chapter 2

Physical Setting and Geomorphology of Coastal Lagoons Eric C.F. Bird Department of Geography, University of Melbourne, Parkville, Vic. 3052, Australia

Coastal lagoons are areas of relatively shallow water that have been partly or wholly enclosed from the sea by the formation of depositional barriers. They vary in size and shape in relation to antecedent morphology, the form of the enclosing barrier, and the extent of erosion and deposition since their initial formation. There are variations related to geological, hydrological, climatic and ecological factors. Some have been shallowed by sedimentation; others have shrunk as the result of bordering accretion, including swamp land encroachment; others have become rounded, and in some cases segmented by the growth and coalescence of cuspate spits. Salinity regimes, related primarily to the interaction of fresh water from rain and rivers with salt water from the sea, related to the dimensions and variability of marine entrances, condition their ecology, and determine the extent of mangroves, salt marsh and transitions to freshwater vegetation, which in turn influence swamp development and encroachment. Examples are given of lagoon dynamics related to each of these factors, and to recent man-induced changes.

Introduction Coastal lagoons may be defined as areas of relatively shallow water that have been partly or wholly sealed off from the sea by the formation of depositional barriers, usually of sand or shingle, built up above high tide level by wave action. This definition excludes lagoons enclosed by coral reefs,either within atolls or between fringing or barrier reefs and the mainland, because these are essentially marginal marine environments linked with the open sea at high tide. Aspects of the geomorphology of coastal lagoons have been discussed by Day (1951),Emery and Stevenson (19571, Gierloff-Emden (1961,19801,Bird (1967a),Castaiiares and Phleger (19691,Phleger (1969,19811,Colombo (19771, Barnes (19801, UNESCO (1980) and Kjerfve (1986).

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Physical Setting and Geomorphology of Coastal Lagoons

Coastal lagoons, thus defined, are found in a variety of environments, ranging from arctic to equatorial, and from arid to humid (Lasserre, 1979; Guilcher, 1981), and on a variety of scales from over 10,000 km2 (Lagoa dos Patos, Brazil) down to less than a hectare. They may have one or more natural entrances from the sea, which may be permanent or intermittent gaps through the enclosing barriers. In some cases artificial entrances have been cut, usually to facilitate navigation or hasten the discharge of floodwaters to the sea. The definition of a coastal lagoon implies that the entrances are narrow compared with the coastwise extent of the enclosing barriers. The European Wadden Sea, behind the Frisian barrier islands, is not usually considered a coastal lagoon because the combined width of the openings t o the sea (including the broad gap east of Wangeroog) is more than one-third of its coastwise extent: it can be classified an ‘open lagoon’ (Lasserre, 1979). The same is true of the area behind the sandy barrier islands (‘sea islands’) of the coast of Georgia, in the United States. In general, the term coastal lagoon is applied where the width of marine entrances at high tide is less than one-fifth (20%)of the total length of the enclosing barrier (Bird, 1982). In geomorphological terms, coastal lagoons have usually formed where valley mouths or lowlands have been submerged by the sea during the later stages of the world-wide Late Quaternary marine transgression, which on tectonically stable coasts brought the sea up to approximately its present level about 6000 years ago. Some of the coastal inlets and embayments thus formed have been enclosed by the growth of spits across their mouths, or of barriers washed up by wave action. They have a variety of shapes and sizes, related to the configuration of the pre-existing coastline and the enclosing spits and barriers, as modified by internal erosion and deposition. The smallest and simplest lagoons are found where the mouth of a river has been ponded back by a wave-built barrier. Such a barrier may be breached from time to time by storm waves, or when river floods pour over it after heavy rain, but it is usually soon rebuilt by wave action when fine weather returns. Lagoons of this type are common on oceanic coasts, where beaches and barriers have been built across drowned valley mouths by the action of strong swell. Some lagoons are long and narrow, parallel to the coast and separated from the sea by barriers built up in front of the former coastline: the Coorong in south-eastern Australia is an example (Fig. 2.1). Others show a branched configuration, elongated at an angle to the coastline, formed where river valleys have been submerged, then enclosed by a depositional barrier built up across their mouths: an example is Lake Tyers in south-eastern Australia (Fig. 2.2). At Orbetello, on the west coast of Italy, a lagoon has been enclosed between the twin barriers that form a double tombolo, attaching a former island to the mainland (Sacchi, 1979). The largest and most complicated lagoon systems are found where broad embayments have been sealed

E. C.F. Bird

11

Fig. 2.1. The Coorong, in South Australia, is a long narrow lagoon, cut off from Encounter Bay by a sandy barrier. Dunes mobilized after the clearing and burning of the native vegetation cover are spilling over into the lagoon.

Fig. 2.2. Lake Tyers, in south-eastern Australia, shows the branched configuration of incised river valleys partly submerged by the Late Quaternary marine transgression, then enclosed by a Holocene sand barrier. The islets inside the entrance are parts of an inwashed tidal delta.

12

Physical Setting and Geornorphologyof Coastal Lagoons

Fig. 2.3. The Gippsland Lakes, Victoria, Australia, enclosed by an outer barrier along the Ninety Mile Beach, and retaining remnants of earlier (Pleistocene) inner and prior barriers. Inset: section through the barriers.

off from the sea by successive depositional barriers, as in the Gippsland Lakes in south-eastern Australia (Fig. 2.3). Geomorphological studies of the Gippsland Lakes (Bird, 1978, 1983) indicated the range of factors that must be taken into account in dealing with the evolution and dynamics of coastal lagoons. It is necessary to seek evidence of the geological and geomorphological history of a n area, and the sequence of changes in the levels of land and sea which have resulted in coastal submergence, forming the inlets and embayments. The growth of coastal barriers must then be explained in terms of the effects of waves, tides and currents in delivering sediment, shaping the depositional morphology, and determining the position and dimensions of entrances from the sea. Once formed, coastal lagoons are modified by erosion and deposition. Infilling by accumulation of inwashed sediment, organic deposits such as peat or shells, and precipitated material, results in the shallowing and shrinkage of lagoons. It is possible to relate changes in configuration to the effects of wind-generated waves and the currents produced by rivers, wind action and tides within the lagoon. Ecological conditions, particularly water salinity and temperature, are important in the geomorphological evolution of coastal lagoons, for they control the extent to which vegetation can colonize lagoon shores, impeding erosion, promoting patterns of sedimentation, and generating organic deposits.

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These various processes have generally reduced the areas of many coastal lagoons since their original enclosure within the past few thousand years. Some have disappeared completely within aggraded coastal plains, as in the Anzio district in Italy. On parts of the Gulf and Atlantic coasts of the United States, enclosing barriers have been driven landward to narrow, and in places extinguish, the lagoon. Elsewhere, marine erosion has cut back the seaward margins of stationary barriers, such as the Ninety Mile Beach in south-eastern Australia, and some lagoons have been reopened as marine embayments following the breaching of enclosing barriers: Robe Bay and Guichen Bay in South Australia have been cited as examples of this (Bird, 1984). Coastal lagoons are thus transitory features, subject to relatively rapid geomorphological evolution and presenting dynamic environments. Lagoons are most extensive on low-lying coasts. They are poorly developed on coasts dominated by high retreating cliffs, as in the Great Australian Bight; on the steep and rocky fiord coasts of Norway, British Columbia, Chile, and southern New Zealand; on the ice-girt Antarctic and Greenland coasts; and on the rapidly-emerging coasts of northern Canada and the Gulf of Bothnia. They are also rare on macrotidal coasts, such as the Bay of Fundy in Canada or the Baie de Mont Saint Michel in France. Coastal lagoons around the world show great contrasts, but the same processes have operated in similar situations. It is possible to explain contrasts between lagoons in terms of their geomorphological history and the influence of geological, climatic, hydrological and ecological factors, taking account of the effects of human activities. It would be premature to attempt a comprehensive analysis of coastal lagoon evolution and dynamics when so many lagoons have been so little studied, but it will be useful at this stage to describe and illustrate the various factors that have influenced coastal lagoon geomorphology, and the processes at work in coastal lagoons (Table 2.1). Geological Background

The evolution of the world’s coastline has been much influenced by changing levels of land and sea, bringing marine processes to work upon the great variety of rock formations that outcrop in coastal regions. The physical setting of coastal lagoons is the outcome of the submergence of continental margins during Late Quaternary times, when a world-wide marine transgression accompanied the Late Quaternary amelioration of global climate and the consequent melting of snowfields, glaciers and ice sheets and glaciers in polar and mountain regions. The immediate result was the submergence of valley mouths to form inlets and estuaries and of wider lowlands to form embayments between promontories of higher ground. This marine transgression was the last of a series of oscillations of sea level through Pleistocene times, when the sea reached higher levels and submerged the coast during interglacial phases, and fell away to lower levels,

14

Physical Setting and Geomorphology of Coastal Lagoons

TABLE 2.1 Factors influencing the distribution and dynamics of coastal lagoons

Antecedent geomorphology The configuration of the coastline and the immediate hinterland determine first the extent to which barrier formation can occur, and then the initial shape of the enclosed lagoons. Coastal lagoons are rarely found on straight, steep coastlines, even where there were valley-mouth inlets, but some indented cliffed coastlines have developed valley-mouth lagoons as the result of barrier formation nourished by the erosion of intervening promontories (e.g. New England coastline). Most coastal lagoons are found on low-lying coasts where Holocene marine submergence has been accompanied by the generation of barrier formations in front of straight or indented coastlines. Materials for barrier formation Barriers enclosing lagoons are typically formed by longshore drifting of sediment derived from the adjacent coastline (e.g. from eroding cliffs or material delivered to the mouth of a river) and/or by shoreward drifting of material derived from the sea floor. Barriers are more likely to be formed where the coastal, hinterland and nearshore geology includes material that is unconsolidated, o r readily disintegrated or decomposed into beach-sized sediment (sand or gravel); they are less likely where these areas have massive rock outcrops, or where the sediments or weathering products are fine-grained (silt, clay). Sediment supply to enclosed lagoons Once enclosed, lagoons are re-shaped by erosion and deposition around there shores, by the accumulation of material washed or blown over the enclosing barriers, and by the accretion of in-flowing river sediment. These accumulating sediments result in shallowing and shrinkage of coastal lagoons, and their eventual natural reclamation as infilled plains.

Tectonics Lagoons developed on coastlines that are rising tectonically will become shallower, and may eventually give place to plains formed by emergence and accretion. Where tectonic subsidence is continuing, lagoons will be maintained, and may increase in depth and area, with the possibility of breaching and removal of the enclosing barriers t o reopen coastal inlets and embayments. Tide range Where the tide range is relatively large, strong transverse currents will develop through entrances maintained as barriers form. For this reason, coastal lagoons on macrotidal coasts remain as 'open lagoons'with persistent tidal entrances between barrier islands, and remain subject to tidal ventilation. Such features as 'tidal deltas' and radiating ebb and flood scour channel systems are characteristic of the entrances t o such lagoons. Where the tide range is small, tidal entrances are less likely t o be maintained, and barriers may become complete, excluding tidal influences from the enclosed lagoons. Climate In arid regions, where evaporation exceeds rainfall, coastal lagoons tend to become hypersaline, and may dry out as evaporite plains. In more humid regions, rainfall and river inflow reduces salinity, and in well-enclosed systems may become entirely fresh. Salinity regimes influence the nature and pattern of sedimentation (fine-grahed sediments tend to flocculate in brackish water) and ecological conditions(salt marshes and mangroves in the more brackish areas pass laterally to reed and rush vegetation where the water is relatively fresh. This,in turn influences patterns of sedimentation and swamp encroachment on lagoon shores.

E.C.F. Bird

15

so that the continental shelves emerged, during each glacial phase. The Late Quaternary marine transgression began about 18,000 years ago,

and brought the sea up to approximately its present level about 6,000 years ago, but the sequence of relative changes of sea level in Holocene times around the world’s coastline has shown variations, partly related to accompanying upward or downward movements of the land margin. Thus the Gulf and Atlantic coasts of the United States have had a continuing gradual rise of sea level relative to the land, whereas elsewhere oscillations have been registered. In southern Australia, South Africa, and south-east South America, for example, there is evidence of an episode of slightly higher Holocene sea level, followed by an emergence. The outcome is that the barriers enclosing coastal lagoons on the southern and eastern seaboards of the United States are in many places transgressive, the lagoons having a long history of landward migration, whereas in Brazil the barriers are more stable, and lagoons such as the Lagoa de Araruama are bordered by emerged shore features. Where coastal uplift has continued, as in northern Scandinavia, coastal lagoons are rare; where they persist, as at Brusand in Norway, they show emergent features (Bird and Klemsdal, 19861, but generally there are stairways of beach ridges rather than barrier lagoons. Most coastal lagoons post-date the Late Quaternary marine submergence, but it is possible that some formed originally during earlier episodes of submergence in Pleistocene interglacial phases, when barriers were built across the mouths of coastal inlets and embayments. In South Australia, the Coorong is the latest in a series of long, narrow lagoons, enclosed by successive barriers on a coast that has been uplifted during Quaternary times. Its predecessors are marked by tracts of lagoon and swamp (now largely drained and reclaimed) lying between successive emerged sand barriers in the country behind Encounter Bay. Lagoons that were enclosed by Pleistocene barriers drained out during the ensuing Last Glacial low sea-level phase, leaving subaerial basins that were re-flooded when the sea rose again. The Gippsland Lakes are an example of a lagoon system that drained and revived in this way. The majority of coastal lagoons, however, are simply the product of barrier and spit deposition across inlets and embayments that formed in Holocene times. Climatic and Hydrological Factors Coastal lagoons exist in several climatic zones. In polar regions they are frozen in winter and thaw in summer. Rapid summertime erosion of tundra bluffs, which consist largely of morainic or glacifluvial debris, has provided abundant material for the formation of spits and barriers, as in Alaska. In the humid temperate zone coastal lagoons are freshened by rainfall and runoff from their catchments, and typically have estuarine regimes. There are seagrasses on mudflats exposed at low tide and salt marshes

16

Physical Setting and Geomorphology of Coastal Lagoons

passing into freshwater reed and rush swamps in the less brackish parts. In the humid tropics, lagoons receive large quantities of fluvial water and sediment, especially clay and silt, and are generally turbid, with much sediment in suspension. The fringing vegetation includes mangroves as well as salt marshes. It should be noted that mangroves actually extend well into the temperate zone in the southern hemisphere: to southern Brazil, south-eastern Australia, and the Bay of Plenty in New Zealand. In arid areas, whether temperate or tropical, lagoon salinity increases, and may become hypersaline, with salinity exceeding that of sea water (i.e. about 35 parts per mille). Apart from these climatic factors, the hydrological characteristics of a coastal lagoon are determined partly by its configuration and the nature and dimensions of entrances from the sea, and partly by the balance between precipitation, fresh-water inflow from rain and rivers, and evaporation on the one hand, and salt water inflow from the sea, related to tide range and tidal ventilation of the lagoon, on the other. Lagoons with entrances that are permanently open, permitting unrestricted inflow of sea water and outflow of river floods, are essentially estuarine, with fresh water from rain and rivers meeting and mixing with salt water from the sea. Typically there is a transverse salinity gradient from sea water at the entrance t o fresh water at the mouths of inflowing rivers, as in the Peel-Harvey Inlet in Western Australia (Hodgkin, 1981). Lagoons with more restricted or temporary entrances are less influenced by tidal movements and more protected from the effects of waves from the open sea. Geomorphologically they may resemble inland lakes. In New South Wales, for example, Lake Macquarie is essentially a marine lagoon, with almost tideless shores, a barrier that excludes ocean waves, and water salinity similar to that of the open sea, except in small areas of dilution near the mouths of inflowing streams. Tidal influences are also much reduced in lagoons where the entrance is in the form of a long, winding channel through the enclosing barrier, as in the Myall Lakes, also in New South Wales (Thom, 1965). Usually the water is fresh or slightly brackish, but during droughts salinity increases as sea water spreads in along the connecting channel. Some coastal lagoons have been sufficiently cut off from the sga by enclosing impermeable barriers to become fresh water lakes. Slapton Ley, in south-west England (Fig. 2.41, is an example (Mercer, 19661, while the Murray-mouth lagoons in South Australia (Fig. 2.5) have become fresh after their entrances were artificially sealed by barrages (Bird, 1962). In New Zealand, the Waitangitaona River changed its course during a flood in 1967 to flow to the mouth of Okarito Lagoon, which was thus freshened. The cutting or enlarging of an entrance to a lagoon that was previously fresh or slightly brackish results in a salinity increase that can have geomorphological as well as ecological consequences. This has happened in the Gippsland Lakes (see below), and two West African lagoons: Lake

E.C.F. Bird

17

Fig. 2.4. Slapton Ley, a freshwater lagoon fringed by reedswamp behind a coastal barrier of sandy gravel on the coast of south-west England.

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Fig. 2.5 (Oppositepage).The Murray-mouth lagoons, separatedfromEncounter Bay by a sandy barrier. Artificial barrages built to prevent sea water entering through the five natural entrances were completed in 1940, and Lakes Alexandrina and Albert have since become freshwater systems, fringed by reedswamp.

Nokou6 in Benin, after the opening of a marine entrance at the port of Cotonou, and the Ebri6 lagoon on the Ivory Coast aRer the cutting of the Vridi Canal through the enclosing barrier in 1950 (Hinschberger, 1985). Reduction of runoff from rivers by reservoir construction or soil conservation schemes in the hinterland can also increase salinity in coastal lagoons, and may have contributed to the recent rise in salinity in the Gippsland Lakes. In drier regions, lagoons completely cut off from the sea are less likely to become fresh because high evaporation increases their salinity. If they lose more water by evaporation than they receive from rainfall and runoff, they become hypersaline, and may dry out altogether as saline flats. The Laguna Madre in Texas (Hedgpeth, 1967) and the Coorong in South Australia (Brown, 1969) are notable examples, and there is an evaporite lagoon behind Israelite Bay, on the west coast of the Great Australian Bight. On the arid coasts bordering the Red Sea and the Arabian Gulf lagoons partly enclosed by barrier spits have become hypersaline (i.e. more saline than the sea),and some have dried out as salt flats, termed sebkhas, with extensive, often chemically zoned, evaporite deposits (Evans and Bush, 1969). Similar features are found bordering King Sound on the arid north-west coast of Australia. Evolution of Coastal Barriers The evolution of spits and barriers has been the subject of a n extensive geomorphological literature (e.g. Schwartz, 1972, 1973; Leatherman, 1979; Zenkovich, 1969). They have formed by the deposition of sediment derived from hinterland, alongshore or offshore sources and delivered to the coast. On the global scale, these depositional features are mainly built of sand, but some consist of gravel, or mixtures of sand and gravel; most beach gravels consist of well-rounded water-worn pebbles, termed shingle. Spits may grow along the coast to become barriers enclosing lagoons, but many barriers have developed as the result of shoreward movement of sediment accumulated from the sea floor. Some have formed by a combination of longshore and onshore sediment supply. In arctic environments, periglacial processes and the erosion of tundra bluffs provided material for the building of extensive barriers that enclose lagoons along the shores of the Beaufort Sea in Alaska (Shepard and Wanless, 1971) and Svalbard in Norway (Klemsdal, 1986). In south-east Iceland, shallow lagoons have been impounded by barriers of sand and shingle built up by waves working across glacio-fluvial outwash deposits produced by recurrent volcanic melting of hinterland glaciers.

20

Physical Setting and Geomorphology of Coastal Lagoons

Spits and barriers formed of material eroded from cliffs cut in glacial drift deposits enclose lagoons on the shores of Puget Sound: the intricate Dungeness Spit in Washington (Schwartz et al., 1987) is an example, and there are others on Rhode Island, in Massachussets, on the southern shores of the Gulf of St. Lawrence, as well as in equivalent parts of the coastline of northern Europe, notably the southern Baltic and the Danish archipelago, and eastern Asia, particularly on Kamchatka and Sakhalin. Erosion of the soft sandy cliffs on the southern shores of Point Reyes in California has contributed to the barriers that enclose Drake’s Estero and the Limantour lagoons (Shepard and Wanless, 1971). On the South Island of New Zealand, Okarito Lagoon is enclosed by gravelly barriers built by waves re-working partly submerged and locally cliffed deposits of glacifluvial outwash, supplemented by similar material delivered to the coast by rivers during floods. Fluvial sediment has supplied sands and gravels to build spits that have grown alongshore to enclose lagoons on deltaic shores: especially on the Ebro, the Danube and the Nile deltas. Mixed with sediment carried in from the sea floor, fluvial sands have built barriers impounding lagoons around the Caribbean, notably in Venezuela, Honduras, and Mexico (Lankford, 19761, and along the Gulf coast from Texas to Florida (Shepard, 19601. On the shores of the Azov Sea barriers are composed of molluscan shells washed in from the sea floor, and in the Caspian Sea similar formations have been built from marine oolites washed onshore (Zenkovich, 1969). Sediment from the sea floor has also been built into barriers and spits on the shores of the Red Sea and the Arabian Gulf. On oceanic coasts, sand washed in from the continental shelf is prominent in barriers enclosing lagoons. There are many examples on the southern and eastern coasts of Australia, from Perth around to Fraser Island, on the Cape Coast in South Africa, the Gulf of Guinea coast in West Africa, the coastlines of Brazil, southern India and Sri Lanka, and the north coast of Hokkaido. On Chatham Island, east of New Zealand, a group of rocky islands has become linked by barriers of sand washed up from the sea floor in such a way as to enclose Te Whanga lagoon (Fig. 2.6). Often the sediment washed in from the sea floor is mixed with material derived from cliff erosion or river yields. On the coast of New South Wales north from the Hunter estuary at Newcastle, the inwashed barrier sands have been augmented by fluvial sediment delivered to river mouths and distributed alongshore (Bird, 1967a). Most of these spits and barriers have been built during and since the world-wide Late Quaternary marine transgression They attained their present outlines within the past 6000 years, when on most parts of the world’s coastline the sea has been close to its present level relative to the land. Longshore growth of spits has often occurred in stages marked by successive terminal recurves, and spits that have grown into barriers may retain relics of such recurves protruding into the lagoon, as on the long

E.C.F. Bird

21

I

A

I? O

B

m

M

kn

C Fig. 2.6. The evolution of Te Whanga lagoon, Chatham Island, New Zealand, as the result of the formation of sandy barriers between islands.

barrier spit known as the Langue de Barbarie on the West African coast, which has deflected the mouth of the Senegal River 30 km southward in front of an elongated lagoon (Guilcher and Nicolas, 1954). Similar recurves are seen behind the barrier spits that have grown northward on the west coast of Sri Lanka to enclose lagoons at Negombo, Chilaw and Puttalam (Swan, 1982).

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Physical Setting and Geomorphology of Coastal Lagoons

Many barriers and barrier island chains originated on alignments that lay off earlier coastlines, enclosing the intervening water areas as lagoons. Such barriers may have been initiated by coastal emergence, when wave action in shallowing water began to re-shape the sea floor in such a way as to pile up sand or gravel on an alignment some distance off (and parallel to) the former coastline. Offshore barriers appeared in this way off parts of the northern Caspian coast during the phase of falling sea level that occurred between 1930 and 1977 (Leontiev et al., 19781, but coastal emergence is not essential for barrier initiation. Providing nearshore waters are shallow, and there is a n abundance of unconsolidated sediment on the sea floor, wave action can build up a barrier while sea level remains stable, or even during a slow marine transgression. On the Atlantic coast of the United States the Outer Banks of North Carolina, enclosing the broad lagoon known as Pamlico Sound, formed in this way, as did the similar barrier islands on the Texas coast, fronting lagoons such as Galveston Bay, Matagorda Bay and the Laguna Madre. The barriers on the Gulf and Atlantic coasts of the United States were initiated when the sea stood 5 to 10 m below its present level relative to the land, and have been built upward and driven landward during the ensuing submergence (Kraft and John, 1979). The shingle barrier of Chesil Beach in southern England (Fig. 2.7) formed during the Late Quaternary marine transgression, and has subsequently migrated landward in such a way as to prevent waves from the open sea reaching the mainland coast, so that the gentle slopes of the embayed shoreline behind the Fleet lagoon lack marine clifing (Steers, 1953). The Mehechkyn shingle barrier on the Soviet Bering Sea coast is of similar origin (Zenkovich, 1969). Parts of the sandy barriers on the Gulf and Atlantic coasts of the United States are still moving intermittently landward as the outcome of storm ovenvash and the drifting of dunes driven by onshore winds. In places they consist largely of coalescent ovenvash fans formed by successive storm surges and trimmed back by wave action along the seaward margin (Leatherman, 1979, 1981). Elsewhere, the barriers have remained in position parallel to the pre-existing coastline, and some have been widened seaward by progradation resulting from the arrival of more sandy sediment on their shores. On Galveston Island in Texas and St Vincent Island in Florid&such stages in progradation are marked by successively built dune ridges. On the Pacific coast of Mexico the Nayarit barrier enclosing Laguna Agua Brava has prograded to form a beach ridge plain up to 15 km wide (Curray et al., 19691, and similar accretion has taken place on the barrier fronting Laguna Guerrero Negro in Sebastian Vizcaino Bay to the north (Phleger, 1969). In south-eastern Australia a history of intermittent barrier progradation is shown by the numerous successively-formed parallel dune ridges behind the Ninety Mile Beach (Bird, 1978). The Gippsland Lakes (Fig. 2.3) were

E.C.F. Bird

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Canal cut 1908-09, sed 1915-30 and reopened with sluice

KILOMFTRES

I

Fig. 2.7. Ringkobing Fiord, a lagoon on the west coast of Jutland, Denmark. Had a natural outlet which migrated southward. An artificial canal has been cut through the enclosing barrier at Hvide Sande.

24

Physical Setting and Geomorphology of Coastal Lagoons

first enclosed by a spit prolonged across the mouth of a n embayment, which was then widened by accretion of sand on the seaward side. This became a n inner barrier when a chain of barrier islands which subsequently formed a mile or so offshore were linked to form a n outer barrier, backed by a long, narrow lagoon (Lake Reeve), and bordered on the seaward side by the Ninety Mile Beach (Bird, 1978). The inner barrier is largely a n inheritance from Late Pleistocene times, and the outer barrier is of Holocene age, constructed as the Late Quaternary submergence came to an end. Borings through these coastal barriers have shown that they consist of unconsolidated sediment, banked against a partly submerged Pleistocene land surface (inset, Fig. 2.3) (Thorn, 1984).In the absence of cliffs and rivers opening to the sea, the barrier sands have been derived from the sea floor. The Gippsland barriers are not being driven landward in the manner of shingle barriers such as Chesil Beach on the south coast of England; once formed, they have remained in position, the seaward margin prograding when the waves deposited sand and retrograding during phases of marine erosion. The shores bordering the Gippsland Lakes show promontories of various kinds, some of which were terminations of recurved spits, while others formed where mobile dunes spilled over into the lagoon, or where sand was washed through low-lying parts of the barrier by stormy seas or exceptionally high tides. Waves and currents generated within the lagoon have modified the configuration of these in-facing barrier shores. The small tide range (less than 2 m) on much of the coast of south-eastern Australia has facilitated the formation of such barriers. Large tides generate strong ebb and flow currents which maintain tidal entrances through barriers; smaller tides produce weaker currents, the effects of which are more easily overcome by constructive wave action, so that lagoon entrances are deflected or sealed off by sand deposition. Barriers of Pleistocene age occur in association with many coastal lagoons. The barrier of calcareous sand that lies seaward of the Coorong lagoon on the coast of Encounter Bay in South Australia is the last of a series of Pleistocene barriers which are preserved as uplifted calcarenite ridges in the hinterland. The origin of the outermost barrier is here the outcome of partial submergence, dissection and re-working of earlier calcarenite barriers that existed seaward of the present coastline (Bird, 1973). Relics of Pleistocene barriers also occur in lagoons on the Zululand coast in south-east Africa (Orme, 1972; Hobday, 19751, in the depositional lowlands around Lagoa dos Patos in southern Brazil (Delaney, 1963), and interspersed with lagoons and swamps on the coasts of West Africa (Le Bourdiec, 1958), Pacific Central America (Gierloff-Emden, 19611, and Sardinia (Guilcher, 1983). This wide distribution of relict Pleistocene barriers indicates that many lowland coasts have had similar histories of submergence by marine transgressions, formation of barriers enclosing lagoons, emergence when the

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lagoons drained out and the barriers were dissected, and renewed phases of submergence and outer barrier development culminating in the Holocene. Most of the barriers that now enclose coastal lagoons have been shaped in Holocene times, although they often incorporate re-worked Pleistocene sediments, and are in many cases underlain by older barrier sediments that date from the Pleistocene. Changes may continue on the spits and barriers that enclose lagoons (Bird, 1982). Some spits are still growing, reducing the dimensions of a marine entrance to a coastal lagoon, or causing that entrance to migrate alongshore. Spit growth may result in a lagoon becoming sealed off from the sea, at least temporarily. Progradation of enclosing barriers has little effect on coastal lagoons, except to increase their separation from the sea, but in recent decades the seaward margins of most barriers have been retreating because of a prevalence of marine erosion (Bird, 1985a). As the barriers become narrower there is an increasing chance of sediment being swept across them by ovenvash during storm surges. Blowouts that form in vegetated dunes grow landward under the influence of onshore winds, and may eventually spill over the inner shore into the lagoon (Fig. 2.1).Such erosion can be followed by the breaching of new marine entrances, and if erosion continues the barrier may be completely removed, re-opening the lagoon as a coastal inlet or embayment. Alternatively, the barrier may be driven landward by ovenvash processes and dune drifting, so that it encroaches upon the lagoon, which is thus narrowed, subdivided and eventually extinguished as the barrier sand or shingle comes to rest against its inner or landward margin as a fringing beach. Chesil Beach, on the shores of Lyme Bay in southern England, has been driven landward in such a way as to overrun lagoon marsh deposits, which now outcrop on its seaward side (Bird, 1972). South-east of Abbotsbury Chesil Beach is still a barrier in front of the Fleet lagoon, but to the north-west it has reached the mainland, and forms a beach extending past residual lagoon segments (e.g. Burton Mere) to the cliffs of West Bay. Chesil Beach, up to 14.7 m high, is still occasionally overwashed by storm surges, and comparison of surveys made in 1852 and 1969 indicated that landward movement had continued near the south-eastern end (Carr and Gleason, 1972). Lagoon Entrances

Some lagoons have been completely cut off from the sea by barriers, but most have at least one marine entrance. These are sometimes called tidal inlets, but this term is inappropriate where the tide range is very small, as in the Baltic region. The entrance to a coastal lagoon may be bordered by a spit or paired spits, as in the estuarine lagoons on the south coast of

26

Physical Setting and Geomorphology of Coastal Lagoons

England (Robinson, 1955); or there may be several entrances separating barrier islands, as in the Dutch, German and Danish Wadden Sea. Some lagoon entrances are residual gaps that have persisted between spits or barrier islands where the lagoon has never been completely sealed off from the sea. Others are the outcome of breaching, either by storm waves or floodwaters spilling out of the lagoon. The Outer Banks of Carolina have had a long and complicated history of breaching, enlargement, reduction and closure of tidal inlets to Pamlico Sound (Dunbar, 1956). Many lagoon entrances are now artificial, having been excavated and maintained, usually between bordering breakwaters. The configuration of a lagoon entrance is the outcome of interactions between waves from the sea and inflowing tides, which tend to wash sediment into the gap, reducing it and eventually sealing it off, and currents generated by tidal ebb and the discharge of floodwaters from the lagoon, which tend to keep it open (O’Brien, 1969). Currents are generated in several ways. There are tidal currents produced by tides entering and leaving the lagoon, their strength increasing with tide range; there are currents due to outflow from rivers, particularly after heavy rain, when floods build up the level of the lagoon so that water pours out through the entrance; and there are currents generated by wind action when onshore winds drive sea water into the lagoon, or offshore winds drive lagoon water out through the entrance. The velocity of these currents is related to the external tide range, the ratio of lagoon volume to cross-sectional area of the entrance, and the configuration of the lagoon and its entrance. Tidal inlets are larger, more numerous, and more persistent on barrier coastlines where relatively large tide ranges generate strong currents, as on the German North Sea coast, than where tidal action is weak, as on the long sandy barriers which fringe the southern shores of the Baltic Sea. Lagoon entrances often show seasonal variations, being shallowed or sealed in dry seasons when outflow is weak, and reopened, widened or deepened when the wet season brings greater volumes of water outflow from the lagoon. This sequence is well known on the Zululand coast, where most lagoons are sealed off by sand deposition in the relatively dry winter season (May through August) and entrances are reopened in the summer when rains in the hinterland increase fluvial discharge into the lagoons until t4e water spills over into the sea. In south-eastern Australia lagoon entrances are usually reduced in size or sealed off altogether in dry summer periods, then reopened or enlarged in the wetter winters. When a barred entrance has persisted for several seasons, local people may artificially reopen it by digging an outflow channel, which widens and deepens as the head of water disperses. Lagoon entrances are generally backed by partly or wholly submergent shoals or fans of inwashed sediment, termed thresholds or ‘tidal deltas’. Examples have been documented from the South Coast of New South Wales

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(Bird, 1967b)and the entrance to the Murray-mouth lagoons (Bourman and Harvey, 1983). Such features are less common on the seaward side, where stronger wave action disperses outwashed sediment offshore or alongshore. Some lagoon entrances remain in position; others migrate to and fro alongshore in response to interactions of waves and currents (Bruun and Gerritsen, 1961).Persistent lagoon entrances are often found alongside rocky headlands: an example of this is seen on the north coast of New Zealand where the entrance to the lagoon at Tauranga, behind a sandy barrier (Matakana Island), is alongside the promontory of Mount Maunganui (Healy, 1977). Others are in the lee of islands or reefs where wave action is weakened and the ebb and outflow currents are sufficient to maintain a gap. The entrance to Lake Illawarra is protected by Windang Island immediately offshore, while the entrances to several lagoons on the South Coast of New South Wales are situated at the southern end of sandy bays, close to rocky headlands, where the dominant southeasterly ocean swell is much refracted, and therefore weakened (Bird, 1967b). Migration of lagoon entrances has occurred on many barriers, and is well illustrated on the Danish North Sea coast where the barrier enclosing the lagoon known as Ringkobing Fiord (the term fiord includes lagoons in Danish terminology) has had an entrance in various positions since 1650, with a tendency to migrate southward (Fig. 2.8). This variable entrance has been replaced by an artificial canal at Hvide Sande, maintained between stone jetties: a sluice was added in 1931 t o reduce sea water inflow. In New Jersey curving channels lead from lagoons into the rear of the sandy barrier towards entrances that have been deflected along the coastline by longshore drifting of sand, and in the Gippsland Lakes similar curved channels lead towards the sites of tidal entrances that have since been sealed off by sand deposition. When explorers arrived at the Gippsland Lakes in the eighteen-forties they found a small natural entrance at Cunninghame Arm near the eastern end of the lagoon system, which was sealed in calm weather, when onshore and longshore drifting of sand overcame the effects of transverse currents. It used to remain closed until heavy rains flooded the rivers and raised the level of the lakes so that water spilled out over the barrier. The difficulties of navigating so variable a passage led t o a local demand for an artificial entrance, and in 1889 a gap was cut through the outer barrier a t what is now Lakes Entrance. This gap, bordered by stone jetties, is a permanent artificial entrance, maintained by currents; a looped sandbar offshore testifies to the efforts of wave action to seal it off. After 1889, the natural entrance fell into disuse, and is now permanently closed off. In the Venice region, entrances to the lagoon have been artificially stabilized by breakwaters up to 2 km long, the intervening barrier islands having been armored by large sea walls (Zunica, 1987). Lagoon entrances thus show a variety of features. At one extreme is the

28

Physical Setting and G'eomorphology of Coastal Lagoons

Fig. 2.8. Chesil Beach, a shingle barrier on the shores of Lyme Bay, southern England, borders the Fleet lagoon. Inset as sea level rose, this barrier was driven landward by overwashing storm surges.

permanent entrance, natural or man-made, which allows a perennial unhindered exchange of water, sediment, dissolved materials and organisms between the lagoon and the open sea. The lagoon then shows estuarine characteristics, such as a transverse salinity gradient declining towards the mouths of inflowing streams and an inwardly diminishing tidal ventilation. Tide range within a lagoon diminishes rapidly away from an entrance, the more remote sections of large lagoon systems being unaffected by marine tides. In the Gippsland Lakes the range of spring tides at Lakes

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29

Entrance is slightly less than a meter, but at Metung it is less than 30 cm, and tides are not perceptible in Lake Victoria and Lake Wellington. There are, however, irregular changes of level due to heavy rain or river flooding, and marked oscillations during and after periods of strong wind. Similar variations have been observed in other coastal lagoons. At the other extreme is the completely enclosed lagoon with a n impermeable barrier preventing exchanges with the sea. As has been noted, such lagoons tend to become fresh water in humid environments (Slapton Ley), and hypersaline in arid regions (Laguna Madre). However, shingle barriers are not impermeable: sea water forced through Chesil Beach into the Fleet lagoon during high tides and stormy periods maintains relatively brackish conditions (Bird, 1972). Between these extremes are many lagoon entrances that vary in form, dimensions and location, and are sometimes completely sealed off. Such variations modify tidal ventilation, flood water levels, marine incursion, salinity regimes, and related ecological conditions, within the lagoon system, and can influence patterns of sedimentation and geomorphological change. Sedimentation in Lagoons Sediment is carried into coastal lagoons by rivers, by tidal currents entering from the sea, and by winds. Deposits in lagoons also include material of organic origin, such as shells, guano and peat, and in arid regions chemically precipitated salt, calcite and dolomite (Aldermann et al., 1957). In one way or another, most lagoons are being gradually filled in and replaced by depositional coastal plains. Lagoons fed by rivers receive sediments ranging from coarse sand to silt and clay. The coarser material is deposited as the river enters the lagoon, and may be added to lagoon beaches and spread around the shore by wave action; the finer sediment is carried out into the lagoon and deposited on the floor, progressively reducing the depth. Brackish water is a n electrolyte which coagulates and precipitates fine-grained sediment, so that many lagoons show a transition from turbid fresh water to clearer brackish water towards the marine entrance. Rates of fluvial sediment yield to lagoons may be accelerated by the reduction of vegetation cover and the onset of soil erosion in the river catchment. Deforestation and agricultural development in the catchment of the Citanduy River in southern Java greatly increased the flow of sediment from that river into the Segara Anakan lagoon, which is rapidly silting (Bird and Ongkosongo, 1980). On the other hand, there could be a reduction of sediment yield to a coastal lagoon where a dam constructed on a n inflowing river is intercepting sediment in the reservoir, or by successful soil conservation works in the hinterland.

30

Physical Setting and Geomorphology of Coastal Lagoons

In the vicinity of tidal entrances, sand washed in from the sea forms thresholds or tidal deltas exposed at low tide, and on the inner shores of the barrier, sand blown across, or washed over in storms or very high tides, forms fans of sediment, which are spread along the shore by waves. Sediment also circulates within a lagoon system, material eroded from some parts of a lagoon shore being deposited elsewhere on prograding sectors, or on the lagoon floor, while a certain amount may be carried out to sea through the entrance by outflowing floods or ebbing tides. Subsidence of coastal regions, as in the northern Adriatic or along the Gulf and Atlantic coasts of the United States, may deepen and maintain coastal lagoons, delaying their infilling. Cavazzoni (1983) concluded that recent erosion on the floor of the Venice lagoon was due partly to subsidence, and partly to changes in water circulation. On the north coast of New Guinea the 1907 earthquake in the Torricelli Ranges resulted in local subsidence on the coastal plain and the formation of a new lagoon a t Sissano (Bird, 1985b). Patterns of sedimentation are thus related t o sources of sediment and processes at work in and around lagoons. In the Gippsland Lakes relatively coarse sediment is found on the bordering beaches and in channels leading from the river mouths towards the tidal entrance, while finer sediment occupies intervening areas of the lagoon floor less subject t o wave and current turbulence. Deltas built of silt and clay project into the lagoon at the mouths of rivers, those of the Latrobe, Avon and Tambo Rivers being cuspate in form, while the Mitchell delta, built into the more sheltered water in the northern part of Lake King, consists of elongated ‘silt jetties’. The rivers that have built these deltas all show a marked seasonal regime, the deltas being channel-side levees built up and prolonged into the lagoon by sedimentation during floods. Other lagoons have different sedimentary patterns. In some the sediment deposited by inflowing rivers is dominant, and the lagoon will eventually become a deltaic plain. In others sedimentation from the seaward side has been more rapid, and the lagoon will eventually become a coastal plain of marine deposits.

Swamp Land Encroachment Shore vegetation has a strong influence on patterns of sedimentation in coastal lagoons. Salt marshes and mangrove swamps border the more tidal and saline shores, particularly in the vicinity of entrances, and sediment carried into these halophytic communities on the rising tide is filtered out by the vegetation and retained after the tide has fallen. Mangrove swamps are better developed in less enclosed lagoons,because of the greater tide range and wider intertidal zone. Swamp land encroachment initiated by mangroves

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31

has been described from Lagos lagoon in Nigeria (Webb, 1956) and the Segara Anakan, an estuarine lagoon in southern Java (Bird and Ongkosongo, 1980). Salt marshes can also spread in from tidal lagoon shores. In less brackish water salt marshes give place to reeds, rushes, and sedges, which can grow in water up to 1.5 m deep, and promote sedimentation by trapping silt and floating debris and contributing organic matter so that new land is built up. Swamp land encroachment has been an important process on the shores of the Gippsland Lakes, where it is accompanied by a vegetation succession, the bordering reedswamp being invaded by swamp scrub communities, and then swamp forest, as the substrate aggrades (Fig. 2.9). Swamp land encroachment is only possible where ecological conditions are favorable. If the reeds and rushes are not present, sedimentation near the lagoon shore is reduced, and the lagoon becomes shallower instead of shrinking. Strong waves or current scour impede reedswamp encroachment, the reed fringe becoming broader on sheltered parts of a lagoon shore and thinning out where exposure is greater. Reedswamp can be reduced by cutting, or damaged by boat scour or water pollution, but the limiting factor in coastal lagoons is usually salinity: the reed fringe is best developed in fresh water around river mouths, and thins out, disappearing towards marine entrances as the lagoon becomes more saline.

Fig. 2.9. Swamp land encroachment on the shores of the Gippsland Lakes (above)gave place to erosion when the reed fringe died back (below)as the result of increasing salinity following the opening of an artificial entrance to these coastal lagoons.

32

Physical Setting and Geomorphology of Coastal Lagoons

Changes resulting from the modification of a lagoon entrance have been documented from the Gippsland Lakes (Bird, 1978). In a humid temperate environment, and fed by five substantial rivers and several smaller streams, these lagoons were relatively fresh, and their shores were extensively fringed by encroaching reedswamp, until an artificial entrance was cut (Bird and Lennon, 1989). Once established, this permanent entrance allowed both rapid outflow of river floodwaters in wet periods and unrestricted inflow of sea water in dry seasons. Over subsequent decades, salinity increased in the Gippsland Lakes, and changes have taken place. The reedswamp fringe has largely disappeared, surviving only in relatively freshwater areas close to river mouths, and without its protection the lagoon shores, including the silty deltaic jetties built by inflowing rivers, have been eroded (Fig. 2.9), and sediment removed from them deposited offshore. The change to more brackish water has led to the Gippsland Lakes becoming larger and shallower. By contrast, the coastal lagoons at the mouth of the Murray River in South Australia (Lakes Albert and Alexandrina) were naturally brackish, estuarine systems with several natural entrances from the sea, when Europeans settled this region in the mid-nineteenth century. The climate here is sub-arid, and prolonged droughts occur. Under natural conditions, sea water used to invade the Murray-mouth lakes to compensate the high evaporation losses in dry seasons, and the lagoons became strongly saline. In the droughts that occurred during the nineteen-thirties brackish water spread upstream into the lower reaches of the Murray River, affecting a developing irrigation area. In consequence, barrages were built to seal off the five marine entrances in such a way as to permit the outflow of river floodwaters but prevent marine incursion (Fig. 2.5). Since these were completed in 1940 the Murray-mouth lakes have freshened, and the salt marshes that formerly bordered their shores have been replaced by reedswamp, which is spreading forward into the lagoons and trapping sediment that would otherwise have been dispersed or swept out to sea. The effects of sealing off natural entrances to the Murray-mouth lakes have thus reversed the geomorphological sequence seen in the Gippsland Lakes. Similar changes are taking place in the Kalametiya lagoon in southern Sri Lanka following the building of a sluice at the entrance t o keep out sea water, and the increased inflow of fresh water discharged from irrigated rice fields in the hinterland (Mahinda Silva, 1986). Rounding and Segmentation

As lagoons become enclosed by barriers, waves and currents are generated by winds blowing over their waters. These are determined by the direction and strength of winds and the length of fetch (open water) across which they are effective. Where there is no shore vegetation, the lagoon

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shore may be cliffed by wave attack, and beach sediments drift alongshore, the drifting being strongest where the waves arrive at a n angle of about 45". Sediment moved along the shore is built into spits and bay barriers, rounding and smoothing any initial irregularities in configuration. Zenkovich (1967)illustrated this process from the Shagany lagoon on the Soviet Black Sea coast. Growth of spits into cusps and cuspate forelands also changes the shape of a lagoon, as shown by Fisher (1955)from Koozata Lagoon on St. Lawrence Island, Alaska. The spits may grow to such a n extent that a lagoon which was at first long and narrow becomes divided into a series of smaller, round or oval lagoons, linked by narrow straits or cut off completely. Price (1947) termed this process segmentation, and it was described and analyzed by Zenkovitch (19591,with examples from Russia. Figure 2.10 shows stages in the segmentation of a long, narrow lagoon. With equivalent winds from all directions, the segmented lagoons produced by wave action would become circular; they are more often oval, with a long axis parallel to the direction of the prevailing winds. In the more advanced

Fig. 2.10. Segmentationof a coastal lagoon my the erosion of embayments and growth of spits and cuspate forelands in response to waves generatedby local winds.

Physical Setting and Geomorphology of Coastal Lagoons

34

stages of segmentation, wind-driven currents smooth the curved shorelines of lagoons and maintain connecting straits. Segmentation takes place most readily in tideless lagoons, or in parts of lagoons where the tide range is small, for tidal currents interfere with the wave processes and modify the outlines of growing spits and forelands so that they trail towards, or away from, the point of tidal entry, and the coalescence of opposing spits is prevented. Tidal changes of level prevent continuous wave action at a particular shoreline level, so that a neat adjustment of shorelines to wave resultants is less likely. In the Gippsland Lakes segmentation is shown by the growth of a recurved spit on the eastern shore of Lake Wellington, which has been almost isolated from Lake Victoria, except for the link maintained by currents through McLennan Strait (Fig. 2.3). The oval outline of Lake Wellington reflects the prevalence of westerly winds. Erosion of embayments and growth of intervening cuspate forelands on the shores of Lake Victoria are further signs of segmentation in progress (Fig. 2.11).

0

5 km

Fig. 2.11. Segmentation likely to occur as the result of growth and migration of cuspate spits such as Storm Point (S), bordering Lake Victoria, in the Gippsland Lakes (see Fig. 2.3).

Another form of segmentation occurs where tidal inflow from more than one marine entrance meets behind barrier islands. Deposition in the meeting zone can produce a n intertidal watershed, and eventually a land isthmus segmenting the lagoon, as in the Laguna Guerrero Negro, Mexico (Phleger, 1969). Summary and Conclusions

The geomorphology of a coastal lagoon system thus depends on a range of hydrological, geological, climatic and ecological factors. The configuration is determined initially by the shape of the inlet, embayment or drowned valley formed during submergence, and the shape of the barriers that develop

E.C.F. Bird

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across its mouth. Its subsequent evolution may take one of several courses. If the barrier is complete or the entrance small or impermanent, so that marine influences (ocean waves, tidal movements and salinity) are excluded, the lagoon will develop in the manner of a fresh water lake. Sediment carried in will be deposited on the lagoon floor, and if reedswamp develops around the shores, swamp land encroachment will lead to a contraction of the water area, accompanying the reduction in depth. The shape of contracting lagoons is related to wave conditions, and in turn to the winds that generate the waves on the lagoon, swamp land encroachment being greater on sheltered parts of the lagoon shore. More brackish but relatively tideless lagoons have little or no shoreline vegetation, and are bordered by beaches, spits and cuspate features built and shaped by the action of lagoon waves. The growth of these depositional features may eventually divide the lagoon into a chain of smaller segments, each with a configuration related to wind and wave patterns, the segmented lagoons being elongated in the direction of the prevailing winds. Lagoons which have permanent entrances from the sea and are affected by tidal movements are usually bordered by salt marsh or mangrove swamps in the zone between high and low tides. Their floors are marked by shoals, which may be partly emerged at low tide, and intervening channels shaped by the ebb and flow of tidal currents. Such lagoons show features similar to those of river estuaries, and may be regarded as estuarine lagoons. Many lagoons are estuarine in the vicinity of a tidal entrance, brackish and less tidal in a central zone, and fresh, dominated by the effects of inflowing rivers, away from the tidal entrance. The three modes of geomorphological evolution may thus be represented within a single lagoon system (Fig. 2.12), as in the Gippsland Lakes. The close relationships between ecological conditions and geomorphological development are best seen in the estuarine zone, where salt marsh and mangrove vegetation influence sedimentation, and in the freshwater zone, where encroaching reedswamp

I

ZONE OFREEDSWAMP ENCROACHMENT

1

ZONE OF SPIT GROWTH AND SEGMENTATION

I

TIDAL ZONE WITH SALT MARSH

Salinity and tide range increasing

1

ENTRANCE

Fig. 2.12. Schematic map of the typical zones of a coastal lagoon.

I

36

Physical Setting and Geomorphology of Coastal Lagoons

influences sedimentation. The relationships become obvious where a change in the ecological conditions, perhaps due to human interference, results in a change in geomorphological development: a freshened coastal lagoon develops bordering reedswamp and begins to contract in area, while a lagoon that becomes more brackish loses its reedswamp fringe, has eroding shorelines, and becomes segmented as bordering spits and embayments develop. Coastal lagoons are thus dynamic systems, within which changes are continuing in response to geomorphological processes. Some general principles and relationships have been deduced, but only a few of the world’s coastal lagoons have been investigated in detail, and much more research is needed. References Aldermann, A.R., Skinner, H. and Catherine, W. 1957 Dolomite sedimentation in the south-east of South Australia. American Journal of Science 255, 561-567. Barnes, R.S.K. 1980 Coastal Lagoons. Cambridge University Press. Bird, E.C.F. 1962 The utilization of some Australian coastal lakes. Australian Geographer 8,199-206. Bird, E.C.F. 1967a Coastal lagoons of south-eastern Australia. In Landform studies from Australia and New Guinea (Jennings, J.N. and Mabbutt, J.A., eds). 365-85. Bird, E.C.F. 1967b Depositional features in estuaries on the south coast of New South Wales. Australian Geographical Studies 5, 113-124. Bird, E.C.F. 1972 The physiography of the Fleet. Proceedings of the Dorset Natural History and Archaeological Society 93, 119-124. Bird, E.C.F. 1973 Australian coastal barriers. In Barrier Islands (Schwartz, M.L., ed). 410426. Bird, E.C.F. 1978 The Geomorphology of the Gippsland Lakes Region. Ministry for Conservation, Victoria, Publication 186. Bird, E.C.F. 1982 Changes on barriers and spits enclosing coastal lagoons. Oceanologica Acta, 45-53. Bird, E.C.F. 1983 Shoreline changes in the Gippsland Lakes, 1957 to 1983. Proceedings of the Royal Society of Victoria 95, 227-235. Bird, E.C.F. 1984 Coasts. Blackwell, Oxford. Bird, E.C.F. 1985a Coastline changes. Wiley, Chichester. Bird, E.’C.F. 1985b Papua New Guinea. In The World’s Coastline (Bird, E.C.F and Schwartz, M.L. eds). Van Nostrand Reinhold, New York, 887-897. Bird, E.C.F. and Klemsdal, T. 1986 Shore displacement and the origin of the lagoon at Brusand, southwestern Norway, Norsk Geografisk Tidsskrift 40, 27-35. Bird, E.C.F. and Lennon, J.L. 1989 Making an entrance. Yeates, Bairnsdale. Bird, E.C.F. and Ongkosongo, O.S.R. 1980 Environmental changes on the coasts of Indonesia. United Nations University, Tokyo. Bourman, R.B. and Harvey, N. 1983 The Murray mouth flood tidal delta. Australian Geographer 15,403406.

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Brown, R.G. 1969 The Coorong Lagoon, South Australia. In Lagunas Costeras (Castaiiares, A. and Phleger, F.B., eds). un Simposio, 191-192. Bruun, P. and Gerritsen, F. 1961 Stability of coastal inlets. Proceedings of the 7th Conference on Coastal Engineering, 386-417. Carr, A.P. and Gleason, R. 1972 Chesil Beach, Dorset and the cartographic evidence of Sir John Coode. Proceedings of the Dorset Natural History and Archaeological Society 93, 125-131. Castaiiares, A. and Phleger, F.B. (eds) 1969 Lagunas Costeras, un Simposio, UNAMUNESCO, Mexico. Cavazzoni, S. 1983 Recent erosive processes in the Venetian Lagoon. In Coastal Problems in the Mediterranean Sea (Bird, E.C.F. and Fabbri, P., eds). Bologna, 19-22. Colombo, G. 1977 Lagoons. In The Coastline (Barnes, R.S.K., ed). 63-81. Curray, J.R., Emmel, F.J. and Crampton, P.J.S. 1967 Holocene history of a strand plain, lagoonal coast, Nayarit, Mexico. In Lagunas Costeras, un Simposio (Castaiiares, A. and Phleger, F.B., eds). pp. 63-100. Day, J.H. 1951 The ecology of South African estuaries, I: General considerations. Transactions of the Royal Society of South Africa 33,53-91. Delaney, P.J.V. 1966 Geology and Geomorphology of the coastal plain of the Rio Grande do Sul, Brazil and Northern Uruguay. Coastal Studies Institute, Baton Rouge, Technical Report 18-B. Dunbar, G.S. 1956 Geographical history of the Carolina banks. Coastal Studies Institute, Louisiana State University, Technical Report 8. Emery, K.O. and Stevenson, R.E. 1957 Estuaries and lagoons. Memoirs of the Geological Survey of America 67,673-750. Evans, G. and Bush, P.R. 1969 Some oceanographical observations on a Persian Gulf Lagoon. In Lagunas Costeras, un Simposio (Castaiiares, A. and Phleger, F.B., eds), pp. 155-170. Fisher, R.L. 1955 Cuspate spits of St. Lawrence Island, Alaska. Journal of Geology 63, 133-142. Gierloff-Emden, H.G. 1961 Nehrungen and Lagunen. Petermanns Geographischen Mitteilungen 105,81-92 and 161-176. Gierloff-Emden, H.G. 1980 Geographie des Meeres: Ozeane und Kusten. Berlin. Guilcher, A. 1981 Les etangs littoraux: azonalitb d'ensemble e t modalitds zonales. Bulletin de la Socie'te' Langue'docienne de Ge'ographie 15,3-10. Guilcher, A. 1983 Twin barrier-lagoon systems in Sardinia. In Coastal Problems in the Mediterranean Sea (Bird, E.C.F. and Fabbri, P., eds). Bologna, 77-82. Guilcher, A. and Nicolas, J.P. 1954 Observations sur la langue de Barbarie et les bras du Senegal aux environs de St. Louis. Bulletin d'lnformation Ockanographique, Etude Cbtes 6,227-242. Healy, T.R. 1977 Progradation of the entrance to Tauranga Harbor, Bay of Plenty. New Zealand Geographer 30,90-91. Hedgpeth, J.W. 1967 Ecological Aspects of the Laguna Madre, a hypersaline estuary. In Estuaries (Lauff, G.H., ed). 408419. Hinschberger, F. 1985 Ivory Coast. In The World's Coastline (Bird, E.C.F. and Schwartz, M.L., eds). Van Nostrand Reinhold, New York, 585-589. Hobday, D.K. 1975 Quaternary sedimentation and development of the lagoonal complex, Lake St Lucia, Zululand. Annals of the South African Museum 71,93-113. Hodgkin, E.P., Birch, P.B., Black R.E. and Humphries, R.B. 1981 The Peel-Harvey

38

Physical Setting and Geomorphology of Coastal Lagoons

estuarine system study (1976-1980). Department of Conservation and Environment, Western Australia, Report No.9. Kjerfve, B. 1986 Comparative oceanography of coastal lagoons. In Estuarine Variability (Wolfe, D.A., ed). Academic Press, 63-81. Klemsdal, T. 1986 Lagoons along the coast of the Svalbard archipelago and the island of J a n Mayen. Norsk Geografisk Tidsskrif? 4 0 , 3 7 4 4 . KraR, J.C. and John, J.J. 1979 Lateral and vertical facies relations of a transgressive barrier. Bulletin of the American Association of Petroleum Geologists 63, 2145-2163. Lankford, R.R. 1976 Coastal lagoons of Mexico: their origin and classification. In Estuarine Processes (Wiley, M., ed). 2, 189-215. Lasserre, P. 1979 Coastal Lagoons: sanctuary ecosystems, cradles of culture, targets for economic growth. Nature and Resources 15,2-21. Leatherman, S.P. (ed). 1979 Barrier islands: from the Gulf of St. Lawrence to the Gulf of Mexico. Academic Press, New York. Leatherman, S.P. (ed). 1981 Overwash processes. Dowden, Hutchinson and ROSS, Stroudsburg, Pennsylvania. Le Bourdiec, P. 1958 Aspects de la morphogenese plio-quaternaire en basse CBte d’Ivoire. Revue de Ge‘omorphologieDynamique 9 , 3 3 4 2 . Leontiev, O.K., Maev, E.G. and Rychagov, G.I. 1977 Geomorphologiya beregov i dna Kaspiiskogo morya. Moscow State University. Mahinda Silva, A.T. 1986 Ecological and socio-economic aspects of environmental changes in two mangrove-fringed lagoon systems in southern Sri Lanka. In Man in the Mangroves (Kunstadter, P., Bird, E.C.F. and Sabhasri, S., eds). United Nations University, Tokyo, 79-86. Mercer, I.D. 1966 The natural history of Slapton Ley nature reserve. Field Studies 2, 385-405. O’Brien, M.P. 1969 Dynamics of tidal inlets. In Lugunas Costeras, un Simposio (Castaiiares, A. and Phleger, F.B., eds). pp. 397406. Orme, A.R. 1976 Barrier and Lagoon systems along the Zululand Coast, South Africa. In Coastal Geomorphology (Coates, D.R., ed). 181-215. Phleger, F.B. 1969 Some general features of coastal lagoons. In Lugunas Costeras, u n Simposio (Castaiiares, A. and Phleger, F.B., eds), pp. 5-26. Phleger, F.B. 1981 A review of some general features of coastal lagoons. In Coastal Lagoon Research, Past, Present and Future. UNESCO Technical Papers in Marine Science 33,7-14. Price, W.A. 1947 Equilibrium of form and forces in tidal basins of the coast of Texas and Louisiana. Bulletin of the American Association of Petroleum Geologists 31, 16191663. Robinsgn, A.H.W. 1955 The harbour entrances of Poole, Christchurch, and Pagham. Geographical Journal 121,33-50. Sacchi, C .F. 1979 The coastal lagoons of Italy. In Ecological processes in coastal environments (Jeffi-ies, R. and Davy, A.J., eds). 593-601. Schwartz, M.L. 1972 Spits and bars. Dowden, Hutchinson and ROSS, Stroudsburg, Pennsylvania. Schwartz, M.L. 1973 Barrier islands. Dowden, Hutchinson and Ross, Stroudsburg, Pennsylvania. Schwartz, M.L., Fabbri, P. and Scott Wallace, R. 1987 Geomorphology of Dungeness Spit, Washington, USA. Journal of Coastal Research 3,451-455.

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Shepard, F.P. 1960 Gulf coast barriers, Recent sediments, north-west Gulf of Mexico, Tulsa, Oklahoma, 197-220. Shepard, F.P. and Wanless, H.R. 1971 Our changing coastlines. McGraw Hill, New York. Steers, J.A. 1953 The sea coast. Collins, London. Swan, S.B. 1982 Sri Lanka. In The world’s coastline (Bird, E.C.F. and Schwartz, M.L., eds). Van Nostrand Reinhold, New York, 749-759. Thorn, B.G. 1965 Late Quaternary coastal morphology of the Port Stephens Myall Lakes area. Proceedings of the Royal Society of New South Wales 98,23-36. Thorn, B.G. 1984 Sand barriers of eastern Australia: Gippsland - a case study. In Coastal Geomorphology in Australia (Thorn, B.G., ed). Academic Press, 99-121. UNESCO. 1980 Coastal Lagoon Survey. Technical Papers in Marine Science 31. Webb, J.E. 1958 The ecology of Lagos Lagoon. Philosophical Transactions of the Royal Society, London 241, 307-318. Zenkovich, V.P. 1959 On the genesis of cuspate spits along lagoon shores. Journal of Geology 76, 169-177. Zenkovich, V.P. 1967 Processes of Coastal Development. Edinburgh. Zenkovich, V.P. 1969 Origin of barrier beaches and lagoon coasts. In Lagunas Costeras, u n Simposio (Castaiiares, A. and Phleger, F.B., eds). pp. 27-38. Zunica, M. 1987 Lo spazio costiero Ztaliano. Valerio Levi Editore, Rome.

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Coastal Lagoon Processes edited by B. Kjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

41

Chapter 3

Geological History of Coastal Lagoons Louis Martina and Jose Maria Landim Dominguezb aCentre ORSTOM, 70-74 Route d Aulnay, F-93140 Bondy, France, bInstituto de Geoci&ncias,PPPG, Universidade Federal d a Bahia, Rua Caetano Moura 123, FederaCBo, CEP-40.210, Salvador, Bahia, Brazil

This paper discusses the geological processes controlling the origin and evolution of coastal lagoons. The study of a lagoon is intimately connected with the barrier island enclosing it - one cannot exist without the other. Three main factors control the origin and maintenance of sandy barriers and thus determine the ultimate fate of coastal lagoons: (1) sea-level history; (2) shoreface dynamics; and (3) tidal range. The history of infilling of coastal lagoons is controlled primarily by rates of sediment supply relative to rates of sea-level rise. In areas of high relative sediment supply, such as on mesotidal coasts or where rivers empty directly into lagoons, the lagoon floor is rapidly brought to the intertidal level. Thus, sediments deposited in the intertidal zone will dominate the sedimentary record of the lagoons. In areas of low sediment supply, lagoons will be characterized by unobstructed water bodies and the lagoonal sedimentary record will be dominated by sediments deposited in sub-tidal environments. The effects of sea-level history and rates of sediment supply on the evolution and infilling of ‘fossil’coastal lagoons along the eastern coast of Brazil are documented and discussed for three major scenarios: (1) lagoons formed on a low-relief coastal plain during a sea-level rise; (2) lagoons formed on a low-relief coastal plain during a sea-level drop; and (3) lagoons formed on a high-relief coastal plain during a sea-level rise.

Introduction

The purpose of this paper is to discuss the geological processes controlling the origin, evolution, and ultimate fate of coastal plain lagoons. Our discussion will be restricted to lagoons dominated by terrigenous siliciclastic sediments with only minor additions of locally produced skeletal remains. We will draw heavily on studies from the coast of Brazil, not only because our experience is mostly restricted to this geographic region, but for two other fundamental reasons: (1)along the 3,000 km-long eastern coastline of

42

Geological History of Coastal Lagoons

Brazil, many different lagoon-forming processes were operative during the Holocene; and (2) the Brazilian coastline has been subjected to a particularly interesting Holocene sea-level history, involving a rise until 5 ka, followed thereafter by an overall sea-level drop (Martinet al., 1980b; Suguio et al., 1980).This allows the study of how different sea-level scenarios effect the origin and evolution of coastal lagoons. The complex Brazilian sea-level history has been extensively documented by mapping, radiocarbon dating, and vibracoring (Dominguez, 1987,1989; Martin and Suguio, 1986; Martin et al., 1980a, 1988). The Origin and Evolution of Coastal Lagoons The existence of coastal lagoons is intimately connected with the barrier enclosing it -one cannot exist without the other (Barnes, 1980).Therefore, to understand how lagoons form and evolve requires an understanding of the mechanisms that produce and maintain the sandy barriers separating the lagoon from the open sea. It is our opinion that three main factors control the origin and maintenance of sandy barriers and thus determine the ultimate fate of coastal lagoons. These are: (1)sea-level history; (2) shoreface dynamics; and (3) tidal range. Sea-Level History Sea level is controlled by complex mechanisms that act on different time scales and magnitudes, and include both local and global components. During the Late Quaternary, the most important mechanism controlling global sea-level variations was the advance and retreat of ice-sheets modulated by Milankovitch cycles. Until about 7,000 years ago, rates of sea-level rise associated with deglaciation were so high (10 m per 1,000 years) that the effects of other eustatic and local sea-level components were suppressed. After 7 ka, deceleration in glacio-eustatic rates of sea-level rise allowed mechanisms such as hydroisostasy, local and regional tectonism, and geoidal deformation to influence sea level. For this reason, varying trends of sea level for the last 7 ka have been reported for different regions of the world. Examples of contrasting sea-level histories are reported for the eastern coast of Brazil and the Atlantic and western Gulf coasts of the United States. Along the eastern Brazilian coast, Holocene sea-level history is characterized by a maximum level about 5.1 ka, which was 5 m above present level (Martin et al., 1980b; Suguio et al., 1980). Since that time, the overall trend has been a sea level fall (Fig. 3.la and b). On the other hand, along the United States Atlantic and western Gulf coasts, sea level has been continuously rising during the last 5,000 years (Fig. 3.lb).

43

L. Martin and J.M.Landim Dominguez

m +5

/

0

7

4

5

6

,

I

1

7//

,

.

I.

6

5

3

1

.

2

1

4

0

xl000 yrr

0.R

4

Fig. 3.1. (A) Relative sea-level curve for the east coast of Brazil (after Martinet al., 1979). (B) Schematic curves showing contrasting sealevel histories for the east coast of Brazil (upper curve) and the United States Atlantic and western Gulf coasts (lowercurve after Suguioet al., 1984).

These contrasting sea-level histories have exerted a fundamental control on coastal sedimentation (Dominguez, 1984;Suguio et al., 1984).Under conditions of sea-level rise, barrier islandAagoonal systems become important environments of sedimentation. Rivers do not directly reach the inner shelf, but tend to build deltas in protected environments such as lagoons, bays or estuaries. Beach-ridge plains are virtually absent. This morphology is characteristic of the United States Atlantic and western Gulf coasts (Fig. 3.2). By contrast, sea-level fall poses highly unfavorable conditions for the genesis'and maintenance of barrier islandAagoona1systems. Lagoons and bays become emergent. Beach-ridge plains rapidly prograde, giving origin to regressive sand sheets, and rivers empty directly onto the inner shelf. This is the case along the east-southeastern coast of Brazil, where narrow, elongated barrier islands separated from the mainland by a lagoon are virtually absent today, but were abundant in the past (Fig. 3.3). Why do barrier islands form preferentially under conditions of sea-level rise? According to Bruun's rule (Bruun, 1962), if the coastal profile is accepted as an equilibrium response of the sea floor to the coastal fluid

44

Geological History of Coastal Lagoons

Fig. 3.2. Simplified geological maps of representative sectors of the North American coastal zone. (A) San Antonio bay, Texas (modifiedfrom LeBlanc and Hodgson (195911, (B) Apalachicola Riverarea, Florida (modifiedfrom Schnable and Goodwell, 1968),(C) Cape Kennedy area, Florida (modifiedfrom Shepard and Wanless (1971)and Stapor and May(1988)),(D)Galveston Bay area, State of Texas (modified from LeBlanc and Hodgson (1959). 1: Pleistocene beach ridges; 2: bamer islands; 3: alluvial-deltaic plain and coastal marshes; 4: Pleistocene (undifferentiated).

power expenditure, then the effect of a sea-level rise could be deduced as a landward and upward translation of the profile (Fig. 3.4a). Thus, as sea level rises along a low-relief coastal plain, the beach and dune are nourished by the longshore drift and grow upwards at the same rate of sea-level rise, following Bruun’s rule. The swale behind the dune, however, remains at the same altitude and, as sea level rises, becomes a lagoon. Straight or nearly straight barrier islands will form and be separated from the mainland by a

L. Martin and J.M. Landim Dominguez

45

Fig. 3.3. Simplified geological maps of representative sectors of the eastern Brazil coast. (A) Caravelas strandplain, State of Bahia (modified from Martin et al., 1980a1, (B) Sao Joao strandplain, State of Rio de Janeiro (modified from Martin and Suguio, 19891, (C) Valenca strandplain area, State of Bahia (modified from Martinet al., 1980a),(D) Rio Real strandplain, State of Sergipe (modified from Bittencourt et al., 1983). 1: Pleistocene beach ridges; 2: Holocene beach ridges; 3: alluvial-freshwaterswamps and mangrove swamps; 4 Pre-Quaternary (undifferentiated).

lagoon of varying width (Fig. 3.5). This mechanism of barrier island formation, known as ‘mainland beach detachment’, is described in detail by Hoyt (1967) and Hails and Hoyt (1968). This model is accepted as the predominant mode of barrier island formation on low-relief coastlines of unconsolidated sediments. Lagoons associated with barriers formed through a

Geological History of Coastal Lagoons

46

_.

INITIAL

gN ::J:N

PROFILE TRANSPOST

Fig. 3.4.The Bruun’s rule of translation of the shoreface profile (Bruun, 1962).During a rise in sea level erosion prevails in the upper part of the profile causing the shoreline t o recede. During a fall in sea level the inverse is true (after Dominguez, 1987).

BELCH OR DUNE RIDGES

1

m SEA LEML-;

-9 BARRIER . ISLAND

2,

1

------

8. BEACH DETACHMENT

C.

BARRIER

RETREAT

Fig. 3.5.“Mainland beach detachment” mechanism for barrier island formation as a low-relief coast passes from stillstand to transgression (modified from Hoyt (1967)and Swift (1975)). See text for details.

L. Martin and J.M. Landim Dominguer

41

‘mainland beach detachment’ process will tend to have the long axis oriented parallel to the shoreline. When sea level falls, the inverse of Bruun’s rule applies (Fig. 3.4b), resulting in a seaward and downward translation of the coastal profile, and shallow back-barrier lagoons eventually become emergent. Shoreface Dynamics Open-ocean shorelines possess an innate tendency towards two-dimensionality (Swift, 1975). When a coastline of appreciable relief undergoes transgression, it is characterized initially by alternating promontories and bays (Fig. 3.6). The tendency of the shoreface to maintain its lateral continuity will act to prevent discontinuities, sealing off the bays. Shorefaces will be first incised into the seaward faces of promontories, and will propagate in the downdrift direction, giving origin to spits (Niedoroda et al., 1985).

Fig. 3.6. “Coastwisespit progradation”mechanism for barrier island formation on a high-relief coast during transgression (modified from Johnson, 1919). See text for details.

48

Geological History of Coastal Lagoons

These spits may eventually be segmented during storms, producing typical barrier islands (Fig. 3.6). This mechanism of barrier island formation is named “coastwise spit progradation” (Gilbert, 1885).Lagoons originating in this manner will probably have the long axis perpendicular to the shoreline and will tend to have greater water depths. Coastwise extension of sand spits is also a n important mechanism for the origin of elongate lagoons of reduced dimensions, forming on the downdrift side of river mouths or associated with abrupt changes in coastline orientation (Fig. 3.6). These lagoons are short-lived because they commonly form in areas of high sediment supply. They also form preferentially under conditions of stable or slowly changing sea levels. Tidal Range Studies of the world-wide occurrence of barrier island and lagoonal systems show that barrier islands are restricted to those coastal plains with tidal ranges less than approximately 4 m (Hayes, 1965; Hayes and Kana, 1976). This is caused by the reduced effectiveness of wave action h e . , waves cannot break in a concentrated area for a long period of time), and increased tidal current activity as the vertical tidal range increases (Hayes, 1979). As a result, on macrotidal shorelines (tidal range greater than 4 m) sand deposits are restricted to linear sand shoals, or tidal-current ridges in the offshore areas (Fig. 3 . 7 ~ ) As . tidal range decreases, wave effectiveness

Fig. 3.7. Influence of tidal range on barrier island morphology. Microtidal: long, narrow barrier islands with abundant washover fans and a paucity of tidal inlets. Mesotidal: stunted drumstick-shaped barrier islands with abundant tidal inlets. Back-barrier areas are characterized by a complex network of tidal channels, salt marsh/ mangrove swamp areas. Macrotidal: barrier-islands absent;numerous linear sand ridges present offshore (after Hayes, 1979).

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49

increases and barrier islands become a common feature of coastal plains experiencing transgression (Figs. 3.7a and 3.7b). On mesotidal coasts (tidal range 2-4 m), barrier islands are short, stunted, and dissected by numerous tidal inlets with well-developed ebb deltas (Fig. 3.7b). On microtidal coasts (tidal range 0-2 m), barrier islands are long and linear with a predominance of storm washover features (Fig. 3.7a). Tidal inlets and tidal deltas are relatively minor features. As pointed out by Hayes (1979), microtidal barriers are frequently overwashed by storms, probably because of a lack of conduits in the form of tidal inlets, which allow the rising water of storm surges to flow past the barrier. These contrasting dynamics of micro- vs. mesotidal barriers partially control the modes of sedimentation in back-barrier lagoons. Back-barrier areas on mesotidal coasts are characterized by a complex network of tidal channels, salt marsh or mangrove swamp, and open-lagoon areas. The extensive intertidal areas in these mesotidal back-barrier zones apparently result from high sedimentation rates of detritus imported from the inner continental shelf through numerous tidal inlets. Conversely, microtidal back-barrier lagoons are normally characterized by open water bodies, probably because sediment influx to lagoons is largely restricted to washover processes. This brief review shows that sea-level history is undoubtedly a hndamental control in determining the appearance and fate of coastal lagoons. During sea-level rise, barriers (and back-barrier lagoons) will form on micro- and mesotidal coasts through ‘mainland beach detachment’ and ‘coastwise spit progradation’ processes on low- and high-relief coastal plains, respectively. Barrier islands will not form during falling sea level, except as a result of ‘coastwise spit progradation’ processes. Coastwise spit extension will be common only in areas downdrift of major river mouths or associated with major changes in shoreline orientation. Lagoons that originated during a previous episode of sea-level rise will become emergent during the succeeding sea-level fall. Bays located between promontories will be sealed off by longshore drift and the shoreline will prograde rapidly because of enhanced sediment supply during episodes of falling sea level. The increased sediment supply is because (a)rivers discharge directly onto the inner shelf; and (b) BrGun’s rule requires sediment to be eroded from the lower shoreface and deposited in the upper shoreface during a drop in sea level (Fig. 3.4b). Geological History of Coastal Lagoons: Brazilian Examples

As already mentioned, relative sea level has been dropping along the east coast of Brazil during the last 5,000 years, after reaching a 5 m highstand around 5.1 ka (Fig. 3.la). Sea-level fall from that highstand was interrupted by high-frequency oscillations as large as 2-3 m acting on time scales of no

Geological History of Coastal Lagoons

50

more than 200-300 years (Fig. 3.la) (Martin et al., 1980b; Suguio at al., 1980). This sea-level history has exerted a fundamental control on the origin and evolution of coastal lagoons and associated sedimentary plains located on the eastern coast of Brazil (Fig. 3.8). During the 5.1 ka highstand, these plains were drowned and the barrier island-lagoonal systems associated with intralagoonal deltas were the dominant environments of deposition During the subsequent drop in sea level the lagoons (Figs. 3.8b and 3.8~). became emergent and the coastline rapidly prograded, giving origin to regressive sand sheets (Fig. 3Ad). In this section we document and discuss the effects of sea-level history and rates of sediment supply on the evolution and infilling of ‘fossil’ coastal lagoons occurring along the eastern coast of Brazil. Three major scenarios will be considered: lagoons formed on (1)a low-relief coastal plain during a sea-level rise; (2) a low-relief coastal plain during a sea-level drop; and (3) a high-relief coastal plain during a sea-level rise. The eastern coast of Brazil is a microtidal coast with a tidal range of approximately 1-2 m. Thus, tidal influences controlling the modes of sedimentation in back-barrier lagoons will be ignored.

c

= + -==Jz

i

S T A G E I:

STAGE

,,Y

-.-.

Y

-

PLEISTOCENE STRANDPLAINS (SEA LEVEL DROP)

IlI: INTRALAGOONAL

FRESHWATER FLUVIAL

DELTAS

STAGE I[. (MAXIMUM

BARRIER ISLAND / LAGOON SEALEVEL RISE 1

STAGE I V :

HOLOCENE STRANDPLAINS (SEALEVEL DROP I

SWAMP

HOLOCENE

.... ....... . ... . . . . .. .... ........ . .. .. . .

PLEISTOCENE

BEACH BEACH

RIDGE RIDGE

Fig. 3.8. Evolutionary scheme proposed for the strandplains associated with the major rivers emptying along the east coast of Brazil (after Dominguez et al., 1987). See text for details.

L. Martin and J.M. Landim Dominguez

51

Lagoons Formed on a Low-Relief Coastal Plain During Sea-LevelRise Two cases were selected, both located in areas influenced by major river discharge, the Jequitinhonha and the Doce River strandplains (Fig. 3.9). These two cases illustrate the relationship between the rate of sea-level rise and sediment supply, in controlling the evolution and facies distribution in back-barrier lagoons. The Jequitinhonha and the Doce strandplains are located on the eastern coast of Brazil (Fig. 3.91, and have areas of 800 km2 and 2,500 km2, respectively. Paleogeographic reconstructions of these two plains show that around 5.1 ka, extensive barrier island lagoonal systems had developed in those areas (Fig. 3. lo). Barrier islands (and their associated back-barrier lagoons) probably formed through a mechanism of ‘mainland beach detach-

Fig. 3.9. Location and simplified geologic maps of the Jequitinhonha and the Doce strandplains (modified from Dominguez, 1987).

52

Geological History of Coastal Lagoons

-ISLAND

Fig. 3.10. Paleogeographic reconstructions for the Jequitinhonha and Doce strandplains during the 5.1 ka highstand. Note that both strandplains were drowned and replaced by extensive barrier-islandhagoonal systems. Areas of the strandplain where paleo-lagoonal deposits were studied in detail and are discussed in text are enclosed by small rectangles (modified from Dominguez, 1987).

ment’ because of the low relief of these plains. These back-barrier lagoons became emergent when sea level dropped aRer 5.1 ka. Currently, freshwater swamp deposition dominates these former lagoonal areas. The sedimentary fill of the paleolagoonal depressions formed during this episode of sea-level rise has been documented in detail by Dominguez (1987) for the northern and the southern sectors of the Jequitinhonha and the Doce strandplains, respectively (Fig. 3.10). The substrate beneath the paleolagoon deposits in these two areas are dark brown, coarse to very coarse sands cemented by humic acids and iron oxides (coffee rock) which represent remnants of the Pleistocene beachridge terraces that border the paleolagoons in their western sides. We briefly discuss below the geological history of the paleolagoons formed at the Jequitinhonha and the Doce strandplains. The contrasting evolutionary

L. Martin and J.M.Landim Dominguez

53

histories of these two areas have resulted primarily from differences in sediment supply. The Jequitinhonha Strandplain

Dominguez (1987) identified six sedimentary facies in the paleolagoonal infill at the northern part of the strandplain based on (1) sediment color, (2) sediment texture, (3) types of plant remains, (4)molluscan assemblages and (5) physical and biological sedimentary structures (Figs. 3.11 and 3.12).

1311

FRESHWATER MANGROVE

-.-.

SWAMP SWAMP

FLUVIAL

(5.30'

BEACH-RIDGE TERRACE HOLOCENE BEACH-RIDE TERRACE

VIBRACORING SITE

-0

5Km

Fig. 3.11. Detailed map of the northern part of the Jequitinhonha strandplain showing the paleolagoon formed during the period 7.0-5.1 ka (now freshwater swamps) and location of vibracoring sites (after Dominguez, 1987).

54

Geological History of Coastal Lagoons

These facies are interpreted as being the result of deposition in the following environments (Fig. 3.12): (1)

Mangrove Swamp Facies - dark gray, very plastic organic-rich muds with abundant plant remains (roots, rootlets, pieces of wood

Fig. 3.12.Schematic cross-section across the paleolagoon located at the northern part of the Jequitinhonha strandplain, showing facies distribution and environmental interpretations. Vibracoring sites are shown in Figure 3.11 (modified from Dominguez, 1987). 1: whitish vertical roots; 2: long narrow leaves; 3: large flat leaves; 4: mangrove rootlets; 5: wood; 6: shells; 7: reddish-brown massive roots; 8: cross-bedding; 9:sand; 10:mud; 11:vibracoring site.

L. Martin and J.M. Landim Dominguez

55

and leaves) deposited in intertidal areas vegetated by mangrove trees. Well-marked organic-rich intervals composed of fibrous rootlets, wood fragments and oyster beds are very conspicuous in this facies. Radiocarbon dating shows that those organic-rich intervals occurring in the same stratigraphic position are contemporaneous. These intervals were interpreted as representing surfaces developed under a mangrove cover during a pause in sea-level rise as has been previously proposed by Thom et al. (1975)for the Cambridge-Ord river of Western Australia. Open Lagoon Facies - greenish to olive black muds with layers of detrital organic matter deposited in quiet subtidal environments where leaves and detrital organic matter would settle out. Sandy Lagoon Margin - interbedded muds and medium to coarse sand, exhibiting crossbedding and planar horizontal bedding deposited in a shallow subtidal to intertidal environment exposed to current activity. Tidal Channel Facies - interbedded coarse sand, mud, shell, and detrital organic matter beds forming cross-bedded and horizontally bedded deposits in a tidal channel environment. Washover Fan Facies - burrowed, well-sorted, coarse, micaceous sand occurring in the back-barrier area and interpreted as deposition associated with washover activity. Freshwater Swamp Facies - light gray muds with long whitish roots and grass-like leaves occurring in vertical position, similar to those thriving in the freshwater swamp today. Integration of (1)areal and vertical facies distribution with (2) radiocarbon dating and (3) the sea-level history for the eastern coast of Brazil allowed detailed reconstruction of the history of infilling of this paleolagoon, which is summarized below (Dominguez, 1987)(Fig. 3.13). The first signs of marine invasion in the paleolagoon are dated at 7,lka. At this time the paleo-barrier island was located seaward of its present position. It is assumed that the paleolagoon was an open body of water with mangrmes restricted to its margins (Fig. 3.13a). Prior to 7 ka, rates of sea-level rise were very high (1-2 m per 100 years). River valleys were flooded and river mouths were still retreating landward. Not much sediment was available to aggrade the lagoon floor. Because of the rapid rates of sea-level rise and scarcity of sediment to build u p the lagoon floor, mangrove-swamp sedimentation could not keep up with the sea-level rise. After 7.1ka the slowing rate of sea-level rise allowed the fluvial systems to recover, increasing the supply of sediments to the lagoon. Mangroveswamp sedimentation could then keep u p with sea-level rise, particularly

Geological History of Coastal Lagoons

56

PRESENT

f

MANGROVE SWAMP FRESHWATER SWAMP PLEISTOCENE BEACH-RIDGE

TERRACE HOLOCENE BEACH-RIDGE TERRACE

Fig. 3.13. Evolutionary history of the paleolagoon formed during the period 7.0-5.1 k a at the northern part ofthe Jequitinhonha strandplain. Black circles show locationofvibracoringsites (after Dominguez, 1987). See text for details.

at the mainland margin of the lagoon (Fig. 3.13b). Location of the open lagoon areas appears to be controlled by the paleodrainage carved into the Pleistocene beach-ridge terrace during the last glacial period. These areas remained as open water bodies throughout most of the lagoon history (Fig. 3.13~).Around 5.2-5.4 ka most of the lagoon floor built up to the intertidal level and was occupied by extensive mangrove forests. Open water bodies were restricted to tidal channels (Fig. 3.13d). A prominent organic-rich interval is present in most cores, suggesting maximum development of a mangrove cover by that time (Fig. 3.12). During the time interval preceding 5.2 ka, the barrier island has migrated slowly landward. After deposition of the 5.2-5.4 ka organic-rich interval sea level rose quite rapidly (Fig. 3.la). This rise translates into a replacement in some cores of the sandy lagoon-margin facies by the open lagoon facies. At the more distal (seaward) portions of the paleolagoon, the 5.2-5.4 ka organic-rich interval is buried under barrier island sediments or washover deposits. This last

L. Martin and J.M.Landim Dominguez

57

situation could only be the result of the barrier island migrating over this organic-rich interval in response to rising sea level. The effect of this rapid rise were probably restricted to the margins of the tidal channel (Fig. 3.13e). As a result of the drop in sea level that followed the 5.1 ka highstand, the mangrove-swamp environment was rapidly replaced by a freshwater swamp. Mangrove-swamp sedimentation was probably restricted to the margins of the tidal channels (Fig. 3.130. Radiocarbon dating of wood fragments collected in the tidal channel facies indicates that the channel remained active a t least until 3.2 ka. What was the source of the sediment which kept the lagoon floor at a depth appropriate for mangrove colonization throughout most of its history? A paleochannel comparable t o the present Pardo River can be seen in aerial photographs flowing into the paleolagoon (Fig. 3.11). This paleochannel almost connects with the present river course suggesting that early in the Holocene, the Pardo River might have emptied into the paleolagoon. Additional sources of fine sediments were provided by the small streams that flow in the valleys carved into the Tertiary alluvial fans.

The Doce Strandplain Five sedimentary facies have been identified in the paleolagoon located in the southern section of the Doce strandplain (Figs. 3.14 and 3.15) (Dominguez, 1987). Four facies (mangrove swamp, open lagoon, sandy lagoon margin and freshwater swamp) are similar to those occurring at the Jequitinhonha strandplain and will not be described here. However it should be noted that important differences in areal distribution of the mangrove swamp and open lagoon facies occur. In the Doce strandplain, the mangrove swamp facies is restricted to the basal portion of the paleolagoon infill which rests on top of a Pleistocene ‘coffee rock’. Also the open lagoon facies is more widespread in the Doce strandplain than in the Jequitinhonha strandplain. A fifth sedimentary facies named the “intralagoonal prodelta” is restricted t o the Doce strandplain. This facies is characterized by light olive gray well-laminated muds. Very micaceous, very fine sand laminae are common. This facies is interpreted as deposition in a prodelta environment as indicated by (1)the absence of mollusks and bioturbation, (2) the sedimentary characteristics indicating a calm subaqueous depositional environment with periodic introduction of coarse sediment (micaceous layer), and (3) the close proximity of the vibracoring sites to a former distributary of the Doce river intralagoonal delta (Fig. 3.14). The evolutionary history of the paleolagoon formed in the Doce strandplain was quite different from that observed in the Jequitinhonha strandplain. These differences result in part from the much larger area (at least 3 times larger) of the paleolagoon in the Doce strandplain and a lower

Geological History of Coastal h g o o n s

58

9.35'-

5 Krn

I 3 -: - .

19045'-

FLUVIAL HOLOCENE BEACH-RIDGE TERRACE

\ \

PALEOCHANNEL TERTIARY ALLUVIAL FANS

I . ]

VIBRACORING SITE

Fig. 3.14. Detailed geologic map of the southern part of the Doce strandplain showing the paleolagoon formed during the period 7.0 - 5.1 ka (now freshwater swamps) and location of vibracoring sites (after Dominguez, 1987).

apparent rate of sediment supply as compared with the Jequitinhonha strandplain (see Fig. 3.10). The first signs of marine invasion in the paleolagoon of the Doce strandplain are indicated by the presence of the mangrove swamp facies resting directly on top of the Pleistocene strandplain sands. Radiocarbon dating shows that this event took place about 7.6 ka in the more distal (seaward) parts of the paleolagoon and around 6.9 ka in the more proximal (landward) parts. The mangrove swamp environment existed briefly and was rapidly drowned by the continuously rising sea level, being replaced by a n open lagoon environment. Holes drilled by Petrobras in the alluvial valley of the Doce River penetrated at least 32 m of estuarine muds below coarse fluvial

59

L. Martin and J.M. Landim Domingwz

1-1. ....,:

lml

SAND

VIBRACORING

SITE

%

MUD

-

MANGROVE ROOTLETS

m]

LONG NARROW LARGE FLAT TUBULAR

WHlTElSH VERTICAL ROOTS REDDISH BROWN MASSIVE ROOTS

LEAVES LEAVES

ROOT EPIDERMIS

SHELLS

+ I]

F d

mxx, VERY MCACEOUS LAMINA

Fig. 3.15. Schematic cross-section across the paleolagoon located at the southern part of the Doce strandplain showing facies distribution and environmental interpretations.Vibracoring sites are shown in Figure 3.14 (modified from Dominguez, 1987).

sediments (Bandeira Jr. et al., 1975). Parts of the lake bottoms that fill the valleys carved into the Barreiras Formation (Fig. 3.9) are today well below sea level (-8 to -31 m). These observations suggest that much of the sediment load of the Doce River initially filled the estuary and proximal (landward) sections of the paleolagoon and that little sediment reached the distal (seaward) areas where vibracoring was performed. The decreasing rate of sea-levelrise after 7 ka allowed the river to reverse from a trend of landward river-mouth retreat to one of progradation. After infilling the estuary, the Doce River emptied into the paleolagoon and began depositing the intralagoonal delta. Prodeltaic muds were deposited in the

60

Geological History of Coastal Lagoons

distal (seaward) parts of the paleolagoon, as the delta prograded. Subsequently the intralagoonal prodelta environment was replaced by a sandy lagoon margin environment. The sources of sand during deposition of this facies were probably (1) washover activity and (2) erosion of the remnants of the Pleistocene beach-ridge terrace outcropping at the western margin of the paleolagoon. Radiocarbon dating indicates ages around 5.65.8 ka for deposition of the sandy lagoon-margin facies. The drop in sea level that followed the 5.1 ka highstand resulted in replacement of the paleolagoon by a freshwater swamp environment. In comparing the two strandplains, the sediment supply t o the Jequitinhonha strandplain was sufficient to keep most of the paleolagoon surface in the intertidal zone (Dominguez, 1987). As a result, mangrove-swamp sedimentation prevailed during the period 7-5 ka. In contrast, sediment supply to the distal (seaward) parts of the paleolagoon in the Doce strandplain was much reduced. The mangrove environment present around 7 ka was rapidly drowned and most of the lagoon became an open body of water. Consequently, facies deposited in subtidal environments dominate the lagoonal infill sequence of the Doce strandplain. The absence of sediments deposited in a mangrove swamp environment, indicates that sediment supply was not sufficient to raise the paleolagoon into the intertidal zone. Lagoons Formed on a Low-Relief Coastal Plain During Sea-Level Drop We have chosen the downdrift portion of the Doce strandplain to illustrate this scenario. Paleogeographic reconstruction of the barrier island system shows that around 5.1 ka the shoreline was sub-divided into convex and concave seaward sections (Fig. 3.10b). The tendency of the shoreface to maintain its lateral continuity acts to prevent discontinuities of the shoreline (Swift, 1975). Therefore, when the shoreline curves landward into an embayment such as at the northern part of the barrier island chain, the coastal dynamics will work to seal it off. This sealing off took place according to the following mechanism (Fig. 3.16). At the concave section of the shoreline, the barrier island chain inflected abruptly landward giving origin to a broad embayment. The shoreface, nourished by the northward directed longshore drift that predominates in the region and trying to maintain its lateral continuity, then began extending into this coastal void (Fig. 3.16a). As a result of this process, sand spits formed and continued extending northward into the open ocean far away from the shoreline in the concave section. As the spit extended coastwise, refraction of the S-SE waves and waves impinging from E-NE drove it landward until its northern extremity welded to the shoreline in the concave side. In several cases, this process gave origin to an extensive elongate lagoon separated from the open ocean

-61

L. Martin and J.M.Landim Dominguez

v

SEA-LEVEL

FLOODING

1-1

MANGROVE

FALL

TREES

PLEISTOCENE BEACH RIDGE TERRACE

-

Fig. 3.16.Evolutionary model for the northern part of the Doce strandplain during (1)the drop in sea level from 5.1 to 3.8 ka (A), and (2) the rise in sea level from 3.8 to 3.5 ka (B). During the drop in sea level lateral extension of sand spits gave origin to elongate lagoons which after h a l closure evolved into freshwater swamps or lakes (A). When sea level rose during the period 3.8-3.5 ka the sea drowned the river mouth of a Doce river distributary and invaded the low-lying areas which became interconnected bays (after Dominguez, 1987).

by the spit itself. During the enclosure process, the elongate lagoon trapped suspended sediments. If sedimentation was sufficient to bring the lagoonal floor to the intertidal zone, mangroves thrived. Otherwise after total closure, the elongate lagoon evolved to a freshwater lake (Fig. 3.16a). Repetition of this process has resulted in extensive progradation of the northern

62

Geological History of Coastal Lagoons

part of the strandplain which was characterized by low-lying areas (former lagoons) separating beach-ridge sets (former spits). This entire episode of progradation of the shoreline took place during a fall in sea level that persisted from 5.1 to 3.8 ka (Fig. 3.la). The rapid rise in sea level that took place afterwards (3.8-3.6 ka) (Fig. 3.la) caused inundation of these low-lying areas in the northern part of the strandplain, which then became interconnected bays (Fig. 3.16b). Part of the discharge of the Doce river began emptying into the lagoon that resulted from the interconnection of the individual elongate lakes. At those individual lagoons located closer to the river mouth sediment supply was enough to bring the lagoon floor to the appropriate level for mangrove colonization. Those lagoons located farther away from the riverine source did not receive enough sediment and most of the lagoon floor remained subtidal. Finally with the drop in sea level after 3.5 ka, freshwater swamp deposition ensued. The sedimentary fill of the elongate lagoons is characterized by a regular succession of facies that records from base to top the enclosure of the lagoon by coastwise spit progradation during a period of sea-level drop (5.1-3.8 ka) and the subsequent invasion by the sea when sea level rose very rapidly at 3.8-3.5 ka. A vibracore taken in one of these elongate lagoons (Fig. 3.17) shows that the lagoonal substrate is composed of light olive gray coarse silt to very fine sand deposited in a lower shoreface environment (Facies A, Fig. 3.17). It represents portions of the shoreface isolated during the coastwise progradation of the sand spit. Immediately above these lower shoreface sands, well-bedded very plastic muds accumulated, reflecting the deposition of suspended sediments during enclosure of the lagoon (Facies B, Fig. 3.17). In these sediments the number of mollusk species and the diversity indices (Shannon-Wiener Index) calculated for the molluscan assemblages steadily decrease upwards. The complete disappearance of mollusks at the top of facies B also reflects the final closure of the elongate lagoon with the onset of freshwater or very low-salinity conditions. The invasion of the sea during the sea-level rise episode of 3.8-3.5 ka is indicated by an increase in diversity and number of mollusc species (Facies C, Fig. 3.17). Evidence for river discharge into the lagoon is indicated in facies C by the presence of laminae of yellowish gray mud, micaceous coarse silt and very fine sand, similar to the intralagoonal prodelta facies described in the previous section. Deposition of these sediments brought the sediment surface to the appropriate depth for mangrove colonization (Facies D, Fig. 3.17). A radiocarbon date of a Crussostreu rhizophorue shell from these sediments provided an age of 3.5 ka which coincides with the 3.8-3.5 ka rapid rise depicted in the sea-level curve (Fig. 3.la). Finally with the drop in sea level after 3.5 ka, freshwater swamp deposition ensued. Freshwater peats presently cap the paleolagoonal sequence (Facies E, Fig. 3.17).

L. Martin and J.M. Landirn Dominguez CORE

63

RD-i5

ml ml

LONG NARROW LEAVES LARGE FLAT LEAVES WHlTElSH VERTICAL ROOTS

MUD

*d la l

BURROWS

v)

w

DIVERSITY

0 4 Y

Fig. 3.17. Representative core log of a vibracore of the sedimentary fill of the elongate paleolagoons formed at the northern part of the Doce strandplain (after Dominguez, 1987). See text for details.

Lagoons Formed on a High-Relief Coastal Plain During Sea-Level Rise

By the end of the Pliocene, climate on the eastern coast of Brazil was characterized by semi-arid conditions which resulted in the deposition of the extensive alluvial fans, of the Barreiras Formation. Today these coalesc-

64

Geological History of Coastal Lagoons

ing alluvial fans form extensive tablelands along the coastal zone, with mean altitudes around 100 meters. These tablelands are cut by large valleys that can be several kilometers wide and have steeply inclined walls. The boundary between these Tertiary alluvial fans and the Quaternary strandplains is a line of fossil sea-cliffs with heights varying from a few meters to more than 50 meters. These valleys were inundated, during the 5.1 ka highstand, giving origin to bays and estuaries some of which reached lengths of the order of tens' of kilometers. Later these bays and estuaries were partly closed by sand spits anchored in the sea cliffs giving origin to lagoons. These lagoons had their major axis perpendicular to the overall trend of the shoreline. When sea level dropped after the 5.1 ka highstand

57 BRAZIL

Fig. 3.18. Simplified geological map of a sector of the State of Alagoas (northeast Brazil) showing different evolutionary stages of lagoons formed during transgression on a high- relief coastal plain (modified from Barbosa et al., 1986). 1: Pleistocene beach ridges; 2: Holocene beach ridges; 3: mangrove swamps;4 alluvial/freshwater swamps;6:Pre-Tertiary;6:Tertiary alluvial fans.

L. Martin and J.M. Landim Domingwz

65

the fate of the lagoons was controlled by sediment supply to the lagoon itself and along the open ocean shoreline. Where major rivers were present, progradation of bay-head deltas completely filled the lagoons which are now emergent. In these areas sedimentation presently takes place in fluvial and freshwater swamp environments. In areas of low sediment supply to the lagoon, they either evolved to freshwater lakes if sediment supply along the open-ocean shoreline was sufficient to seal it off, or they maintained their status as coastal lagoons if a n inlet is still present today. Along the northeastern coast of Brazil all gradations exist between these extreme situations and are illustrated in Figure 3.18. Summary

The origins and geological histories of coastal lagoons are intimately associated with the mechanisms that give origin to and maintain the sandy barriers that separate the lagoon from the open ocean. Because barrier islands form favorably during rises in sea level, coastal lagoons will be a common feature in coasts experiencing sea-level rise (e.g. U.S.Atlantic and western Gulf coasts). Lagoons are relatively rare features on coasts experiencing a drop in sea level. During sea-level rises barrier islands and back-barrier lagoons will form on low-relief coastal plains through a mechanism of “mainland beach detachment”. Therefore the substrate of the lagoon will be subaerial deposits. On high-relief coasts, barrier islands and back-barrier lagoons will form essentially through a mechanism of ‘coastwise extension of sand spits’. During stable to slowly changing sea levels, barrier islands and back-barrier lagoons will form essentially through a mechanism of ‘coastwise extension of sand spits’ both on high and low relief coastal plains. The history of infilling of coastal lagoons is controlled primarily by rates of sediment supply relative to rates of sea-level rise. In areas of high relative sediment supply such as on mesotidal coasts or where rivers empty directly into lagoons, the lagoon floor is rapidly brought up to the intertidal level, and mangrove swamps (low latitudes) or salt marshes (high latitudes) thrive. Thus, sediments deposited in the intertidal zone will dominate the sedimentary record of the lagoons. In areas of low relative sediment supply lagoons will be characterized by unobstructed water bodies and the lagoonal

Geological History of Coastal Lagoons

66

sedimentary record will be dominated by sediments deposited in sub-tidal environments. (8)

Because lagoons are generally shallow, they are very sensitive to fluctuations of sea level; small rises and falls translate respectively into widespread inundation and emergence of coastal lagoons. Thus coastal lagoons during their geological history may be affected by multiple episodes of invasion and emergence.

Acknowledgments We are grateful to Dr. Bjorn K j e f i e for inviting us to write this contribution. We thank Dr. David Krantz for his thorough review of the paper. His comments greatly enhanced and clarified the text. J.D.thanks the National Research Council of Brazil (CNPq) for a research fellowship. References Bandeira Jr., A.N., Petri, S. and Suguio, K. 1975 Projeto Rio Doce. Petroleo Brasileiro (Petrobras) S.A., Internal Report, 203 pp. Barbosa, L.M., Bittencourt, A.C.S.P., Dominguez, J.M.L. and Martin, L. 1986 Mapa Geologico do Quaternario Costeiro do Estado de Alagoas. In Mapa Geologico do Estado de Alagoas, Escala 1:250.000(Dantas, J.R.A. and Souza, E.P., eds.). Departamento Nacional da Producao Mineral, Brazil. Barnes, R.S.K. 1980 Coastal Lagoons, The Natural History of a Neglected Habitat. Cambridge University Press, 107 pp. Bittencourt, A.C.S.P., Martin, L., Dominguez, J.M.L. and Ferreira, Y.A. 1983 Mapa Geologicodo Quaternario Costeiro do Estado de Sergipe. In Mapa Geologico do Estado de Sergipe, Escala 1:250.000(Bruni, M.A.L. and Silva, H.P., eds.). Departamento Nacional da Producao Mineral, Brazil. Bruun, P. 1962 Sealevel rise as a cause of shore erosion. American Society of Civil Engineers Proceedings, Journal of Waterways and Harbors Division, 88,117-130. Dominguez, J.M.L. 1984 Sealevel history: a dominant control on modern coastal sedimentation styles (abstract). Society of Economic Paleontologists and Mineralogists, First Midyear Meeting, Sun Jose, California, 26 pp. Dominguez, J.M.L. 1987 Quaternary sealevel changes and the depositional architecture of beach-ridge strandplains along the east coast of Brazil. Doctoral Dissertation, University of Miami, U.S.A., 288 pp. Dominguez, J.M.L., Martin, L. and Bittencourt, A.C.S.P. 1987 Sea-level history and quaternary evolution of river-mouth associated beach-ridge plains along the eastsoutheast Brazilian coast: a summary. In Sealevel Fluctuation and Coastal Evolution (Nummedal, D., Pilkey, O.H. and Howard, J.D., eds.). Society of Economic Paleontologists and Mineralogists, Special Publication, 41,pp. 115-127. Dominguez, J.M.L. 1989 Ontogeny of a strandplain -evolving concepts on the evolution of the Doce river strandplain (east coast of Brazil). International Symposium on Global Changes in South America During the Quaternary, INQUA, Brazil pp. 235240.

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Emery, K.O. and Uchupi, E. 1972 Western North Atlantic Ocean: topography, rocks, structure, water, life, and sediments. American Association of Petroleum Geologists Memoir 17,532. Gilbert, G.K. 1885 Lake Bonneville. U.S. Geological Survey Monograph 1,438 p. Hails, J.R. and Hoyt, J.H. 1968 Barrier development on submerged coasts: problem of sealevel changes from a study of the Atlantic coastal plain of Georgia and part of the East Australian coast, Zeitschrift fur Geomorphologie 7,224-255. Hayes, M.O. 1965 Sedimentation on a semi-arid, wave-dominated coast (south Texas); with emphasis on hurricane effects. Ph.D. Dissertation, University of Texas, 350 pp. Hayes, M.O.and Kana, T.W. 1976 Terrigenous clastic depositional environments: some modern examples. American Association of Petroleum Geologists Field Course Guidebook and Lecture Notes, Technical Report, No. 11-CRD, Coastal Research Division, Department of Geology, University of South Carolina, Part I, 131 pp. Hayes, M.O. 1979 Barrier island morphology as a function of tidal and wave regime. In Barrier Islands, from the Gulf of St. Lawrence to the Gulf of Mexico (Leatherman, S.P., ed.). Academic Press, pp. 1-27. Hoyt, J.H. 1967 Barrier island formation. Geological Society of America Bulletin 78, 1125-1136. Jonhson, D. 1919 Shore Processes and Shoreline Development. Hafner, New York (1965 facsimile), 584 pp. LeBlanc, R.J. and Hodgson, W.D. 1959 Origin and development of the Texas shoreline. Transactions of the Gulf Coast Association of Geological Societies 9, 197-222. Martin, L., Flexor, J.M., Vilas Boas, G.S., Bittencourt, A.C.S.P. and Guimaraes, M.M.M. 1979 Courbe de variations du niveau relatif de la mer a u cours des 7000 dernieres annees sur un secteur homogene du littoral bresilien (nord de Salvador, Bahia). In Proceedings 1979International Symposium on Coastal Evolution in the Quaternary, Sao Paulo, Brazil (Suguio, K., Fairchild, R.R., Martin, L. and Flexor, J.M., eds.). pp. 264-274. Martin, L., Bittencourt, A.C.S.P., Vilas Boas, G.S. and Flexor, J.M.1980a Mapa Geologic0 do Quaternario Costeiro do Estado da Bahia, Escala 1:250.000.Secretaria de Minus e Energia, Govern0 do Estado da Bahia, Brazil. Martin, L., Suguio, K., Flexor, J.M., Bittencourt, A.C.S.P. and Vilas Boas, G.S. 198Ob Le Quaternaire marin bresilien (littoral pauliste, sud-fluminense and bahianais). Cahiers Office de la Recherche Scientifique et Tecnique &Outre-Mer, Serie Geologie 10, 95-124. Martin, L. and Suguio, K. 1986 Excursion route along the coastal plains of the states of Parana and Santa Catarina. International Symposium on Sealevel Changes and Quaternary Shorelines, Sao Paulo, Brazil, Special Publication No. 1, 124 pp. Martin, L. and Suguio, K. 1989 Excursion route along the Brazilian coast between Santos (State of Sao Paulo) and Campos (northern State of Rio de Janeiro). International Symposium on Global Changes in South America During the Quaternary Past Present -Future, Sao Paulo, Brazil, 136 pp. Martin, L., Suguio, K., Flexor, J.M. and Azevedo, A.E.G. 1988 Mapa geologic0 do Quaternario costeiro dos Estados do Parana e Santa Catarina. Departamento Nacional da Producao Mineral, Brazil, Serie Geologica no. 28. Niedoroda, A.W., Swift, D.J.P. and Hopkins, T.S. 1985 The Shoreface. I n Coastal Sedimentary Environments (Davis Jr, R.A., ed.). Springer-Verlag, New York, pp. 533-624.

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Schnable, J.E. and Goodwell, H.G. 1968 Pleistocene-Recent stratigraphy, evolution and development of the Apalachicola coast, Florida. Geological Society ofAmerica, Special Paper 112,72 pp. Shepard, F.P. and Wanless, H.R. 1971 Our Changing Coastlines. McGraw-Hill Book Company, 579 pp. Stapor, F.W. and May, J.P. 1983 The cellular nature of littoral drift along the northeast Florida coast. Marine Geology 51,217-237. Suguio, K., Martin, L. and Flexor, J.M. 1980 Sea level fluctuations during the past 6000 years along the coast of the state of Sao Paulo (Brazil). In Earth Rheology, Isostasy and Eustasy (Morner, N.A., ed.). John Wiley and Sons, New York, pp. 471486. Suguio, K., Martin, L., Bittencourt, A.C.S.P., Dominguez, J.M.L. and Flexor, J.M. 1984 Quaternary emergent and submergent coasts: comparison of the Holocene sedimentation in Brazil and southeastern United States. Anais d a Academia Brasileira de Ciencias 56, 163-167. Swift, D.J.P. 1975 Barrier-island genesis: evidence from the central Atlantic shelf, eastern U.S.A.. Sedimentary Geology 14, 1-43. Thom, B.G., Wright, L.D. and Coleman, J.M. 1975 Mangrove ecology and deltaic estuarine geomorphology - Cambridge Gulf-Ord river, Western Australia. Journal of Ecology 63,203-232.

Coastal Lagoon Processes edited by B. yjerfve (Elsevier Oceanography Series, 60) Q 1994 Elsevier Science Publishers B.V. All rights reserved

69

Chapter 4

Water, Salt and Heat Balance of Coastal Lagoons Ned P. Smith Harbor Branch Oceanographic Institution, Fort Pierce, FL 34946, USA

Water, salt and heat balance studies are surveyed and summarized within the context of coastal lagoons. The most important geomorphologicalfactors influencing these physical processes are inlet configuration and dimensions, lagoon size and orientation with respect to prevailing winds, and water depth. Results of past water balance studies show that advective transport dominates gains and losses by rainfall, evaporation, surface runoff and groundwater seepage. All terms, however, show a distinct seasonality in response to seasonally changing winds, wet and dry periods and higher evaporative losses during summer months. Current measurements are decomposed into steady and non-steady components to show that in coastal lagoons the time varying, low frequency fluctuations are often dominant. When lagoon-shelf exchanges are restricted, tidal currents are often important only in the vicinity of inlets,and advection is primarily in response to local wind forcing. The wind driven circulation includes a downwind drift that produces a surface slope and drives a near-bottom return flow. A density driven current can be significant even in well mixed lagoons. Residual tidal flow can be important in the long term, but over shorter time intervals the net flow in response to local wind forcing is usually dominant. Salt balance is discussed in terms of transport through a transverse cross section, and in terms of local gains and losses at a given location. Transport occurs by advection and diffusion. Long term balances maintain brackish conditions in the lagoon; short term imbalances can change the salinity distribution significantly. Results from previous studies in lagoons and well mixed estuaries indicate that salt transport is primarily in response to advection associated with freshwater outflow or the slow draining and filling forced by changes in coastal sea level, tidal diffusion, and to shear effects arising from spatial correlations of vertical and especially transverse deviations in salinity and current speed. Heat balance, like water balance, is a response to local processes and advection. Heat energy gains and losses occur primarily over annual and diurnal periodicities, and over time scales on the order of 1-2 weeks in response to synoptic scale weather patterns. Previous studies indicate that heating is dominated by incoming solar radiation; cooling is primarily in response to outgoing long-wave radiation and latent heat fluxes. Advective heat fluxes are especially difficult to quantify because of strong spatial gradients in both temperature and velocity. Advection is of particular importance in the vicinity of inlets, because of lagoon-shelf differences in the rates of warming and cooling.

70

Water, Salt and Heat Balance of Coastal Lagoons

Introduction

The subjects of water, salt and heat balance are central to the understanding of the biology, chemistry and physics of estuaries of all kinds, including coastal lagoons. Fluxes of water control the flushing of a lagoon, thereby maintaining water quality, and they provide a mechanism for the import or export of planktonic plants and animals. Salt fluxes determine the overall estuarine characteristics of the lagoon, and heat fluxes control warming and cooling. These three topics are logically combined in a single chapter, because they are interrelated in several ways. Latent heat fluxes, for example, involve a water loss as much as a heat loss. Similarly, advective transport of salt and heat necessarily involves a simultaneous transport of water. There are conceptual similarities as well. The governing equations for heat and water balances consist of a group of local processes plus an advective term. While a considerable body of literature has accumulated on these three subjects, the majority of studies have been conducted in drowned river valley estuaries, rather than in coastal lagoons. Results from well mixed estuaries are often applicable to lagoons, however, and additional information from partially stratified estuaries can be gained by inference and extrapolation. At the very least, a compilation of the existing literature from prior water, salt and heat balance studies in coastal lagoons serves to identify gaps, suggest analytical methodologies and form hypotheses for follow-up and/or comparative studies. A few general remarks are in order at the outset regarding the relationship between geomorphology and the processes that control water, salt and heat budgets of a coastal lagoon. Within the lagoon, physical processes are influenced primarily by five geomorphologic factors: inlet configuration, lagoon size and orientation with respect to prevailing wind directions, bottom topography and mean depth. Inlet dimensions control the exchange of water, including dissolved and suspended material. Lagoon-shelf exchanges, in turn, determine flushing rates and residence times, and thus water quality. Wind effects will be enhanced in larger lagoons, especially when the longitudinal axis of the lagoon is aligned with the prevailing wind direction. The bottom topography of the lagoon, including both natural and man made channels, plays an important role in guiding the tidal and non-tidal circulation and thus controlling advective transport. Mean depth may be the most important of the factors listed above. Lagoons are characteristically shallow, with a large surface area to mean depth ratio, and several hydraulic and hydrographic features arise as a direct consequence. First, shallow water is especially responsive t o heating and cooling processes. Distinct temperature changes are recorded even over diurnal time scales. Second, significant wave action and thus wave mixing can extend to the bottom. This acts to destroy vertical stratification, and as a result

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density gradients in lagoons are characteristically greater along the longitudinal and lateral axes than in the vertical. Third, in shallow water the bottom friction layer will extend to the surface except where currents are consistently weak, or where vertical stratification is unusually strong. This further mixes the water column, and currents may be damped in a matter of hours once forcing ceases. To maintain well defined circulation patterns forcing must be both continuous and local. Tidal forcing is continuous in a periodic sense, but tidal motions arise from exchanges with adjacent continental shelf waters, and as noted above inlets may have a significant constricting effect. Tidal motions may be significant only in the immediate vicinity of the inlet. Wind forcing is local, but it is intermittent in time, and variable in speed and direction. Thus, near inlets, tidal currents are often dominant, while in the interior of the lagoon, wind forcing is primarily responsible for maintaining the circulation of the lagoon. Finally, shallow water enhances the residual, or net motion that occurs over any complete tidal cycle. The objective of this chapter is to survey water, salt and heat balances in coastal lagoons within the context of physical processes that regulate them. The chapter begins with a discussion of water balance, summarizing the processes which add, remove and redistribute water within the lagoon. This leads directly to the following section on salt balance, which involves advective and diffusive processes, and which explains the mechanisms by which salt is gained or lost, thereby maintaining or altering the estuarine characteristics of the lagoon. The chapter concludes with a description of heat balance, which summarizes warming and cooling by both local and advective processes. A familiarity with these cause-and-effect relationships is a prerequisite for a thorough understanding of the ecology and proper management of a lagoon. Water Balance

The basic objective in an investigation of the water balance of a coastal lagoon is a n accounting of the gains and losses of water - for the lagoon as a whole, or for some sub-area of interest - and a description of the mechanisms by which water is redistributed within the lagoon. A thorough study of water balance evaluates all the processes which contribute or remove water. These processes are represented by terms in the hydrologic equation, or storage equation. This is an equation of continuity that explains volume changes in terms of processes that involve both salt water and fresh water. Consider, for example, a lagoon connected to the adjacent continental shelf by a single inlet, and a transverse section across the inlet. The time rate of change of volume in the lagoon is given by

Water, Salt and Heat Balance of Coastal Lagoons

72

AV -At

-P-EtR+G+A

(4.1)

where V is the volume of the lagoon, P and E are the spatially integrated precipitation and evaporation rates, R is surface runoff, G is groundwater seepage and A is the advective gain or loss of water. All terms are commonly expressed in units of m3/sec. In the case of a cross section at the inlet, advection through the transverse cross section describes lagoon-shelf exchanges. Alternately, the area chosen for study may be within the lagoon and describe the gain or loss of water from a particular study area. Studies that attempt to evaluate all of the terms in Eq. 4.1 are relatively rare. Pritchard (1960), for example, described exchange rates in Chincoteague Bay after assembling historical water level data and estimating freshwater runoff, precipitation and evaporation. More recently, Smith (1991) used weather records to quantify precipitation and evaporation for the northern segment of Florida’s Indian River lagoon. Freshwater runoff records, combined with groundwater seepage estimates, were added to the data base and model results provided advective gains and losses. Results from a half-year study of the northern segment of Florida’s Indian River lagoon indicated that evaporative losses of about 16 m3/s were roughly in balance with gains from precipitation (13 m3/s),surface runoff (5 m3/s) and groundwater seepage (0.5 m3/s).In contrast, model results for the same time and study area indicate that the standard deviation of advective gains and losses through a transverse cross section was 555 m3/s. At certain times, or under certain conditions, some terms in the hydrologic equation will be small and can be neglected with justification. Even under simplified conditions, however, evaluating the terms in Eq. 4.1 can be a formidable task, requiring assumptions and interpolation. Spatially averaged precipitation, for example, can be estimated using a network of rain gauges surrounding the lagoon, with individual readings weighted according to the fraction of the lagoon they represent (Thiessen, 1911). Nonlinear interpolation techniques can be used, but it is rare to find more than a few rain gages operating in the vicinity of a given lagoon. When estimates require an extrapolation from inland weather stations, allowances must be made for significant spatial gradients which often occur perpendicular to the coast. For example, precipitation data from the Texas coastal zone in the vicinity of Laguna Madre indicate a seaward decrease in annual rainfall of approximately 0.2 cmkm, in addition to a north to south decrease of approximately 0.1 cmkm. Similar data collected in the vicinity of Indian River lagoon on the east coast of Florida indicate a seaward decrease in mean annual precipitation of approximately 1.4 c m k m over the northern part of the lagoon. In the southern part of the lagoon, however, mean annual rainfall decreases landward by approximately 0.4 cmkm. Apart from precipitation, fresh water entering the lagoon will be by

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surface runoff, including the discharge from rivers and canals, and groundwater seepage. The relative importance of these two terms varies inversely. In lagoons with few natural streams or drainage canals, groundwater will contribute a substantial fraction of the total. Freshwater discharge values are obtained from gauges on major freshwater canals and rivers emptying into the lagoon. Total amounts may be increased if relationships can be established with short term studies to quantify the contributions of small creeks and streams. Groundwater seepage is an integral part of lagoon hydrography, but it is usually difficult to quantify and thus has received relatively little attention. Pandit and El-Khazen (1990) used a finite elem e n t model to estimate flow into Florida’s Indian River lagoon; Bokuniewicz (1980) made in situ measurements to record flow through the sediments of Great South Bay, along the south shore of Long Island, New York. In the latter study, groundwater seepage decreased rapidly with distance from the shore into the bay. Seepage was greatest on the landward side of the bay and in a band several tens of meters wide running parallel to the shore. Evaporation estimates require a measure of humidity along with wind readings that are representative of over-water conditions. Hsu (1981,1986) has made a study of the relationship between overland and over-water wind measurements and suggested corrections as a function of overland wind speed only. Richards et al. (1966) have made similar studies in the Great Lakes and developed relationships based upon wind speed, fetch and atmospheric stability in near surface layers. Complicating the matter is the possibility that inland weather stations will record calm wind conditions during late night and early morning hours more frequently than will a coastal weather station, where a radiation inversion is less likely to form. Special attention is often given to the advection term, because of the efficiency with which it transports dissolved and suspended matter. The advective transport of water is conceptually straightforward, but it is especially complex and difficult to estimate because gradients of current speed in both vertical and horizontal directions are significant. In practice, one measures the flow from top to bottom at several stations in one or more transverse cross sections. By interpolation and extrapolation, one can then estimate the total flow and transport through the cross section. Comparison of adjacent cross sections reveals convergence and divergence within the enclosed area, and thus the change in the storage term. Alternately, if the objective is to understand transport processes, careful analysis of repeated flow measurements through a given transverse cross section provides information about the mechanisms by which water moves through the study area. The temporal variability of the individual terms in Eq. 4.1 is considerable, and this must be taken into account when choosing the sampling frequency and the length of time over which a field study is conducted. Precipitation varies greatly over time scales of hours to months, but seasonal variations

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Water, Salt and Heat Balance of Coastal Lagoons

Fig. 4.1. Along-channel component currents recorded half-hourly in Florida's Indian River lagoon (27"46.3'N,8Oo25.8'W),approximately 7.5 km south ofsebastian Inlet, April 27 to May 28, 1977. Positive values indicate northerly flow.

are repeated in a general way from one year to the next. Evaporation may show significant seasonality, too, in addition to a diurnal variation in response to wind speed and humidity. Weather data obtained near the northern end of the Laguna Madre of Texas (Smith, 1989) indicate that evaporative water losses during the winter months of December-February are only 23% of those calculated for the summer months of June-August. Surface runoff shows considerable variation over time scales ranging from days to seasons. High frequency fluctuations are damped somewhat, however, as rainwater percolates through soil en route to the canal or river. Temporal fluctuations in groundwater seepage are damped more than fluctuations in surface runoff, because of the greater distances involved when water percolates all the way to the lagoon. The advective movement of water within the lagoon is the most rapidly fluctuating term, because shallow waters respond to both tidal and wind forcing, as noted above. Significant changes in current speed and direction are commonly recorded over time scales on the order of a few hours. This is superimposed onto much slower variations in response to weather systems moving through the area, and t o seasonally changing wind patterns. Figure 4.1 shows a time series of half-hourly along-channel component currents recorded during late spring, 1977, in Florida's Indian River lagoon just south of Sebastian Inlet. Both a periodic tidal signal and a quasi periodic non-tidal component appear in the plot. The focal point in water balance investigations may vary considerably from one study to the next. In this regard, it is important to distinguish between more general water balance studies and more highly focused mass flux, or mass transport studies. In the former case, the emphasis is gener-

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75

ally on effects, rather than causes, and AV/At is evaluated by assembling water level records and multiplying the change in water level by the surface area of the lagoon. Mass transport studies intentionally take a more limited approach and concentrate specifically on the advection term to describe transport processes. One of the first papers to summarize the governing equations (Pritchard, 1958), describes water balance in terms of mass or volume continuity. Over the following three decades, the majority of studies have focused on the relative importance of individual transport processes. This emphasis in previous work will be reflected here, and the remainder of this sub section will survey advective mass transport processes in coastal lagoons. The circulation of a coastal lagoon, and thus mass transport processes specifically can be subdivided in any of several ways. One of the most basic distinctions is that separating tidal and non-tidal transport. Tidal currents are as dependable and predictable as they are periodic, thus they should be quantified in any comprehensive study of lagoonal circulation and water balance. Non-tidal circulation can be subdivided further by distinguishing between barotropic and baroclinic forcing. A barotropic pressure gradient arises from a slope in the surface of the lagoon. Such a slope may in turn be created by the wind driven setup or setdown of water levels, or it may occur in response to surface runoff entering the lagoon at some point. A baroclinic pressure gradient arises most commonly from longitudinal salinity gradients and the resulting density gradients. Baroclinic pressure gradients may be created in subsurface layers at the inclined interface between salt water and brackish water near an inlet. A salt wedge is not a prerequisite for a baroclinic pressure gradient, however, and a well defined pressure gradient can exist even in a vertically mixed lagoon. The circulation within a coastal lagoon can also be thought of as arising in response to local and non-local, or ‘remote’ forcing. Local forcing is dominated by wind stress, as discussed above. Both tidal and low frequency variations in coastal sea level result in the non-local forcing of lagoonal circulation. Semidiurnal and diurnal shelf tides force the ebb and flood of water through inlets. The low frequency rise and fall of coastal sea level in response to changes in surface pressure and wind conditions produce a similar inflow and outflow, but over time scales on the order of a few days or longer. Longer period tides occur in shelf waters, and thus in the lagoon, but they are characteristically of lesser importance than low frequency non-tidal water level variations. Within the lagoon, the measured flow at a given place and time may be a complex mix of tidal and non-tidal currents, and at the same time a response to both local and non-local forcing. The various current components that may be superimposed at a particular time and place can be sorted out to some degree when observations in a time series are decomposed into quasi steady, periodic and high frequency aperiodic components. One common practice is to decompose short time

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76

series of measured current speeds into means and departures from the means. The means may involve a temporal or a spatial averaging. When temporal averages are computed, it is customary to compute tidal cycle means over a n integral number of tidal cycles. One tidal cycle may be sufficient where the tide is clearly semidiurnal or diurnal. Where the tide is mixed, a n even number of tidal cycles is needed to minimize the effect of a diurnal inequality. Furthermore, repeated studies may be required to quantify distinctly different conditions at times of spring and neap tide. As a n example, one may decompose the measured current speed, u, into the steady flow and the time varying motion, and then expand the time varying flow into its periodic and turbulent parts. Thus, for each measurement, u, in a time series, we can write u = (u)

+ u + u’

(4.2)

where (u) is the mean over the time period of the study, U is the sinusoidal ebb and flood of the tide and u’ is the non steady, non periodic residual. In many, if not most coastal lagoons, the tide is not dominated by a single diurnal or semidiurnal constituent. Thus, to describe tidal currents in detail, a careful harmonic analysis of the data must be conducted. Ideally, a 29-day time series is obtained to include a complete lunar synodic month. When several tidal constituents contribute to the observed tidal current,

A isin(wit + qi)

U=

(4.3)

i

where A is the amplitude of the ith constituent, o is its angular frequency and @ is its phase angle. Depending upon the length of the study and the existence of low frequency non-tidal motion, u’ may be restricted to relatively high frequency turbulent fluctuations, or it may include low frequency, wind driven motions as well, if the total time interval is in excess of several days. Furthermore, flow that appears quasi steady in a given direction for a shorter study would appear in the ( u ) term, while the same flow in a longer study would be quasi periodic and appear in the u’ term. Figure 4.2 is a composite of total, tidal and non-tidal motion, using a 55-day current meter record from a study site at the southern end of Florida’s Banana River lagoon, just west of Cape Canaveral. Parts (a), (b) and (c) correspond to u , U and u’ in Eq. 4.2, respectively. During this particular study in 1983, the mean flow, (u), was a relatively small -1.8 c d s , indicating a n outflow from the lagoon. A decomposition according to Eq. 4.2 is an oversimplification even when the tidal term is represented by several constituents, and when the study is long enough to calculate a representative mean. The underlying, time varying non-tidal residual current may represent both high frequency turbulence and low frequency meteorological forcing. In such cases, Eq. 4.2

77

N.P.Smith 100,

v)

b

\

E

50-

Y

W 0

w n

0-

v)

5

-50-

w cf

5

-100-

50

0

-50 - 1004 14 21 JANUARY

2R

4 11 FFRRIJARY

I 18

75

I

4 MA9-H

'

Fig. 4.2. Composite of (a) total, (b) tidal and (c) non-tidal along-channel currents recorded at the southern end of Florida's Banana River lagoon (28"36.4'N,28"09.2W),January 15 to March 8,1983. Positive values indicate northerly flow into the lagoon.

could be expanded to subdivide the time varying non-tidal term into a high frequency term, u', and a low frequency term, u". Figure 4.3 shows u,U and u" from a study site in Indian River lagoon. Data were collected during a 49-day period of time in 1986 at a study site 23 km north of Sebastian Inlet. The high frequency non-tidal term is not shown because little information is conveyed in a plot of u". In coastal lagoons having restricted exchanges with adjacent shelf waters, the ebb and flood of the tide may be relatively unimportant, and most of the instantaneous observation will be comprised of the quasi steady mean and a non-tidal deviation from that mean. Within the context of mass transport, the two most significant terms are often the low frequency non-tidal flow in the relatively short term, and the steady flow in the longer term. A careful decomposition of the available current data is necessary for determining the nature of the advective gain or loss of water within the lagoon. Depending upon the questions being addressed and the physical characteristics of the lagoon under investigation, the appropriate decomposition of

Water, Salt and Heat Balance of Coastal Lagoons

78

I

h

-1

a

"1

\ Fig. 4.3. Composite of (a) total, (b) tidal and (c) low frequency non tidal currents recorded in Florida's Indian River lagoon (28"03.3'N,80'34.6W) approximately 23 km north of Sebastian Inlet, October 2 to November 19,1983. Positive values indicate flow toward the north.

the measured current may be quite different. For example, it may be desirable to consider deviations from cross sectional means rather than temporal means. Or it may be preferable to consider deviations from vertical means if vertical shear is significant. Additional techniques will be described in the following section. Although the instantaneous ebb and flood is the most prominent feature of the tidal circulation, the associated residual flow is especially important as a baseline measure of the circulation. The residual flow of water past a study site resulting from unequal ebb and flood transport during a complete

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tidal cycle arise from the phase relationship of tidal fluctuations in currents and water level, from nonlinear interactions between the tidal flow and bottom topography, from density gradients, wind stress and from net freshwater discharge. Van de Kreeke (1976) has shown that the tide induced residual flow is proportional to the ratio of tidal amplitude to water depth. Thus, a significant residual flow requires either a large amplitude tide or shallow water. Coastal lagoons generally do not have prominent tides, but they do provide shallow water and thus exhibit residual flow. A recent study by Smith (1990a) examined the magnitude and nature of residual tidal flow in the southern segment of Florida’s Indian River lagoon. A water column mean current speed of 0.8 c d s is significant as a potential flushing mechanism, but even under normal wind conditions the non-tidal wind drift is capable of dominating residual tidal flow (Smith, 1990b). Residual motion is made up of two components, relating to the steady flow of water and to oscillatory motions. The first is referred to as “Eulerian residual flow,” and is simply the average of the individual current measurements made over a time interval such as a tidal period. Eulerian residual flow is given by

RE= (u> (4.4) where u is the current speed and the diamond brackets indicate a temporal average over a tidal cycle or other appropriate time interval. The current speed may be a vertical mean, or it may apply to a specific layer of the water column. Eulerian residual flow is used to quantify freshwater outflow, all long period wind driven transport and other non-tidal flow in response to lagoon-shelf exchanges onto which tidal oscillations are superimposed. The second component, termed the “mass transport Stokes drift,” arises from the correlation of the rise and fall in water level with the ebb and flood of the current. A net transport of water will occur if the tide is of the partially progressive type (Dyer, 1974; Tee 1976). The mass transport Stokes drift for a long, narrow lagoon is given by (4.5) Rs = (iifi)I (H) here ii = u - (U) (see Eq. 4.2) and H = H - (IT)i.e. , the differences between the individual measurements and the temporal mean values. A Stokes drift in the direction of tidal wave propagation will arise when over half of the flood tide coincides with water levels above the tidal cycle mean. While Rs is especially well suited for describing transport in a co-oscillating current system (Uin Eq. 4.21, the Eulerian residual current ( ( u )in Eq. 4.2) generally has to be evaluated as well. Ianniello (1977)has noted that a mass transport Stokes drift commonly coexists with a n Eulerian residual flow. The net drift arising from unequal flood and ebb transport produces a tidal pumping of water into an estuary. To maintain a steady-state condition, this inflow is compensated for by an opposing Eulerian outflow. In lagoons with both

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Water, Salt and Heat Balance of Coastal Lagoons

quasi steady and oscillatory currents, the mass transport residual current,

RL = RE + Rs,is calculated to quantify the total transport. Alternately, Robinson (1983) defines an Eulerian residual flow in terms of both water depth and current speed as

where q is the tidal height relative to the mean water column depth. The number of inlets serving a given lagoon may have a significant bearing on whether the vertically integrated Eulerian and Lagrangian residual flows act in concert with one another, or in opposition. In lagoons with a single inlet, the Eulerian and Stokes residual motions may transport water in opposite directions, as noted above. In lagoons with two or more inlets, residual tidal motions may work in concert. In the two inlet situation, residual tidal flow arises as a result of differences in mean water level, tidal amplitudes and phases at the inlets, and differences in the inlets themselves. Van de Kreeke and Dean (1975) have shown that the residual flow will be toward the inlet at which the tidal amplitude is smaller, or at which the tidal phase is lagged, all other factors being equal. Alternately, a residual transport will be directed toward the inlet of greater length, narrower width or greater depth - again, all other factors being equal. Liu and Aubrey (1990) have combined model results with field measurements in an estuary with multiple tidal inlets. Their work shows that the residual tidal flow will be in the direction of decreasing mean sea level and toward the inlet with the lower tidal amplitude. In contrast, the response to differences in phase is relatively small and inconsistent. Inlet dimensions may differ in many ways, thus it is difficult to predict the direction and magnitude of a residual flow. But the existence of residual tidal motion must be recognized and sampling must be designed to detect and quantify it. The vertical variation of residual tidal flow is a significant issue when dissolved or suspended matter is unequally distributed through the water column. Regardless of the local or non local nature of the forcing producing the circulation, strongest current speeds will be found in near surface layers furthest removed from the bottom friction layer. This will be the case for a logarithmic current profile characteristic of the ebb and flood of the tide, as well as for wind driven currents, which decrease exponentially with increasing distance below the surface. The pattern is more complex for residual tidal flow, however. Smith (1990a), for example, has quantified vertical variations in residual flow individually for Stokes and Eulerian residual motion in Florida’s Indian River lagoon at the midpoint between two inlets. Residual flow in the uppermost quarter of the water column was nearly twice that in the lowest quarter, and both Eulerian and Lagrangian components decreased with increasing depth. Ianniello (1977) on the other hand,

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considered a one inlet estuary of constant breadth and depth, and found that while the Eulerian residual flow was directed seaward at all depths, the Lagrangian residual was landward in the upper part of the water column. I n lagoons with sufficiently restricted exchanges the local wind driven circulation will dominate all other forms of locally or remotely forced nontidal circulation. An important feature of the local wind driven current, and one that can set it apart from the ebb and flood of the tide, stems from the seasonality in wind forcing. Tidal circulation is dominated by semidiurnal and diurnal constituents which vary only slightly in magnitude or in phase over the course of a year. Seasonally changing wind patterns, however, will result in corresponding changes in the wind driven circulation, and thus in net transport. It is common for wind forcing to be stronger during fall, winter and spring months, then decrease significantly during summer months. A recent study by Smith (1990b) compared wind forcing with the residual tidal flow in Florida’s microtidal Indian River lagoon. Results suggest that even relatively light summer winds of as little as 2-3 d s e c can negate or double the Stokes drift calculated under calm conditions. Local forcing by wind stress produces a direct downwind transport regardless of the wind direction. The initial response will be a set up of water levels along the downwind shore, a setdown along the upwind shore, and thus a n upwind directed slope in the free surface of the lagoon. Even for relatively light winds of 5-10 m s-l, the slope of the lagoon surface will be on the order of 0.5 cm km-l. The rise or fall in water level at the downwind and upwind ends of the lagoon will be especially pronounced if the lagoon is elongated and the wind direction parallels the longitudinal axis of the lagoon. The response to wind forcing can exceed the rise and fall of the tide in a lagoon having restricted exchanges with shelf waters, and a longitudinal axis on the order of several tens of kilometers. The secondary response to the wind, and the primary response to the barotropic pressure gradient established by the set up and set down condition, is an upwind directed return flow. This occurs at near bottom levels, and in the deeper waters along the middle of the lagoon. An upwind return flow may be aided by navigational channels. Both natural and man made channels serve as conduits by locally expanding the layer between the downwind directed wind stress itself, and the bottom frictional force which resists the upwind return flow. This has been discussed by Pitts (1989)for Florida’s Indian River lagoon, and modeled by Smith (1990a). The pressure at any depth, z , is given by (4.7) 11

where p is water density and g is gravity. Density can vary significantly

82

Water, Salt and Heat Balance of Coastal Lagoons

along the longitudinal axis of the lagoon as well as vertically. Alternately, the surface of the lagoon is influenced by the wind set up, as noted above, fresh water entering the lagoon, or tidal or non-tidal exchanges through the inlet. Thus, a horizontal plane at some intermediate depth z can experience a pressure gradient because of a slope in the free surface, or because of spatial variations in density. The pressure gradient along the longitudinal axis of the lagoon therefore expands into (4.8) The first right hand side term in the final form of the equation describes the baroclinic pressure gradient driving the density driven circulation. The baroclinic pressure gradient will always be directed toward regions of lower salinity, assuming that the effect of temperature on density gradients is minimal. Thus, in a brackish water lagoon, the baroclinic pressure gradient will tend to force near bottom water into the lagoon. In a hypersaline lagoon, on the other hand, baroclinic forcing will act to force high salinity water out the inlet. Noting that 6 z / 6 x is equivalent to 6q/6x, where q is the surface elevation relative to mean water level, H , the second term describes the barotropic pressure gradient in response to the surface slope. An evaluation of the two terms on the right hand side of Eq. 4.8 suggests that baroclinic forcing, involving density gradients, will generally be of lesser importance. It is important to note, however, that the circulation associated with density gradients is directly related to depth. Thus, it will increase in importance at intermediate and especially near bottom layers. Fischer (1972) has noted that this may explain a transverse shear in longitudinal flow, with a net inflow into the estuary in deeper layers in mid channel, and a compensating outflow through the shallow waters nearer either shore. The barotropic pressure gradient is not a function of depth, and thus it will be felt equally at all sub surface levels. An understanding of lagoon shelf mass exchanges is important for two primary reasons. First, an active exchange of water with the adjacent shelfhas an important effect on lagoon hydrography and water quality. Adjacent shelf waters are relatively stable in terms of temporal variability in temperature and salinity. Exchanging water with the lagoon imports that stability and reduces environmental extremes in the lagoon. This same exchange of water provides a flushing effect which maintains or restores water quality. The second reason involves the transport of migrating animal species. Shelf waters are the source of much of the biota of lagoons, particularly larval forms which colonize lagoons. Adult animals leaving lagoonal nursery areas migrate seaward through inlets to live in continental shelf waters. Because of the fundamental importance of lagoon-shelf exchanges, Kjerfve (1986) has suggested a classification scheme in which coastal lagoons are

N.P.Smith

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sub divided according to the nature of the exchanges between lagoons and inner shelf waters. ‘Leaky’ lagoons have a relatively free exchange. At the other extreme, ‘choked’lagoons exhibit little or at times no exchange. Those with an intermediate degree of exchange are termed ‘restricted’. To the extent that exchanges are restricted, coastal lagoons will exhibit a degree of isolation that sets them apart from other types of estuaries. The forcing that effects lagoon-shelf exchanges is conceptually straightforward. Tidal exchanges are forced by high and low water conditions occurring along the inner shelf, as tidal waves of ocean basin scale move along the coast. Non-tidal exchanges occur when coastal winds directly or indirectly raise or lower coastal sea level. Coastal sea level responds in a direct way to cross shelf winds. Water level is set up and set down when winds are directed landward and seaward, respectively. Coastal sea level also responds to along shelf winds, although indirectly. Along shelf winds drive an along shelf circulation. In an equilibrium condition in the Northern Hemisphere, the Coriolis force will be balanced by an opposing pressure gradient, and sea level will slope upward to the right, looking downstream. Thus, for along shelf winds and currents with the coast on the left, coastal sea level will fall, and water will be drawn out of the lagoon. When the wind and current reverse direction, coastal sea level will slope upward toward the coast, water level will rise and the lagoon will fill. The resulting inflow into, or outflow from the lagoon is much like a tidal exchange, although it characteristically occurs over time scales on the order of a week, as synoptic scale weather patterns move through the study area. Inlet morphology plays a central role in the lagoon-shelf exchange process. The constriction of an inlet acts as an exponential filter in the sense that the damping effect it has on exchanges is directly related to the frequency of the rise and fall in coastal sea level. For a sufficiently constricting inlet, tidal amplitudes in a lagoon may be greatly reduced below amplitudes found nearby on the continental shelf, yet low frequency variations over time scales in excess of a few days may be nearly identical in lagoon and coastal waters. DiLorenzo (1988)has described the modification of tidal conditions by inlets for coastal bays in which water levels rise and fall uniformly in space. This ‘Helmholtz’, or ‘pumping mode’ response is characteristic of many smaller bays and lagoons. The Helmholtz frequency, F h , is defined by (4.9)

where g is gravity, A, is the mean cross sectional area of the channel connecting the lagoon and the ocean, L is the channel length and A, is the mean surface area of the lagoon. For low frequency ocean forcing, e.g., less than about 0.1 F h , the rise and fall of the lagoon surface will approximate that of the adjacent ocean closely. As the frequency of ocean forcing ap-

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Water, Salt and Heat Balance of Coastal Lagoons

proaches F, however, the morphology of the connecting channel become more important in determining the lagoon response. A channel damping coefficient, D,is defined by

(4.10) where I is a measure of the channel impedance, and HOis the amplitude of the ocean tide. For large damping coefficients, the response of the lagoon is a damped version of the ocean forcing. For low damping coefficients, however, a significant amplification may occur. This is especially true for overtides, which are amplified or damped approximately twice as much as the astronomical constituents are. The principal diurnal and semidiurnal tidal constituents have been compared for shelf waters of the northwestern Gulf of Mexico and for the Laguna Madre of Texas. Overtides were not physically significant in this case. The damping coefficient for Aransas Pass was high enough to damp all astronomical tidal constituents, but in every case semidiurnal constituents were preferentially damped relative to diurnal constituents. This can be quantified with the ‘form number’ defined by the sum of the amplitudes of the two principal diurnal tidal constituents (usually KI+ 01) divided by the sum of the amplitudes of the two principal semidiurnal constituents (usuallyMz+ S2).Form numbers computed from water levels along the Gulf coast of Texas are 2.0 to 2.5,while values computed from measurements made in Laguna Madre are generally between 6.0 and 10.0.Figure 4.4 shows this with energy density spectra computed from water level time series recorded at Aransas Pass on the Texas Gulf coast and at the northern end of Laguna Madre (see Smith, 1988).Long period energy density levels are nearly identical. At diurnal and semidiurnal tidal periodicities, however, energy density levels in the lagoon are decreased by approximately one and a half and two and a half orders of magnitude, respectively. The concept of water balance is fundamental in calculating two commonly used measures of lagoon flushing - hydraulic replacement time, Th,and freshwater residence time, Tf. The hydraulic replacement time is given by V T .- ’- Qf

(4.11)

where Vis the mean volume of the lagoon, and Qfis the total net freshwater discharge. The freshwater replacement time is defined by

(4.12) where So and S1are the mean salinities of adjacent inner shelf waters and the lagoon, respectively. In practice, surface runoff is used for Qf. This may

N.P. Smith > W

n

$W Z W

85

5

'1

3

I

OLI "'100 I

I

24

12

8

6

5

PERIOD(Hours)

Fig. 4.4. Energy density and coherence spectra computedfrom water level time series recorded at Aransas Pass (27"50.3'", 97"03.0W)on the Texas Gulf coast (light line) and at the northern end of Laguna Madre (27"37.1", 97"14.3'W),January 29 to June 27,1974.

be a poor approximation, because strictly speaking net freshwater discharge must be the sum of the surface runoff, groundwater seepage and direct precipitation (see Eq. 4.1). Equally important, this total must be corrected for evaporation. This is especially true for shallow lagoons with a large surface area to total volume ratio. Failure to evaluate these processes and incorporate all the significant terms can lead to substantial over or underestimates in the flushing rate. Unpublished data from Corpus Christi Bay, Texas, for example indicate a freshwater residence time of 0.27 years if freshwater discharge is uncorrected for evaporation. Similar calculations indicate a hydraulic replacement time of 1.87 years. If calculations are corrected for evaporation, however, the freshwater residence time increases to 1.03 years, and the hydraulic replacement time increases to 7.34 years.

Salt Balance The importance of understanding the salt balance of a coastal lagoon stems not only from the role salt plays in determining the estuarine characteristics of the lagoon, but also from the fact that salt serves as a convenient natural tracer, and as such it can be helpful in water quality studies. Carefully conducted salt balance studies will indicate the magnitude and thus the relative importance of a variety of tidal and non tidal transport processes. Detailed studies can explain observed spatial patterns of salinity, as well as temporal variability occurring over time scales ranging from hours to seasons.

86

Water, Salt and Heat Balance of Coastal Lagoons

Generally speaking, salt balance studies are designed to investigate: (1) the rate at which salt moves through a cross section at a n inlet or within the lagoon; (2)the mechanisms most responsible for salt transport through a given cross section; or (3) the rate of change of salinity within the lagoon as a whole, or within a particular segment of interest, resulting from spatial variations in salt transport. The most basic distinction in salt transport mechanisms is between advective transport and non-advective, diffusive processes. Over time scales longer than several days, at least, lagoons with a sufficiently strong and consistent freshwater outflow will have a quasi steady seaward directed advective transport and a landward directed salinity gradient that drives a n upstream diffusion of salt. Over the longest time scales, a balance of these two processes maintains the brackish water conditions in the lagoon. Over shorter time scales, however, meteorological forcing in one form or another can effect significant imbalances, redistribute salinity within the lagoon and produce significant variations in salinity patterns. Any of the mass transport mechanisms noted previously can advect salt as well. Thus, the salt balance of a coastal lagoon is intimately related to the water balance. Pritchard's (1960) investigation of the salt balance of Chincoteague Bay is noteworthy in this regard, because the approach was to integrate hydrological and meteorological data into a n analysis of historical salinity data to predict the annual variation in salinity in terms of freshwater outflow. From this data base, distinctly different information becomes available. Specifically, by relating water balance with salinity conditions, Pritchard was able to estimate a volume renewal rate of 7% per day for the bay. This approach may be appropriate for utilizing the considerable amount of hydrographic data that have been accumulated and archived through monitoring programs in many coastal lagoons. For several reasons, salt balance studies i n coastal lagoons are distinctly different from similar studies in drowned river valley estuaries. First, tidal processes may be of secondary importance. The semidiurnal or diurnal time scales are often dominated by quasi periodic variations in current speed and salinity over time scales on the order of 1-2 weeks. This makes it difficult to select a n appropriate time period for defining the mean flow. Second, in contrast with the dominant longitudinal salinity gradient characteristic of most drowned river valley estuaries, elongated lagoons lying landward of a barrier island may have well defined lateral salinity gradients as well. Lateral salinity gradients occur when fresh water enters through several rivers, streams or drainage canals on the landward side of the lagoon; or especially when groundwater seepage contributes a significant fraction of the total freshwater inflow. No general equation similar to Eqs. 4.1 and 4.21 (next section) exists for the salt balance of a coastal lagoon as a whole, because local sources and

N.P. Smith

87

sinks of salt are negligible. Local gains and losses of fresh water affect salinity to be sure, but not the salt content of the study area. Instead, one quantifies transport, whether by advection or diffusion, and then, depending upon the objectives of the study, expresses the time rate of change of salinity at a point in terms of spatial gradients of advective and/or diffusive transports. For an elongated lagoon that can be described with one-dimensional flow, the instantaneous advective transport through a cross section of area A is given by

T,=

dA

us

(4.13)

A

where u is the component of the current velocity normal to the cross section and s is the salinity. In a tide dominated estuary, it is common practice to make calculations over an integral number of complete tidal cycles, then to define the mean flux in terms of tidal cycle averages:

(T,)=

us t

(4.14)

dAdt

A

where P is the tidal period. Allowances must be made for diurnal inequalities in high and low water in coastal areas with a mixed tide, and for dominant non-tidal time scales where meteorological forcing is of comparable or greater importance than tidal forcing. Also, in coastal regions where spring and neap tide conditions differ appreciably, measurements may have to be repeated at weekly intervals. In practice, advective processes are quantified aRer the current speed, salinity and water level time series are decomposed in terms of their temporal and/or spatial variability. Both the current speed and salinity in the first term on the right hand side of Eq. 4.13 can be decomposed into mean and time varying components to investigate local salt transport. Recalling Eq. 4.2,we can decompose a time series of salinity exactly as we decomposed current speed. The salinity measured at a particular time and place can be thought of in terms of steady, periodically varying and turbulent components. Each instantaneous measurement from a given level in the water column is comprised of a temporal mean value and both tidal and non-tidal deviations from the mean: s = (s) +

s + s’

(4.15)

where (s)is the time average, S is the tidal period variation of salinity, and s’ is the turbulent fluctuation. The product on the right hand side of Eq. 4.14 then expands into us = (u)( s ) + (U)S + (u)s‘ +

U(S)

+ us + Us’ + u’(s) + U’S + u’s’

(4.16)

Water, Salt and Heat Balance of Coastal Lagoons

88

When the cross products are integrated over one or more tidal periods, several terms will drop out and the equation is simplified considerably. Dyer (1973)has discussed this at length. In some cases, individual terms are not correlated with each other; in others, the sum of the products is zero. For example, both the (u)s’ and u’(s) terms should drop out over a tidal cycle because the turbulent fluctuations will have a zero mean. Similarly, it is unlikely that the tidal period variation in salinity is correlated with the mean velocity, or that the tidal current is correlated with the mean salinity, and (u)Sand U(s)drop out. Thus, for quantifylng advective salt transport, Eq. 4.14may be expressed as

(T,)=

$

( ( u )(s) + US

+ u’s’)dA dt

(4.17)

t A

Additional salt transport results from a variety of dispersive mechanisms, however. Neglecting molecular diffusion, dispersion terms occur in the presence of spatial gradients in current speed and salinity. The upstream diffusion of salt in the direction of the salinity gradient and opposing the advective salt transport is treated mathematically by introducing a coefficient of longitudinal dispersion which Bowden has called the effective eddy diffusion coefficient because it represents mixing conditions over a complete tidal cycle. Salt transport by diffusion is quantified by multiplying the salinity gradient by the effective eddy diffusion coefficient:

(4.18) In a well mixed, one-dimensional lagoon, the effective eddy diffusion coefficient can be calculated if the corrected freshwater outflow from Eq.4.1,the mean longitudinal salinity gradient, 6(s)/&, and the cross sectional area, A, are known. Then,

(4.19)

Dyer (1973)has summarized values of K, found in several previous studies. Typical values are on the order of lo6 cm2/s.Bowden (1963)has found that in alternating tidal flows of amplitude Uo,K, = 0.15 UO h. Once K, is known for a given study area, one may use Eq.4.18to estimate salt transport by diffusion. The temporal variation in salinity, whether for the lagoon as a whole, or for a specific sub basin, takes the concepts of advective and diffusive transport one step further. To increase or decrease salinity, there must be a convergence or divergence in salt transport, respectively. This is ex-

N.P. Smith

89

pressed mathematically by taking the x-derivatives of the transport equations. Dyer (1973) has discussed this for the three-dimensional case. For simplified, one-dimensional flow, this is written as 6s - -6(us) ---+6t 6x

K*62(s) 6X2

(4.20)

The introduction of x-derivatives will often eliminate additional terms in the expansion of the advection term (Eq. 4.16).For example, the tidal period velocity and salinity variations may be highly correlated, but unless this correlation changes along the axis of the lagoon, &US)/& will be negligible. Similarly, while it is logical to assume that larger turbulent fluctuations might be embedded in stronger tidal currents, it does not follow that they will be consistently correlated, or that a longitudinal gradient in this correlation will exist. Finally, an increase in the amplitude of the tidal current might produce both a larger tidal variation in salinity and larger turbulent fluctuations, but the longitudinal variation in this relationship may be negligible. Only field measurements will determine which terms in Eq. 4.16 must be retained and which can be neglected. In a steady-state condition, salinity will be constant in time, and the two right hand side terms in Eq. 4.20 will balance. While it is tempting to simplify the expression in this way, in practice it is rare to find steady-state conditions, except over the longest time scales. Most studies are not continued beyond several consecutive tidal cycles. Salinity fluctuates significantly over time scales ranging from hours to seasons, and it is generally preferable to assume a non steady, time varying system, and use a less abbreviated form of Eq. 4.20 to explain observed temporal variability in the salinity field over shorter time scales. Apart from the matter of decomposing salinity, velocity and perhaps cross sectional area into steady and time varying components, one must also select sampling stations and levels in a manner appropriate to resolve both transverse and vertical gradients. Stations and layers can be chosen in such a way as to emphasize either transverse (see Fischer, 1972) or vertical deviations (see Hughes and Rattray, 1980) from the cross sectional mean. Park and James (1990) have noted that the partitioning procedure must be selected in accordance with the hydrodynamics of the estuary under investigation. In a well mixed coastal lagoon, transverse variations in salinity are generally more significant than vertical variations. The number of layers and stations need not be large to resolve gradients. Park and James (1990) found, for example, that four stations and seven layers were sufficient. This is in good agreement with results of Kjerfve et al. (1981). The sampling grid can be defined in such a way that layers are of uniform thickness, or proportional to local depth (sigma coordinates). Rattray and Dworski (1980)and an anonymous reviewer have recommended against the

90

Water, Salt and Heat Balance of Coastal Lagoons

common practice of proportional spacing of vertical layers, because salt fluxes can be erroneously attributed to the transverse coordinate. Park and James (19801, however, have conducted a n error analysis using both transverse and vertical partitioning and concluded that either method can be used. Sampling in the vertical is usually from top to bottom at a given location, resulting in profiles of temperature, salinity and current velocity. Rattray and Dworski (19801, however, have noted that sampling at a given depth can be at locations spaced proportional to the width of the cross section at that depth. Once the transverse cross section has been gridded appropriately, velocity and salinity observations can be expressed in terms of vertical, transverse and/or cross section means, and in terms of departures from these means. The definitions of means and departures can become quite involved. A notable exception is the technique used by Murray and Siripong (1978) who express transverse and vertical effects in terms of the deviations of lateral and vertical means from the cross sectional means. Analysis of variance techniques were then used to correlate cross products and quantify the relative importance of vertical and lateral fluxes in the salt balance of a well mixed estuary. A more complex approach may be required to investigate more specific transport mechanisms. Results of a considerable number of salt balance studies can be summarized to identify, or at least infer the primary mechanisms for the salt balance of a coastal lagoon. First, and generally most important, is the advective transport associated with river discharge, tidal co-oscillations or associated with the slow non-tidal filling and draining that occurs as a consequence of low frequency fluctuations i n coastal sea level. Second, a tidal diffusion contribution involves the correlation between tidal period fluctuations in current speed, salinity and depth. Finally, ‘shear effects’ arise in response to spatial correlations of the transverse and vertical deviations in salinity and current speed. Listing the principal salt transport mechanisms in order of their importance is relatively difficult, because they vary considerably from one estuary to the next, often from one location to the next in a given estuary, and perhaps from one study period to the next due to seasonally changing wind or precipitation patterns. Also, the decomposition techniques employed from one study to the next can make comparison difficult. Nevertheless, some generalizations are possible. There seems to be widespread agreement that the advective transport associated with the mean flow is a dominant term. Pritchard‘s (1958) study of the salt balance of the James River Estuary concluded that the mean horizontal advection and the vertical diffusion of salt were the two most important terms. Park and James (1990) found that the most important term when averaged over one or more tidal cycles was the product of the cross sectional mean salinity and the cross sectional mean velocity. The instantaneous salt transport was dominated by the product of the tidal period mean salinity and the cross sectional mean

N.P. Smith

91

deviation from the tidal period mean velocity. Dyer (1974) has compared the salt balance of three estuaries with differing stratification. Results from the two partially mixed estuaries are relevant within the context of coastal lagoons. There, the lateral contribution to the total salt flux was similar in magnitude to the vertical contribution. Logically, in vertically mixed lagoons having significant lateral gradients, the lateral and longitudinal advection and diffusion terms are dominant. Murray and Siripong (1978), working in a shallow well mixed estuary near Guayaquil, Ecuador, quantified terms responsible for both upstream and seaward fluxes. Results confirmed that the longitudinal flux due to lateral gradients was greater than that associated with vertical mixing. In vertically mixed coastal lagoons, it is probable that lateral gradients in salinity, arising from groundwater seepage and freshwater runoff, combine with tidal and low frequency co-oscillations in the longitudinal flow to enhance the upstream dispersive flux of salt.

Heat Balance Warming and cooling in coastal lagoons is a straightforward result of a net gain or loss of heat energy through the air-water interface, a n advective gain or loss by tidal and non-tidal circulation, and to a lesser extent, gains and losses by conductive exchanges with the underlying sediments. Heat budget studies are conceptually similar to water balance studies in the sense that the heat budget equation, like the hydrologic equation (Eq. 4.1), contains terms representing both local and advective processes. The relative importance of local processes may be considerably greater in heat budget studies than in water budget studies. This is especially true for lagoons which have restricted exchanges with adjacent shelf waters. The heat budget of a coastal lagoon is summarized by the well known heat budget equation:

AQ

-=

At

(Qg

- Qb) + (Qh

-

QJ

+ Qm + QB

(4.21)

where AQ/At is the temporal change in total heat energy stored in the water column, Qsrepresents heating by incoming solar radiation (insolation), Qb is the net long wave radiative energy loss, Q h is the sensible heat flux, Qe is the latent heat flux, Qm represents conductive heat exchanges with the underlying sediments, and Qa is the advective heat flux. Qsand Qb have been combined because they are often considered together. Both are radiation terms, and both vary inversely with cloud cover. An overcast sky, for example, reflects insolation and thereby decreases heating. Cloud cover also insulates the lagoon against long wave radiative energy

92

Water, Salt and Heat Balance of Coastal Lagoons

losses. Similarly, Qh and Qe are often considered together, because both respond to the overlying air mass, and both are directly proportional to wind speed. Warm, humid maritime tropical air, for example, will inhibit sensible heat losses as well as evaporative cooling. Qm has received the least attention of all the heat budget terms, because instantaneous fluxes into and out of underlying sediments are characteristically small, and because the net gain or loss over the longest time scales is negligible. QB is the most difficult term to quantify, as discussed below, and in many studies it is calculated as a residual - the difference between what can be estimated for the other terms, and the actual temperature change that has been observed at the study site. Miller (1981) and Reed (1976a, 1978) discuss the calculation of insolation over a water surface. Several expressions are available for estimating solar heating. Basically, they consist of a correction to the solar constant to account for seasonal variations in the earth sun distance and the zenith angle at the latitude of the study site, a second correction to account for the sun angle between sunrise and sunset, a third for atmospheric turbidity, a fourth for cloud top reflection, and a fifth correction for surface reflectivity at the air-water interface. The result is a value for the insolation entering and thus heating the lagoon. Net outgoing long wave radiation is based upon the Stefan-Boltzmann equation, but again corrections are needed to account for the insulating effect of humidity and cloud cover. Reed (1976b) has discussed this at some length. A recent paper by Gardashov et al.(1988)discusses the increase in both emissivity and the reflectivity of thermal atmospheric radiation as a function of wind speed and hence the wave distortion of a water surface. Applying this correction increases radiative heat losses on the order of 5-1096. Sensible and latent heat fluxes can be investigated in a variety of ways, including: (1)cross correlation methods which require simultaneous measurements of wind components and air temperature; (2) aerodynamic methods which use exchange coefficients; (3) aerodynamic profile methods; and (4) bulk aerodynamic methods based upon air-water differences of potential temperature and specific humidity. The bulk aerodynamic method is probably the most used for lagoonal heat balance studies (see Hasse, 1970; Hsu, 1978; and Priestley and Taylor, 1972). Although sensible heat fluxes can either warm or cool a lagoon, in practice cooling is considerably more efficient than heating. When warm air comes in contact with relatively cool water, both the lower atmosphere and the upper layers of the water column will stratify as they approach a common temperature, and further heat transfer is reduced significantly. The most common form of the expression used to evaluate the sensible heat flux term includes the air-water temperature difference, wind speed at a standard height above the lagoon and a bulk aerodynamic coefficient for heat as a proportionality factor to relate the wind speed and temperature gradient to the heat flux that they produce.

N.P.Smith

93

Hsu (1978)has summarized values of the bulk aerodynamic coefficient found in several previous studies. Overland wind measurements can be substituted for over water applications, if an appropriate correction is applied (see Hsu, 1981,1986;Richards et al., 1966). Latent heat fluxes are also much greater as heat losses by evaporation than as heat gains by condensation. Expressions for latent heat fluxes are similar to those for sensible heat fluxes. Latent heat flux can be evaluated from wind speed, a measure of atmospheric moisture and the bulk aerodynamic coefficient for water vapor. Here, too, one must correct overland measurements t o approximate over water conditions, if local wind data are not available. Conductive heat exchanges with underlying sediments can be evaluated from the product of the thermal conductivity of the sediment and the vertical gradient in temperature (Sellers, 1965). In many cases, bottom sediments are layered, and either a mean value or a depth varying value must be used in the calculations. Substituting representative values for conductivity (see Geiger, 19651, calculations suggest that temperature changes at depths 2-3 m below the water-sediment interface may be small enough to neglect, even over the longest time scales. Heat fluxes through the water-sediment interface are characteristically 1-2 orders of magnitude smaller than fluxes through the air-water interface (Smith, 19821, because at the surface individual processes may act to warm and cool the water column simultaneously, and their net effect may be small. At most, sediments act only to store heat over seasonal and sub seasonal time scales. Over seasonal time scales, temperatures may penetrate a few meters; over time scales on the order of a few days, heat fluxes penetrate only a few tens of centimeters. The special case of heat fluxes from sediments that are alternately heated at low tide, then inundated by the flood tide has been described by Vugts and Zimmerman (1975)but will not be considered here. Advective heat fluxes are computed from the product of the water temperature, T,density, p, specific heat, cp,and current speed, u:

(4.22) where u is directed normal to the cross section. In practice, vertical temperature gradients (and thus density gradients) will be slight, because wave mixing in shallow water will act to redistribute heat vertically as it arrives through the air-water interface. Heath (1977)has taken advantage of the mixing effect of waves and turbulence to use instrumentation from a single intermediate level to quantify advective heat transport through the channel connecting Pauatahanui Inlet with the Tasman Sea. Within a lagoon, however, vertical and lateral temperature gradients may be significant, and both lateral and vertical current shear in the longitudinal flow may be

94

Water, Salt and Heat Balance

of

Coastal Lagoons

complex if the current responds to both wind and tidal forcing. It is rare that one can resolve adequately the spatial variability in both temperature and velocity structure. Thus, one either interpolates based upon limited available measurements, approximates, calculates this term as a residual, or ignores it altogether and focuses on local fluxes. Temperature data obtained over sufficiently long study periods indicate that net heat fluxes and the resulting warming and cooling occur over three distinct time scales. A diurnal rise and fall in temperature occurs as a logical response to heating by incoming solar radiation during daylight hours counteracted by relatively steady evaporative and long wave radiative cooling throughout the day. Cooling by evaporation may in fact show a subtle, though distinct periodicity because of the diurnal variation in wind speed. Over time scales on the order of 1-2 weeks, warming and cooling occurs as a response to synoptic scale weather systems passing over the coastal zone, affecting cloud cover, wind speed, and perhaps air temperature and humidity. Finally, seasonal warming and cooling resulting from variations in the zenith angle of the sun produce a n annual cycle onto which all higher frequency fluctuations are superimposed. Figure 4.5 contains two time series obtained from Florida’s Indian River lagoon during midsummer (upper) and midwinter (lower) months. The separation of the two curves emphasizes seasonal variations; the higher frequency rise and fall of the 24-hour mean values indicates the day to day variations about seasonal means that occur in response to weather systems moving through the area.

E

24-

14 --

I2

~

10-

DEC

1

JAN

FEB

MAR

Fig. 4.5. Daily mean temperatures during a 110-daytime period from June 16 to October 3, 1979 (top) and from December 16, 1977 to April 4, 1978 (bottom) at a study site in Florida’s Indian River lagoon (27”32.0”, 80°21.0W).(After Smith (1983);used with permission).

N.P. Smith

95

Results of heat budget studies in coastal lagoons can be summarized relatively easily. Lagoonal heat budget studies show that without exception insolation is the dominant heating term (Smith, 1981a, 1981b); latent heat fluxes and net outgoing long wave radiation are the dominant cooling processes. The latter are generally of the same order of magnitude, however in tropical settings Qe may be somewhat larger because of higher saturation vapor pressures in warm air. Conversely, at higher latitudes, lower atmospheric vapor pressure permits greater radiative cooling. Sensible heat fluxes may be significant over time scales on the order of several days, as weather systems move through the study area. However, over longer time scales sensible gains and losses tend to cancel, leaving a much smaller seasonal variation. In most settings, because lagoons respond to warming and cooling processes rapidly by virtue of their shallow depth, they will warm more quickly than shelf waters in spring and cool more quickly in autumn months. Thus tidal and low frequency exchanges produce a n advective cooling and warming that is relatively well defined during the period of spring warming and fall cooling, respectively. Heath's (1977) study of Pauatahanui Inlet revealed advection to be a significant term in the heat budget of the inlet, however it is probable that the advection term becomes significantly less important in the interior of a larger lagoon. Transient perturbations of the seasonal cycle in advective heat fluxes occur as synoptic scale weather systems move across the coastal zone, and more persistent local anomalies may occur in response to upwelling events. For example, Alvarez-Borrego and Alvarez-Borrego (1982) have described a situation along the coast of Baja California in which coastal upwelling during summer months provide a source of cool water that is drawn into San Quintin Bay and Estero de Punta Banda on the flood tide. As a result, strong seaward directed temperature gradients and advective cooling of the lagoons persist well into midsummer months. An example of a heat budget study conducted in a subtropical lagoon in winter months is shown in Figure 4.6. Only local fluxes were considered in this study. Insolation and outgoing long wave radiation are combined at the top of the plot. Insolation averaged 236 Watts m-2during the study, but day to day variations in the daily means occurred in response to changes in cloud cover. Outgoing long wave radiation, in contrast, is relatively stable in time. Sensible and latent heat fluxes, shown in the middle of the plot, are similar in many respects, as a result of their response to the overlying air mass. Water-sediment conduction, at the bottom of the plot, is relatively insignificant. The mean over the study period was only 3 Watts m-2. While the plot and the mean values are study site specific, the relative magnitudes of the local heat flux terms are more generally representative. On average, for this winter-spring study, the heat balance of this coastal lagoon involves a gain by insolation balanced by losses due to latent heat fluxes, outgoing long wave radiation, and to a lesser extent sensible heat flux.

96

Water, Salt and Heat Balance of Coastal Lagoons

- 100

$l-&d-L-

I

Water-Sediment Cmductm

-100

20

DEC

I

15

JAN

I

15

FEB

I

15

MAR

I

15

APR

Fig. 4.6. Composite of five local energy exchange processes simulated for a study site in Florida's Indian River lagoon (27"32.1'", 8O021.OW),December 20, 1979 to April 16, 1980. (After Smith (1982);used with permission).

A recent study of Corpus Christi Bay, Texas (Smith, 1989)quantified local heat energy fluxes over time scales ranging from hours to seasons. An important, though expected finding was that individual terms can vary significantly over the course of a year. For example, mean insolation increases by over a factor of two between winter (December-February), and summer (June-Augus t) months; evaporative cooling increases by over a factor of four. Mean net outgoing long wave radiation and sensible heat fluxes are relatively constant from one season to the next. Seasonality of another kind appears in low frequency variability over time scales on the order of several days (Fig. 4.7). Frontal passages in late fall, winter and early spring months increase the day to day variability in outgoing long wave radiation and especially in sensible and latent heat fluxes. A significant problem in many heat budget studies arises from the use of weather data collected some distance from the study site. In the coastal zone, it is common for significant spatial gradients to exist - usually oriented normal to the coast, rather than parallel to it. Cross shelf gradients in air temperature, wind speed and perhaps cloud cover are most pronounced. During summer months, the diurnal cycle of air temperature can be significantly greater over a land surface than over a water surface. The resulting variations in density produce a pressure gradient which drives the

97

N.P. Smith Incorning Solar Radiation

5001 400 A

300

nl

E

> *0

200 100

L1

z

01

I

I

1

I

I

I

I

I

I

I

I

I

f

I

I

I

Outgoing Longwave Radiation

O1

Sensible Heat Flux

3002

Latent Heat Flux

I

-20

I

I

I

I

I

I

I

I

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Fig. 4.7. Composite of daily average local heat energy gains and losses calculated for Corpus Christi Bay, Texas (27"50'N,97"15W),1974. (After Smith (1989); used with permission).

familiar land and sea breezes. Furthermore, stronger surface heating will increase convective activity and thus cloud cover over a land surface. Conversely, during winter months, cloud cover affecting the heat budget of a coastal lagoon may be greater than that recorded at an inland weather station, because of the heat and moisture put into the atmosphere over the continental shelf. As noted above, radiation inversions forming over land surfaces, but not over water, can result in significantly different surface

98

Water,Salt and Heat Balance of Coastal Lagoons

wind conditions between the study site and the nearest airport. For all these reasons, observations at a n inland weather station may be unrepresentative of conditions at the coast to which the lagoon is responding. This matter has been discussed by Smith (1985),who concludes that errors introduced by spatial gradients tend to cancel over extended simulations. Thus, routine weather observations should be satisfactory for many types of investigations of heat budgets over time scales in excess of several days. Concluding Remarks

The literature survey conducted during the preparation of this chapter indicated convincingly that coastal lagoons as a group are less extensively studied than are drowned river valley estuaries and perhaps even fjords. In many cases, the importance of a given physical process in a lagoon setting can only be inferred from findings obtained in investigations of well mixed drowned river valley estuaries. In view of the broad geographical distribution of lagoons, one must conclude that the dearth of lagoonal studies reflects the spatial distribution of investigators as much as the distribution of lagoons. The literature survey also revealed a paucity of information on processes involving water-sediment interactions. For example, groundwater seepage is poorly documented, and its relative importance in the hydrologic balance of a coastal lagoon remains problematic. Also, water-sediment heat energy exchanges have received little attention in most heat budget studies. Investigators who have quantified this term, however, are in agreement that it is relatively unimportant as a warming and cooling process over longer time scales. Both terms deserve more attention i n future studies. A comparison of several processes acting in concert becomes difficult when analytical methodologies are inconsistent from one study to the next, especially in salt balance investigations. Differences between estuaries require different methodology in some cases. In other cases, however, the methodology selected represents only a n attempt to explore new analytical techniques. While this is commendable in and of itself as a contribution to marine science, it would be helpful if investigators would make a n attempt to broaden their analyses wherever possible to incorporate common methodology, facilitate comparisons and in that way make sorely needed generalizations regarding physical processes in coastal lagoons. References Alvarez-Borrego, J. and Alvarez-Borrego, S. 1982 Temporal and spatial variability of temperature in two coastal lagoons. CalCOFZ Report 23, 188-197. Bokuniewicz, H. 1980 Groundwater seepage into Great South Bay, New York. Estuarine and Coastal Marine Science 10,437-444.

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99

DiLorenzo, J.L. 1988 The overtide and filtering response of small inlethay systems. In Hydrodynamics and Sediment Dynamics of Tidal Inlets (Aubrey, D.G. and Weishar, L., eds). Lecture Notes on Coastal and Estuarine Studies, vol. 29, Springer-Verlag, New York, pp. 24-53. Dyer, K.R. 1973 Estuaries: A Physical Introduction. John Wiley and Sons, London. 140 PP. Dyer, K.R. 1974 The salt balance in stratified estuaries. Estuarine and Coastal Marine Science 2, 273-281. Fischer, H.B. 1972 Mass transport mechanisms in partially stratified estuaries. Journal of Fluid Mechanics 53,671-687. Gardashov, R.G., S h i f i n , K.S. and Zolotova, J.K. 1988 Emissivity, thermal albedo and effective emissivity of the sea at different wind speeds. Oceanologica Acta 11, 121-124. Geiger, R. 1965 The Climate Near the Ground (Revised Edition). Harvard University Press, Cambridge. 611 pp. Hasse, L. 1970 On the determination of the vertical transports of momentum and heat in the atmospheric boundary layer at sea. Technical Report Number 188, Deptartment of Oceanography, Oregon State University, Corvallis. 55 pp. Heath, R.A. 1977 Heat balance in a small coastal inlet Pauatahanui Inlet, North Island, New Zealand. Estuarine and Coastal Marine Science 6,783-792. Hsu, S-A. 1978 Micrometeorological Fluxes in Estuaries. In Estuarine Transport Processes (Kjerfve, B., ed.). University of South Carolina Press, Columbia, S.C., pp. 125-134. Hsu, S-A. 1981 Models for estimating offshore winds from onshore meteorological measurements. Boundary-Layer Meteorology 20,341451. Hsu, S-A. 1986 Correction of land-based wind data for offshore applications: a further evaluation. Journal of Physical Oceanography 16,390494. Hughes, F.W. and Rattray, M. 1980 Salt flux and mixing in the Columbia River Estuary. Estuarine and Coastal Marine Science 10,479-493. Ianniello, J.P. 1977 Tidally induced residual currents in estuaries of constant breadth and depth. Journal of Marine Science 35,755-786. Kjerfve, B. 1986 Comparative oceanography of coastal lagoons. In Estuarine Variability (Wolfe, D.A., ed.). Academic Press, New York, pp. 63-81. Kjerfve, B., Stevenson, L.H., Proehl, J.A., Chrzanowski, T.H. and Kitchens, W.M. 1981 Estimation of material fluxes in an estuarine cross-section: A critical analysis of spatial measurement density and errors. Limnology and Oceanography 26,325-335. Liu, J.T. and Aubrey, D.G. Tidal residual currents through multiple tidal inlets. In Formation and Evolution of Multiple Tidal Inlet System (Aubrey, D.G. and Giese, G.S., eds). Springer-Verlag, New York (in press). Miller, D.H. 1981 Energy at the Surface of the Earth. Vol. 27, International Geophysics Series. Academic Press, New York. 516 pp. Murray, S.P. and Siripong, A. 1978 Role of lateral gradients and longitudinal dispersion in the salt balance of a shallow, well-mixed estuary. In Estuarine Transport Processes (Kjerfve, B., ed.). University of South Carolina Press, Columbia, S.C., pp. 113-124. Pandit, A. and El-Khazen, C. 1990 Groundwater seepage into the Indian River Lagoon at Port St. Lucie. Florida Scientist 53, 169-179. Payne, R.E. 1972 Albedo of the sea surface. Journal ofAtmospheric Science 29,959-970. Pitts, P.A. 1989 Upwind return flow in a coastal lagoon: seasonal-scale barotropic

100

Water, Salt and Heat Balance of Coastal Lagoons

transport. Estuaries 12,92-97. Priestley, C.H.B. and Taylor, R.J. 1972 On the assessment of surface heat flux and evaporation using large-scale parameters. Monthly Weather Review 100, 81-92. Pritchard, D.W. 1955 Estuarine circulation patterns. Proceedings of the American Society of Civil Engineering 81, 71711-717111. Pritchard, D.W. 1958 The equations of mass continuity and salt continuity in estuaries. Journal of Marine Research 17,412-423. Pritchard, D.W. 1960 Salt balance and exchange rate for Chincoteague Bay. Chesapeake Science 1 , 4 8 4 7 . Rattray, M. and Dworski, J. 1980 Comparison of methods for analysis of the transverse and vertical circulation contributions to the longitudinal advective salt flux in estuaries. Estuarine and Coastal Marine Science 11,515-536. Reed, R.K. 1976a An evaluation of cloud factors for estimating insolation over the ocean. NOAA Technical Memo ERL PMEL-8. Seattle, Washington, 20 pp. Reed, R.K. 1976b On estimation of net long-wave radiation from the oceans. Journal of Geophysical Research 81,5793-5794. Reed, R.K. 1978 Cloud effects on insolation over tropical Pacific Ocean. Journal of Applied Meteorology 17,401404. Richards, T.L., Dragert, H. and McIntyre, D.R. 1966 Influence of atmospheric stability and over-water fetch on winds over the lower Great Lakes. Monthly Weather Review 94,448-453. Sellers, W.D. 1965Physical Climatology. University of Chicago Press, Chicago. 272 pp. Smith, N.P. 1974 Intracoastal tides of Corpus Christi Bay. Contributions in Marine Science 18, 205-219. Smith, N.P. 1981a Energy balance in a shallow, coastal lagoon: winter conditions. Limnology and Oceanography 28,482-491. Smith, N.P. 1981b Local energy exchanges in a shallow coastal lagoon: summer conditions. Atmosphere-Ocean 19,307-319. Smith, N.P. 1982 An investigation of the heat budget of the Indian River lagoon, Florida, during winter months. Oceanologica Acta, Proc. Znternational Symposium on coastal lagoons, SCORIIABOI UNESCO. pp. 185-189. Smith, N.P. 1983 A comparison of winter and summer temperature variations in a shallow bar-built estuary. Estuaries 8,2-9. Smith, N.P. 1985 The suitability of routine weather data for estimating local estuarine heat energy fluxes. Estuaries 8, 270-278. Smith, N.P. 1988 The Laguna Madre of Texas: Hydrography of a Hypersaline Lagoon. In Hydrodynamics ofEstuaries (Kjerfve, B., ed.). CRC Press, Boca Raton, Florida, pp. 3140. Smith, N.P. 1989 The heat budget of a coastal bay. Contributions in Marine Science 34, 1-16. Smith, N.P. 1990a Computer simulation of residual tidal transport in a coastal lagoon. Journal of Geophysical Research 95, 18, 205-18,211. Smith, N.P. 1990b Wind domination of residual tidal transport in a coastal lagoon. In Residual Currents and Long-term Transport (Cheng, R.T., ed.). Coastal and Estuarine Studies 38, 123-133. Smith, N.P. 1990 Longitudinal transport in a coastal lagoon. Estuarine, Coastal and Shelf Science 31: 835-849.. Tee, K. 1976 Tidal-induced current, a 2-D nonliner numerical tidal model. Journal of

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Marine Research 34,603-628. Thiessen, A.H.1911 Precipitation for large areas. Monthly Weather Review 39,10821084. van de Kreeke, J. 1974Tide-induced mass transport in lagoon-inlet systems.In Proceedings of the X N International Conference on Coastal Engineering. Copenhagen, pp. 2290-2301. van de Kreeke, J . 1976 Tide-induced mass transport: a flushing mechanism for shallow lagoons. Journal of Hydraulic Research 14,6147. van de Kreeke, J. and Dean, R.G.1975 Tide-induced mas8 transport in lagoons. Journal of Water Resources, Harbors and Coastal Engineering Division, ASCE 101,393- 402. Vugts, H.F. and Zimmerman,J.T.F. 1975 Interaction between the daily heat balance and the tidal cycle. Nature 255, 113-117.

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Coastal Lagoon Processes edited by B. Kjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

103

Chapter 5

Modeling of Circulation and Dispersion in Coastal Lagoons Malcolm L. Spaulding

Ocean Engineering, University of Rhode Island, Kingston, Rhode Island 02881, USA

A review of strategies to model the circulation and pollutant transport in inletcoastal lagoon systems is presented. Simplified procedures based on correlation analysis, analytic solutions to the ordinary linearized differential equation describing the system, and a numerical model that solves the governing equation for multi-inlet, multi-basin systems are described. The models predict the inlet flow characteristics, basin surface elevation response, and pollutant concentrations for cases where the basin responds in a simple pumping or Helmholtz mode. To describe circulation and pollutant movement when the pumping mode assumption is inappropriate two-dimensional, vertically-averaged models based either on finite element or finite difference solution methodologies are presented. This approach is also employed when the specific interest is circulation or pollutant transport within the basin. A unique hybrid model, coupling a one dimensional inlet model with a two-dimensional, vertically-averaged model for the basin, is described. The use of boundary fitted coordinate models is explored exploiting their ability to accurately describe areas with disparate spatial scales. A procedure is presented to assist in model selection for inlet-lagoon systems. To demonstrate model application for management decision making a simplified approach is applied to predict the impacts of inlet channel modification on the circulation, flushing dynamics, and salinity in the Charlestown, R.I. pond system.

Introduction Coastal lagoons or ponds are ecologically and economically important because of their high productivity and their intensive use for aquaculture facilities, recreation, and waste disposal. The high productivity in part derives from retention of material in these systems. Unfortunately this retentive capacity and typical high development pressures on the land surrounding the lzgoons result in poor flushing and large pollutant loads to the system. The end result is usually degraded water quality and decreased

Modeling of Circulation and Dispersion in Coastal Lagoons

104

productivity. Because of the conflicting demands on these areas to serve simultaneously as breeding and nursery grounds for fisheries, waste receptacles and recreational areas, it is necessary to understand the circulation and mixing dynamics and how they might be altered under a variety of management (structural or policy) scenarios. The present chapter reviews models of the circulation and mixing dynamics of lagoon systems, proceeding from simple correlation analyses to full scale numerical hydrodynamic and pollutant transport models. Model selection and application guidelines are presented. Application of simplified models to a selected lagoon system is then given to illustrate the use of the models to assess the impact of various management strategies. Simplified Circulation and Pollutant Transport Models for Inlet-Lagoon Systems Hydrodynamics

A typical inlet-lagoon system (Fig. 5.1) normally consists of one or more relatively narrow (branching) channel(s) connecting one or several basins, lagoons or ponds to the ocean. The seaward side of the lagoons are typically formed by a barrier beach or otherwise narrow strip of land. The simplest of models for such systems focus on the inlet channels since these passages, usually narrow, control the transport of water and salt (and other substances) between the lagoon and the ocean. The lagoon is typically treated as a water storage basin in these models and the surface elevation assumed to vary uniformly in a pumping or Helmholtz mode response. The governing equation for a one-dimensional inlet is written as

au -+ at

au

ah Y UI UI ax h

u-=-g---

ax

(5.1)

where U is the cross-sectionally averaged along channel velocity, g is gravity, h is the water depth in the inlet and y is the bottom friction coefficient. The bottom frictional losses are expressed by a quadratic law where the absolute value sign is used to assure the appropriate sign of the friction. Integration of Eq. 5.1 along the channel axis yields a n expression relating the sea surface level to that in the basin. Assuming the inlet is finite in length, L , Eq. 5.1 can be integrated to yield

(5.2) where h, and h b are the surface elevations in the ocean and bay, respectively, and It represents the combined entrance, It,,, and exit, It,,, losses as the flow enters and leaves the channel (k = It,, + k,3. Assuming that the

M.L. Spaulding

105

Plan View --

BAY SEA

Fig. 5.1. Schematic of inlet-lagoon system.

basin oscillates in a pumping mode (uniform spatial response) the mass conservation equation for the basin becomes

(5.3) where A, is the cross-sectional area of the inlet and Ab the surface area of the basin. Equation 5.3 assumes that changes in Ab and A, due to depth variations are small over a tidal cycle. Eliminating U in Eq. 5.2 by use of Eq. 5.3,we obtain (5.4)

Equation 5.4 can now be non-dimensionalized to determine the parameters

106

Modeling of Circulation and Dispersion in Coastal Lagoons

characterizing inlet behavior. The dimensionless variables, noted by a n asterisk, are defined as

(5.5) where T is the forcing period and a, is the sea level range in the ocean. Using Eq. 5.5 to define the dimensionless quantities, Eq. 5.4 becomes

Equation 5.6 has three dimensionless parameters.

GI, GS and Gs characterize the temporal acceleration, convective acceleration, and frictional dissipation terms in the governing equation (Eq. 5.1), respectively. Based on order of magnitude analysis for typical inlet-lagoon systems the frictional loss term normally is large compared to the temporal or convective acceleration terms. G3 is therefore a n appropriate choice to analyze inlet-lagoon response. To examine the usefulness of parameterizing the system employing G3, data on existing inlet-bay systems and laboratory experiments were collected. Only those inlet-basin systems having a well defined single inlet where the bay oscillates in a pumping mode were chosen. The data (Table 5.1) were collected on actual systems (Mota Oliviera, 1970; Vincent and Corson, 1980; Shemdin and Forney, 1970; O'Brien and Clark, 1974; Seelig et al., 1977; Isaji et al., 1985; Bruun et al., 1978; and Baines, 1957) and from laboratory experiments by Mayor-Mora (1973,1974). Figures 5.2,5.3,and 5.4 show the dimensionless amplitude response, high water phase lag and low water phase lag versus the logarithm of G, defined as the inverse square root of G3, respectively. The dimensionless amplitude response is defined as the sea surface elevation range in the bay, a b , divided by the value in the ocean, a,. The phase lags are non-dimensionalized by the period of the primary forcing function in the ocean. For small values of G the inlet acts as a filter, markedly decreasing the amplitude response and increasing the phase shift between the driving ocean sea surface elevation and that in the lagoon. As G increases, the filtering effectiveness of the inlet decreases. When G is on the order of 1 or larger sea surface elevation perturbations in the ocean travel through the inlet unimpeded.

TABLE 5.1 Tabulated data for prototype lagoon systems Q

System (Reference)

AC (m2)

Macquaire Harbor Inlet (Mota-Oliveira, 1970)

4200

7000

Rockway Inlet (Bruun et al., 1978)

5580

Lake Wyman Inlet (Shemdin and Forney, 1970)

Boca Raton Inlet (Shemdin and Forney, 1970)

L (m)

a0

T

Ub

EH

(m)

(hr)

(m)

(hr)

EL (hr)

6

0.9

24

0.44

4.0

4.0

7050

6.4

1.42

12.42

1.576

0.77

0.77

156

2235

2.4

1.03

6.3

0.91

0.46

0.23

130

814

2.1

1.19

6.3

1.03

0.41

0.76

Burrard Inlet (Baines, 1957)

4508

1830

12.2

4.38

24

4.42

0.3

0.49

Masonboro Inlet (Seeliget ul., 1977)

1171

830

3.35

1.31

12.42

1.305

0.5

0.4

42

1000

1.5

0.88

12.42

0.18

2.8

3.48

5920

5750

12.5

1.13

12.42

1.22

0.3

0.48

3300

10100

1.25

12.42

0.21

3.226

3.226

Charlestown Inlet (Isaji et al.,1985)

San Diego Inlet

h (m)

Ab (m2)

(O'Brien and Clarke, 1974)

Fire Island Inlet (O'Brien and Clarke, 1974)

4.27

5 5

z

Modeling of Circulation and Dispersion in Coastal Lagoons

108 1.6

1.4 1.2 W VI C 0

1 .o

a Lo W

rY W -0

3

c .-

n

E

0.8 0.6

0.4

Q

0.2 0.0

I

I

I

I

I

-3

-2

-1

0

1

-0.2

Log (GI

Fig. 5.2. Dimensionless bay amplitude versus log (GI for prototype and laboratory data. Lines show results of correlation analysis.

0.40

1

0.35

I

0

Experimental Data

A

Inlet Data

-

0.30

I

I

I'

0.25

1st Order Curve 2nd Order Curve 3 r d Order Curve Slqmold Curve

0.15 0.10

-

0.05

-

0.00

-

0

I

-0.05 -0.10

\ I

-3

-2

I

-1

I

0

1

Log ( G I

Fig. 5.3. Dimensionless high water phase lag versus log (GI for prototype and laboratory data. Lines show results of correlation analysis.

While geometric constraints are important in establishing the value of G it also depends on the forcing period. As the forcing period decreases, G decreases and the inlet filtering effectiveness increases.

M.L. Spudding 0.40

0.35

109

'

0.30 T1

0 ._ L

0

0.25

a \

0.20

0 2 0

0.15

-

0.10

-

0.05

A

0.00 -0'05 -0.10

t

--

.....

I

-3

Inlet Data

1 s t Order Curve 2 n d Order Curve 3 r d Order Curve Sigmoid Curve I

-2

I

I

I

-1

0

1

Log (G)

Fig. 5.4. Dimensionless low water phase lag versus log (G) for prototype and laboratory data. Lines show results of correlation analysis.

A least-squares regression analysis was performed for amplitude response and higMow water phase lag versus log G for first, second and third order polynomials and a sigmoid function. The sigmoid function was selected because it displays asymptotic behavior at high and low values of G which are consistent with observations. The general form of the polynomial power series is y = co + c1x

+ c2x2 + c3x3

(5.7a)

The sigmoid function is given by (5.7b) where x is equivalent to log G and y is the amplitude response and higMow phase lags. The coefficients, Cj, are given in Table 5.2. The regression curves are plotted in Figs. 5.2,5.3, and 5.4. Comparison of the sigmoid regression curves to the field (inlet) and experimental (laboratory) data show fair to reasonable agreement. The exceptions occur mainly in the field data. The polynomial representations are less appealing since they do not correctly approach acceptable limits as G becomes greater than 1.0 or less than 0.01. Attempts to perform regression between the three response variables and GIshowed poor agreement. Correlations with Gz were better but not as good as with GB.

Modeling

110

of

Circulation and Dispersion i n Coastal Lagoons

TABLE 5.2 Coefficients of polynomial power series and sigmoid curve fit to the amplitude response and higMow water phase lag Functional form

co

Amplitude Response Polynomial 1st order Polynomial 2nd order Polynomial 3rd order Sigmoid

1.0391 1.0337 0.9784 -1.0

0.4600 0.4448 0.0464 5.0

High Water Phase Lag Polynomial 1st order Polynomial 2nd order Polynomial 3rd order Sigmoid

0.0444 0.0154 0.3374 -0.2

-0.0589 -0.1182 0.0140 -3.0

-0.0168 0.1579 -0.8589

0.0587 0.2

0.0427 0.0344 0.0439 -0.3

-0.1281 -0.1516 -0.0830 -4.0

-0.0144 0.0794 -0.8326

0.0306 0.3

c1

CZ

c3

-

-0.0073 -0.5339 -1.0881

-

-0.1769 1.0

-

Low Water Phase Lag Polynomial 1st order Polynomial 2nd order Polynomial 3rd order Sigmoid

Based on this analysis it is concluded that for inlet-bay systems with well defined inlets, where the lagoon oscillates in a pumping mode, operating at frequencies removed from resonance, and where frictional effects dominate, the responses obtained by correlations with G are fairly accurate. G is a particularly useful parameter since it is based solely on quantities that are easily measurable in the field or laboratory. It does not require estimation of friction coefficient or entrance or exit loss coefficients. Several investigations have directly solved Eq. 5.4, or a variant, to determine the solution for the surface elevation in the lagoon. Once this is obtained, Eq. 5.3 can be solved to determine the flow rate and velocity through the inlet. Solutions, assuming sinusoidal forcing, are presented by Shemdin and Forney (19701, Huval and Wintergerst (19771, Mayor-Mora (1973), King (19741, Goodwin (19741, King and Shemdin (1975) and Seelig and Sorensen (1977). Baines (19571, Freeman et al. (1974) and Kondo (1975) present solutions by linearizing the frictional and entrancdexit loss terms. Van de Kreeke (1967), Keulegan (1967) and Mota Oliveira (1970) neglected the inertial terms while Dean (1971) and Brown (1928) neglected inertial terms and linearized dissipation.

M.L.Spaulding

111

One of the most fully developed solutions to Eq. 5.4 that is valid for the entire amplitude and frequency range and incorporates both friction and inertial terms was developed by Ozsoy (1977) and summarized i n Mehta and Ozsoy (1978). Assuming that the ocean and basin sea surface elevation responses are sinusoidal Ozsoy (1977) linearized the inertial and frictional terms by use of a truncated Fourier series (1st order). The solution to the governing equation is then given as

ab 42[(1-

a214

+ p211'2 - 2 ( 1 -

a2)2

CL2

a0

(5.8)

where p=-

16pa2

37c

is a n effective dissipation factor expressed in terms of

(5.9)

a is the dimensionless forcing frequency specified as 0

(5.10)

and w is the frequency of forcing (w = 2dT).Note that a equals 1corresponds to resonant conditions. The phase lag, E, between the ocean and the bay is given as E = tan-'

[

CL

[%)I

2 ( 1 - a2)

(5.11)

a0

For low damping near the resonant frequency (a= 1) the tide is amplified in the basin compared to its ocean level (Fig. 5.5). The amplitude response decreases with increased damping. When a is below 0.1 the response, independent of damping, is about 1. As a increases beyond 0.1, the inlet serves as a n increasingly effective filter resulting in a continuing decrease of the amplitude response. For a > 5 the amplitude response is small (<0.05) and asymptotically approaches zero as a increases. For low damping and low frequencies, a c 1, the ocean and basin sea surface elevation variations are in phase, while for a > 1 they are 180" out

112

Modeling of Circulation and Dispersion in Coastal Lagoons

DIME NSlON LESS FREQUENCY OL

Fig. 5.5. Dimensionless bay amplitude versus dimensionless frequency predicted by Mehta and Ozsoy’s (1978) model.

of phase (Fig. 5.6). As damping increases the relative phase lags increase and form a smooth, continuous transition between frequencies above and below resonance. If we take the limit as p becomes large (systems dominated by inlet dissipation), then the amplitude response reduces to the simple expression

M.L.Spaulding

113

5

1

DIMENSIONLESS FREQUENCY

o(

Fig. 5.6. Phase lag, e, versus dimensionlessfrequency predicted by Mehta and Ozsoy’s (1978) model.

(5.12)

The dependence of udu, on o,A,, Ah, and L predicted in Eq. 5.12 is exactly that determined by the dimensionless correlation analysis presented at the beginning of this section. The disadvantage of using Eq. 5.12 however is that one must estimate p which requires determination of entrance and exit losses and bottom friction dissipation. Speer and Aubrey (1985) have extended the simplified analyses to address the problem of a surface elevation dependent inlet cross sectional area. They studied several analytic forms to describe the channel side walls. When the cross sectional area showed large variations over the tidal cycle strong nonlinearities were generated in the flow. The characteris tic signature was flood-ebb tidal asymmetry (duration and amplitude) and the presence and amplification of tidal harmonics and compound tides. The models described above have approximated the system by an ordinary (linearhon-linear) differential equation to predict the surface elevation response. In most cases the equation has been linearized to simplify its solution. To address a wider class of problems (multiple inlet, multi-basin), including increased definition of the 1ateraVlongitudinal variability in the inlet channel and a basin surface area that varies with sea surface eleva-

114

Modeling of Circulation and Dispersion in Coastal Lagoons

tion, Seeliget al. (1977) developed a simple numerical model that solves the area averaged momentum equations for the inlets and the continuity equations for the basins. The model assumes that: The bay responds in a pumping mode; Bay water surface area and inlet channel cross-sectional area are functions of bay water level; Wind stress and Coriolis acceleration can be neglected; The storage of water in the inlet is negligible compared to that in the bay; The local water level slope in the inlet is assumed to be linearly related to the local friction loss along the inlet between the sea and bay level; At least one inlet must continuously connect the bay and the sea, although some areas of inlets may go dry during the water level cycle, and one or more inlets may go dry as long as one inlet contains water. A flow net, consisting of a series of laterally partitioned interconnected channel segments, is used to represent the inlet(s). Along and across channel width and depth variability can be accurately represented using this approach. Branching, reconnecting, and multi-passage channels can readily be accommodated using the flow net representation. Cross-sectional areas of the channel and the bay surface area may be selected as a function of sea level. Inflow into the bay from sources other than the inlet may be a function of time or bay surface elevation. Frictional dissipation is approximated by the Manning formulation and is quadratically dependent on the cross-sectionally averaged channel velocity. The lateral flow distribution across each longitudinal section of channel is described by a weighting function. This function is selected such that the frictional dissipation in the inlet(s) is minimized. Convective accelerations are evaluated in terms of empirical loss coefficients (kenand k,3. The non-linear differential equation describing the system is solved by a fourth order Runge-Kutta-Gill method to predict the time-dependent surface elevations in the bay and the inlet discharges and velocities. The method is self-starting, extremely stable, converges quickly, uses reasonably long time steps, and is computationally inexpensive. The model code is well documented (Seelig, 1976; Seelig et al., 19771, widely distributed and has received considerable use.

Simplified Pollutant Transport Models Isaji and Spaulding (1981a) proposed a box model for estimating the steady-state concentration of dissolved constituents in a completely mixed single inlet-multiple basin system with a steady inflow of fresh water. This model was extended by DiLorenzo et al. (1989) to incorporate a mass loading

M.L. Spaulding

115

term. The final constituent mass balance equation is written as PCo+M=(P+R)Cb

(5.13)

where C, and c b are the ocean and basin constituent concentrations, respectively. P is the basin tidal prism, R is the freshwater volume input per tidal cycle and M is the constituent mass loading per tidal cycle. Rearranging to solve for the dimensionless concentration gives (5.14) This is a standard tidal prism formulation modified to account for external loading, M. It assumes complete mixing within the basin or sub-basins and no recycling of ebb discharged water on the following flood tide. The tidal prism volume, assuming that the basin operates in a pumping mode and that the inlet surface area is small compared to that in the basin, is given as

P = UbAb

(5.15)

Following DiLorenzo et al. (1989)M and R can be expressed in dimensionless form as

(5.17) where r and m are the nondimensionalized freshwater input and mass loading rates, respectively. R is the integrated freshwater input volume from river discharge, surface runoff and ground water sources. Substituting Eqs. 5.16 and 5.17 into Eq. 5.14 gives ab

-+m c b c 0

- a0

(5.18)

ab

-+r a0

The ada, ratio can be obtained directly from the correlation relationship (Eq. 5.7b), the results of Mehta and Ozsoy (1978) (Eq. 5.8 or 5.12),both discussed earlier, or from data collected for the system ofinterest. DiLorenzo et al. (1989) have generalized the constituent mass balance equation to simulate periodic tidal variations in the constituents with a linear, first-order decay.

Modeling of Circulation and Dispersion in Coastal Lagoons

116

Since the solutions are rather complex the reader is referred to DiLorenzo et al. (1989)for a more thorough discussion. Two-dimensional Vertically Averaged Models of InletLagoon Systems Hydrodynamics

While the inlet-basin models described above provide a reasonable approach to describe the basic response of the lagoon to external forcing they provide no information on the circulation dynamics within the lagoon itself. In addition, if the basin does not respond in a simple pumping mode more complicated bathymetrically controlled flows are generated within the basin. This condition is observed when the dimensionless forcing frequency, a, is less than 0.1 a nd o r the basin dimensions are a large fraction of the wavelength of the external forcing. For these cases a n alternate method is needed to describe the circulation. Observing that most lagoon systems are shallow and well mixed, the conservation of mass and momentum equations are normally vertically integrated to represent the flow. This procedure effectively reduces the dimensionality of the system of equations and substantially facilitates their solution. The two-dimensional, vertically-averaged equations typically have the form Conservation of mass a17 a H u -+-+-at ax

aHv

ay

(5.19)

-0

Conservation of momentum aHu -+-+-

aHu2 a H W a17 1 -fv = - g + - (zs, ax JY ax P

aHv -+-+-

a H w

at

at

ax

+ zb,)

(5.20)

am2-fu = - g 317 1 + - (zsy + Thy)

(5.21)

JY

aY

P

where U,V are the vertically-averaged velocities in the x and y directions, respectively, f i s the coriolis parameter, 77 is the free surface elevation, H is the total water column depth and zs, ~b are the surface and bottom stresses, respectively, with the subscript indicating the direction. The surface and bottom stresses are normally approximated by quadratic formulations in the forcing velocity with a drag and ChezyNanning coefficient, respectively, to parameterize the proportionality constant. A thorough derivation of the vertically-averaged governing equations is presented in Leenderste (1967) and Harris and Bodine (1977).

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Because of the non-linear nature of these governing equations and the complex geometry and bathymetry of most lagoon systems these equations are solved by numerical methods. The primary solution methodologies are finite difference or finite element techniques. These can include explicit, implicit or semi-implicit representations in the time domain. Numerous variations of solution methodologies are summarized in the papers reviewed by Dronkers (19691, Hinwood and Wallis (1975a,b), Gordon and Spaulding (19771,Abbott (19761, Pinder and Gray (19771, and ASCE (1980). Specific examples of the application of two-dimensional models to lagoon systems are given in Short et al. (19741, Butler and Raney (19761, Harris and Bodine (19771, Chen and Hembree (19771, and Masch et al. (1977) for finite difference techniques and in Wang (1975) and Isaji et al. (1985) for finite element techniques. This list of references is meant to be illustrative, not comprehensive. The disparate (horizontal) scales between inlet channels and the basins in most lagoonal systems often make it computationally impractical to perform two-dimensional vertically-averaged simulations for the entire system. This occurs because the computational time-step for solutions of the shallow water wave equations are controlled by a stability limit or accuracy criteria which are directly proportional to the grid size. As the grid size reduces so does the allowable model time step. While it is possible through the use of rectangular or simply stretched grids to ameliorate this impact the computational burden still remains significant. To overcome this problem Isaji et al. (1985) developed a hybrid model. This model couples Seelig et al.’s (1977) one-dimensional channel model for the inlet with a two-dimensional, vertically-averaged finite element model for the basin. Careful attention to the interface between the two grid systems allows good representation of the flow. The finite element triangular grid system used in this model is well suited to represent complex areas and interfacing needs since the element sizes can be locally varied. The hybrid model was successfully applied to predict the circulation in the Charlestown Pond lagoon system (Isaji et al., 1985). The hybrid model provides relatively accurate prediction of the flow fields for the entire system at reasonable computational costs. A second promising development that can address the disparate spatial scales between inlet and basin is the use of boundary fitted coordinate techniques (Spaulding, 1984; Swanson, 1986; Willemse et al., 1986).In this approach the study area boundaries define the coordinate system and a set of quasi-linear, coupled elliptic equations are employed to map the physical plane to a corresponding transformed space. Nonhomogeneous terms can be added to the coordinate generating equations to allow control of the interior grid lines. Additional control can be exercised by altering the spacing of the grids along the boundaries of the model domain. The approach does not require that the grid lines be locally orthogonal as do conventional curvi-lin-

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ear grid models (Blumberg and Herring, 1987; Wanstrath et al., 1976). Given a definition of the coordinate system the hydrodynamic equations,

mapped to the transformed space, are solved using standard finite difference methods on square grids. Interpolation or extrapolation of boundary values are totally avoided with this method. Spaulding et al. (1989) have employed this technique to study circulation in Mt. Hope Bay, simultaneously incorporating a small tidal embayment with the lower reaches of a major river which discharges into it. The model grid system readily addresses the large variations in spatial scales between the narrow river and the broad bay. To date there have been no applications, of which the author is aware, of boundary-fitted hydrodynamic models to inlet-lagoon systems. The technique however appears ideally suited for such applications given its proven ability to predict currents and sea surface elevation fields over widely varying spatial scales.

Pollutant Transport Models The transport of pollutants in inlet-lagoon systems, assuming vertically well-mixed conditions, are given by the vertically-averaged conservation of constituent mass equation

acH aucH avcH a -+ay ax at ax

(5.22)

where the U and V are the vertically-averaged velocities determined by solving Eqs. 5.19,5.20, and 5.21. H i s the total water column depth. D, and Dyare the horizontal dispersion terms. S is the sinWsource of the constituent and I is a reaction matrix defining growth/decay or interaction of the constituent with other parameters. Equation 5.22 can be generalized by solving repeatedly for various interacting water quality parameters. Water quality parameters of primary interest include coliforms (total or fecal), dissolved oxygen-biochemical oxygen demand (with/without nutrients), suspended sediments and metals. The basic solution methodology (finite difference-finite element) selected to solve the pollutant transport equation is normally the same as that used for the hydrodynamic model. This matching of solution techniques and grids, while not strictly necessary, is done as a matter of convenience since it eliminates many of the major problems with transferring the current data from the hydrodynamic model to the pollutant transport model. If different solution techniques are employed great care must be exercised to assure that water mass conservation is maintained when the two models are interfaced. In either case the linkage should be tested and mass conservation characteristics verified.

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Representative finite difference and finite element solutions to the polluta n t transport equations are given by Leenderste (1970), Isaji and Spaulding (1981b1, and Leimkuhler (1974). Spaulding and Liang (1989) present solutions for the boundary-fitted coordinate case. While development activity in three-dimensional models have been vigorous in the past decade they have seen little application for coastal lagoon problems because of their complexity, high computational cost and lack of an adequate data base to support their calibration and verification. In addition most lagoons are sufficiently shallow and well mixed to allow accurate representation with two-dimensional vertically-averaged models. Model Selection Process In the above presentation two general modeling approaches are outlined: (1) a series of simplified approaches ranging from algebraic to numerical solutions of the one-dimensional, non-linear, ordinary differential equation describing the system and (2) numerical solutions to the two-dimensional, vertically-averaged governing equations. The model selection process is principally driven by the problem at hand, the availability of observational data on the system and the desired accuracy or level of analysis. As a simple starting place, in the absence of reliable field data to accurately describe the inlet-lagoon characteristics, the first step in model selection is to estimate the effective dissipation factor, p (Eq. 5.91, the size (maximum horizontal dimension) of the basin relative to the primary ocean forcing wavelength, and the dimensionless forcing frequency a (Eq. 5.10). The size scales and forcing frequency are readily determined by knowing the geometry of the system and the primary forcing period. The dissipation factor is hard to estimate because it depends on entrance and exit loss coefficients and bottom friction, all three of which are difficult to determine accurately. If the maximum dimension of the basin is small ( 4 0 %of the forcing wave length) then the basin responds in a pumping mode and a simplified analysis is appropriate. If this condition is not satisfied a two-dimensional, vertically-averaged approach is required. For the simplified approach the next step is to determine p and a. Following Mehta and Ozsoy (1978) k,, = 1. Entrance losses vary from 0.05 to 0.25 according to Dean (1971). For smooth entrances losses are low and ken = 0.05, for inlets with exposed jetties losses are higher because flows must bend sharply to enter the channel and ken = 0.25. The value for y depends on the flow characteristics and geometry of the inlet. Typical values range from 0.002 to 0.005. An extensive discussion on specification of frictional dissipation is contained in Mehta and Ozsoy (1977). As a first approximation a value of 0.0035 is recommended.

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Mwleling of Circulation and Dispersion in Coastal Lagoons

With values for Ken, K,, and y specified and information on the system geometry the effective dissipation of the inlet, p, can be defined. Equation 5.8 is then used to calculate the amplitude response factor for the system and Eq. 5.11 the phase lag. If the frictional dissipation is large, p > 100, as it is for many inlets, the simplified version of the amplitude response given in Eq. 5.12 can be used directly. If the inlet is frictionally dominated the correlation relationship given in Eq. 5.7b provides an alternate procedure to determine the response. If the simplified model predicted amplitude response factor is less than one, the pumping mode response is confirmed by field observations and the surface area is independent of tidal range then this simple approximation is a very reasonable first order estimate of the inlet-lagoon system behavior. Equation 5.18 can then be used to predict the steady-state water quality in the system, assuming complete flushing. For multi-inlet, multi-basin systems with complicated inlet channel geometry and surface areas that vary with sea surface elevation it is recommended t o use Seelig et d ’ s (1977) numerical model. This procedure is well developed with good documentation (Seelig, 1976) and requires minimal computational resources. Numerous case examples are presented (e.g., Pentwater Lake, Michigan; Masonboro Inlet, North Carolina; Indian River Inlet, Delaware; and Cabin Point Creek, Virginia) and the model is robust. To predict water quality distributions within the basin Eq. 5.18 can be used again if we assume that the constituent concentrations in the channels are of limited interest and tidal prism flushing is appropriate. When the amplitude response factor is on the order of one the inlet is no longer an effective filter and sea surface elevation fluctuations propagate unimpeded into the lagoon. In this case the assumption of a pumping mode response in the basin is inappropriate unless the basin scale is small compared to the forcing function wavelength. For this situation or when one is particularly interested in the circulation or pollutant transport within the lagoon, a two-dimensional, vertically-averaged modeling strategy should be used. If the inlet has rectilinear flow then a hybrid model, such as developed by Isaji et aZ.(19851, can be successfully employed. Provision can be made in this hybrid model to accommodate dryindflooding boundary conditions and hence to represent changes in bay surface area with sea surface range. For basins with very wide inlets and significant topographic variations, a two-dimensional representation of the inlet proper may be appropriate. The choice of finite difference versus finite element solution approaches is a matter of modeling experience and taste. Finite element models allow greater flexibility in grid design and mesh refinement but are harder to use, less available, and not as well documented in comparison to finite difference techniques. Once a solution strategy is selected for the circulation the same methodology should be used in the pollutant transport calculations. The two-dimensional, vertically-averaged model simulations are accurate

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but computationally intensive and hence costly. They should be used only if absolutely necessary to address the problem of interest. As a final note no model, either simple or complex, should be used to assess management strategies until calibrated and validated with field observations. Model Application to Assess Management Strategies An application of a simplified model to predict the impact of various inlet modification scenarios on the Charlestown lagoon is given to illustrate model use in a management framework. The Charlestown lagoon (Fig. 5.7), located on the southern coast of Rhode Island in the northeastern United States, system consists of two major ponds: Charlestown Pond (CP) (also called Ninigret Pond) (6 km long and 1.4 km wide) and Green Hill Pond (GHP) (1.5km long and 1.4km wide). The surface area of Charlestown Pond is 6.23 x lo6 m2 with a mean low water volume of 7.91 x lo6 m3 and an average depth of 1.27 m. The surface area of Green Hill Pond is 1.55 x lo6 m2with a mean low water volume of 1.22x lo6 m3and an average depth of 0.79 m. The lagoon lies behind a narrow barrier beach. Charlestown Pond is connected to Block Island Sound (BIS) through Charlestown Breachway (30 m wide, 1,000 m long, 1-2 m deep), while Green Hill Pond is connected to Charlestown Pond through a narrow inlet ( 5 m wide, 20 m long, 1.0 m deep). Historically, the breachway connecting BIS to the ponds has opened and closed in response to both seasonal and long term sediment transport along the barrier beach (Lee, 1980). A permanent breachway was constructed at the present location in 1952.Several small streams discharge into the lagoon, contributing about 30%of the total freshwater inflow. The major freshwater source appears to come from ground water, although accurate estimates of the exact sources and their flow rates are not available. Isaji et al. (1985)and Spaulding (1993)have studied the circulation and salinity dynamics of the pond using a combination of field observations and numerical modeling techniques. Their studies show that the lunar semi-diurnal (Mz) tidal range is reduced by a factor of 5.5 from Block Island Sound to Charlestown Pond and another factor of 3 from Charlestown to Green Hill Ponds. High tide occurs 2.5 hrs and 4.5 hrs after high tide in Block Island Sound for Charlestown and Green Hill Ponds, respectively. The circulation in the pond is dominated by tidal forcing. Low frequency (> 2 days) wind induced sea level fluctuations however are observed. They travel unimpeded from offshore through the breachway into the ponds. The salinity distributions in CP and GHP are determined by a balance between the salt water entering the pond through Charlestown Pond Inlet, the freshwater entering as runoff or groundwater and the mixing between these two water types. With the present permanent breachway allowing relatively large amounts of salt BIS water (31.5 ppt) into CP, the annual

122 Modeling of Circulation and Dispersion in Coastal Lagoons

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mean salinity in CP is 28.5 ppt and 23.3 ppt in GHP. Because of the extremely shallow depths of the pond most of the pond water (>95%) is flushed every tidal cycle. One of the critical management questions for the lagoon is assessing whether simple modifications of the breachway-channel systems connecting Charlestown Pond to Block Island Sound and Green Hill Pond to CP could be made to (Lee, 1980; Olsen and Lee, 1982): (1)

allow safe and easy passage of pleasure boats and small commercial craft between GHP, CP, and BIS;

(2)

decrease the sedimentation rate of CP;

(3)

increase the range of salinity variations and decrease mean salinity in CP to enhance brackish water fisheries;

(4)

increase the flushing rate of GHP and selected areas of CP to decrease local pollution problems.

Spaulding (1990) provides a detailed assessment of the impact of various management alternatives (selective dredging, breachway closurdopening and combinations of the two) on the pond uses. This section however explores only the effect of systematic reduction in the width of Charlestown Pond Inlet (CPI) on the tidal range, tidal discharge volume, maximum inlet velocity and salinity in the ponds to illustrate model use in management decision making. Given the pumping mode response of the ponds to tidal forcing the inlet hydrodynamic model developed by Seelig et al. (1977) and modified to incorporate salinity dynamics by Spaulding and Isaji (1981a)was employed. After calibration against field data (Isaji et al., 1985; Spaulding, 1993) the model was applied to assess the impact of reducing the width of the landward half of the fixed portion of the Charlestown Breachway by 50%, 62.5%, 75% and 87.5%. In each case lunar semi-diurnal tidal forcing was used since field observations show that this tidal constituent dominates current and sea elevation energy distributions in the inlet. Results of the simulations, in terms of tidal range, maximum inlet crosssectionally averaged velocity, tidal discharge volume, and salinity, are shown in Table 5.3. The present conditions are given as a convenient reference. The percent change caused by the proposed alterations, relative to present conditions, is also given. The effect of partially closing the breachway is to reduce the tidal range, tidal discharge volume, flushing rate, and mean salinity (increases the salinity variation however) and to increase the maximum inlet velocity compared to the present condition (Table 5.3). The larger the closure, the larger the changes. From a detailed review of the table it is seen that by reducing the width of the Charlestown Pond inlet one can readily optimize

TABLE 5.3 Effect of inlet width reduction on selected parameters for Charlestown Pond System Inlet Modification (% width reduction) Location

Present

50%

Semidiurnal tidal range (m) Charlestown Pond Green Hill Pond

0.137 (O)* 0.037 ( 0 )

62.5%

75%

0.099 (-28%)

0.076 (44%)

0.054 (-61%)

0.027 (-81%)

0.032 (-12%)

0.030 (-23%)

0.028 (-23%)

0.021 (43%)

2.79 (53%)

3.03 (66%)

3.10 (70%)

1.35 (-18%)

1.26 (-23%)

0.94 (-43%)

Maximum inlet cross-sectionallyaverage velocity ( d s e c ) Charlestown Breachway 1.82 ( 0 ) 2.66 (46%) Green Hill Pond Inlet 1.64 (0) 1.45 (-12%)

87.5%

Tidal discharge volume (x106 m3) Charlestown Breachway Green Hill Pond Inlet

0.778 ( 0 ) 0.047 ( 0 )

0.567 (-27%) 0.041 (-12%)

0.447(-43%)

0.324 (-58%)

0.166 (-79%)

0.038 (-18%)

0.036 (-23%)

0.027 (-43%)

salinity (%)** Central Basin West Basin

29.8 ( 0 )

29.2 (-2%)

28.7 (-4%)

27.9 (-6%)

25.8 (-13%)

Fort Neck Green Hill Pond

27.6 ( 0 )

26.3 (-5%)

25.2 (-9%)

23.2 (-15%)

18.4 (-34%)

28.3 (0)

27.2 (-4%)

26.1 (-8%)

24.4 (-13%)

20.1 (-29%)

23.3 ( 0 )

22.2 (-5%)

21.4 (-8%)

20.5 (-12%)

17.4 (-25%)

* Percent change referenced to existing conditions. ** Reference salinity in Block Island Sound 31.5%.

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the salinity regime in the pond. Salinity control can be achieved in this manner to permit or enhance a given fishery or aquaculture operation. This result is achieved however at the expense of decreased flushing rates of both ponds and, therefore, enhanced pollution problems. Recreational boating in the ponds, becomes increasingly untenable since width reductions of CPI produce extremely high inlet velocities and safe boating becomes impossible. Conclusions

A review of numerical models to predict circulation and pollutant transport in inlet-lagoon systems are summarized. Assuming single inlet-single basin systems and assuming that sea surface elevation in the basin responds in a pumping mode solutions to the inlet momentum and basin continuity equations are formulated to predict the flow characteristics of the inlet. Reference is made to the numerous authors who have solved this problem for a wide variety of assumptions regarding the importance of various terms in the governing equations. In the majority of cases the equations are linearized to facilitate analytic solution. The most thoroughly developed simplified hydrodynamic model was developed by Ozsoy (1977) and allows prediction of the inlet-basin response for the full range of conditions observed. Solutions are presented in terms of a dimensionless forcing frequency and dissipation parameter. The model successfully reproduces the behavior of inlet-basin systems based on comparison to observations and accurately predicts the response near resonant conditions. For more complex inlets (e.g., multiple branching channels with lateral and longitudinal variations in bathymetry) a numerical solution to the governing equations is described (Seelig et al., 1977). The inlet geometry is described by a flow net and the basin's surface area in a functional form which includes dependence on sea level in the basin. The model includes fully non-linear frictional dissipation and can be used to study multi-inlet, multi-basin systems. The model is well documented, includes test cases and has been applied to numerous systems. Simplified pollutant transport or water quality models are generally based on extensions of Isaji and Spaulding's (1981a) tidal prism flushing approach which provides estimates of the steady-state concentrations in the basin. This method was extended by DiLorenzo et al. (1989) to estimate pollutant concentrations from a specified mass loading rate and first order decay for various tidal stages. The model parameterizes the hydrodynamics in terms of basin amplitude response and non-dimensional mass and freshwater loading rates. For inlet-lagoon systems where the inlets are sufficiently large to allow sea surface fluctuations to pass relatively unimpeded and the basin does not respond in a pumping mode or where the primary interest is in circulation

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Modeling of Circulation and Dispersion in Coastal Lagoons

and pollutant transport within the lagoon, two-dimensional, vertically-averaged circulation and pollutant transport models are used. The verticallyaveraged approximations are used since most lagoon systems are shallow and well mixed by winds and tides. These models are well developed, widely available and routinely used. Finite difference and finite element techniques are the basic solution methodologies normally employed. Through careful selection of grid size and time-step and calibration and validation with observational data these models can provide accurate predictions of flow and water quality in the inlet-lagoon systems. The models however are computationally intensive and therefore expensive compared to the simplified procedures. To reduce the computational cost while still maintaining a capability to predict the flow and pollutant concentrations within the lagoon, a hybrid approach can be used. In this procedure one-dimensional model&) of the inlet(s) are linked to a two-dimensionally, vertically-averaged model(s) of the lagoon. This approach has received limited application to date but is a promising compromise between the simplified models and the fully two-dimensional representations. As a n approximate guideline, selection of appropriate models to use for a particular system depend on the problem of interest and the characteristics of the lagoon (e.g., pumping response). The basic strategy in model selection is to determine the validity of the pumping mode response of the lagoon and degree of filtering afforded by the inlet. Simplified models work best when filtering effects are significant, inlet geometry is simple and the basin responds in a pumping mode. Simpler models that capture the key physical processes are preferred when possible. Simplified models for circulation and salt were employed to predict the change i n inlet flow and velocity and the change in flushing of the Charlestown pond system for a variety of inlet modification scenarios. Model results provide data to evaluate the tradeoffs of the various inlet modification plans as they impact pollutant flushing, aquaculture operations, and recreational boating. Acknowledgements

The field observations and hydrodynamic modeling studies of Charlestown Pond and Green Hill Pond reported here were part of a large interdisciplinary effort sponsored by the University of Rhode Island Sea Grant Program from 1979 to 1982 to study Rhode Island’s coastal ponds. The correlation analysis was performed by Dr. J. Craig Swanson, Applied Science Associates, Inc. (ASA). Typing of the manuscript was performed by J a n Hopp, ASA.

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References American Society of Civil Engineering Task Committee on Computational Hydraulics 1980 Sources of computer programs in hydraulics. Journal of Hydraulics Division of the American Society of Civil Engineering 106,915-923. Abbott, M.B. 1976 Review models of tidal waters. Journal of Hydraulic Division 102, 1145. Baines, W.D. 1957 Tidal currents in constricted inlets. Proceedings of 6th Coastal Engineering Conference, 1957, 545-61. Blumberg, A.F. and Herring, H.J. 1987 Circulation modelling using orthogonal curvilinear coordinates. In Three-Dimensional Models of Marine and Estuarine Dynumics (Nihoul, J.C.J. and Jamart, B.M., eds). Elsevier, New York, NY,pp. 55-88. Brown, E.I. 1928 Inlets on sandy coasts. Proceedings of the American Society of Civil Engineers 54,505-553. Bruun, P., Mehta, A.J. and Johnson, I.G. 1978 Stability of Tidal Inlets: Theory and Engineering. Elsevier Scientific Publishing Company. Butler, H.L. and Raney, D.C. 1976 Finite difference schemes for simulation flow in an inlet-wetland system. Proceeding of the 1976 Army Numerical Analysis and Computer Conference, ARO Report 76-3. U.S. Army Research Office, Research Triangle Park, NC, pp. 393-411. Chen, R.M. and Hembree, L.A., Jr. 1977a Comparison of numerical and physical models, Masonboro Inlet, North Carolina. Appendix 3 - Numerical Simulation of Hydrodynamics, GITI Report 6. U.S. Army Engineers Coastal Engineering Research Center. Chen, R.M. and Hembree, L.A., Jr. 1977b Numerical simulation of hydrodynamics (TRACOR). Appendix 3, GITI Report 6. U.S. Army. Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, VA, and U.S. Army Engineers Waterways Experiment Station, Vicksburg, Miss, June 1977. Dean, R.G. 1971 Hydraulics of inlets. Department o f Coastal and Oceanographic Engineering, University of Florida, COEUUFL-71/019. Dean, R.G. and Taylor, R.B. 1972 Numerical modeling of constituent transport in bay systems. Proceedings of the Thirteenth Coastal Engineering Conference. American Society of Civil Engineers, Vancouver, B.C., Canada 4, 2217-2240. Dean, R.G. and Taylor, R.B. 1972 Numerical modeling of hydromechanics of Biscayne Bay I Card Sound System, Part I and 11:Dispersive and non-dispersivecharacteristics. Department of Coastal and Oceanographic Engineering, University of Florida, July. DiLorenzo, J.L. 1986 The overtide and filtering response of inletlbay systems. Thesis presented to the State University of New York at Stony Brook, New York, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Dihrenzo, J.L. 1988 The overtide and filtering response of small inlethay systems. Lecture Notes on Coastal and Estuarine Studies 29. In Hydrodynamics and Sediment Dynamics of Tidal Inlets (Aubrey, D.G. and Weisher, L., eds). Springer-Verlag, NY, pp. 24-53. DiLorenzo, J.L., Huang, P.S. and Najarian, T.O. 1989 Water quality models for small tidal inlet systems. ASCE Journal of Environmental Engineering 115, 192-209. Dronkers, J.J. 1969 Tidal computations for rivers, coastal areas and seas. Journal of Hydraulics Division, Proceedings of American Society of Civil Engineers 96,29-77. Escoffier, F.F. 1975 Unpublished notes.

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Modeling of Circulation and Dispersion in Coastal Lagoons

Escoffier, F.F. 1977 Hydraulics and stability of tidal inlets. G.Z.T.Z.Report 13,August. U.S.Army Corps of Engineers Coastal Engineering Research Center, Ft. Belvoir, Virginia. Escoffier, F.F. and Walton, T.L., Jr. 1979 Inlet stability solutions for tributary inflow. Journal of the Waterway, Port, Coastal and Ocean Division, ASCE 105,341-355. Freeman, N.G., Hamblin, R.F. and Murty, T.S. 1974 Helmholtz resonance in harbors of the Great Lakes. Seventeenth Conference on Great Lakes Research, International Association for Great Lakes Research, Hamilton, Canada, August 12-14,399411. French, J.L. 1960 Tidal flow in entrances. Technical Bulletin No. 3, January. U.S. Army Corps of Engineers, Waterways Experiment Station, Committee on Tidal Hydraulics, Vicksburg, Mississippi. Goodwin, C. 1974 Physical parameters which control propagation of tidal waves in estuaries, verified for three significantly different Oregon estuaries. Proceedings of the Fourth Annual Technical Conference on Estuaries of the Pacific Northwest. Oregon State University Engineering Experiment Station Circular No. 50, March, Corvallis, Oregon, 37-49. Gordon, R. and Spaulding, M. 1974 A bibliography of numerical models for tidal rivers, estuaries and coastal waters. Marine Technical Report 32, University of Rhode Island, 55 pp. Harris, D.L. and Bodine, B.R. 1977 Comparison of numerical and physical hydraulic models, Masonboro Inlet, North Carolina. GZTZ Report 6. U.S. Army Corps of EngiJune 1977. neers, (including Appendices la), Hinwood, J.B. and Wallis, I.B. 1975a Classification of models of tidal waters. Journal of the Hydraulics Division, 1315. Hinwood, J.B. and Wallis, I.B. 1975b Review of models of tidal waters. Journal ofthe Hydraulics Division, 1405. Huval, C.J. and Wintergerst, G.L. 1977 Simplified numerical (lumped parameter) simulation. Appendix 4, GZTZ Report 6. U S . Army Corps. of Engineers, Coastal Engineering Research Center, Fort Belvoir, VA, and U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., June 1977. Isaji, T. and Spaulding, M.L. 1981a A simplified model for assessing the impact of breachway modifications on coastal pond circulations and flushing dynamics. Proceedings of OCEANS 81,824-828. Isaji, T. and Spaulding, M.L. 1981b Numerical modeling of entrainment and far field thermal dispersion for the NEP I and I1 power station, Charlestown, RI. Department of Ocean Engineering, University of Rhode Island, Kingston, RI, Prepared by the Oak Ridge National Laboratory, Oak Ridge, Tennessee for the Department of Energy, Contract # W-7405-eng-26.ORNyTM-7590. Isaji, T., Spaulding, M.L. and Stace, J. 1985 Tidal exchange between a coastal lagoon and offshore waters. Estuaries 8,203-216. Joshi, P.B. 1982 Hydromechanics of tidal jets. Journal of the Waterway, Port, Coastal, and Ocean Division, Proceedings ofAmerican Society of Civil Engineers 108,239-253. Keulegan, G.H. 1967 Tidal flow in entrances, water-level fluctuations of basins in communication with seaa. Technical Bulletin No. 14, Committee on Tidal Hydraulics, Waterways Experiment Stations. U.S. Army Corps of Engineers, Vicksburg, Miss., July 1967. Keulegan, G.H. and Hall, J.V., Jr., 1950 A formula for the calculation of the tidal discharge through a n inlet. U.S. Army Corps of Engineers, Beach Erosion Board

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Bulletin 4, 15-29. King, D.B. 1974 The dynamics of inlets and bays. Technical Report No. 22, Coastal and Oceanographic Engineering Laboratory, University of Florida, Gainesville, Florida, March 1974. King, D.B. and Shemdin, O.H. 1975 Modeling of inlet-bay systems in relation to sand trapping. Proceedings of the Symposium on Modeling Techniques, September 3-5. American Society of Civil Engineers, San Francisco, CA, pp. 1623-1637. Kondo, H. 1975 Depth of maximum velocity and minimum flow area of tidal entrances. Coastal Engineering in Japan, Committee on Coastal Engineering 18. Japan Society of Civil Engineers, Tokyo, Japan, pp. 167-183. Lee, V. 1980 An elusive compromise: Rhode Island coastal ponds and their people. University of Rhode Island, Coastal Resources Center, Marine Technical Report 73, 82 PP. Leendertse, J.J. 1967 Aspects of a computational model for long-period water-wave propagation. RM-5294-PR, Rand Corporation, Santa Monica, CA. Leendertse, J.J. 1970 A water-quality simulation model for well mixed estuaries and coastal seas. Principles of computation 1.The Rand Corporation, R.M.-6230-RC. Leimkuhler, W. 1974 A two-dimensional finite element dispersion model. M.S. Thesis, Department of Civil Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts. Masch, F.D., Brandes, R.J. and Reagan, J.D. 1977 Numerical simulation ofhydrodynamics (WRE). Appendix 2, GITZ Report 6. U.S. Army Corps of Engineers Coastal Engineering Research Center, Fort BelvoSir, VA, and U.S. Army Engineers Waterways Experiment Station, Vicksburg, Miss., June 1977. Mayor-Mora, R. 1973Hydraulics of tidal inlets on sandy coasts. University of California, Hydraulic Engineering Laboratory, Berkley, California, Technical Report HEL-2416. Mayor-Mora, R. 1974 Hydraulics of tidal inlets on sandy coasts. Proceedings of the 14th Coastal Engineering Conference, ASCE 2,1524-45. Mehta, A.J. and Hou, H.S. 1974Hydraulic constants of tidal entrances ZZstabiZity ofLong Island inlets. Technical Report No. 23, Coastal and Oceanographic Engineering Laboratory, University of Florida, Gainesville, November. Mehta, A.J. and Ozsoy, A.E. 1978 Inlet hydraulics. In Stability of Tidal Inlets - Theory and Engineering (Brunn, P., ed.). Elsevier Publishing Co., New York, NY,96 pp. Mehta, A.J. and Zeh, T.A. 1980 Influence of a small inlet on a large bay. Coastal Engineering 4,157-176. Mota Oliveira, I.B. 1970 Natural flushing ability in tidal inlets. Coastal Engineering 3, 1827-1845. O’Brien, M.P and Clark, R.R. 1974 Hydraulic constants of tidal entrances. Coastal Engineering 2, 1546-65. Olsen, S. and Lee, V. 1982 Inlet modification: an example of an holistic approach to lagoon management. Oceanollogica Acta (SpecialEdition), 373-382. Ozsoy, E. 1977 Flow and mass transport in the vicinity of tidal inlets. Technical Report No. TR-036, Coastal and Oceanographic Engineering Laboratory, University of Florida, Gainesville, FL. finder, G.F. and Gray, W.G. 1977 Finite element simulation in surface and subsurface hydrology. Academic Press, New York, New York. Seelig, W.M. 1976 Computer program documentation - Inlet 2. U.S. Army Engineers,

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Modeling of Circulation and Dispersion in Coastal Lagoons

Coastal Engineering Research Center, Vicksburg, Mississippi. Seelig, W.M. 1977 A simple computer model for evaluating coastal inlet hydraulics. Technical Aid #77-1,July 1977.US.Army Corps of Engineers, Coastal Engineering Research Center. Seelig, W.M., Harris, D.L. and Herchenroder, B.E. 1977A spatially integrated numerical model of inlet hydraulics. GZTZ Report 14.U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, VA, and U.S.Army Engineer Waterways Experiment Station, Vicksburg, Miss. Seelig, W.M. and Sorensen, R.M. 1977Hydraulics of Great Lakes inlets. Technical Paper No. 77-8,July 1977,U.S. Army Corps of Engineers, Coastal Engineering Research Center. Shemdin, O.H. and Forney, R.M. 1970 Tidal motion in bays. Coastal Engineering Proceedings 3,222542. Short, F.T., Nixon, S.N. and M a t t , C.A. 1974Field studies and simulations with a fine grid hydrodynamic model in a n environmental study of a nuclear power plant at Charlestown, RZ. University o f Rhode Island, Marine Technical Report. No. 33,pp. 1-27. Sorensen, R.M. 1975 Procedures for preliminary analysis of tidal inlet hydraulics and stability. Coastal Engineering Technical Aid. U.S. Army Corps of Engineers, Coastal Engineering Research Center, Ft. Belvoir, Virginia. Sorensen, R.M. and Seelig, W.N. 1976 Hydraulics of Great Lakes inlet-harbor systems. Proceedings of the Fifteenth Coastal Engineering Conference, July. Amererican Society of Civil Engineers, Honolulu, Hawaii, vol. 2,pp. 1646-1665. Spaulding, M.L. 1984 A vertically-averaged circulation model using boundary-fitted coordinates. Journal of Physical Oceanography 14,973-982. Spaulding, M.L. 1993 Inlet hydrodynamics, Chapter 5. A case study in science andpublic policy: Rhode Island's coastal lagoons. Oxford University Press, New York, New York (in preparation). Spaulding, M.L., Huang, W. and Mendelsohn, D. 1989 Application of a boundary fitted coordinate hydrodynamic model. ASCE Specialty Conference, Estuarine and Coastal Modeling, 15-1 7 November 1989.Newport, RI. Spaulding, M.L. and Liang, S.J. 1989 Boundary fitted estuarine water quality model. ASCE Specialty Conference, Estuarine and Coastal Modeling, 15-17November 1989. Newport, RI. Speer, P.E. and Aubrey, D. 1985 A study of the non-linear tidal propagation in shallow inledestuarine systems. Part 11. Theory. Estuarine, Coastal and Shelf Sciences 21, 185-205. Swanson, J.C. 1986 A three-dimensional numerical model system of coastal circulation and water quality. Ph.D. Thesis, Department of Ocean Engineering, University of Rhode Island, Kingston, RI. Taylor, R.B. and Dean, R.G. 1974 Exchange characteristics of tidal inlets. 1974 Proceedings, Coastal Engineering Conference, ASCE 3,2268-89. Uncles, R.J. and Kjerfve, B. 1986.North Inlet, South Carolina. Estuaries 9,3942. Unluata, V.A. and Ozsoy, E. 1974 Tidal jet flows near inlets. Hydraulics in the Coastal Zone, ASCE, 1977,90-98. Van de Kreeke, J. 1967 Water level fluctuations and flows in tidal inlets. Journal of the Waterways and Harbor Division, American Society of Civil Engineers 93,97-106. Van de Kreeke, J. 1972 A numerical model for hydromechanics of lagoons. Coastal

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Engineering, Proceedings 3,2251-2263. Van de Kreeke, J. and Cotter, D.C. 1974 Tide-induced mass transport in lagoon-inlet systems. Coastal Engineering, 1974 Proceedings 3, 2290-2301. Vincent, C.H. and Corson, W.D. 1980 The geometry of selected U.S. tidal inlets. GZTZ Report 20. U.S. Army Corps of Engineers, May 1980. Walton, T.L., Jr. and Escoffier, F.F. 1981 Linearized solution to inlet equation with inertia. Journal of the Waterway, Port, Coastal and Ocean Division, Proceedings of American Society of Civil Engineers 107, 191-195. Wang, H-P. 1975 Modeling an ocean pond. Marine Technical Report No. 40, University of Rhode Island, Kingston, RI. Wanstrath, J.J., et al. 1976 Storm surge simulation in transformed coordinates. TR 76-3, U.S. Army, Corps. of Engineers, Coastal Engineering Research Center, Fort Belvoir, VA, November 1976. Willemse, J.B.T.M., Stelling, G.S. and Verboom, G.K. 1986 Solving the shallow water equations with an orthogonal coordinate transformation. Delft Communication No. 356. Presented at International Symposium on Computational Fluid Dynamics, Tokyo, Japan, September 9-12,1985.

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Coastal Lagoon Processes edited by B. yjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V.All rights reserved

133

Chapter 6

Mass Balance of Nutrient Fluxes in Coastal Lagoons Stephen V. Smithaand Marlin J. Atkinsonb aDepartment of Oceanography, University of Hawaii, Honolulu, Hawaii 96822 and bHawaii Institute of Marine Biology, University of Hawaii, P.O.Box 1346, Kaneohe, Hawaii 96744, USA

Net mass balance analysis of material inputs to an ecosystem minus outputs from that system provides a measure of how that system is biogeochemicallycoupled with adjacent systems. Some materials, such as total salts (most often expressed as salinity), usually show little or no evidence of net uptake or release in coastal marine systems; such materials are said to be ‘conserved‘in the system. Materials which do show release are termed ‘nonconservative’.The dissolved nutrient elements phosphorus, nitrogen, and carbon are among the most conspicuous and ecologically important nonconservative materials in aquatic environments. This paper presents a methodology for evaluating nonconservative fluxes in coastal lagoons. The methodology, borrowed from studies of material fluxes in bays and estuaries, has apparently not been used in coastal lagoons. At steady state in a hydrographically simple system, any material, Y,which is conserved with respect to salinity will be distributed along a straight line on a graph of salinity versus Y. The position of the straight line is fixed by the composition of water at either salinity extreme of the system. Both temporal variation in end-member water composition and hydrographic complexity can cause deviations from this simple straight-line behavior. With proper attention to these factors, however, mixing models can be used to evaluate the degree of departure of any material from a simple conservative relationship with salinity. If a water budget is available for the system of interest, then the rate of net nonconservative flux can be calculated. The model has been most frequently applied to systems showing net freshwater input; the model is readily applicable and somewhat more robust in systems showing net evaporation. The mathematical derivation for this model is presented. Nonconservative fluxes of dissolved inorganic phosphorus, nitrogen, and phosphorus are largely related to primary production minus respiration (i.e., to net metabolism) in aquatic systems, although each of these elements has complications in its flux pathways. Nonconservative carbon flux is complicated by both calcium carbonate precipitation-dissolution reactions and gas exchange across the air-water interface. Nitrogen flux is complicated by nitrogen fixation and denitrification. Acknowledging that phosphorus flux can be associated with various abiotic sorption and precipitation reactions, we nevertheless consider nonconservative phosphorus flux to be the best measure

134

Mass Balance of Nutrient Fluxes in Coastal Lagoons

of net ecosystem metabolism. Systems which import more dissolved phosphorus than they release are apparently net autotrophic; systems which export more dissolved phosphorus than they import are apparently net heterotrophic. There is very little annual-average information on net nutrient fluxes in coastal marine systems, but a number of studies describe the primary production and respiration of major system components. We can infer that most of these systems are likely to export dissolved phosphorus. We believe, based on analogy with estuaries and limited lagoonal data, that most coastal lagoons are likely to be net heterotrophic. This expectation requires further evaluation.

Introduction Coastal lagoons are coupled to adjacent land and ocean ecosystems by the exchange of water and particulate and dissolved materials. Materials t h a t enter lagoons via stream runoff or groundwater flow react in the lagoons, accumulate in the sediments there, or are removed to the ocean without reaction. The lagoon-ocean coupling is more complex. Tidal ebb and flow at passes of the lagoons result in bidirectional exchange and mixing, with either net import of water from the ocean to the lagoon to match net evaporation, or net export from the lagoon to the ocean to accommodate freshwater outflow (Kjerfve and Magill, 1989). Studies of material dynamics within systems are commonly made i n aquatic ecology. For example, ecologists typically measure primary production, respiration, and benthic nutrient fluxes. These within-system studies are, of course, very useful in their own right. However, another scale of dynamics is not well addressed by such studies. Component flux studies are generally poor analytical techniques to address questions regarding exchanges of materials between adjacent ecosystems (e.g., between a lagoon and the adjacent ocean). Mass balance analyses of biogeochemically reactive materials for whole systems can be used to answer questions regarding whether lagoons are sources or sinks of materials. If a material moves through a lagoonal system in direct proportion to its abundance relative to some non-reactive material such as salinity, then this material is said to be ‘conservative’with respect to salinity. Passage through the lagoon exercises no control on the fate of that material. If, on the other hand, the material of interest moves through the system disproportionally relative to a conservative tracer, it is said to be ‘nonconservative’ with respect to salinity. Materials that are nonconservative with respect to salinity are likely to be biogeochemically and ecologically interesting. Although mass-balance budgets have been undertaken i n other shallow water systems (including estuaries, bays, and coral atoll lagoons), we have found no direct nonconservative mass balance analyses for coastal lagoons (as defined by Kjerfve, 1986; Kjerfbe and Magill, 1989). Nor has a survey of published papers on coastal lagoons yielded data sets which are readily amenable to such budgetary analysis.

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The plant nutrients, phosphorus, nitrogen, and carbon, are essential to metabolism, so these elements comprise a particularly important class of nonconservative materials. Nitrogen and phosphorus tend to be present in unpolluted water in low concentrations, lending further interest to their dynamics. There are several papers dealing with nutrient chemistry in coastal lagoons. Some relevant papers are summarized by Nixon (1982), although it should be noted that Nixon’s summary includes several sites which are not geomorphologically ‘coastal lagoons’. Several papers which do describe aspects of the nutrient chemistry of coastal lagoons include Okuda (19691, Boutiere et al. (19821,Vaulot and Frisoni (19861, Cossu et al. (1987), and Yaiiez-Arancibia and Day (1988). Unfortunately, this literature on the water composition of coastal lagoons is difficult to use for mass balance analysis, at least as presented here. Reported data for conservative properties (e.g., salinity) and nutrients are annual or seasonal averages, rather than being individual measurements with concurrent salinity and nutrient data. Nonconservative mass balance analysis according to the analysis approach presented here requires current measurements of a conservative water mass tracer and the nonconservative materials of interest. The purpose of this paper is therefore rather different from most other papers in this volume. Instead of relying primarily on a review of extant literature about coastal lagoons for discussion of net C-N-P mass balance, this paper lays out a general methodology of non-conservative mass balance analysis in semi-confined water bodies in general, using water and salt budgets. With that general framework in place, a series of cautionary caveats is then offered to the application of such mass-balance computations in lagoons or other coastal systems. We present some alternative approaches which might be used, indicating why we believe the salt-waternutrient balance approach which we advocate to be the most robust. Finally, we consider net production minus respiration of a number of bays, estuaries, and lagoons; net flux of the nutrient elements must be related to organic production minus respiration. We reason that net lagoona1 metabolism is similar to that of other coastal water bodies. This tentative conclusion needs direct evaluation with data from coastal lagoons. We hope that one outcome of the present paper will be a reassessment of data sets which are either presently unpublished or are published in summary forms which are not amenable to this budgetary analysis. Computational Model

The computational model used in this analysis is based on the conservation of water, conservation of a dissolved non-reactive material (usually salinity), and any dissolved reactive substance. The model is most simply applied in systems in which mixing in the longitudinal direction is much

136

Mass Balance of Nutrient Fluxes in Coastal Lagoons

Fig. 6.1. Schematic diagram of one dimensional embayment. Important variables labeled on the diagram are as follows: x denotes distance inward from the bay mouth (length).A is the cross sectional area at the bay mouth (len h2). F is freshwater inflow; R is rainfall; E is evaporation (F,R, and E all in units of length /time). u is the flow rate into the bay (lengthhime) and is evaluated as u = (E - R - F)/A.K is the longitudinal mixing coefficient (length2/time). Further variables needed for mass balance analyses include the salinity (S)and composition of any material Y of water a t the mouth of the bay, average bay water, and inflowing water from land; S and Y for rainfall and evaporation are assumed t o be equal to 0. The local gradient of Y with respect to S at the bay mouth is also needed for calculations as discussed in the text.

P

slower than cross-sectional or vertical mixing. Moreover, the model is most readily applied if mixing with the ocean occurs through a single cross-sectional area. These factors allow one-dimensional analysis of the general conservation equations for reactive and non-reactive materials (Fig. 6.1). Lagoons classified as ‘choked’ or ‘restricted‘ in the terminology of Kjerfve and Magill (1989) are most amenable to the treatment presented here. Conservation of water is established from a water budget. The water volume averaged over time is assumed to remain constant. Net volume transport through the system equals evaporation minus the sum of rainfall, runoff, and groundwater inflow. Hereafter, runoff and groundwater inflow are treated as a single term. Additional flows generated by tides, winds, or gravitational currents sum to 0. Conservation of salt (S) at position x in a one-dimensional system can be written as:

where A is the cross sectional area at longitudinal position x along the system (positive into the system); u is the water velocity normal to A and positive into the system; K is the longitudinal mixing, or dispersion, coeffi-

S.V.Smith and M.J. Atkinson

137

cient through A; aSlat and aslax are partial derivatives for the conservative quantity, S. S,A, K , and u are all functions of x and t . A similar conservation equation can be written for any material Y, which is also a function of x and t . This equation contains an additional term, B , for sources or sinks of Y at position x and time t:

-fmav\l

A-l

I

(6.2) Now let us consider any finite volume within the system, between x = 1and 2. We now deal with the total derivatives of S and Y within the volume V:

VdS (uAs),+(UAS),+ dt

(6.la)

and (6.2a) These equations are similar to the derivation of Officer and Lynch (1981), and different from more frequently presented steady state derivations (e.g., Boyle et al., 1974; Officer, 1979). The equations as presented here allow the possibility for temporal variations in both S and Y. If we consider an entire lagoonal system, rather than a n arbitrarily bounded volume in the lagoon, then there is no mixing at the land end. Water flow (e.g., stream runoff) occurs at both the ocean ( x = 1)and land ( x = 2) ends of the system, but mixing only occurs at the ocean end of the system. Equations 6.la and 6.2a simplify accordingly:

VdS dt

+ (UAS), +

(6.3)

V d Y - (uAY),+(UAY),+ dt

(6.4)

-= - (UAS),

and

Let us briefly consider how these equations are used in practice. V, A , S,and Y are estimated for the system of interest. The product UA in the above equations is estimated from a water budget, as the net (tidally averaged) volume transport of water past any particular cross section. As presented

138

Mass Balance of Nutrient Fluxes in Coastal Lagoons

by Officer (19791, this term is minus river flow (-F; negative because of the notation used here; i.e., x is positive towards land); in the more general case considered here, uA at the ocean end of the system includes evaporation and rainfall, each summed over the map area of the system ( & I = E - R - F);uA at the land end only includes freshwater runoff (uA2 = Note, as previously stated, the values for uA are the small residual flows associated with conserving water volume within the system, not the generally much larger reversing flows associated with tides and other current generating forces. K , which includes mixing associated with currents moving water to and from the system, can be derived as the unknown in Eq. 6.3. This value for K is then used to derive B by rearrangement of Eq. 6.4. Substituting for uA1 and uA2and rearranging, we derive B for the system as a whole:

-n.

In most instances, salinity at the land end of the system can be treated as 0, so Eq. 6.5 simplifies: (6.5a)

J A positive value for B indicates that there is a net source of Y within the system, while a negative value indicates that there is a net sink. Note that K and A drop out as explicit terms in Eq. 6.5. The importance of canceling K out in this fashion is that, if this budgetary approach to nonconservative flux calculations is employed, mixing need not be measured directly in order to evaluate the source or sink term, B. Caveats t o the Application of this Model

A number of cautions should be offered to the application of this model. Boyle et al. (1974) pointed to two important cautions. In the first place, Ys which are conservative with respect to salinity can yield data which deviate from a straight mixing line if three (or more) water masses are mixing; thus interpretation of data deviating from a straight mixing line as resulting from uptake or release between two end members needs to be treated with some caution. This possibility is one of several reasons for deliberately analyzing hydrographically simple, rather than complex, systems in the study of nonconservative fluxes. Those authors also observed that the exact rate of non-conservative flux inferred along the mixing line is sensitive to the characteristics of the curve

S.V. Smith and M.J. Atkinson

139

D 1

m

15f-l 10

*

5

I

I

l." 0

0 1020304050 SALINITY

5u

00 1 0 2 0 5 0 4 0 5 0 WNITY

Fig. 6.2. Schematic mixing diagrams, assumed to represent steady-state conditions. The solid line represents conservative mixing; the dotted line represents data, distributed from the ocean (S = 35)into the coastal water body; the dashed line (where illustrated) represents the slope of the data at the oceanic end of the system. (A) System dominated by freshwater flow, as a sink for Y. (B) System dominated by freshwater flow, as a source for Y. (C) Source of Y near system mouth exceeds sink at lower salinities. (D)Sink for Y near a salinity of 20, with mixing dominating in the salinity regions between 0 and 20, and 20 and 35. (E)System dominated by evaporation, as sink for Y . (F) System dominated by evaporation, as a source for Y.

used to describe the data. Concave upward distribution of data on mixing diagrams is often taken to indicate that the system is a net sink of Y (Fig. 6.2a), while convex upward distribution is taken to indicate a net source (Fig. 6.2b). As lucidly described by Officer (19791,the more rigorously correct reference variables on a two-component mixing diagram are the concentrations which represent the composition of the freshwater and seawater end members and the slope of the nonconservative variable relative to the conservative variable at the oceanic end of the diagram. Effort should therefore be devoted to obtaining robust values for these three variables on the diagram.

140

Mass Balance of Nutrient Fluxes in Coastal Lagoons

This point is at once rather subtle and very important. Many systems examined in toto show concave upward distribution of properties along much of their salinity range, indicating uptake - but convex upward . distributions near the oceanic end of the salinity range (Fig. 6 . 2 ~ ) This convexity at the ‘oceanic toe’ of the distribution diagrams, often overlooked, may in many instances be sufficient for the system as a whole to be a net source, rather than a sink, of the material in question. Many data sets have too few or too scattered data points to reconcile this possibility, especially if the analytical techniques being employed are not sensitive to low concentrations and small variations in concentration at the oceanic end of the distribution. Let us briefly consider simplified versions of Eq. 6.5a in order to see why Officer’s (1979) conclusion is valid. For this discussion, we will make the following assumption: The system is assumed to be at steady state, so the term involving time derivatives drops out. We initially consider the case where evaporation and rainfall are 0 (equivalent to Officer’s analysis). Equation 6.5a becomes:

then the data are distributed along the conservaIf [-Sl(dY/dS)l + Yll = Yz, tive mixing line; B = 0. If [-Sl(dY/dS)l+Yll < Yz,then the system (at salinities <S1) is a net sink of Y (Fig. 6.2a); if that term is greater than Yz, then the system is a net source of Y (Fig. 6.2b). Concavity or convexity at intermediate salinities is not the determining factor (Fig. 6 . 2 ~ ) .For a system with F = 0 and R > E , the term YZis ordinarily assumed to be 0 (note, however, that this assumption need not be made); otherwise the reasoning is the same. Next consider the less frequently analyzed net evaporative system (discussed by Smith and Jokiel, 1978; Smith and Atkinson, 1983; Smith and Veeh, 1989; Smith et al., 1987, 1989). We assume, for simplicity, that F = 0 and E >> R. Equation 6.5a becomes:

Water removed by evaporation is assumec, to have S = 0 anc Y = 0. In this case, the condition of zero nonconservative flux is described by data whose slope at the oceanic end of the diagram extrapolates to the origin of the graph ([Sl(dY/dS)1-Yd = 0 ) .If [(S,(aY/dS)1) - Y1)< 01, then the system is a sink for Y (Fig. 6.2e). If this term > 0, then the system is a source for Y (Fig. 6.20.

S.V.Smith and M.J. Atkinson

0

n

0.05

0.00 35

141

,

3

. .-.-.

-Gr--.

0

--

.’.-_.

- 4 -

37

39

41

43

-

~

,

5

SALINITY ( o / o o )

Fig. 6.3. Salinity versus phosphate (DIP)in Spencer Gulf,Australia, a net evaporative system. The solid line represents the conservative mixing line (i.e.,it extrapolates through the origin from the mean oceanic condition).The system is interpreted to show a sink near 37.5%, and two salinityregions (36-37.5;37.5-44)dominated by mixing(datafromSmith andVeeh, 1989).

Equation 6.5 is most frequently solved with the assumption of steady state (i.e., dS/dt, dYldt = 0). Examples of this derivation include Boyle et al. (19741, Wollast (19781, Officer (19791, Kaul and Froelich (1984), Smith et al.(1987), and Smith and Veeh (1989).With this assumption, a straight line between any two points on a plot of S versus Y implies that the net biogeochemical rate, B, is insignificant relative to the rates of hydrographic processes in accounting for variation in water composition over the salinity region in question (Figs. 6.2d, 6.3). This is the classical interpretation of a so-called ‘mixing diagram’ (Officer, 1979). Both Loder and Reichard (1981) and Officer and Lynch (1981) pointed out that varying inputs can result in curvature on S versus Y diagrams, even though Y may be non-reactive. An explicit way to deal with this possibility is presented by Smith et al. (1991). Data are collected over time, allowing dSldt and dYldt to be evaluated numerically; then the assumption of steady state is not required. This point is illustrated later, in the section on considerations in site selection. In principle, more complex hydrodynamical models with terms for calculation of non-conservative flux can be derived for any water body. However, the more complex the model must be in order to describe circulation, the more dependent the calculated nonconservative fluxes are on details of the hydrodynamic model being applied; the results can be said to be ‘model-dependent’. Serious problems of interpretation can arise from such model dependence, because composition changes in response to nonconservative fluxes of nutrients are often small relative to changes due to advective and mixing fluxes. Therefore small errors in the assessment of advection and mixing in complex systems, insignificant to the hydrographic analysis itself, can result in large proportional errors in nonconservative flux estimates.

142

Mass Balance of Nutrient Fluxes in Coastal Lagoons

In contrast, very simple systems can be described by the simple and relatively robust analysis presented above. I n these simple systems, errors in estimates of net advection and mixing will affect the calculated nonconservative rates. However, these errors will not introduce disproportional errors among different nonconservative fluxes being compared. Thus, if a major purpose of a particular study is to understand general biogeochemical and ecological controls on nonconservative fluxes (as opposed, for example, to simply characterizing the mass balance of a particular system), then it is advisable to choose systems carefully. For a robust biogeochemical assessment, careful choice of systems for analysis is a legitimate and important consideration in experimental design. It can be seen by inspection of Eq. 6.5 that for any given nonconservative flux ( B ) , as the hydrographic terms (E, R , F ) grow larger, the absolute concentration deviations from ‘conservative behavior’ grow smaller; in other words, rapid hydrographic flux tends to obscure the nonconservative biogeochemical signals. The long-term data set by Peterson et al. (1985) for northern San Francisco Bay, sorted both seasonally and interannually by intensity of stream flow, shows considerable variation in the degree of curvature of material distributions on the mixing diagrams. It is likely that some of this variation in curvature reflects variable freshwater inflow, rather than variable net system sources or sinks. Considerationsin Site Selection

Water budgets and nutrient and salinity distribution data are available for several estuaries, bays, and coral atoll lagoons. Examination of net water flow driven by freshwater (rain plus river) inflow minus evaporative removal in these systems gives insight into analysis strategies. Table 6.1 summarizes net water flow ( E , R , F ) for 26 shallow marine systems; 10 of these systems are coastal lagoons. I n order to compare among systems of varying size, the original volume flow rate data were divided by map area in order to yield units of lengthhime (expressed here in units of m d d ) . The rainfall and evaporation data are not available in all cases; in any event, they are significant only if F is small. The median ( E - R - F ) value for all the systems is about 17 m d d ; the lagoonal median does not differ significantly from the overall value. Note that four of the systems (including one coastal lagoon) listed are marginally net evaporative. For the systems with runoff and rainfall in excess of evaporation, there is a thousand-fold variation in net flow rates per unit area. If biogeochemical reaction rates were constant across these systems (undoubtedly not true, but useful for illustrative purposes), then the degree of curvature in salinity versus Y plots of the data sets would vary accordingly. Nonconservative nutrient budgets have been calculated for three of the net evaporative

S.V. Smith and M.J.Atkinson

143

TABLE 6.1 Net water flow to maintain constant depth in selected marine and estuarine systems. For ease of comparison among systems, water volume flow rate has been divided by system area and expressed in the units m d d . Expressed as E - R - F, as explained in text. E is evaporation; R is rainfall; F is streamflow. It is rare that E - R exceeds an absolute value of 3 to 4 m d d . Therefore, for most systems with E - R - F more negative than about 10 m d d , it is reasonable to assume that F is the only term. Data from various sources (modified from Smith, 1991; Kjerfve, 1986). Sites denoted by * are considered to be coastal lagoons. System

Canton Atoll, Kiribati Shark Bay (E. gulfl, Aust. Spencer Gulf, Aust. Coorong, Aust. Lag. Joyuda, Puerto Rico South River estuary, USA Lake St. Lucia, S. Africa Christmas Atoll, Kiribati Belize Lag., Belize

Area (km')

E-R-F (mmd-l)

Comments

40

7 4 3 2 0

F-O,E>R F - 0, E > R F-O,E>R F - 0 , E > R* F - 0, E - R* F only, E , R unavail. F>E>R* F - 0, E < R F>R>E* F>>R,E F>R>E* F>R>E* F only, E , R unavail. F only, E , R unavail. F only, E , R unavail. F >> R z E,winter; E > F > R, summer F>>R>E* F >> E > R* F only, E , R unavail. F>>R>E* F>>R,E F >> R> E * F only, E , R unavail. F only, E , R unavail. F only, E , R unavail. F only, E , R unavail.

4,300 21,700 260 1 25 310 140

-4

-5 -5

12,700

-6

31 1,630

-9 -11

1,040

-14

25 11,300

-16 -17

1,940 18

-20 -20

Terrninos Lagoon, Mexico

2,500

Gippsland Lakes, Aust. Narragansett Bay, USA Lagoa dos Patos, Brazil San Francisco Bay, USA Mississippi Sound, USA Ochlockonee Bay, USA Neuse River estuary, USA Columbia River estuary, USA

340

-22 -25

328

-28

Kaneohe Bay, USA Lake Ponchartrain, USA Lake Songkla, Thailand Newport River estuary, USA Chesapeake Bay, USA Delaware Bay, USA Tomales Bay (inner), USA

Fourleague Bay, USA

10,360

-35

1,240 2,130 24

-40

-58 -225

400

-375

380

-1,250

93

-5,300

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Mass Balance of Nutrient Fluxes in Coastal Lagoons

systems listed (Canton Atoll, Republic of Kiribati: Smith and Jokiel 1978; Shark Bay, Australia: Smith and Atkinson 1983; Spencer Gulf, Australia: Smith and Veeh 1989) and for three of the systems with net freshwater input (Christmas Atoll, Kiribati: Smith et al., 1983; Tomales Bay, U.S.A.: Smith et al,. 1987,1989, and 1991; and Ochlockonee Bay, U.S.A.: Kaul and Froelich, 1984). Of these sites, Ochlockonee Bay has by far the most rapid freshwater input (225 mm/d); this site still yields interpretable nonconservative flux estimates. All of the lagoons show lower net water flux than this value. Several of the other sites (both lagoonal and non-lagoonal) are known or suspected to have nutrient gradients which are amenable to non-conservative budgetary analysis; we would expect, however, that the flow rates of the Columbia River estuary, U.S.A. (1250 mm/d), or Fourleague Bay, U.S.A. (5300 mm/d), might not be amenable to such analysis. There is also a specific hydrographic advantage to budgetary analysis in net evaporative systems (Wolanski, 1985; Lennon et al., 1987; Smith et al., 1991). Water in evaporative systems is often warmer than in the adjacent ocean. Despite elevated salinity, the water in such systems can be less dense than adjacent ocean water. Exchange with the ocean becomes ‘plugged’, or impeded by the combination of density decrease into the systems and slight residual water inflow to balance evaporative water loss. Either seasonally changing temperature (Lennon et al., 1987) or changing runoff (Smith et al., 1991) can break this plug in one of two ways. Seasonal cooling of water in the system will reverse the horizontal density gradient. If topography permits, this reversed horizontal density gradient will induce gravitational flow of saline water along the sea floor and out of the system. Surface inflow will occur to offset the deep outflow. Seasonal occurrence of freshwater inflow will reverse the direction of residual flow, from inward to outward, and set up classical estuarine circulation with surface outflow and deep inflow. In both cases, horizontal shear between the surface and bottom water will increase vertical mixing and accelerate exchange of dissolved materials. A consequence of density plugs originating from warm, hypersaline water and low residual inflow rather than outflow is that sewage or other pollutants introduced into systems exhibiting such impeded flow will tend to become trapped for long periods of time. All of the sites which were discussed above as having been used for nonconservative flux analysis have relatively simple geometry. Size itself is not a major issue as long as sampling logistics are available; the studied sites have ranged in area from 18 km2 (inner Tomales Bay) to 22,000 km2 (Spencer Gulf). In fact, all else being equal, a large size is more likely to yield simple data because of extended space and time for net chemical reactions to occur. Sampling at a large spatial scale also has the advantage of reducing errors associated with sampling patches of nonconservative substances (Atkinson et al., 1987). In that paper it was recommended that sampling

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Fig. 6.4. Salinity (A), phosphate (B), and nitrate (C) distributions in Tomales Bay, California, as functions of month and distance from the bay mouth. All three variables show seasonal variation, so the steady-state version of Eq. 6.6 is inappropriate for evaluating fluxes (data from Smith et al.,1991). The system is a source for phosphate and a s i n k for nitrate.

should be done at intervals of about 10 times the maximum patch length, in order to get robust estimates of mean changes at the scale of whole systems. Horizontal patch length in shallow, tidally dominated water bodies tends to be on the scale of 100s to 1000s of meters, so sampling at scales of kilometers to 10s of kilometers is most appropriate for system-scale mass balances. The systems in Table 6.1range between about 10 and 300 km in length, so it is easy to space sample locations over appropriate size scales. Considerations of the temporal frequency of sampling are also important if the nonsteady state model described here is to be used. That model was originally developed for Tomales Bay, a system in which seasonal variations in inputs and water composition are important. Figure 6.4illustrates the temporal and spatial variation in salinity, phosphate, and nitrate in Tomales Bay over two annual cycles. Sampling was undertaken at approximately 2-month intervals in order to describe the trends illustrated in that figure. Because of the temporal (seasonal) variations of salinity and nutrient concentrations in the system, curvature on S versus Y mixing diagrams at discrete times is not a measure of nonconservative fluxes. Data collected in Ochlockonee Bay over an annual cycle (Kaul and Froelich, 1984)also shows temporal variation. Systems much smaller than Tomales and Ochlockonee bays would be expected to show short-term variations associated with varying freshwater flow, tidal oscillations, and diurnal cycles of organic production and respiration. The interpretation of such high-frequency variation would then be more complex. By contrast, Spencer Gulf and Shark Bay are sufficiently large, with slow water exchange that averages water composition over approximately an annual cycle. The water composition therefore remains relatively constant over time, as exemplified by several sets of salinity versus dissolved inorganic phosphorus data for Shark Bay (Fig. 6.5). Complexity of inputs and outputs can be a serious issue. Some examples of potential complexity are worth brief discussion. Multiple stream inputs

Mass Balance

146

0.35h

r)

I

0.304

0

SEPT 1981

o

DEC 1981

+

UARCH 1982

of

Nutrient Fluxes in Coastal Lagoons

/

E 0.25 0.20

0.151

n in

-"-

I 0

I

/ 1'0

20

30

40

50

60

70

SALINITY (o/oo)

Fig. 6.5. Salinity versus phosphate (DIP) in Shark Bay, Australia, a net evaporative system which has a DIP sink near S = 45. "he solid line represents the conservative mixing line. The system has a water residence time of about one year, and data from three different expeditions describe similar trends of salinity and phosphate (data from Smith and Atkinson, 1983).

can be problematical, potentially yielding the multiple end members discussed by Boyle et al. (1974). Tomales Bay (Smith et al., 1987, 1989, and 1991) has streams entering both ends of the system. However, the stream draining into the outer end of the system enters a region which exchanges rapidly with the ocean. The modeled portion of Tomales Bay is inside of that stream, so inner Tomales Bay can be considered to represent a simple one-dimensional system with stream inflow at one end and exchange by mixing with the 'ocean' (including the oceanward stream) at the other end, Nutrient sources or sinks revealed by budgetary analyses are not limited to chemical reactions within the system. In some instances, budgets may point to 'hidden' material sources or sinks (such as groundwater flow, atmospheric exchanges, or sewage discharge) which might not be readily revealed by hydrographic analysis of water, salt, or heat budgets. The Scheldt River estuary, Belgium (Wollast, 19781, and Delaware Bay, U.S.A. (Sharp et al., 1982), are geometrically simple estuaries, with river flow at one end and oceanic exchange at the other. Both the Scheldt River Estuary and Delaware Bay have received heavy loading of domestic and industrial sewage. As long as these internal loadings are well defined and not so large that they totally dominate the fluxes, net biogeochemical reaction sources or sinks can still be assessed. Geometries of some water bodies are not well approximated by a one-dimensional model. Shark Bay (Smith and Atkinson, 1983, 1984) is comprised of two gulfs; modeling has been restricted to the eastern gulf, which is separable hydrographically from the western gulf. San Francisco Bay, U.S.A. (Peterson et al., 19851, a system for which a great deal of water composition data exists, consists of two separate subestuaries joined by a central basin

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which exchanges with both the ocean and each of the subestuaries; any model of San Francisco Bay would either have to restrict itself to one or the other of these subestuaries or be complicated by including exchanges among these three basins. Chesapeake Bay, U.S.A., is the site of perhaps more estuarine oceanographic and marine biological studies than any other U.S. coastal ecosystem. This bay consists of a ‘mainstem estuary’ with numerous subestuaries at numerous scales joining into this main stem. One might consider that modeling each subestuary by the simple two-part mixing model would be easier than modeling the entire estuary - which is receiving time-varying inputs from each of these subestuaries. However, the ‘oceanic’as well as the riverine end of each subestuary varies substantially in composition over time. Taft et al. (1978) developed a box model with two vertical layers and five or six horizontal sectors to calculate nitrogen fluxes for the mainstem of the Chesapeake. Strong horizontal stratification of salinity and nutrients can further complicate interpretation of net fluxes in coastal water bodies. ’ho-layer box models can deal with such stratification, potentially lending insight into the vertical distribution of nonconservative sources and sinks. Officer (1980) discusses general strategies of developing such box models, and Taft et al. (1978) provide a n example for Chesapeake Bay. An alternative, applicable in situations where total-system flux is of primary interest, is to use vertically averaged property distributions. Most coastal lagoons are sufficiently shallow and weakly stratified that this strategy is acceptable (e.g., Kjerfve, 1986; Kjerfve and Magill, 1989). All of the above complications describe separate water bodies which could, in principle, be modeled individually. A quite different complication can be ascribed to a number of coastal lagoons. Lagoonal systems behind barrier beaches (i.e., ‘leaky’lagoons) typically have multiple passes leading between the lagoon and ocean; the positions of these passes may be ephemeral (Kjerfve, 1986). Therefore choice of lagoons with stream input and oceanic exchange well removed from one another would optimize application of this mixing model. Preferably the position of the passes between the lagoon and ocean would be relatively stable. Alternative Models for Net Nonconservative Flux Analysis

We have presented one model for analysis of nonconservative fluxes. That model is a one-dimensional hydrographic flux model. As we have pointed out, the model can in principle be extended to two or three dimensions. We have also pointed out that the usual steady-state assumption used in the application of this model can be relaxed. In this section, we briefly explore several other methods for estimating nonconservative fluxes.

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Mass Balance of Nutrient Fluxes in Coastal Lagoons

One alternative method is to measure material inputs and outputs at the entrance (or entrances) to the system of interest over one or more tide cycles, and to attribute the net nonconservative flux to the difference between inputs and outputs. To do this correctly requires intensive sampling. For example, Kjerfve and Proehl(1979) used 125 workers sampling 10 stations across a 320 m cross section at 30-minute intervals for three tide cycles to demonstrate the complexity of water flow to and from North Inlet, South Carolina. Subsequent, detailed ‘input-output’ studies of nutrient flux have relied on data collections over 30-40 tidal cycles during a year, and then have used detailed data smoothing techniques and multiple regression analysis to assess the data. Wolaver and Spurrier (1988) and Wolaver et al. (1988) provide examples of such analyses for phosphorus and nitrogen fluxes, respectively, between the salt marsh and a small tidal creek within North Inlet. An obvious problem in this input-output approach to nonconservative mass balance analysis is that a large amount of data is needed in order to establish meaningful patterns of net import or export. This is evident in the large number of individual flux studies (tide cycles) during which net nutrient fluxes in the system were near 0. Less intensive efforts might have yielded data which were analytical artifacts or which did not represent average conditions. A second approach, used by Nixon (1987) in calculating nutrient fluxes for Chesapeake Bay, is to compare loadings from land with sedimentation rates for these nutrients. The difference between loading and burial is attributed to export. Nixon concluded that about 95% of the total terrigenous nitrogen loading and 84% of the total terrigenous phosphorus loading to Chesapeake Bay is exported in order to balance the terrigenous loading with the sediment accumulation. The inferred rate of P and N export would be difficult to ascertain by tidal input minus output measurements a t the mouth, as demonstrated by the conclusion drawn by previous authors that the bay is a n efficient nutrient trap. Calculations such as Nixon’s obviously require accurate baywide estimates of loading and sedimentation rate. Moreover, these estimates give long-term system performance, not necessarily shorter term variations in nonconservative fluxes. Nevertheless, such substantial discrepancies between loading and sedimentation are persuasive evidence of net flux. A third approach to estimating nonconservative nutrient fluxes is to estimate component fluxes (uptake and release) and sum them. This approach has been used, for example, by Hopkinson (1985), in estimating net nonconservative fluxes in the Georgia Bight. This approach is conceptually attractive if evaluating the contributions of individual component fluxes to the total flux is a major goal of the investigation. However, as argued by Smith et al. (1987 and 1991), the accumulation of errors associated with each component flux contributing the whole is likely to make such summations unreliable estimators of net flux unless the net flux deviates strongly from 0.

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Nutrient Dynamics in Coastal Systems

The mass balance model developed in this paper is applicable to any material which might be chemically reactive. We restrict our remaining discussion to the dissolved inorganic nutrients carbon, nitrogen, and phosphorus. If lagoons are net producers of organic matter, then these systems might be expected to take up dissolved inorganic nutrients from terrestrial delivery and/or oceanic exchange. If these systems are net consumers of organic matter, then net oxidation should release dissolved inorganic nutrients, which will then be transported to the ocean. When we look at the individual nutrients, the actual pattern of flux is more complicated than such simple oxidation-reduction considerations might suggest. Thus, such budgetary analyses for nutrients are useful exercises to assess production or consumption of materials within ecosystems, hence characteristics by which these biogeochemically interesting materials are buried or consumed within systems or exchanged between adjacent systems (e.g., Smith and Veeh, 1989; Smith et al., 1991). In the discussion to follow, it is important to keep in mind a particular aspect of community metabolism. Net metabolism (either net production or net consumption) in nearshore ecosystems is ordinarily a small fraction of gross production and respiration (Table 6.2). This small net metabolism, not gross metabolism, is measured by the mass balance model presented here. Various recent papers (e.g., Nixon and Pilson, 1984; Hammond et al., 1985; Smith and Atkinson, 1983, 1984; Smith et al. 1987, 1989 and 1991; Smith and Veeh, 1989; Smith, 1984 and 1991) have adopted a strategy of evaluating ‘net ecosystem stoichiometry’. Simple, approximately known chemical reactions are assumed to govern the scaling between carbon, nitrogen, and phosphorus fluxes for the entire ecosystem. Such inherently simple stoichiometry is the underlying basis for the so-called ‘Redfield Ratio’, one of the most useful conceptual constructs in biological and chemical oceanography. The general Redfield Ratio to describe the C:N:P ratio in organic matter or a locally appropriate modifications thereof is used to construct net chemical reactions (e.g., Richards, 19651, in order to partition carbon, nitrogen, and phosphorus fluxes among organic carbon metabolism, nitrogen fixation -denitrification, CaC03 reactions, and gas flux across the air-water interface. It is important to remember, however, that the ‘local Redfield Ratio’ can differ substantially from the general ratio. This caution is particularly appropriate if benthic plants contribute significantly to community metabolism, because the composition of these plants tends to differ dramatically from the classical Redfield Ratio (Atkinson and Smith, 1983). Let us briefly consider chemical reactions which involve the net uptake or release of the nutrient elements carbon, nitrogen, and phosphorus. Uptake or release of phosphorus can be interpreted to follow the general trend of

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Mass Balance of Nutrient Fluxes in Coastal Lagoons

TABLE 6.2 Total-system annual mean primary production (PI,respiration (R) and net system metabolism (P - R) for selected shallow-water sites. Rates in g C ,-'year-' (modified from Smith, 1991). Sites designated by * are considered be coastal lagoons. Site

Reference

P

R ~

Bissel Pond, RI Nixon and Oviatt (1973) Georgia coast Hopkinson (1985) El Verde Lag., Mex. Flores-Verdugo et al. (1988) Estero Pargo, Mex. Day et al. (1988) Tomales Bay, CA Smith et al. (submitted) Narragansett Bay, RI Nixon and Pilson (1984) North Atlantic Bight Rowe et al. (1986) Terminos Lag., Mex. Day et al. (1988) S. Kaneohe Bay, HI Smith et al. (1981) Newport R. Est., NC Kenney et al. (1988) MERL control microcosms Frithsen et al. (1985) South San Francisco Bay, CA Hammond et al. (1985) Spencer Gulf, Aust. Smith and Veeh (1989)

960 539 521 345 328 310 230 219 217 211 177 118 92

980 759 599 405 361 230 280 219 294 242 180 110 84

-20 -220 -78 -60 -33 80 -50 0 -71 -3 1 -3 8 9

0.98 * 0.71 0.87 * 0.85 * 0.91 1.35 0.82 1.00 * 0.74 0.87 0.98 1.07 1.10

Average k std. dev.

328 233

365 264

-37 I0

0.94 0.17

nutrient uptake or release during oxidation of organic matter (e.g., Smith and Atkinson, 1983; Smith and Veeh, 1989; Smith et al., 1987, 1989 and 1991). Thus, net production of organic matter takes up phosphorus, while net consumption releases phosphorus. There is, however, potential interference of abiotic reactions such as adsorption-desorption or coprecipitation (Morris et al., 1981;Atkinson, 1987; Froelich, 1988; de Jonge and Villerius, 1989). Sharp et al. (1984) suggest that most such 'geochemical flux' of phosphorus occurs in the very low salinity portion of estuaries, associated with changes in ionic strength, and that phosphorus flux further into the estuaries can probably be largely attributed to biochemical reactions. We assume the same to be true for coastal lagoons. Nitrogen flux, although usually interpreted to arise from simple production or consumption of organic matter (e.g., Taft et al., 1978; Kaul and Froelich, 19841, is actually made more complex because of the additional reactions of gaseous Nz fixation into organic matter, or conversion of fixed nitrogen to gaseous Nz by denitrification. Apparently these processes which are minor to gross nitrogen turnover can dominate the net nonconservative

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flux of nitrogen. Smith and Veeh (1989 and Smith et al. (1987, 1989, and 1991) have made this argument on the basis of mass balance analysis. The conclusions those authors have derived are consistent with smaller scale biochemical assays of nitrogen fixation (e.g., Capone, 1983)and denitrification (e.g., Seitzinger, 1988) in coastal systems. Net system nonconservative carbon flux is complicated by both CaC03 reactions and C02 gas flux across the air-water interface (e.g., Smith and Jokiel, 1978; Smith and Atkinson, 1983; Smith and Veeh, 1989). In the references cited above, nonconservative uptake of total alkalinity has been used as a measure of CaC03 reactions. While this procedure is apparently satisfactory in clearly calcifymg systems like coral reefs (Kinsey, 1978), it does not work in systems with rapid sulfate reduction. Sulfate reduction is probably an important pathway of organic oxidation in many sedimentdominated aquatic systems containing a significant fraction of seawater (to supply sulfate ion) and exhibiting relatively high rates of metabolism in the sediments. Under such circumstances, there is nonconservative alkalinity production, rather than consumption. If reactions involving carbonate minerals appear minor, then alkalinity production can be used as a n estimate of the production rate of sulfide minerals (Smith et al., 1991). This net rate will inevitably be lower than the gross rate of sulfate reduction, because much of the hydrogen sulfide produced by sulfate reduction will subsequently back-react to sulfate. C02 gas flux across the air-water interface can be estimated according to either of two procedures. If a n estimate of the gas exchange coefficient is estimated for the system, then the gas flux can be estimated from calculations or measurements of the water COZ partial pressure (Smith and Atkinson, 1983; Smith and Veeh, 1989). Alternatively, a stoichiometric procedure can be used to estimate carbon metabolism from phosphorus flux; gas flux is then estimated to balance the carbon budget. Smith et al. (1987, and submitted) have applied this procedure and then have calculated the gas exchange coefficient to check that the derivations from the budgetary analyses are physically reasonable. From the summary data which appear in Table 6.2, it appears that shallow water marine ecosystems tend to be slight net consumers of organic material. These systems are net sinks, rather than sources, of organic matter. Net heterotrophy without a n external source of organic matter can only be a transient phenomenon - until biomass carbon or buried organic carbon is consumed. We assume that the apparent tendency of coastal ecosystems to be net heterotrophic does not represent a wholesale transient phenomenon characterizing most shallow water ecosystems. The conclusion that shallow-water systems tend towards net heterotrophy might initially seem at odds with the widely held belief that such systems are often ‘highly productive’. There is no contradiction; systems can at once exhibit high primary production and at the same time exhibit even

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Mass Balance of Nutrient Fluxes in Coastal Lagoons

higher respiration - as long as there is an adequate supply of organic matter. It is well established that organic loading t o the ocean exceeds burial of organic matter in the ocean (Smith and Mackenzie, 1987), and the most plausible region for the net heterotrophy would appear to be the coastal zone. This pattern of net heterotrophy therefore implies that the metabolism of these systems is largely supported by the delivery of organic matter, rather than by inorganic nutrients. Nutrient budgets should tend t o indicate that such systems export dissolved inorganic phosphorus, with nitrogen and carbon fluxes being influenced by additional processes such as denitrification and gas exchange. We assume that, in general, the degree of heterotrophy will increase as organic loading increases. A caution needs to be given to this interpretation of net organic consumption, rather than production, by coastal systems, based on the data in Table 6.2. For reasons discussed by Smith (1991), Smith et al. (1987, and 1991), and the section on alternative methods of net nonconservative flux analysis, net metabolism data derived from the difference between measurements of primary production and respiration are not as robust statistically as are data derived from nutrient budgets. If nutrient budgetary data for net metabolism and measurements of primary production are available, then internally consistent estimates of system respiration can be calculated by difference. The only systems in Table 6.2 evaluated from nutrient budget and primary production data are Tomales Bay and Spencer Gulf, so these estimates should be the most robust estimates summarized in this table. We have been able to find no available net nutrient budgets for coastal lagoons, nor data which we could use t o construct such budgets. Note, however, that four of the systems for which production and respiration data are listed in Table 6.2 are coastal lagoons (Bissel Pond, U.S.A.; and El Verde Lagoon, Estero Pargo, and Terminos Lagoon, all in Mexico). These lagoonal systems apparently tend to conform to the generality of net heterotrophy for shallow-water systems; their median P/R ratio is not different from the P/R ratio of the overall data set. Coastal lagoons therefore do not appear t o be metabolically different from the other shallow-water ecosystems listed in that table. This conclusion should be tested further by direct evaluation of the metabolic performance of a number of coastal lagoons. Acknowledgements

This work was done as part of NSF grant numbers OCE8816709 and OCE8914833 (Stephen V. Smith, principal investigator), Sea Grant number NA89AADSG063 (Marlin J. Atkinson, principal investigator). Contribution Number 3028 of the School of Ocean and Earth Science and Technology, University of Hawaii.

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Kjerfve, B. and Proehl, J.A. 1979 Velocity variability in a cross-section of a well-mixed estuary. Journal of Marine Research 37,409-418. Lennon, G.W. and others. 1987 Gravity currents and the release of salt from an inverse estuary. Nature 327,695-697. Loder, T.C. and Reichard, R.P. 1981 The dynamics of conservative mixing estuaries. Estuaries 4 , 6 4 4 9 . Morris, A.W., Bale, A.J. and Howland, R.J. 1981 Nutrient distributions in an estuary: evidence of chemical precipitation of dissolved silicate and phosphate. Estuarine and Coastal Marine Science 12, 205-216. Nixon, S.W. 1982 Nutrient dynamics, primary production, and fisheries yields of lagoons. Oceanologica Acta, Proceedings of the international symposium on coastal lagoons SCORI lAB0 I UNESCO, Bordeaux, France. pp. 231-242. Nixon, S.W. 1987 Chesapeake Bay nutrient budgets -a reassessment. Biogeochemistry 4, 77-90. Nixon, S.W. and Oviatt, C.A. 1973 Ecology of a New England salt marsh. Ecological Monographs 43,463:498. Nixon, S.W. and Pilson, M.E.Q. 1984 Estuarine total system metabolism and organic exchange calculated from nutrient ratios: an example from Narragansett Bay. In The Estuary as a Filter (Kennedy, V.S., ed.). Academic, New York, pp. 261-290. Officer, C.B. 1979 Discussion of the behavior of nonconservative dissolved constituents in estuaries. Estuarine and Coastal Marine Science 9,91-94. Officer, C.B. 1980 Box models revisited. In Estuarine and wetland processes (Hamilton, P. and MacDonald, K.B., eds). Plenum, New York, pp. 65-114. Officer, C.B. and Lynch, D.R. 1981 Dynamics of mixing in estuaries. Estuarine, Coastal and Shelf Science 12, 525-533. Okuda, T. 1969 Estudio comparativo del las condiciones hidrograficas del las lagunas de Unare y Tacarigua, Venezuela. In Lagunas Costera, un Simposio. Mem. Simp. Intern. Lagunas Costeras (Castaiiares, A.A. and Phleger, F.B., eds). UNAM-UNESCO, Mexico, pp. 291-300. Peterson, D.H. and others. 1985 Interannual variability in dissolved inorganic nutrients in northern San Francisco Bay. Hydrobiologia 129,37-58. Richards, F.A. 1965 Anoxic basins and fjords. In Chemical Oceanography v.1, 1st ed. (Riley, J.P. and Skirrow, G., eds). Academic, New York pp. 611-645. Rowe G.T. and others. 1986 Do continental shelves export organic matter? Nature 324, 559-561. Seitzinger, S.P. 1988 Denitrification in freshwater and coastal marine ecosystems: ecological and geochemical significance. Limnology and Oceanography 33, 702-724. Sharp, J.H., Culberson, C.H. and Church, T.M. 1982 The chemistry of the Delaware estuary. General considerations. Limnology and Oceanography 27,1015-1028. Sharp, J.H., Pennock, J.R., Church, T.M., Tramonto, J.M. and Cifuentes, L.A. 1984 The estuarine interaction of nutrients, organics, and metals: a case study in the Delaware estuary. In The estuary as a filter (Kennedy, V.S., ed.). Academic, New York, pp. 24 1-258. Smith, S.V. 1984 Phosphorus versus nitrogen limitation in the marine environment. Limnology and Oceanography 29,1149-1160. Smith, S.V. 1991 Stoichiometry of C:N:P fluxes in shallow-water marine ecosystems. In: Comparative Analyses of Ecosystems: Patterns, Mechanisms and Theories (J. Cole, G. Lovett and S. Findlay, eds.). Springer-Verlag, New York, pp. 259-276.

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Smith, S.V. and Atkinson, M.J. 1983 Mass balance of carbon and phosphorus in Shark Bay, Western Australia. Limnology and Oceanography 28,625-639. Smith, S.V. and Atkinson, M.J. 1984 Phosphorus limitation of net production in a confined aquatic ecosystem. Nature 307,626-627. Smith, S.V., Hollibaugh, J.T., Dollar, S.J. and Vink, S. 1989 Tomales Bay California: a case for carbon-controlled nitrogen cycling. Limnology and Oceanography 34,37-52. Smith, S.V., Hollibaugh, J.T., Dollar, S.J. and Vink, S. 1991 Tomales Bay metabolism: C-N-P stoichiometry and ecosystem heterotrophy at the land-sea interface. Estuarine, Coastal and Shelf Science 33,223-257. Smith, S.V. and Jokiel, P.L. 1978 Water composition and biogeochemical gradients in the Canton Atoll lagoon. Atoll Research Bulletin 221, 15-53. Smith, S.V. and Mackenzie, F.T. 1987 The ocean as a net heterotrophic system: implications from the carbon biogeochemical cycle. Global Biogeochemical Cycles 1,187-198. Smith, S.V. and Veeh, H.H. 1989 Mass balance of biogeochemically active materials (C, N, P) in a hypersaline gulf. Estuarine, Coastal and Shelf Science 26, 195-215. Smith, S.V. and others. 1981 Kaneohe Bay sewage diversion experiment: perspectives on ecosystem responses to nutritional perturbation. Pacific Science 35,279-395. Smith, S.V. and others. 1984 Chemical stoichiometry of lagoonal metabolism: preliminary report on an environmental chemistry survey of Christmas Island, Kiribati. University of Hawaii SeaGrant Report No.UNIHI-SEAGRANT-CR-84-02. Smith, S.V. and others. 1987 Stoichiometry of C, N, P, and Si fluxes in a temperate-climate embayment. Journal of Marine Research 45,427-460. Taft, J.L., Elliot, A.J. and Taylor, W.R. 1978 Box model analysis of Chesapeake Bay ammonium and nitrate fluxes. In Estuarine Interactions (Wiley, M.L., ed.). Academic, New York, pp. 115-130. Vaulot, D. and Frisoni, G.-F. 1986 Phytoplanktonic productivity and nutrients in five Mediterranean lagoons. Oceanologica Acta 9, 57-63. Wollast, R. 1978 Modelling of biological and chemical processes in the Scheldt Estuary. In Hydrodynamics of estuaries and fjords (Nihoul, J.C.J., ed.). Elsevier, Amsterdam, pp. 63-77. Wolanski, E. 1985 An evaporation-driven salinity maximum zone in Australian tropical estuaries. Estuarine, Coastal, and Shelf Science 22,415424. Wolaver, T.G. and Spurrier, T.G. 1988 The exchange of phosphorus between a euhaline vegetated marsh and the adjacent tidal creek. Estuarine, Coastal and Shelf Science 26,203-214. Wolaver, T.G. and others. 1988 Bly Creek ecosystem study - nitrogen exchange within a euhaline salt marsh basin of North Inlet, South Carolina. Marine Ecology Progress Series 49, 107-116. Yaiiez-Arancibia, A. and Day, Jr., J.W. 1988 Ecological characterization of Terminos Lagoon, a tropical lagoon+stuarine system in the southern Gulf of Mexico. InEcology of coastal ecosystems in the southern Gulf of Mexico: The Terminos Lagoon region (Yaiiez-Arancibia, A. and Day, Jr., J.W., eds). Inst. Cienc. del M a r y Limnol. UNAM Coast Ecol. Inst. LSU, Editorial Universitaria, MBxico, DF, pp. 1-26.

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Chapter 7

Sediment Transport Processes in Coastal Lagoons Maynard M.Nichols and John D. Boon Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, VA 23062, USA

A review of sediment transport in lagoons provides a better understanding of how processes act to modify, retain and accumulate sediment. The lagoon transport system is examined as a series of processes that distribute fine-grained sediment between sources and sinks. The processes cycle sediment from one part of a lagoon to another with small amounts being added intermittently from diverse sources to balance amounts removed from the system or that go into storage. Residual transport of fine suspended sediment is regulated by tidal pumping, shear transport or time-flow asymmetry. During transport and recycling, fine particles are modified by aggregation, break-up and reforming. After deposition, benthic fauna further modify the sediment by changing its stability, geotechnical properties, and erosion resistance. Additionally, wind waves winnow fines from shoals thus modifying textural characteristics, while tidal mechanisms have selective effects on the particle composition and size distributions. Lagoon sinks incorporate a number of fill components reflecting multiple sources and fluctuations in energy dissipation interacting on the sediment supply. Climate mainly influences the source material and the sediment character of intertidal zones. Although sediments are extensively modified, recycled and reworked, especially by storms, lagoons primarily function as net sediment sinks in which the accumulation rates adjust to submergence. Sediment processes are a crucial link to understanding the fate of materials in lagoons since they modulate the chemical reactivity and biological productivity of lagoons. Our knowledge, however, is still imperfect and sediment processes therefore warrant increased study and scrutiny.

Introduction Sediments transported into a lagoon enter a remarkably complicated environment t h a t is inadequately understood. They may undergo repeated cycles of erosion, transport a n d deposition by wave action or ebb-flood tidal currents and b e resuspended many times prior t o accumulation. Some sediment may be sorted by wave action, aggregated into flocs or eaten by

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Sediment Transport Processes in Coastal Lagoons

animals, thus changing its physical form or composition. Other sediments may be transported by fluctuating river flows and either ‘filtered’ from lagoon water or flushed into the sea. Consequently, the kind and amount of sediment discharged, or accumulated in a lagoon, may differ markedly from the sediment supplied. How, then, are sediments modified by the widely variable combination of transport processes in a lagoon? And, how do these processes lead to retention, or release, of sediment from a lagoon? Answers to these questions are a fundamental requisite for sharpening our understanding and for predicting how lagoons function. Because both man and nature are continually changing lagoons, a knowledge of transport processes is a requisite for astute management of lagoonal resources. Without an understanding of transport processes, i t is difficult to predict where substances, such as dredged material, sewage discharges, or toxic chemicals, will go or accumulate. Our knowledge, however, is still imperfect; the gaps reveal where much work remains. This chapter reviews the status of selected sediment transport processes and describes how these processes act in lagoons to modify, retain and accumulate sediment. The information comes from many sources: numerous published papers, unpublished reports, as well as from the experience and observations of the authors. The chapter emphasizes clastic sediments in mid-latitude lagoons along the US.Atlantic and Gulf coasts. It synthesizes what seem to be important features of lagoons, introduces provocative ideas, and provides generalizations for future study in a n area where much work remains to be done. Historical Development

Knowledge of sediment processes in lagoons has evolved through four phases of inquiry with major changes in approach and thinking in the light of new results (Fig. 7.1). The directions of research have been diverse rather than systematic. Nonetheless, knowledge has reached a level that warrants treatment as a unified body knowledge. Early ideas came from morphologic studies of evolution relating to contiguous reefs, marshes and barriers. Darwin (1842) recognized that coral reef lagoons fill with sediment during slow ‘subsidence’ (i.e. a rise in sea level relative to the land). The idea of vertical accretion with submergence was strengthened by stratigraphic evidence of Mudge (1862) and Davis (1913) from US. Atlantic coast salt marshes. However, Johnson (1919, 1925), in his classification of shorelines and ‘classic’ theory of barrier islands, noted that lagoons are common features of emergent coasts. This view created much controversy (Shepard, 1948, 1960). Later studies (e.g. Hoyt, 1967; Zenkovitch, 1969) however, showed lagoons are common to coasts of submergence, but discussion continues as a n element of the barrier

M.N. Nichols and J.D. Boon

159

t

a

1840

1920

1930

1940

TIME

-

1950

1960

!/i ti z

1970

1880

1990

Fig. 7.1. Lines of inquiry and historical development of knowledge concerninglagoon sediment processes; schematic.

island controversy. In an early benchmark paper, Lucke (1934) theorized from morphological comparisons that the evolution of lagoon deposits is linked to inlets and in turn, to tidal and longshore currents and distance from headland. Although this model has often been used to illustrate lagoonal fill patterns and resultant facies succession (Fig. 7.2), recent studies, e.g. Halsey (1979) and Oertel et al. (19891, show that antecedent topography can play a significant role in modifylng energy gradients and fill patterns. I n the 1950s and mid-1960s a wave of inquiry focused on characterizing bottom sediments (Fig. 7.1). These efforts aimed: (1)to determine the origin of lagoon sediments, and (2) to describe the sediments in relation to their environment of deposition as a guide for recognizing ancient lagoonal sediments in petroleum exploration. From heavy mineral, grain-size and compositional studies, Dutch workers (e.g. Van Straaten, 1954) traced the main source of fine sediment in the Wadden Sea, The Netherlands, to the North Sea, rather than to a river source as expected in many lagoons. Other studies (e.g. Van Straaten, 1956; Moore and Scruton, 1957; Hedgpeth, 1957; Parker, 1959) revealed the profound effect of benthic biota upon the structure and character of lagoon sediments. The investigation of Shepard et al. (1960) on the U.S. Gulf of Mexico coast highlights the first comprehensive characterization of lagoon sediments on a regional basis. The first significant study of transport processes commenced with Krumbein’s effort in Barataria Bay, LA, U.S.A. (Krumbein, 1939) to relate the distribution of grain size and sorting to the selective effects of tidal currents. Previously (1870-1930) most transport relationships came from regime theory (Kennedy, 1890) or empirical formulations (DuBoys, 1879; ASCE, 1963). The main thrust of process studies came from Dutch workers in the 1960s who attacked the problem of muddy sediment accumulation on flats

Sediment Tmnsport Processes in Coastal Lagoons

160 STAGE 1

[TIDAL FLATS177

STAGE 2

ARSH DEVELOPMEN ON FLOOD-TIDAL DELTA SHOAL

STAGE 3

STAGE 4

Fig. 7.2. Lucke model of lagoon evolution, exhibiting formation of tidal flood deltas from Oertel et al. (1989) modified after Lucke (1934).

of the Wadden Sea, Netherlands, by examining suspended sediment transport and deposition. Postma (1954)initially argued that inward accumulation resulted from settling lag in response to inward diminished tidal current velocity below a critical threshold velocity for suspension. Later Van

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Straaten and Kuenen (1957) modified the settling lag model by taking into account resuspension of fine particles, or scour lag, and multiple events caused by ebb and flood transport directions. They recognized that higher velocities are required to remove particles from the bed than to deposit them. Postma (1961, 1967) confirmed the settling and scour lag process from field observations and discovered a superimposed effect, the timevelocity asymmetry of tidal currents, to account for net landward transport. A different explanation is offered by Phleger and Ewing (1962) in Laguna Ojo de Liebre, Mexico, by considering the inward diminished distribution of turbulence. Process studies expanded in the 1980s through increasing attention to the details of lag effects (e.g. Dyer, 1988), to bed transport of fluid mud layers (e.g. Kirby, 1988) and to particle aggregation dynamics (e.g. Pejrup, 1986a, 1986b;Van Leussen, 1988). As process studies expanded in the early 1960s, it became desirable to organize their systematic study at large time and space scales embracing an entire system; e.g. sources and sinks, material balance, fluxes and exchange with contiguous systems (Fig. 7.1).At about the same time the great variety and complexity of processes recognized created a need to simplify the real world into a frame of reference through modeling. Burns (1963) probably first initiated a system model applicable to sediment transport. Earlier, much impetus in modeling lagoons came from engineers in an attempt to predict the effects of man-made structures on water movement, e.g. diking and closure of the Zuider Zee, The Netherlands (Lorentz, 19261,jettying of Absecon Inlet, NJ, U.S.A. in 1940 (Simmons and Lindner, 1965). With the advent of high-speed computers in the mid-1970s numerical models began to replace hydraulic models. Today the computer’s capability for modeling exceeds the basic understanding of processes being simulated. Accuracy of modeling transport is limited by the ability to reproduce the settling velocity of mud flocs as affected by turbulence and particle aggregation, erosion and resuspension at the bed, especially erosion rates in high concentration gradients of fluid mud. Despite such limitations Sheng (1987) reproduced the net thickness of sediments redistributed by tidal currents in Mississippi Sound, MI, U.S.A. In summary, the broad trends over the past 70 years show that thinking has changed from the idea that lagoons are a passive depositional environment ‘protected’ from ocean wave energy (Twenhofel, 1926) to one of dynamic character having multiple erosion and deposition events induced by the tide (Postma, 1967; Zarillo, 1982), by the wind (Shideler, 19841, or by biogenic activity (Warme, 1971).Concepts of sediment accumulation have changed from simple bathtub trapping mechanisms with particle by particle accumulation (first assumed in French tidal systems) to more elaborate mechanisms based on settling and scour lag or ebb-flood asymmetry (Postma, 1983). Investigations have developed from broad scale descriptive studies (e.g. Johnson, 1919)to detailed quantitative studies in which statis-

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Sediment Transport Processes in Coastal Lagoons

tical analyses (e.g. Klovan, 1966) and numerical modeling (e.g. Sheng, 1987) are considered a necessary adjunct. Formerly, more effort went into study of individual lagoons than into comparing results from different systems that can yield useful generalizations (Emery and Uchupi, 1972). With the advent of satellite imagery and automated, high-speed computer processing, large amounts of detailed data can be acquired over large areas and analyzed quickly. These trends facilitate larger more comprehensive studies involving interdisciplinary efforts in a global context. General Concepts

Sediment processes consist of four components: (1) erosion, (2) transport, (3) deposition and accumulation and (4) diagenesis and consolidation of accumulated sediment. These processes are produced in lagoons by the dissipation of energy from river inflow, tides, waves, density gradients and meteorological forces as wind. As energy is dissipated, sediments are transported, mixed, sorted or accumulated; and the bottom geometry or shore configuration may be changed. The direction and intensity of processes is determined not only by the hydrodynamics but by the behavior of the particles, which varies with their size, shape, density, composition and biological processing. For cohesive sediments the bulk, or geotechnical properties, physiochemical particle behavior and organic changes are of fundamental importance. Although sediment processes can act simultaneously, typically they are transient and intermittent, varying with bursts and sweeps of turbulence, and alternating between storm and fair weather periods or with season. The processes, therefore supply, distribute and store sediment at various time scales, from seconds to years or millennia. Some processes are random while others are periodic. Since most processes are strongly time-dependent, temporal variations provide signatures to detecting a particular process. When processes act, or interact, they give rise to mechanisms or phenomena leading to lagoon-wide sediment movement, or to accumulation; e.g. littoral drift, turbidity maxima or landward transport of fine sediment. In this sense the processes are building blocks for systemwide mechanisms. To depict inter-relationships among diverse and variable processes responsible for modifylng and accumulating sediment in a lagoon, Fig. 7.3 provides a broad framework of essential features. From a simplistic viewpoint the basic model consists of: (1) sources that supply material to the lagoon basin; (2) various forms of energy dissipation (advection and dispersion) that interact with the sources to produce sedimentary processes, i.e. erosion, transport and accumulation (of note, the processes operate within a regime characterized by climatic setting, basinal shape and size of which many features are inherited from a prior cycle of fluvial erosion or lagoon

163

M.N. Nichols and J.D. Boon

EROSION

+ MODIFICATIONS

ACCUMULATION

\

\

---

Feedback

/ 0

Fig. 7.3. Inter-relationships of sediment sources and energy to processes and resultant sediment modifications, export and sinks; schematic. For explanation see text.

deposition; of note too, the old material can become a ‘new’ source of sediment through erosion of the bed and shores); and (3) sinks that accumulate and store material ‘lost’from the lagoon transport system. In the course of lagoon evolution at a geologic time scale (103-104 years), a filled lagoon eventually can be destroyed by marine erosion. In this case the old eroded sink deposits become a ‘marine’ source and feed back into a ‘new’ lagoon system as part of a lagoon cycle, i.e. filling-erosion-refilling. The model components can be linked together and partly quantified when treated as a mass balance in terms of material entering, or mass influx (Mi), the material leaving the system, or mass efflux (Me)and material entrapped or mass accumulated (M&. When a significant amount of sedimentary in a lagoon, these terms biogenic material is produced (PI, or consumed (C), are taken into account. Assuming steady state:

Mi+P (sources)

=

M,+C+M, (losses and sinks)

A mass balance provides a useful means to determine the combined effect of competing processes that are difficult to sort out and parameterize individually. In summary, the sediment processes in a lagoon continually cycle material from one part of a lagoon to another with small amounts being added from diverse sources to balance amounts removed from the system or amounts that go into storage (Dyer, 1988). Accumulation adjusts to long-term submergence, i.e. to relative sea-level rise, which provides space to store sediment. Sources of Sediment

Lagoon sediments are mixtures derived from multiple sources, some external like streams or the sea, others internal from within the system. The term source is used in a broad sense embracing an erosional provenance or marshalling area, and a n input route or pathway into the lagoon. Sedi-

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Sediment Transport Processes in Coastal Lagoons

ment is not supplied uniformly to a whole lagoon. Instead some sediments are injected at point sources like streams; others are dispersed from line sources like barriers. Sediment sources are recognized by textural similarity, e.g. grain roundness, and by mineralogic or chemical similarity. Findings however, are often controversial because different observational and analytical methods of analysis are applied in a single lagoon. Furthermore, the sediments often are reworked from older lagoon deposits having been deposited and eroded many times. Additionally, sediments are not supplied simultaneously. The relative proportions of sediment from different sources not only change with location in a lagoon but with time as the rates of input change. It is often forgotten that the dominance of a particular source may depend as much on availability of a particular source sediment as on the exclusion of other sources. A marine or fluvial source for one sediment component cannot be generalized to have the same origin for all sediment components. Lagoon transport processes can be selective. Fluvial sources dominate in lagoons near deltas or in lagoons backed by large watersheds with steep terrain, rapid runoff and high sediment yield. Atchafalaya Bay, LA, U S A . , for instance receives a large freshwater discharge, averaging about 4,500 m3 s-l annually, and a corresponding large sediment input averaging 89 x lo6 tons year', which is diverted from the Mississippi River (Van Heerden and Roberts, 1988). Typically, the supply of fluvial sediment to a lagoon is intermittent. In US.Atlantic coastal plain watersheds the bulk of the sediment load, more than 80%, is supplied by peak flood inflow in less than 11%of the time (Kennedy, 1964). Although many studies emphasize a fluvial source, others maintain a marine source dominates, particularly in lagoons with active tidal transport through entrances and exchange with the sea. In the 1940s and 1950s, Dutch workers, notably Van Straaten (1954), showed from mineralogical and petrographical analyses within the Dutch Wadden Sea that the sand, as well a s the silt and clay fractions, originated mainly from the North Sea bed. A marine source may prevail in arid or semi-arid lagoons where there is little or no stream discharge into the lagoon. Sediments from Laguna Ojo de Liebre, an arid area of Mexico, for example, reflect a marine source with supply from the barrier or from updrift littoral currents and the ocean shoreface via inlets (Phleger and Ewing, 1962). Inlets are a source of sediment where current scour releases old material from a previous depositional cycle, e.g. in the Wadden Sea (Van Straaten, 1954). Tracing sources of sediment is complicated when different inputs alternate seasonally, a trend reported in monsoonal and northern temperate climatic regimes. Sediments found in Alsea Bay, Oregon, U S A . (Peterson et al., 1982) and in Yaqina Bay, Oregon, U S A . (Kulm and Byrne, 1966) are a mixture of river sands and beach sands. The two sources are very distinctive, differing in both heavy-mineral assemblages, grain roundness, and quartz-feldspar ratios. The relative dominance of fluvial input in winter is

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controlled by seasonal high river discharge whereas in summer when river discharge is low, input of beach and nearshore sand is determined by asymmetrical tidal currents (Peterson et al., 1982). During the last century lagoons have attracted large human populations in developed areas of the world and thus have become receptacles for wastes of industry, commerce and urbanization. Consequently, human materials are an emerging ‘new’ source of sediment in lagoons. These include organic sewage sludge, coal particles, fly ash, building rubble, hydrocarbons, garbage and an array of solid and chemical industrial wastes. In Jamaica Bay, New York, U.S.A., for example, more than 50% of the lagoon has been filled by solid wastes including 1.7 x lo6 tons of garbage in 1965 (National Academy of Sciences, 1971). Additionally, man’s activities can also drastically alter the sediment supply to lagoons through construction of dams and utilization of watersheds for agriculture. Although erosion of lagoon shores and cliffs contributes sediment directly to a lagoon, this source is probably often underestimated. In Pamlico Sound for example, a large open water lagoon with wave fetches of 70 to 100 km, shore erosion rates reach 3 m year’ (Wells and Kim, 1989). The greatest supply of sand (i.e. new material) is from high banks that collapse as slump blocks during storms. Below mean sea level, however, erosion of marshes, flats and beaches, may be considered a n internal source. It is part of a ‘self-digestion’ process that only redistributes sediment within the system without decreasing the total volumetric capacity (Rusnak, 1967). Since lagoons are very productive (Vannucci, 19691, internal biogenic sources are important particularly where inorganic sedimentation is low. Production contributes shells, diatom frustules, foraminifera1 tests, algal material and organic debris, including marsh or mangrove detritus, sub-terrestrial algae, or man-made debris. There is a general lack of data showing the relative abundance of different organic materials; usually total organic content, or carbon, is reported. However, carbodnitrogen ratios can resolve some sources since land plants have a high carbodnitrogen ratio in contrast to marine planktonic material, which has a low carbodnitrogen ratio (Emery, 1969). Additionally, carbon isotopes can serve as tracers since l3C in land plants is depleted relative to marine-produced material. Sackett and Thompson (1963) found a systematic variation in 6l3C in sediments across Biloxi Bay and Mississippi Sound, U.S.A.from the river (averaging -26.3%0) to the barrier islands (-19 to -21%0).Although carbon isotopes may have limited reliability, and few tracer studies have been conducted in lagoons, they are potentially the best method at present for tracing the origin of organic material. An important internal source in arid or semi-arid areas is chemical precipitation of carbonate and evaporite minerals. Whereas some minerals precipitate out directly in ponds or pools, others form in interstitial pores. In near-surface sediment they may expand and raise the sediment surface

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exposing it to reworking by wind-scour and storm floods (Schreiber, 1986). Table 7.1 summarizes lagoon sediment sources and input pathways together with selected examples. TABLE 7.1 Summary of lagoonal sediment sources and associated input pathways with selected examples and literature reference SourcePathway

Lagoon example

Reference

Atchafalaya, LA, USA Lagos-Lekki, Nigeria Wadden Sea, Netherlands Ho BugVGradyb, Denmark Great Sound, NJ, USA Pamlico Sound, NC, USA Galveston, TX,USA Chincoteague, MDNA, USA Chincoteague, MDNA, USA Laguna Madre, TX, USA Charlestown, RI, USA Guerrero Negro, Mexico Laguna Madre,, TX, USA Bahiret et Bibane, North Africa Laguna Sargento, Mexico Ingolfshofi, Iceland Jamaica Bay, NY,USA

Van Heerden and Roberts (1988) Allen (1965) Van Straaten and Kuenen (1957) Bartholdy and Madsen (1985) Kelley (1980) Wells and Kim (1989) Paine and Morton (1986) Bartberger (1976) Bartberger (1976) Dickinson (1971) Boothroyd et al. (1985) Phleger (1965) Rusnak (1966) Pilkey et al. (1989)

Wadden Sea, Netherlands Arcachon, France Lake of Tunis, North Africa Hutt, Western Australia Abu Dhabi, Arabia Sapelo, GA, USA

Van Straaten (1956) Van Straaten (1956) Pilkey et al. (1989) Arakel(1982) Schreibner (1986) Frey and Basan (1985)

Wadden Sea, Netherlands

Van Straaten (1954)

Corpus Christi, TX, USA

White et al. (1983)

1. External Sources

Fluvial, rivers and streams Marine, via entrances from nearshore or shelf Shores, cliff and bank erosion Barrier, via washovers, storm surge channels Barrier, dunes, flats, eolian Desert, eolian Volcanic, flotables, Glacial, streams Anthropogenic, wastes

2. Internal Sources Organic production, shell worm reefs

Chemical precipitates Shores, flat and marsh erosion Erosion of relic deposits: inlet and shore Anthropogenic, dredge and fill

Nichols (1965) King (1956) National Academy of Sciences (1971)

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Fig. 7.4. A. Sources of sediment to Chincoteague, Virginia, U.S.A. with input quantified in percent; data from Bartberger (1976).B. Sources ofsediment to the Ho Bugt/Gradyb,Denmark tidal area. Data quantified in percent; data from Bartholdy and Madsen (1985).

The relative importance of different sediment sources, natural and anthropogenic, can be quantified in a sediment budget. Bartberger (1976) found that about one-half of the total sediment in Chincoteague Bay, Virginia, U.S.A. is mud with 46% of the total sediment derived from shores and 6% of the total input, from streams (Fig. 7.4A). The rest is sand from the barrier with 32% derived via washovers and 16% via wind transport (Fig. 7.4A). The amount derived from the ocean via the inlet is not quantified but believed relatively small. The relative importance of these sources contrasts with those found in the Ho BugtlGradyb, Denmark, where Bartholdy and Madsen (1985) found 85%of the fine-grained sediment comes from the North Sea (Fig. 7.4B). The rest comes from multiple sources including rivers (5-6%), and shores (4%),in addition to a combined input of wastes, atmospheric deposition and production (54%). Because of the limitations of flux measurements in seaward entrances, the marine contributions are often uncertain. Future studies of sediment transport and related budgets could benefit however, by combined study of transport and component composition of various end-members. Pathways

Fine sediment dispersal is broadly organized along a hydrodynamic gradient resulting in a downstream decrease in competence from source to sink,

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Sediment Transport Processes in Coastal Lagoons

Fig. 7.5.A. Lagoon in plan view (upper subfigure) with dispersal pathways (arrows) in three major subsystems (numbered)and their related sources. Cross-sectional profile (lower subfigure) showing vertical distribution of pathways (arrows) and major lagoon zones; schematic. B. Sinks and processes, i.e. erosion and accumulation zones, (upper subfigure). Cross-sectional profile of erosion and accumulation zones, major lagoon zones and mass balance terms Mi, Me, Ms (lower subfigure). For explanation see text, schematic. Pathways approximate (arrows).

e.g. an inward decrease of tidal current velocity from the inlet toward less energetic zones in basins or protected margins. This gradient, besides the intermittency of dispersal, promotes progressive particle sorting and a n inward decrease in particle size. Dispersal within U.S. Atlantic and Gulf coast lagoons consists of a series of interconnected subsystems (Fig. 7.5A):

1.

A deltaic subsystem driven by river inflow involving flow expansion and deceleration upon entering the lagoon from confined channel banks. Where the lagoon is shallow and well-mixed, frictional, rather than buoyancy hypopycnal flow, likely prevails. This causes dispersion of suspended fine sediment into basins, adjacent flats and marshes. Sandy bedload, or flocculated fine sediment, is deposited in delta front shoals that are shaped into middle ground bars with channel bifurcations, e.g. Mobile Bay, Alabama, U.S.A. (Ryan and Goodell, 1972). Where high gradient river inflow creates an estuarine circulation, e.g. in Mississippi Sound, Mississippi, U.S.A. (Phleger, 1969) and in Ebrie, Gulf of Guinea (Gallardo, 19781, dispersal of fine sediment may be expected to follow the two-layered, two-way circulation.

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2.

A barrier subsystem, either aeolian dune dispersal or a washover system, driven by storm winds or storm ovenvash of ocean waves across the barrier crest. Flow through washover channel chutes (breaches) decelerates and expands lagoonward as sheetflow (Leatherman et al., 1977; Schwartz, 1975). Sandy bedload and suspended load is deposited as washover fans or storm surge platforms.

3.

A tidal inlet and channel subsystem driven by the ocean tide. The inlet is a zone of sediment entrainment, except for lag gravel or shell, and rapid transport of fine sediment. Dispersal is distinguished by inward diverging flood flow through bifurcating channels and a seaward ebb flow through the same channels converging in the inlet. Residual transport is flood dominant in the center. Landward flow expansion and deceleration lead to deposition of sandy bedload forming lobate shoals or a flood delta, between the bifurcating channels. In contrast, ebb dominance prevails in side channels and in deep water behind the barrier (Dyer, 1988). Toward the inner channel head tidal current velocities and residual flow diminish as the water shoals and fans out over adjacent shoals. Sediment also circulates around banks, i.e. up flood channels and down ebb channels in inter-fingering patterns.

Sinks for Sediment

Accumulation in lagoons rarely consists of a single fill unit. Instead, it is characterized by a variety of ‘hybrid’ sinks incorporating a number of fill components through vertical or lateral accretion. These are downstream depositional components that are products of sediment movement through the dispersal subsystems. Differences in the type of dispersal subsystems, and the stage of infilling, produce a variety of distinctive components: tidal flood deltas, washover and storm surge platforms, bay-head deltas, basins, marshes and intertidal flats. These are readily recognized by their morphology. Figure 7.5B depicts their spatial arrangement and Table 7.2 summarizes relevant processes and the environmental regime favorable to their formation. Additionally, some lagoons have dunes prograding along margins, e.g. Laguna Madre, Texas, U S A . (Fisk, 1959), Baja California lagoons (Phleger and Ewing, 1962; Phleger, 1965; Fryberger et al., 1990). Other lagoons have cuspate spits, e.g. Chukchi Sea lagoons (Zenkovich, 19671, and Patos Lagoon, Brazil (Delaney, 1962) or reed swamps, e.g. Zululand lagoons, South Africa (Orme, 1973). As lagoons evolve from a n early open water stage, in which the submergence rate exceeds accretion, particle by particle deposition generally prevails with vertical accretion focused in deep, less energetic zones. When the lagoon fills up to a certain level or base level, the accretion surface is more

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Sediment Transport Processes in Coastal Lagoons

TABLE 7.2 Summary of major lagoon sinks and associated dispersaVdepositiona1 processes, environmental regime and selected examples from the U.S. Atlantic and Gulf coast lagoons and references Feature

Transport/ Depositional Process

1. Tidal flood delta

Inlet flow Microtidal, expansion, flood mixed wave dominant timeand tide velocity asymmetry

Ocracoke Inlet and Pamlico Sound, NC, USA Ninigret Pond, RI, USA Essex Bay, MA, USA

2. Washovers

Ocean wave Microtidal, overwash and sheet- storms and flow hurricanes

Laguna Madre, TX, Schwartz (1975) USA Magdalen Islands, Leatherman et Canada al. (1977) Outer Banks, NC, Cleary and USA Hosier (1979)

Storm surge Flow expansion platforms

3. Bay-head delta

4. Basins

Environmental Example Regime

Microtidal, storms and hurricanes

Hubbard et al. (1979) Boothroyd (1985a) Boothroyd et al. (1985b)

Texas coast, USA

Hayes (1967, 1979); McGowen et al. (1970) Rhode Island coast, Boothroyd et USA al. (1985)

Flow expansion and Microtidal, deceleration, deltaic channel bifurcation and dispersion of fines

Atchafalaya Bay, LA, USA

Suspension settling Microtidal, wave protected

Central Texas coast, USA Mobile Bay, AL, USA

5. Marshes and Vegetation flats trapping, binding and stabilization

Reference

San Antonio Bay,

TX,USA

Micro and Sapelo Sound, GA, Mesotidal, USA wave protected Barnstable Harbor, MA, USA

Van Heerden and Roberts (1988) Shepard and Moore (1960); Roy (1984) Shepard and Moore (1960) Ryan and Goodell (1972) Letzsch and Frey (1980) Redfield (1967) Rey and Basan (1985)

M.N. Nichols and J.D. Boon

171

Fig. 7.6. Schematic profile and section along north-south line south of Authie Bay, France showing historically dated time lines cutting diagonally through horizontal strata. From Le Fournier and Friedman (1975). HT = high tide; LT = low tide.

readily attacked by waves and currents. Consequently, lagoon transport becomes more dynamic; erosion and redeposition are active, sediments are redistributed and sorting is improved (Postma, 1980). As sediments fill the space created by submergence a relatively greater proportion of the sediment supply is exported than deposited. Accumulation shifts into remaining intertidal flats, reentrants and marshes. At this stage lateral accretion prevails and can build a vertical succession of superimposed horizontal layers including marsh deposits overlying old subtidal sediments. This is exemplified by lateral accretion behind a n advancing spit at Authie Bay, France where accretion reaches 10 m y e a r ' (Fig. 7.6) (Le Fourneir and Friedman, 1975). Roy (1984) illustrates the sequence of patterns that evolve with deltaic infilling (Fig. 7.7). This proceeds from a relatively smooth shoreline (Fig. 7.7A) to irregular lobate forms with cut-off sub-embayments (Fig. 7.7B) to a sinuous channel with smooth leeve banks (Fig. 7.7C). Other lagoons in late stages of filling, divide into several smaller, rounded lagoons, a process called segmentation (Price, 1947). In the Zululand lagoons, South Africa, reed swamp encroachment segments the lagoons whereas in the Chukchi Sea lagoons (Zenkovich, 1967), Patos Lagoon, Brazil (Delaney, 19621, southeast Alabama, U.S.A. (Shepard, 1960) and Gippsland Lakes, Australia (Bird, 1984, 1991 this volume), cuspate spits, often produced by wave action and longshore drift, lead to segmentation. A different process is observed in Ojo de Liebre, Mexico (Phleger and Ewing, 1962) and in the Wadden Sea, The Netherlands (Carter, 1988)where tidal inflows enter two or more inlets simultaneously, move inward, meet, and cancel each other

172

Sediment Transport Processes in Coastal Lagoons

Fig. 7.7. Accumulation patterns and stages of deltaic infilling in a lagoon system. From Roy (1984)modified by Galloway (1983).For explanation, see text.

about mid-way between inlets. Weak currents in this zone, termed ‘wantide’ (Carter, 19881,encourage deposition. Marsh plants stabilize the shoals thus segmenting the lagoon into two or more parts. Sediment Dynamics in Tidally-Dominated (‘Leaky’)Lagoons

Background The description of tidally-induced sediment transport in the vicinity of coastal inlets with restricted openings to the sea has proceeded with relatively little concern over morphological classifications of the feature at the other end of the inlet, the receiving body of water. Although a few researchers have referred to this body as a coastal lagoon (Mota Oliveira, 1970, e.g., the Aveiro Lagoon in northern Portugal), much of the literature

M.N.Nichols and J.D.Boon

173

refers simply to semienclosed basins having tidal communication with the sea (Keulegan, 1967; Seelig and Sorensen, 1978; Boon and Byrne, 1981) or as well-mixed shallow bays or estuaries (King, 1974; Speer and Aubrey, 1985; Aubrey, 1986; Van de Kreeke, 1988). This is somewhat surprising considering that tidal flow behavior, including such key attributes as flood-ebb tidal asymmetry caused by nonlinear interactions, is determined largely by interior morphological elements. In this section, various relationships between basin and channel morphology and tidal asymmetry will be examined along with their consequent effect on sediment transport in coastal lagoons. We follow the suggestion made in this volume that drowned river valley systems with depths commonly ranging from 2 to 20 m should be referred to as estuaries. Such systems are usually characterized by stratified flow, inclined fluid density gradients and residual sediment transport associated with net motion in distinct advective layers. Coastal lagoons generally have depths less than 2 m and have a vertically-homogenous water column due to mixing by surface winds as well as the tide. ‘Leaky’as opposed to ‘choked’ lagoons (Kjerfve and Magill, 1989) will have sufficient tidal currents to mix the shallow water column completely but advective processes may then outweigh diffusive ones so that horizontal density gradients, produced by river inflow, remain. Coastal lagoons, in contrast to coastal plain estuaries, are commonly elongate with the long axis oriented parallel to the coastline. Where the feature separating the shallow lagoon from the adjacent sea or ocean is a sand barrier (barrier island) of low relief, it is likely to be broken along its length by multiple coastal inlets. The dynamic stability of these inlets has been characterized by Bruun (1977, 1978) in terms of the ratio between tidal prism ( P ) within the lagoon and littoral driR (M) outside. In Bruun’s analysis, a high P I M ratio implies a stable inlet with little outside bar development and an unimpeded entrance channel allowing good flushing of the lagoon. A low P1M ratio implies an unstable, sand ‘bypassing‘ inlet with restricted tidal exchange through choked and frequently shifting channels. Inlet closure, although rarely observed, is a real possibility under the latter conditions. When considering the sediment budget of ‘leaky’ coastal lagoons, it is obviously important to know the long-term magnitude and direction of the tidally-induced material transport occurring via the inlet. Unlike riverine sediment influx, which is always a positive source and tends to fill lagoons, inlet exchange may represent either a positive or negative supply of sediment. Unfortunately, very few field studies have been conducted in these complex systems that provide measures of inlet transport parameters in sufficient detail to characterize long-term behavior. Hence model studies, with certain simplifylng assumptions, have been relied upon to explore the question of net sediment transport. These ordinarily assume that the tide behaves as a shallow water long wave that, after gaining admittance to the lagoon through single or multiple inlets, will not require much time to travel

Sediment Transport Processes in Coastal Lagoons

174

1.oo

h

0.00

-1.00

1

,

,

I

0

3

6

9

I

,

1

I

I

12

15

18

21

24

Time in Hours

B.

M4" - 2M20 = 180'

I

1.00 I

-1.00

I

0

8

3

6

' 9

'

'

'

'

12

15

18

21

'

24

Tme in Hours

Fig. 7.8. Symmetric tidal distortion (equal rise and fall) produced by linear combination of principal lunar semidiurnal (M2) and quarterdiurnal(M4) constituents.A. With relative phase of 0".B. With relative phase of 180".

to the farthest point in the system. This is normally true whenever the ratio L I C is small compared to the tidal period, T , where L is the conveyance channel length and C is the celerity of the long wave (C = where D is channel depth, g is the gravitational constant). Channel geometry and lagoonal morphology are key factors determining the dynamic behavior of tidally influenced lagoons. The length L , as used above, applies to the main inlet channel and the connecting 'feeder' channels that often branch and lead to isolated, semicircular bays that have roughly uniform tidal characteristics (uniform tidal range and phase). Although useful in modeling the gross features of inlet dynamics in lagoon systems, the uniform bay assumption (e.g., King, 1974) per se does not recognize important non-linear tidal deformations that influence fine-scale net tidal behavior in these systems, particularly in the flow regimes found within the interior.

Non-Linear Tidal Deformation

A simple model of the distorted vertical tide, h(t),relative to mean sea level in a tidal lagoon can be expressed by the approximation

M.N. Nichols and J.D. Boon

A.

175

M4' - 2MT = 270'

1.00 I

0

I

3

6

9

12

15

18

21

24

Time in Hours

B.

M4' - 2 M T = 90'

1.00

I

-1.00

I 0

1

I

3

I

6

I

9

I

12

I

I

I

15

18

21

I 24

Time in Hours

Fig. 7.9. Asymmetric tidal distortion (unequal rise and fall) produced by linear combination of principal lunar semidiurnal (M2) and quarterdiurnal (M4) constituents. A. With relative phase of 270". B. With relative phase of 90".

h(t) = Mz cos wt

+ Mq cos(2wt - $)

(7.1) where Mz and M4 are the amplitudes of the lunar semidiurnal constituent and lunar quarterdiurnal (a forced shallow-water) constituent, respectively, o is the M2 tidal frequency, and 41 = M4O-2MZ0 is the relative phase between these constituents. The degree of distortion or asymmetry in the tidal waveform varies as the ratio of the constituent amplitudes, MdM2, while the form of distortion is governed by the phase difference, +.When = 0" or 180", the periods of rise and fall in the vertical tide are equal (Fig. 7.8A,B) but reach their maximum inequality when 41 = 270" (Fig. 7.9A, longer fall) and = 90" (Fig. 7.9B, longer rise). The induced tidal current or horizontal tide, u(t), in the lagoon varies approximately as the time-derivative of the vertical tide or

+

+

dh(t) dt

u ( t ) a -- - o M z

sin o t - 2wM4sin(2ot - $)

(7.2)

in which the amplitude of the quarterdiurnal tidal current is doubled relative to the semidiurnal tidal current and thus tends to enhance whatever asymmetry is present in the vertical tide. Equations (7.1) and (7.2) also

Sediment Tiansport Processes in Coastal Lagoons

176

-

A.

M4' - 2M2" 90'

FLOOD

U

EBB 0

0

0

3

6

9

1

12

15

18

21

24

Time in Hours

B.

-

M4' - 2M2" 270'

FLOOD

U

EBB

1 7

0

I 7

3

1

6

1

1

I

12 15 Time in Hours 9

I

18

.

21

I

24

Fig. 7.10. Maximum flow asymmetry in a tidal current with A. greater ebb maximum produced by relative phase of 90";B. greater flood maximum produced by relative phase of 270".

indicate that u(t) and h(t) are a quarter-wavelength out of phase with one another but otherwise have similar asymmetric forms for a given value of @ as shown in Figs. 7.10 and 7.11. Tidal deformation can occur as either of (or as a combination of) two basic types of asymmetry shown schematically by a distorted sine wave: (1)a maximum-flow asymmetry (Fig. 7.10) featuring unequal flood and ebb current maxima caused by inequalities in flood and ebb phase duration, or (2) a time-flow asymmetry (Fig. 7.11) featuring unequal spacing between slack water times and times of maximum current. Causes and Effects of Tidal Distortion

The asymmetry of tides and tidal currents arises from the forced generation of several overtides and compound tides known as the shallow-water tidal constituents whose mutual interaction yields a complex, time-varying asymmetric pattern (Boon, 1988). Discussions of the origin and form of these distortions have been given by Boon (19751, Boon and Byrne (1981), Aubrey (1986), Dronkers (1986a, 1986b) and Friedrichs and Aubrey (1988). As noted by Dronkers (1986a), unequal flood and ebb maxima mainly affect residual transport of coarse sediment (grain settling velocities: W,> 10 cm s-9 as bedload in high-velocity, channeled flow whereas residual

M.N. Nichols and J.D. Boon

A.

177

M4"-2M20-00

FLOOD

U EBB

0

3

6

9

12

15

18

21

24

18

21

24

Tme in Hours

B.

M4" - 2MP = 180"

FLOOD

U EBB

0

3

6

9

12 15 7ime in Hours

Fig. 7.11. Time-flow asymmetry in a tidal current with: A. Maximum flow acceleration at slack before ebb (SBE) produced by relative phase of 0". B. Maximum flow acceleration at slack before flood (SBF) produced by relative phase of 180".

transport of fine sediment (grain settling velocities: W, < 1 cm s-1) in suspension is strongly influenced by differences in flow acceleration during the two slack water intervals centered on slack before flood (SBF) and slack before ebb (SBE) as illustrated in Fig. 7.11. Using the uniform bay assumption, slacks will occur at the time of high water (SBE) and the time of low water (SBF). However, frictional and inertial effects may cause a slight time delay between the occurrence of tidal height extremes and subsequent slack waters. Within the lagoon interior, the form of tidal asymmetry depends on certain aspects of the general intertidal morphology. Tidal current amplitudes vary roughly in proportion to the local water depth and their directions tend to run parallel to local depth contours (Cheng et al., 1986). Given this tendency, the mean tidal amplitude to depth ratio, rlD, clearly becomes important since it influences channel friction acting on time-varying tidal flow. Also intertidal storage, as indexed by AfIA,, the average ratio of intertidal area (Ad occupied by tidal flats and marshes to channeled area (A,) covered by water at mean low tide, likewise has a direct bearing on flow velocities in channels. Friedrichs and Aubrey (1988)found a clear association between shorter flood durations (greater flood maxima or 'flood domi-

178

Sediment Transport Processes in Coastal Lagoons

nant’) and increasing r/D ratios in modelled systems. They also asserted that a large Af /A, ratio generally causes shorter ebb durations (greater ebb maxima or ‘ebb dominant’). As used here, the term ‘flood dominant’ implies net incoming or landward-directed sediment transport whereas ‘ebb-dominant’ implies net outgoing or seaward-directed transport. Some apparent exceptions to their second assertion may stem from an uncertainty as to whether Ar should include high marsh areas. These become part of the intertidal storage only during maximal stages of extreme spring tides. In addition, a single Af /A, ratio may not be representative of all channels in a system, particularly branching ones whose bed elevations increase significantly as their stream order decreases in the landward direction. Boon and Byrne (1981) presented data showing that basin free-surface area (Af + A,) varies nonlinearly with tidal height (nonlinear basin hypsometry) in marsh-filled lagoons. These often feature a very large increase (decrease) in water-covered area during the late rising (early falling) stage of an extreme spring tide. During such stages, the inner channels, which convey most of the flow reaching the high marsh areas, will experience maximum velocity and assume a flow pattern similar to the one illustrated in Fig. 7.11A.

Bedload Residual Transport According to the argument of Dronkers (1986a), coarse sandy sediments are entrained and placed in motion as bedload only during times when in either the flood or ebb current speeds exceed a certain threshold (U,) direction. The magnitude and direction of the net or residual transport over a tidal cycle may then be determined by a flow asymmetry favoring either a greater flood or a greater ebb maximum current as shown in Fig. 7.10. Channel friction (indexed by r/D ratio) and intertidal storage (indexed by Af/A, ratio) play key roles in determining both the magnitude and the direction of transport favored by this type of distortion. Neglecting riverine sediment and freshwater influx, an ebb dominant bedload transport should be promoted in systems having a relatively high Af/A, ratio (large intertidal storage) and a relatively low r/D ratio (small channel friction) associated with shorter ebb durations in inlet flow. An example of this type of system is the Wachapreague, Virginia, U.S.A. marsh-lagoon complex described by Boon and Byrne (1981)and Price Inlet, South Carolina, U.S.A. described by Fitzgerald and Nummedal (1983). Conversely, systems having a relative low Af /A, ratio and relatively high r/D ratio should experience dominant flood transport. These attributes are reported for Murrells Inlet, South Carolina, U.S.A. (Boon and Byrne, 1981) and Nauset Inlet, Massachusetts, U.S.A. (Friedrichs and Aubrey, 1988). Pertinent values for these systems are:

M.N. Nichols and J.D. Boon Place

rlD

179

AfIAe

MdMz

M4O-2Mzo Flood-Ebb Duration Difference Shorter

Wachapreague

0.10

4.30

0.040

150"

0.4 hours

Ebb

Price

0.14

3.85

0.042

119"

0.5 hours

Ebb

Nauset

0.39

0.49

0.158

297"

3.0 hours

Flood

Murrells

0.27

1.04

0.083

266"

1.4 hours

Flood

The hydrodynamic conditions reflected in these parameters are consistent with Dronker's argument for residual transport of bedload material. They are also consistent with certain morphological features seen in the above inlet-lagoon systems. Both Wachapreague Inlet and Price Inlet have pronounced ebb deltas but lack the flood deltas that are more pronounced at Nauset Inlet and Murrells Inlet. Away from the inlets, it is clear that MdM2 ratios progressively increase toward the interior as the tidal distortions continue to grow. However, it is not at all clear that trends in net bedload transport persist in lagoon interiors because of a general landward decrease in current velocity and an increase in the proportion of fine bottom sediments that are more likely to be transported in suspension.

Suspended Load Residual Transport Fine sediment (W,< 1 cm s-l) will be carried in suspension whenever upward grain movement associated with vertical turbulent diffusion equals or exceeds gravitational settling in the suspending fluid. To the extent that turbulent diffusion within the water column is dependent on the horizontal velocity field, one can postulate that, for decreasing tidal velocity at a fixed height, a minimum velocity is reached below which sediment particles of a given size can no longer be carried in suspension. Rather than being immediately deposited on the bottom upon reaching the minimum velocity, however, the particles continue to be transported in the flow direction for a period of time (settling lag) before reaching the bed. Allowing for bed stress variations related to bottom roughness and stratification, a n accelerating tidal flow reaches and then exceeds the above minimum for a similar period of time (scouring lag) before a velocity is reached that is just sufficient to entrain and resuspend the same particles. In other words, the velocity needed to entrain the material is greater than that needed to keep it in suspension. Using these concepts in combination with a n observed landward decrease

180

Sediment Transport Processes in Coastal Lagoons

in peak tidal flow as well as depth, a well-known qualitative model of fine sediment transport was advanced by Van Straaten and Kuenen (1958)and Postma (1961) to explain net landward transport of material into the interior of the Dutch Wadden Sea. With each flood and ebb cycle, the net effect of the lags is to carry the sediment farther in the landward direction. Such a mechanism was believed necessary to offset a seaward diffusive transport of sediment caused by the observed landward gradient in suspended sediment concentration. Their largely Lagrangian approach, following individual sediment particles coupled with the fluid in time and space, seemed to explain the phenomena but did not lead to quantitative estimates of the net transport over one or more tidal cycles. A semi-quantitative model of the residual sediment transport in the Wadden Sea was later advanced by Groen (1967) who formulated the transport as a n Eulerian sediment flux over a tidal cycle. Groen’s model employed a n alternating tidal flow transporting equal volumes of water but not equal volumes of sediment with each tidal cycle. He demonstrated that such behavior could be explained by a n integration of the time-varying tidal flux calculated as the product of the instantaneous velocity and the instantaneous suspended sediment load (total quantity of sediment suspended in the water column). A harmonic model of the current was used incorporating a time-flow asymmetry with greatest flow acceleration during slack before flood (cf. Fig. 7.11B). The time-varying suspended sediment load was predicted by a differential equation relating the rate of change in the instantaneous sediment load to the difference between the actual suspended load at a given moment and the equilibrium load that would result for a steady current of the same speed as occurs at that moment. Given ‘memory’ of this kind, the time-history of sediment loading in the water column responds with highest levels during the peak flow (flood or ebb) that is preceded by the shortest interval of low-velocity flow. That is clearly the flood peak in the Wadden Sea example given by Groen since it follows SBF whose low-velocity period is very brief because of the maximum flow acceleration experienced at this point. Although Groen’s model does not predict the absolute quantities of suspended sediment engaging in flood or ebb transport, he was able to show that flood displacement of suspended sediment could exceed that of ebb by as much as 38% without requiring any asymmetry in flow maxima. A further step in the quantification of sediment transport in suspension has been made by Dyer (1978) and Uncles et al. (1984, 1985). In their approach, horizontal components of turbulent suspended material flux are neglected and both time-varying and spatially-varying components of the velocity, u , and suspended sediment concentration, c , are considered at points in time and space, along with temporal variations in the cross-sectional area, a , normal to the flow, in deriving the instantaneous flux. These instantaneous components may be expressed by the following sums, respectively:

M.N. Nichols and J.D. Boon

u = (a) + A

181

(7.5)

where the brackets, (), denote time averaging over a tidal cycle and terms with capital letters are tidal oscillatory components having zero mean value, i.e., (U)= (C)= (A)= 0. The subscript A denotes a quantity averaged over the cross-sectional area and the subscript d refers to the local deviation of a point-measured quantity from the area-averaged value so that U = uA+udand c = CA+ cd. Thus, (ud and (cd are space-time means of the velocity and suspended sediment concentration measured in channel crosssections conveying the longitudinal sediment flux. The flux averaged over a tidal cycle as well as all points in the cross-section is then

The first and second terms on the right side of Eq. (7.6) represent flux due to residual flow. This is often the largest flux contribution as shown by measurements in large coastal estuaries such as the Gironde (Dyer, 1978, 1988). Similar measurements unfortunately are lacking for lagoons. The third through the fifth terms involve various correlations among time-varying quantities and have been referred to as tidal pumping (Uncles et al., 1984,1985) as they require addition or loss of suspended material (erosion or deposition) during the tidal cycle. The final two terms involve spatial correlations, particularly those caused by the presence of vertical gradients in velocity and concentration, and include what is referred to as the shear transport. It is clear that both tidal pumping and shear transport play important roles in fine sediment movement in leaky lagoon systems. Tidal distortions of the type leading to pronounced time-flow asymmetry in tidal currents will be most likely to produce residuals in these forms of transport because time-flow asymmetry affects both the total load of sediment and its distribution within the water column over time. As depicted in Fig. 7.12A and 7.12B, the sediment load and its distribution may not vary symmetrically in time (Fig. 7.12A) or in space (Fig. 7.12B) with respect to the slack water points in the velocity field. This is conducive of a residual transport in either the flood or ebb direction, depending upon which maxima (flood or ebb) is preceded by greater flow acceleration (duldt) at the antecedent slack (SBF or SBE).

182

Sediment Transport Processes in Coastal Lagoons

Fig. 7.12. A. Diagram showing tide (h),area-averaged velocity (VA)and suspended sediment concentration over a tidal cycle. Vertical distribution of sediment varies while total load in suspension remains constant, illustrating shear transport. B. Diagram showing tide ( h ) , area-averaged velocity (VA)and suspended sediment concentration over a tidal cycle. Local concentration and total load of sediment in suspension varies with time, illustrating tidal pumping.

Basin Morphology as a Source of Time-FlowAsymmetry

Maximum-flow asymmetry (Fig. 7.10) is largely derived from nonlinear bottom friction and momentum transfer effects in shallow lagoons. These processes are reflected in the ‘growth’ of the shallow water tide in the interior of the lagoon as indexed by the increase in the M4/Mz ratio of the vertical tide proceeding toward the interior. In many lagoons, however, the uniform bay assumption applies and the morphology or basin geometry of the lagoon becomes of key importance. For lagoons connected to the ocean by a single inlet, conservation of mass requires that

M.N. Nichols and J.D. Boon u = -Ab .-

dh A, dt

183

(7.7)

where At, is the free surface area of the lagoon and A, is the cross-sectional area of the connecting channel. From Eq. (7.7) it is clear that strong time-flow asymmetries in u may develop for reasons other than the nonlinear effects (overtides) present in the vertical tide, h. In lagoons with marshes flooded at high tide and extensive tidal flat exposure at low tide, AL,will not remain constant but will show considerable periodic variation as a function of tidal stage. Where the ratio rlD is large, A, will likewise vary periodically about its tidal mean value as a function of h. To show the effect of basin geometry on Ab, ignoring variation in A,, we can assume a linear variation between dimensionless basin area and dimensionless tidal height expressed as

A"= Nlh*

(7.8)

where A*= (At,-A,)/(Ah, - A d is the dimensionless area, and h" = ( h - h,)/r with: A, = lagoon free surface area at mean sea level, A h w = lagoon free surface area at mean high water, Al, = lagoon free surface area at mean low water, h, = mean sea level elevation in lagoon, N I = a slope factor, P = tidal amplitude in the lagoon. Ignoring the shallow water tidal harmonic, M4,and assuming h = h, + M2 cos ot with P = M2, the inlet velocity according to Eqs. (7.7) and (7.8) is u = -o(Ab/A,)M2 sin

ut

= -o(A,/A,)M2 s h o t - o((Ahw- AIw)/A,)NIM2 cosot sincot = -o(A,/A,)M2 s h o t - w((Ahw- AIw)/2A,)N1M2 sin2ot

(7.9)

noting that Ab = A, + A*(Ahw- A d and h' = cosot. The right side of equation (7.9) contains a first harmonic term of the same form as equation (7.2) except that its sine phase relative to the fundamental M2 oscillation is always zero. This contribution to the flow asymmetry due to basin geometry and linear variation in At, corresponds to the phase difference M4"-2M2" = 0" with maximum flow acceleration at SBE as shown in Fig. 7.11A. Alternately, if variations in A, become more important over the tidal cycle, a time-flow asymmetry similar to that shown in Fig. 7.11B (phase difference M4"-2M2" = 180") can result. Letting

A, = A,, + A,, cosot

(7.10)

where A,, is inlet cross-sectional area at msl and A,, is the area added (subtracted) to A,, at high (low) water, we have

Sediment Transport Processes in Coastal Lagoons

184

u = -w(AdA,)Mz sinwt = -wAdMzsinot/(A,,

+ A,,

coswt)

(7.11)

Equation (7.11) is not a linear combination of a fundamental oscillation with its first harmonic but the resulting wave form is similar to that depicted in Fig. 11B in that the flow maxima are shifted toward SBF. The intensity of the distortion depends on the ratioA,dA,, and its form also may depend on the phase lag, if any, between the tide inside the lagoon and the tide at the inlet cross-section. Textural Patterns

In general terms, the distribution of bed sediment texture follows the energy gradients and bathymetry. Phleger (1981) notes the coarsest sediment is in the inlet where current velocities are maximal, and finest sediment is in the innermost reaches where current velocities approach zero. Additionally, the finest material may be selectively transported landward by the tides through settling-scour lag or time-velocity asymmetry effects (Van Straaten and Kuenen, 1957; Postma, 1967). In some lagoons a n inward decrease of wave energy by shoaling or reduced wave exposure, can reduce turbulence and allow accumulation of fine sediment. Three general types of textural patterns are observed: 1.

Mud in central basins and sand on margins. This pattern is common to deep, choked and restricted microtidal lagoons of the US.Atlantic and Gulf coasts, e.g. Mobile Bay, Alabama (Fig. 7.13A) (Ryan and Goodell, 1972) and central Texas bays (Shepard and Moore, 1960). A relatively high river influx of fine sediment into Mobile Bay results in accumulation of mud throughout the central basin below a depth where day-to-day waves lose competency to winnow silt and clay. In contrast, sandy sediment on marginal shoals is determined by wave action that either prevents deposition of fines on exposed shoals, or winnows fine material from mud-sand mixtures. The change from sand to mud often follows a steep bathymetric gradient producing a marked textural transition. Inman and Chamberlain (1955) observed the transition in central Texas bays and recognized fundamental differences between the mode of transport of the sand size material as bedload and of clay and silt carried as suspended load. They suggest the transition can occur when sand is mixed or deposited with fine material in the presence of a weak or partially effective sorting agent.

2.

Sand in central parts and mud along margins. This pattern occurs in shallow, choked or restricted microtidal lagoons where the supply of fine sediment is low and the floor is subject to vigorous wave

M.N. Nichols and J.D. Boon

185

action, e.g. Bogue Sound, North Carolina, U.S.A. (Fig. 7.13B) (Brett, 1963).Wave action deters deposition of silt and clay in central parts. Instead, fine sediments are carried by wind drift currents, or tidal currents, and deposited in protected margins or marshes. Sand is supplied via the inlets or from the barrier. 3.

Coarse sand in entrance channels fining landward with mud on inner tidal flats and margins. This pattern occurs in leaky meso and macrotidal lagoons where tidal currents and wave action decrease landward. Fine sediment may be supplied from the land or sea. The channel system digitates landward into flats and marshes, e.g. Arcachon Bay, France (Fig. 7.13C) (Allen et al., 1973). Whereas, the coarse sand is deposited from bedload carried by strong tidal currents (> 2 m s-l), the fine sediment is deposited from suspension. The pattern is similar to that in the Dutch Wadden Sea (Van Straaten and Kuenen, 1957; Postma, 1967) where fines are supplied from the North Sea. Additionally,benthic filter feeders deposit large amounts of fine material on the flats, especially in summer. Waves, which predominate over currents on the flats, are damped as they pass from deep channels onto inner shoals. Postma (1957) demonstrates from grain size frequency distributions the selective action of currents in channels and of waves on the flats in the Wadden Sea.

Further characterization of texture is likely to yield additional patterns because lagoons exhibit a widely variable combination of bathymetry, vegetation and energy conditions.

IA MOBILE BAY

1

B

BOGUESD.

I C ARCACHON BAY

Fig. 7.13. Three types of textural patterns in lagoons: A. Mud basin and sand margins; from Ryan and Goodell (1972). B. Sand basin and mud margins;from Brett (1963). C. Sand channel and mud inner intertidal; from Allen et al. (1973).

Sediment Transport Processes in Coastal Lagoons

186

Particle Modifications Fine particles in lagoons are either single grains dispersed in suspension or composite particles clustered together as aggregates flocs, pellets or “marine snow”. Some are predominantly inorganic, others organic. The composite particles are significant because their settling velocities range u p to several orders of magnitude greater than single particles. Consequently, composite particles are readily removed from suspended transport, and thus tend to sink and reside within the system. In contrast, fine dispersed particles tend to remain in suspension, recirculate or escape. However, the importance of aggregation processes relative to hydrodynamic agents in retention or escape is controversial. Most information on particle aggregation behavior comes from studies in estuaries. This includes a large body of knowledge on physiochemical action (flocculation), effects of salinity, clay mineral composition and their alterations and effects of turbulence and circulatory patterns (e.g. Whitehouse et al., 1960; Krone, 1962; Owen, 1971; Kranck, 1981; Van Leussen, 1988). Since estuarine conditions develop in certain lagoons, the information from estuaries may apply to lagoons within limits. Lagoons however, have features that affect aggregation processes differently: 1.

In lagoons suspended inorganic sediments are more or less in chemical equilibrium with salinity of the water. Lagoon water is generally more well-mixed vertically by winds than in estuary water. Therefore, physiochemical flocculation by salt contact in strong salinity gradients is less likely and local accumulation zones caused by flocculation are less prominent (Pryor, 1975).

2.

The shallowness of lagoons allows flocs and aggregates to settle to the bed during slack water, thereby increasing the relative importance of aggregation processes, a trend noted by Pejrup (1986a; 198613)in the Ho Bugt-Gradyb, Denmark.

3.

The shallowness of lagoons combined with extensive wind and wave exposure generally promotes greater resuspension of bed sediment than in estuaries. With higher suspended sediment concentrations in the water, floc size increases because of numerous particle collisions. Consequently, flocculation is enhanced overall as shown by results of Pejrup (1989) in the Ho Bugt-Gradyb, Denmark.

4.

The high organic activity in lagoons produces numerous composite particles by filter-feeding, “packaging” or biogenic pelletization into feces or pseudofeces, or by mucal trapping and secretional binding. Carson et al. (1988) found that 70% of the suspensates in Great Sound, NJ, U.S.A. are organic mineral aggregates. Interestingly, they are of relatively low density ( c 1.4 mg/m3) and most abundant

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under fair weather conditions. This contrasts to fecal pellets that are usually more dense (1.7-1.8 mg/m3)and settle more rapidly than the organic-mineral aggregates. Consequently, the low-density aggregates constitute an everpresent background population in suspension whereas the pellets together with single particles, are present in suspension only during fast currents or storm conditions. Thus, the type of aggregation can lead to separate hydrodynamic populations and resultant selective transport and deposition. The more dense fecal pellets, which are hydrodynamically equivalent to coarse silt and fine sand, tend t o codeposit with single grains resulting in poor sorting (Carson et al., 1988). Wells and Shanks (19871,in a study of “marine snow”,i.e. organicrich aggregates greater than 0.5 mm, in Cape Lookout Bight lagoon, North Carolina, U.S.A., show that the abundance and total volume of particles is not related to suspended sediment concentration or to current speed in the 10 to 200 cm s-1range, as in some estuaries. Instead, the abundance of large aggregates accelerates gross flux of sediment to the bed (Wells and Kim, 1989). Van Straaten (1963) argues that the exceptional size of flocs in water of normal salinity in the Zuiderzee, Netherlands, is determined not only by electrolyte concentration but by the mineralogical composition of the clay as well as by the influence of organic substances. The predominance of organic components in lagoon macroflocs means that the particles are more readily broken down by turbulent shear and reformed with new biogenic material. Biological Modifications Benthic organisms continually process, transport, and modify lagoon bed sediments. Two broad categories of opposing processes are recognized: (1) those that bind, protect and stabilize near-surface sediment, and (2)those that loosen and destabilize the sediment. Of primary concern is the stability of the water-sediment interface in response to shear from flow. Processes that destabilize the sediment decrease the critical erosion velocity and vice versa (Rhoads et al., 1978). Stabilization processes bind particles to the bed by secreting mucus or organic films over the surface. Tracks and trails left by gastropods, or bivalves are often floored with mucal binding and thus are more resistant to erosion than surrounding sediment (Nowell et al., 1981). The influence of algae in promoting deposition by both trapping and binding, has been shown in various ways from tidal flats in the Wash, U.K. (Hopkins, 1967; Coles, 1979).If algae are killed by pollution, large amounts of erosion could be expected with consequent sedimentation in channels (Frostick and

Sediment Transport Processes in Coastal Lagoons

188

McCave, 1979; Rhoads and Boyer, 1982). Despite important effects, biogenic stabilizing processes are not included in most sediment transport models (Young, 1982). The degree of binding and cohesion is not simply determined by grain size and there is no good way of measuring it. Thus, the fluid stress required to remove organically bound particles from the bed cannot be accurately predicted. In many lagoons, accumulations of worm tubes, mussels and vascular plants cover the floor. They usually stabilize the bed by cementing mud or trapping sediment between anchor structures, thus providing a tight fabric (Rhoads, 1974; Rhoads and Boyer, 1982). However, when a dense cover is broken by storms, grazers or predators, the remaining low density cover, mounds or beds, can allow flow to accelerate and cause scour (Young, 1982). Destabilization processes destroy bed structure, reduce cohesiveness and increase suspended sediment concentrations above the bed. The most significant modifications are produced by feeding activities of suspension and deposit feeders in the upper 10 to 20 cm thick sediment layer. An individual macrodeposit feeder may ingest a volume of sediment on the order of 100 cm3 yrl.Therefore, in mid-latitude systems, the surface sediment may be processed many times a year (Myers, 1978). As the sediment passes through digestive tracts, organic material is removed, particles can undergo abrasion and certain substances can become more soluble. Mobile organisms or burrowing fish may increase the microtopography of the sediment surface by projecting tubes and fecal mounds. Bed roughness enhances bed resuspension by altering the near-bed turbulent structure, and thus, lowers the critical erosion velocity (Boyer, 1980). From laboratory experiments, Nowell et al. (1981) revealed that destabilization by animal tracks and pelletization lowers the threshold velocity for sediment movement (Fig. 7.14). Bioturbation of sediments by burrowing or deposit-feeders through processes such as irrigation, pelletization and tube construction, usually increases sediment pore space and thus, water content in the upper sediment

*ooc

r

- D

2ot ‘OmL

1

I

I

,a

1

I l l 1

1

I

l

l

\

1

0.1 1.0 GRAIN DIAMETER (mm)

10

-

i/

lo0

Fig. 7.14. Influence of bottom tracks and pellets on the threshold velocity for sediment movement. From Nowell et 4L.(1981).Crosses and encircled crosses indicate values for abiotic and microbially colonized glass beads, respectively.

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layer (-10 to 15 cm) (Rhoads, 1974; Rhoads and Young, 1970). If organic binding is negligible, bioturbation lowers erosion resistance of the surface, and thus destabilizes the bed sediment. The best documented examples are those vertically oriented organisms that feed at depth and defecate at the surface thus generating a “conveyor-belt” transport and sediment mixing (Rhoads and Stanley, 1965; Rhoads, 1974). Intense burrowing active in intertidal deposits of Mugu Lagoon, CA, U.S.A., produced graded bedding (Warme, 1967). The effects of bioturbation are recorded in sedimentary structures. Bioturbation not only mixes sediment but obliterates layers. The rate of bioturbation is usually lower in landward less saline zones where benthic organisms are less abundant and where sedimentation is relatively fast. The bay-head deltaic areas of central Texas bays, U.S.A. are layered whereas deep parts are extensively mottled or homogeneous (Moore and Scruton, 1957). A different trend is found in Doboy Sound, GA, U.S.A. by Howard and Frey (1985) and in Pamlico Sound, NC, U.S.A. by Pickett and Ingram (1969) where the intensity of bioturbation is greater landward. In mid-latitude lagoons, the effects of bioturbation may vary seasonally with temperature and associated biologic activity (Aller, 1980) and with storms and fluvial input (Frey and Basan, 1985). Bioturbation can be important in exchanging particles and pore water nutrients across the sedimenkwater interface as well as through various vertical chemical gradients in the sediments. However, the interactions among chemical processes and physical disturbance remain to be understood and quantified. Response to Sea Level Change

The shallowness of many lagoons and infilling processes like bay-head deltas, swamp encroachment and segmentation, has led to the geomorphic view that lagoons generally are ephemeral features destined to be filled to a marsh or depositional plain in a few thousand years (Fairbridge, 1980; Bird, 1984). The rate of infilling therefore, exceeds submergence. This view is supported by case examples from the central Brazilian coast (Martin et al., 1987) and from New South Wales, Australia (Roy, 1984) where regressive stratigraphic sequences of deltaic fill overlie lagoon deposits. On the Texas coast, U.S.A., former lagoons near the Rio Grande and Brazos rivers are completely buried by river alluvium (Shepard and Moore, 1960). Another view holds that the rate of sediment accretion keeps pace with submergence (Rusnak, 1967; Carson and Ashley, 1988). If sediment accretion equals the rate of relative sea level (RSL) rise, then lagoonal deposits accrete upward with RSL resulting in thick lagoonal deposits while maintaining the same volumetric capacity. In San Antonio Bay, Texas, U.S.A., for example, lagoonal deposits reach 24 m in thickness over the last 9,500 years (Shepard and Moore, 1960).

190

Sediment Transport Processes in Coastal Lagoons

RELATIVE SEA LEVEL RISE, mm/yr

Fig. 7.15. Relationship between accumulation rates and relative sea level rise for selected lagoon systems along the US.Atlantic and Gulf coasts (Nichols, 1989). Short-term rates (decades)are represented by circles, long-term rates (millennia)by dots. Dashed diagonal line indicates panty between accumulation rate and relative sea-level rise. Points lying above the line have a ‘surplus’ accretionary status whereas those below the line have a ‘deficit’accretionary status. Points encircled within the dotted lines have a relatively small accretionary difference, within the range of f1.6 mm year-’ for short-term differences, and within -0.4t o +0.7 mm year-’ for long-term differences; short dashed lines - ranges. Lagoon locations abbreviated as follows: OP, Oyster Pond, MA, CH, Charlestown, RI; PJ, Point Judith, RI;GS, Great Sound, NJ; R, RehobotMndian, DE; C, Chincoteague,MI3NA, W, Wachapreague, VA, P/N, PamlicoNeuse R., NC; NS, AltamahdSapelo, GA;AP,Apalachicola,FL;MB, Mobile Bay, AL,AT, Atchafalaya, LA, G, Galveston, Tx,M, Matagordaavaca, Tx,EM, East Matagorda, Tx,SA, San Antonio, Tx; CIA, CopandAransas, Tx; CC, Corpus Christi, Tx; BB, Baffin Bay, Tx; LM, Laguna Madre, Tx,ML, Mangrove Lagoon, VI;MP, Mandal Pond, VI.

If the relative rate of sea level rise is accelerated during the next century (Barth and Titus, 1984), accretion may not be sufficient to keep pace with, or maintain equilibrium with, the RSL rise. In a study of US.Atlantic and Gulf coast marshes, Stevenson et al. (1986) show that some lagoonal marshes, the near-final stage of infilling, are no longer keeping pace with the recent RSL rise. Therefore, the concepts of lagoon evolution need re-examination with a focus on the interaction of accretion and sea level change. A synthesis of accumulation rates and rates of RSL change in 20 selected US. Atlantic and Gulf coast lagoons (Nichols, 1989) shows a range of accretionary differences, i.e. accumulation minus RSL rise. The differences vary between two end members: (1) a “surplus” lagoon in which accretion exceeds the RSL rise rate, and (2) a “deficit” lagoon whereby RSL rise exceeds the accretion rate. The differences are displayed when accumulation rates are plotted as a function of RSL rise rates (Fig. 7.15). As expected

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lagoons with active deltaic sedimentation, e.g. Apalachicola, Mobile, Atchafalaya and East Matagorda of the Gulf coast, U.S.A., have a high positive accretionary difference, or surplus, mainly in the range 2.8-4.0 mm yr-’ for short-term (decadal) measurements. I n contrast, several lagoons e.g. Matagorda and Galveston of the Gulf coast, U.S.A., have negative accretionary differences, -2.0 to -4.3 mm yr’. These lagoons are affected by subsurface gas, oil or ground water withdrawal. Of note, the majority of lagoons have a near-balance (k1.6 mm yr’) whereby the rate of accretion nearly equals the RSL rise. The short-term differences reflect the direction of the long-term differences (Fig. 7.151, a trend suggesting that geologic processes controlling lagoon evolution continue to the present day except in Galveston and Mobile of the Gulf coast, U.S.A. These latter lagoons shifi from a long-term ((surplus” or balanced status, to a short-term “deficit” status suggesting human intervention in the long-term trends. This likely relates to a n increase in submergence rates in recent years caused by oil, gas or ground water withdrawal at these sites (Gabrysch and Bonnet, 1975). Since the lagoons compared exhibit a range of accretionary differences between two end-members, lagoon evolution should be viewed as a continuum that reflects the resultant effect of accretion and submergence. The concept is illustrated in Fig. 7.16 whereby the accumulation rate is considered as a function of RSL rise rate. For a given lagoon status, the combination of rate of accretion and RSL rise will determine the volumetric capacity of the lagoon, its import-export status, and the resultant evolution and geometry of its deposits. Since a majority of U.S. Atlantic and Gulf coast lagoons attain a near-balance, it remains to consider how a balance is attained. Lagoon Equilibrium An accretionary balance can be attained if (1)the long-term filling rate exactly equals the relative sea-level rise, or (2) the lagoon fills rapidly to capacity (i.e. catches up to the rate of RSL rise) and the accretion surface remains at base level for a long time. Base level approximately parallels sea level within a limited range of water depths. If a balance of sediment accumulation with energy dissipation occurs, the rate of sediment accumulation must equal the rate of RSL rise and the lagoon is in equilibrium. Although base level is a difficult parameter to define in a lagoon, it is taken as the water depth that separates a zone dominated by transportation from one dominated by accumulation. In open-water lagoons, base level is mainly determined by the depth of effective wave base. The accretion surface can persist for a long time at base level because it approaches quasi equilibrium at a depth appropriate to the wave energy, fetch, water depth and basin form. The process is like a negative feedback loop in which

Sediment Transport F’rocesses in Coastal Lagoons

ACCRETIONARY STATUS

\

SURPLUS

\

BALANCE

\

DEFICIT

\ Increasing relative sea level rise

Low High

--

Import > Export

Low

-

VOLUMETRIC CAPACITY

Prograding

Regressive

SEDIMENT SUPPLY knport

9

Expod

ENERGY

DEPOSITIONAL MODE Upward Accretion

Thick Deposits

-

High

Low

* High

Retrograding

Transgressive

Fig. 7.16. Schematic model to illustrate the accretionary status in a lagoon continuum between two end-members;‘surplus’and ‘deficit’lagoon as a function of increasing rate of accumulation and increasing rise of relative sea level. Key features associated with the continuum are presented below the model. Configuration of subsurface lagoon deposits, solid. From Nichols (1989).

sediment accretion raises the accumulation surface to the zone of energy dissipation, thereby decreasing water depth and reducing accumulation rates. As the accretion surface approaches equilibrium, the sediment surface becomes more dynamic, sediment resuspension becomes more active, and part of the sediment supply is transported to the ocean or another site. The process is illustrated in Fig. 7.17 for a hypothetical basin in steadystate whereby, in a mass balance, the input mass, Mi, is equal to the output mass, Me, plus the amount lost to the bed (accumulation), Ms. As the input exceeds the export, the accumulation rate, Ms, may increase at a faster rate than the relative sea-level rise, R. As the accumulation surface is raised and volumetric capacity of the lagoon below base level is reduced, a relatively greater proportion of the input is exported than accumulates.

M.N. Nichols and J.D. Boon

193 3 2 RSL 1

Me

Mi

3 BASELEVEL 2 ACCRETION 1 SURFACE

-

Mi Ms+ Me Mi > Me

R/H INCREASES

Fig. 7.17. Schematic diagram of an open-water lagoon basin showing stages in a rising accretion surface in relation to relative sea-level rise (R), depth below base level (HI,input (Mi), export (Me) and accumulation (Ms).For explanation see text.

The process is exemplified by a mass balance for Mobile Bay on the Gulf coast, U.S.A. (Ryan and Goodell, 1972), a case that has a large short-term accretionary surplus, 3.6 mm yrl. The bay receives a n estimated average input of 4.3 x lo6 tons of sediment each year. Of the total input, 3.0 x 106 tons yr, or about 70%, is accounted for by accumulation in the basin, leaving a difference of 1.3 x lo6 tons yr, or about 30%, that escapes to the sea. The bay therefore, is probably approaching its sediment storage capacity below base level. When filled to this level more material will be transported to the ocean. From analyses of 31 open-water Texas bays, Price (1947) established approximate dynamic equilibrium between form and forces from the ratio of average lagoon width to maximum basin depth. The generic relationship between form and forces is believed by Price to be related to wave base, i.e. the depth of effective marine planation (scour), as a function of average wind fetch, which is determined by average basin width. Price notes that water depths in a number of lagoons have been maintained relatively constant for 100 years. The relative sea-level rise and accumulation have been sufficiently slow to maintain equilibrium of basin form and forces. However, Price recognized that equilibrium can be modified by: (1)floods and hurricanes that produce sudden changes, including washovers, tidal inlet deltas and fluvial deltaic deposits, (2) bottom sediment characteristics that affect the depth and effectiveness of wave scour, and (3)the effects of sediment compaction, which increase average water depth. The exact nature of the temporal coupling between the accretion surface, water depth or base level is difficult to physically measure and statistically analyze. Global Distribution

Lagoons occur all over the world. They are found on every continent except Antarctica (Fig. 7.18). Together with barriers, flats and marshes they cover about 11% of the world coastline. The longest stretch is 2,800 km along the US. Atlantic and Gulf coasts where they occupy about 80% of the

SO'

30'

00

a=

Fig. 7.18. Worldwide distribution of major coastal lagoon districts (solid),climatic zones of Davies (1973),and submergencezones (shaded) derived from tide-gauge records (Emery and Aubrey, 1991). Mid-ocean and barrier reef lagoons excluded. Numbers refer to districts tabulated in Table 7.3; x refers to lack of submergence data for lagoon district; arrows refer to locations of case examples.

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TABLE 7.3 Summary of major coastal lagoon districts and reference studies. Excludes ocean island lagoons. Number refers to location on Fig. 7.18. Number Lagoon District

Reference

1

US.Atlantic

Folger (1972)

2

Gulf of Mexico

Shepard and Moore (1960)

3

Central Brazilian

Martin et al. (1987)

4

Patos

Delaney (1966)

5

Sinola (esteros)

Phleger and Ayala Castanares (1969);Curry et al. (1967)

6

Baja California

Phleger and Ewing (1962,1965)

7

Alaskan

Faas (1969);Naidu and Mowatt (1975)

8

Chukchi

Zenkovich (1967)

9

Sakhalin

Zenkovich (1967)

10

Coorong/Gippsland

Bird (19984)

11

Trucial Coast

Purser and Evans (1973)

12

Madagascar

Gierloff-Emden (1961);Bird and Schwartz (1985)

13

Zululand

Orme (1973)

14

Gulf of Guinea

Webb (1958);Allen (1965);Guilcher (1959)

15

Etangs

Van Andel (1955);Bird and Schwartz (1985)

16

Venice, Lido and Valli

Bortolami et al. (1984)

17

Black Sea (Liman Coast)

Zenkovitch (1967)

18

Sea of Azov (Sivash)

Zenkovitch (1967)

19

Waddens

Postma (1954,1961)

20

Haffen Coast, Baltic

Zenkovitch (1967)

coastline. Lagoons are most widespread on low-lying aggrading coastal plains with a history of submergence during the last 10,000 years. According to Phleger (1965),lagoon formation requires a n abundant supply of sediment for barrier or spit building and a coastal exposure with adequate ocean wave action. Generally, most sediment is supplied by ocean waves from offshore or alongshore via present-day, or former, streams and rivers. Onshore transport is facilitated by a very gentle sloping offshore bed gradient. In some areas, vegetation, biogenic reefs or tectonic structures create a

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Sediment Transport Processes in Coastal Lagoons

lagoon barrier. Other lagoons form behind spits in depressions of shallow river valleys or in depressions flanking large deltas, e.g. on the Niger delta, Gulf of Guinea. In terms of tide range, barrier-lagoon systems are scarce in macrotidal environments (> 4 m range) (Davies, 1973). For one reason, fast entrance currents inhibit barrier building, e.g. on the North Sea coast of Germany (Hayes, 1975). In contrast, microtidal environments allow relatively strong barrier building by waves (Davies, 19731, which is the case on the U.S. Gulf of Mexico coast. It is not surprising therefore to find many lagoons along enclosed tideless seas like the Caspian Sea and Black Sea. When the global distribution of large lagoons (> 100 km2) is plotted on a world map (Fig. 7.18) it is evident that most lagoons occur in units, or districts; the most extensive being long chains behind barrier islands (Table 7.3). This suggests the factors conducive to lagoon formation operate on a large scale. For example, when the global distribution of coastal submergence, which is derived from regression analysis of world-wide tide-gauge stations (Emery, 1980; Emery and Aubrey, 1991), is plotted (Fig. 7.181, it reveals that the major lagoon districts lie in zones of submergence.

Influence of Climate

Lagoon processes are sensitive to climatic variations including sea level change, sediment source supply and intensity of transport processes. Lagoon sediments therefore, are potential climatic indicators. Climate influences terrestrial sediment sources because it controls the intensity and type of rock weathering, either mechanical or chemical and hence, together with relief, the sediment yield. It also controls the availability of source sediment through precipitation and runoff rates or vegetation development. Additionally, the lagoon circulation and the advective and dispersive processes, are keyed to climate through the amount of precipitation, runoff, temperature and evaporation. The basic factor producing climatic variations is the intensity of solar radiation, which varies with latitude (Fig. 7.19A). In turn, radiation determines the amount of light for primary production in lagoons and the production of skeletal biogenic material. Most important for sediment processes are the latitudinal thermal variations that drive atmospheric and hydrologic cycles and result in zones of precipitation excess or evaporation excess (Fig. 7.19B). These zones approximately coincide with humid and arid regions on land. Additionally, the cross-overs near 40" N and 40"s are close to the boundaries between low latitude and mid-latitude shore process zones defined by Davies (1973). Because most lagoon studies are site specific with a limited range of latitude, climatic influence is illustrated by selected lagoons in each climatic zone.

M.N. Nichols and J.D. Boon

I8o

I60

1B

PRECIPITATION & EVAPORATION

197

kppT

I

I

I

I

Fig. 7.19. Variations of climatic factors with latitude in relation to major shore process zones of Davies (1973). High, mid and low latitude boundaries approximate. A. Solar radiation (Holmes, 1957).B. Precipitation and evaporationin the oceans (Fleming, 1957).Zones of excess evaporation over precipitation shaded in relation to arid and humid regions on land (Lisitzin, 1972). C. Trends of erosion rates (sediment yield) on land (Lisitzin, 1972).

High Latitude The high latitude freeze-thaw cycle promotes mechanical weathering of rock in lagoon watersheds and yields coarse material and silt; clay is distinctly subordinate. Additionally, poorly-sorted glacial debris including rock and gravel is common. Illite and chlorite dominate clay mineral distributions. Erosion rates in non-glacial watersheds are relatively low, less

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Sediment Transport Processes in Coastal Lagoons

than 10 tons km-2 (Lisitzin, 1972) (Fig. 7.19C1, because they are ice-bound most of the year. Sediment yield is also low, averaging 42 tons km-2 yr-1 (Milliman and Meade, 1983). Climatic influence is exemplified by processes active in Simpson Lagoon, a shallow system (< 3 m deep) which forms behind a low barrier east of the Coleville River delta, north Alaska, lagoon district 7 (Figs. 7.18 and 7.20A) (Naidu and Mowatt, 1975; Wiseman et al., 1973; Tucker and Burrell, 1977). The coast is characterized by permafrost, low precipitation (< 25 cm y r 9 and low astronomic tides (< 18 cm range). Transport processes in Simpson Lagoon are marked by strong seasonal changes. A long 9-month period of solid ice cover alternates with a short 3-month period of rapid spring thaw, ice break-up and river flooding. During winter freeze-up, frozen shores and tundra streams resist normal erosion. Most lagoon areas are frozen to the bed where depths are less than 2.0 m, and thus are grounded or gouged by moving ice. Consequently the upper 1 m of bed sediment may be mixed and the fauna devastated. Circulation is restricted because ice cover eliminates wind-wave mixing and inlets are blocked by ice. As ice is formed salt is expelled and the remaining lagoon water below the ice cover reaches more than 65%0salinity. Photosynthesis ceases and oxygen may become depleted. When the ice and snow melt in early summer, the most dynamic period, the river ice breaks up several days to weeks before the lagoon ice. Consequently, river-borne sediment, mainly fine sand and silt, is transported over and onto lagoon bottomfast ice by-passing the lagoon-head river mouths. The freshwater fans out seaward at 15 to 30 cm sec-l as a sheet flow inundating lagoon ice up to 1m deep. In seaward zones where lagoon ice is not supported by the floor, the weight of fresh water depresses lagoon ice forming cracks. Sediment-laden meltwater plunges through the cracks and holes (strudels) creating scour depressions on the lagoon floor more than 4 m deep and 15 m wide (Fig. 7.20B) (Reimnitz and Barnes, 1972). Scour depressions, a form of bottom reworking, also occur later as ice lifts off marginal shoals. As floodwater plunges into the lagoon through ice holes, it flows seaward as a freshwater-wedge under lagoon ice and over saline lagoon water. The advance and thickening of the wedge is similar to a large scale process off the Coleville River delta (Fig. 7.20C) (Walker, 1977). As lagoon ice breaks up, inlets are opened by tidal flushing and marine water enters the lagoon. Easterly storm winds in summer can raise water levels up to 2 to 4 m and generate drift currents up to 2 m sec-’ through the inlets. Additionally, wind waves erode tundra shores releasing ice-cemented sediment, a process called “thermal niching”. Contrary to expectations, ice-rafting is not an important transport process. Coarse sand and gravel are scarce except along shores. At the onset of freezing in October, transport competence suddenly drops. Both silt and sand quickly settle to the bed resulting in a n admixture of poorly sorted basin sediment. The textural pattern follows water depth with

Q

R.

Fig. 7.20. A. Configuration of Simpson lagoon, north Alaska coast; east of the Colville delta; from Wiseman et al. (1973). B. Cross section of a lagoon strudel, north Alaska and relevant transport processes; from Reimnitz and Bruder (1972). C. Cross section of freshwater wedge off the Colville delta, north Alaska; from Walker (1977).

8

200

Sediment Transport Processes in Coastal Lagoons

sandy mud and silt deposited in the basin below the zone of ice gouging (approximately 2 m) and sandy material in strudel and shoal zones.

Mid-Latitude Warm temperature and moderate precipitation of the mid-latitude humid climate promote both chemical and mechanical weathering in lagoon watersheds. Chemical leaching and hydrolysis produce soil abundant in clay, particularly kalonite besides montmorillonite and some illite. Despite extensive vegetative cover, erosion rates are substantial, 10 to 50 tons k m - 2 (Fig. 7.19C) (Lisitzin, 19721, while sediment yield is moderate, 10 to 100 tons k m - 2 y r 1 (Milliman and Meade, 1983). This partly depends on the extent of former glaciation and on the degree of chemical weathering. For instance, along the U.S. Atlantic coast, systems north of Chesapeake Bay (latitude 39") receive substantial amounts of river water but relatively little sediment because the rivers drain resistant glaciated terrain. In contrast, southern systems have relatively less river discharge but, formerly substantial fluvial sediment loads because much erodable soil is produced by deep chemical weathering (Meade, 1969). This trend is essentially world-wide (Postma, 1980). The Texas lagoons of the Gulf of Mexico, lagoon district 2 (Fig. 7.18) studied by Shepard and Moore (1960), Fisher et al. (1972), and McGowen et al. (1976), offer an example of climatic influence. The coast is characterized by moderate precipitation, 115 cm annually, and seasonal winds, e.g. onshore (southeast) in summer and offshore (northerly) in winter. Infrequent tropical storms and hurricanes produce extreme water levels and currents. The Texas lagoons lie behind a nearly continuous 500 km-long chain of barrier islands, broken infrequently by narrow entrances. Interestingly, the number and size of entrances along the coast increases northeastward from the semiarid zone of southern Texas to the humid sector in Mississippi. Since the tide range along this coast is small (c 20 cm) and uniform with location, Phleger (1960) attributes this trend to river discharge, which increases from southern Texas to Mississippi. Additionally, short-term (decades) sediment accumulation rates are higher (3.5 to 7.8 mm yr') in the humid lagoons of central Texas than in semi-arid lagoons (1.0 to 2.2 mm yrl). Since all lagoons on the coast have a relatively long freshwater residence (Armstrong, 1982; Nichols, 1989),the higher rates in humid lagoons likely reflect increased fluvial sediment supply with greater fluvial discharge. The central Texas lagoons receive sediment from multiple sources; i.e. clayey material from rivers and sandy material from offshore via inlets and the barriers. Consequently, bed sediments are mixtures of sand, silt and clay with finer fractions in bay-head deltas or deep central parts and coarse sediment around margins or in flood tidal deltas and barrier washovers. Productivity in the bays is relatively high because nutrient-rich freshwater inflows enter from rivers and deltaic marshes (Armstrong, 1982). Conse-

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quently, biogenic activity is important in building oyster reefs, supplying shell, and in biodeposition of suspended sediment loads (Biggs and Howell, 1984). Additionally, benthic organisms destroy laminations by bioturbation resulting in homogeneous or mottled structures. Wind is more important than tides in regulating lagoon transport. This is because tides are small whereas winds cause storm tides of more than 1m amplitude (Shepard and Moore, 1960). In Corpus Christi Bay, onshore winds drive lagoon water landward bringing in less turbid water from adjacent bays; they increase bed sediment resuspension by promoting intense wave action down fetch at the bay head (Shideler, 1984). I n contrast, strong offshore winds produce an opposite effect. Wind direction therefore, not only determines the direction of wind-driven currents and set-up but also the efficiency of water exchange. In turn, wind determines the spatial intensity of sediment resuspension on the lagoon floor. Over the long term the bays probably approach a long-term balance between bay form and energy dissipation by trapping a portion of the river influx and by-passing another portion through wave resuspension and current transport.

Low Latitude, Arid High temperatures and low precipitation in the arid climate promote mechanical weathering in watersheds that yields much sand but little silt and clay. Vegetation is scarce and fluvial input is very low, seasonal or sporadic. Sediment erosion rates are very low, less than 10 tons k m - 2 YI-1 (Lisitzin, 1972) (Fig. 7.190. The excess evaporation over precipitation results in high inner lagoon salinities. This in turn, controls exchange with the ocean: shallow flow in, deep flow out; an anti-estuarine circulation. Lagoons along the Trucial Coast of the Persian Gulf, lagoon district 11 (Figs. 7.18 and 7.21A), described by Evans and Bush (1969) and Purser and Evans (1973) display many processes induced by an arid climate. The Trucial Coast lagoons develop behind barriers formed of cemented and submerged dunes (Fig. 7.21A). These relic dunes are enlarged by accretion of coral reefs, beaches and modern dunes. Longshore transport extends the barriers laterally while aeolian transport extends them landward. The barriers are separated by wide inlets with long tidal channels about 7 to 10 m deep (Fig. 7.21A), which convey sea water and fine sediment into the lagoons. At ends of the channels, flood tidal currents build small flood deltas while ebb tidal currents, which interact with ocean waves, build spectacular accumulations of oolitic sand in large ebb deltas. Lagoon water depths range 2 to 5 m; the tide range diminishes inward from 3.5 m in the gulf to 1.0 m in the lagoons. Salinity ranges 42 to 45%0 in nearshore gulf water and 54 to 72%0in the lagoons. There is essentially no runoff and little detrital terrigenous sediment supply except for quartz sand blown into the lagoons. Most lagoon sediment is produced locally by biological extraction

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SUBTIDAL

-I -

INTERTIDAL - I -

SUPRATIDAL

S

B

fikdej~

’0

pellet muds

and gypsum

emPoriQs

irlondr

Fig. 7.21. A. Morphology of barrier islands and a n d zone lagoons on the central Trucial coast, Arabia. Reproduced from Purser and Evans (1973)with permission from Springer-Verlag publishers. B. Cross sectional profile of a lagoon west of Abu Abyad showing change in sediment character in tidal zones; from Selley (1982). Reproduced with permission of Academic Press.

of calcium carbonate and chemical precipitation. Despite low fluvial input and concentration of nutrients in the lagoons, organic production is very high because light energy is high. Thick mats of algae cover intertidal and supratidal zones or “sabkhas,” while thick masses of seaweed grow on channel sides and shoals. Production results in a n abundance of mollusk shells, ostracod valves, foraminifera tests, pellets, echinoid spines and calcareous algal debris. Calcareous material is broken

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down into sand and fine sediment by mollusks, worms, and crabs. In the process, bed sediments are heavily burrowed and numerous fecal pellets are produced. Despite the abundant production of pellets and carbonate sand, accumulation on the lagoon floor is slow because waves and currents deter deposition. Instead, the sediment is resuspended by waves and transported by currents and wind tides t o intertidal and supratidal flats where it is trapped in algal mats. Wind plays a significant role in redistributing sediment. Strong and persistent “shamal”winds drive skeletal, pelletal and oolitic sands from the gulf nearshore onto barrier dune ridges, which build u p 12 m high. Additionally, the winds produce barchan and parabolic dunes that migrate more than 4 km lagoonward from the barrier. They form trail-like accumulations parallel to the wind and tend to separate lagoon basins (Fig. 7.21A). The high temperatures, evaporation, and high salinity as well as wave and current action, cause chemical precipitation. Oolites form in tidal deltas and calcium carbonate encrusts and cements dune and beach sand. Additionally, interstitial precipitation occurs in pore water of sediments and algal mats on the sabkhas. Marine water is brought into the sabkhas by spray, by tidal and wind-induced flooding, and mainly by intense evaporation, a process known as evaporitic pumping. This process leads to interstitial precipitation of salts, aragonite, gypsum, anhydrite and dolomite as well as cementation of the sediment, and thus enhances intertidal zonation (Fig. 7.21B) Schreiber (1986). The arid climate leaves a strong imprint on intertidal sediments.

Low Latitude, Humid The excess precipitation over evaporation and high temperature, in the presence of carbon dioxide liberated by organic matter, promotes intense chemical weathering of bed rock in lagoon watersheds. Weathering produces thick soil layers of silt and clay with an abundance of kaolinite. Sediment erosion rates are more than 100 tons km-2 (Lisitzin, 1972) (Fig. 7.19C) and sediment yield is more than 500 tons km-2 yr-l (Milliman and Meade, 1983). Fluvial input therefore, is high, seasonal and rich in mud. The Gulf of Guinea lagoons, West Africa, lagoon district 14 (Fig. 7.181, described by Webb (1958), Guilcher (19591, and Allen (19651, exhibit processes induced by a low latitude humid climate. The Gulf of Guinea lagoons extend intermittently along the coast for 1,000 km. They are long and narrow, parallel to the coast, and have elongate arms extending between multiple barrier dune ridges. Inlet channels are infrequent because they are narrowed, or blocked by a strong longshore drift of sand and the coast is prograding seaward. The lagoons are fed during the rainy season by a n enormous freshwater inflow. This raises lagoon water levels high enough to breach the barriers and discharge suspended sedi-

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Sediment Transport Processes in Coastal Lagoons

ment loads directly into the sea. Channel water becomes partly stratified at high inflows and a two-way, two-layered estuarine circulation develops (Gallardo, 1978). By contrast, during the dry season, seawater intrudes more than 32 km landward through inlets. Therefore, seasonality is marked by a wide salinity range, nearly 0 to 30%0.Tidal currents active in inlets transport and sort sediment as well as organic material, but their effectiveness diminishes landward as current speed decreases. Time-velocity asymmetry encourages a greater amount of scour during flood than during ebb and moves bedload landward through the inlets. Accumulation rates are likely much higher than in lagoons of other latitudes because of the long duration and high magnitude of sediment influx and relatively good conditions for entrapment. Sediment is deposited in small lagoon head deltas along landward margins, in algal growths and mangroves, or by settling of fines in basins. Flocculation is favored by the high flood-borne loads rich in organic matter and by the marked salinity gradients. In general, lagoon bed sediments are very muddy, organic rich and poorly sorted except near the inlets. Benthic fauna bioturbates the bed sediment except in central sections. The case examples from different zones illustrate key processes that may be useful climatic indicators. The chief processes are: (1) freezing and thawing including thermal niching, gouging, and ice overflow; (2) sediment availability as controlled by the type and intensity of weathering, watershed erosion and fluvial sediment yield, in addition to (3) the relative mud and sand supply; (4) subaerial aeolian transport and ( 5 ) chemical and biogenic precipitation. Table 7.4 summarizes key process indicators and their relative TABLE 7.4 Summary of key process indicators and their relative importance in different climatic zones

-

PROCESS INDICATOR

HlQH LATITUDE

MID-LATITUDE

LOW LATITUDE

I

ARID

I

nuuiD

1

1-

FREEZING-THAWING (Ice werflow. niming. wginpl ~

1- 1 1- 1

SEDIMENT AVAllABlUN (subanrial nroaion. u d i m yield)

l-

MUDlSANDSUPPLY

AEOLIAN TRANSPORT

CHEMICAL 6 BlOGENiC PREClPlTAllON

lnmse

Moderate

LOW

0 Rare or Absm

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importance in different climatic zones. Other processes such as sediment accumulation, tidal resuspension, and wave erosion are regulated by local energy dissipation, particle modifications or morphologic-energy interactions, rather than climate. In brief, climatic influence is most evident in sediment source material and in intertidal zones. A number of studies indicate that past climates leave an imprint on modern lagoon sediments. Phleger and Ewing (1962) note that sand supplied to arid Ojo de Liebre, Mexico, was delivered during a former Pleistocene rainy period. Van Straaten (1963) traces the observed effeds of a Pleistocene periglacial climate on sub-aerial and coastal sediments of the Netherlands during, and prior to, the Holocene. In glaciated areas such inherited features may be out of equilibrium with the modern energy regime. Storm Effects

Although lagoons are generally net sediment sinks at long time scales, i.e. 100 to 1000 years, with accumulation nearly balancing or exceeding submergence, accumulation is seldom continuous. It is interrupted by storms or river floods in which erosion can exceed accumulation thus reversing the long-term trend. Consequently, storms can change the role of a lagoon from a sink for fluvial or marine sediment to a source of sediment for the shelf. The evidence however, is limited, and thus an area of conflict and controversy. Observations and historical records of hurricanes on the Texas coast, U.S.A. (Hayes, 1967; McGowen et al., 1970; McGowen and Scott, 1975) provide a model illustrating the sequence of events as a hurricane crosses a lagoon. (1)Storm approach is marked by rising tides and piling of water against the lagoon barrier by wind tides and storm surge which attain several meters elevation (Fig. 7.22A).High waves erode sand dunes, breach ridge swales and scour washover channels. Sediment is transported through the channels and deposited on lagoon flats and margins as washover fans or storm surge platforms. Lagoon shorelines are flooded and eroded by large waves. (2) Landfall is marked by an abrupt wind shift, onshore to offshore, and start of a n ebb flow triggered by the sudden release of storm water piled in the lagoon. (3) In the aftermath stage, as the hurricane weakens on land, ebb flow, backed by high stream inflow, drains seaward through inlets and storm breached surge channels (Fig. 7.22B). Sediment is deposited as fan deltas near stream entrances, in the surge channels as well as farther seaward on the inner shelf forming graded sediments. Hayes (1967) argues seaward transport across the shoreface off Laguna Madre, Texas occurs as turbidity currents implying export of lagoon sediment. In contrast, Morton (1981) argues from morphologic evidence and storm discharge records that lagoon discharge is negligible. Instead, the graded deposits are likely produced by large-scale wind-driven shelf currents,

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Sediment Transport Processes in Coastal Lagoons

Fig. 7.22. Schematic diagram of hurricane transport routes in the Texas bays, U.S.A. during: A. Approaching hurricane with breaching ofbarrier and formation ofwashovers. B. Aftermath of hurricane with flushing of lagoons. From Fisher et al. (1972).

or surge pulse, flowing offshore. The lagoons therefore, must function during storms mainly as sinks trapping large amounts of sand in washovers. This is confirmed by Davis et al. (1989) from thick hurricane-deposited layers found

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in Holocene fill of lagoons along the eastern US.Gulf of Mexico, Florida, U.S.A. In contrast, Isphording et al. (1987) in a post-hurricane study of Apalachicola Bay, northern Gulf of Mexico, observed depth changes indicative of pronounced bed scour and seaward transport from the lagoon; currents reportedly reached more than 20 m s-l. As a result of scouring into old sediments, bed texture became less clayey and more silty and sandy. These studies suggest storms cause lagoons to function as both sinks and sources. Generalizations

The review of sedimentary processes focusing on fine sediment with examples mainly from U.S. Atlantic and Gulf coast lagoons leads to generalizations of what to expect in lesser known similar systems.

1.

Lagoon sediments are mixtures derived from multiple sources either external, including rivers, ocean, shores and barrier, or internal, including organic production, chemical precipitates, and erosion of older deposits. The dominance of a particular source depends on supply rates and their seasonality, the exclusion of other sources, the hydrodynamic selectivity of transport processes according to particle size or composition.

2.

Sediment processes are modulated by the sediment supply, the energy available to transport the sediment supplied and the hydrodynamic behavior of sediment particles. The processes are not continuous but intermittent and seasonal. They cycle material from one part of a lagoon to another with small amounts being added from diverse sources to balance amounts removed from the system or stored in sinks.

3.

Fine sediment dispersal is organized into a series of interconnected subsystems: (1) deltaic, (2) barrier, including aeolian dune and washover and (3) tidal inlet and channel.

4.

Lagoon accumulation rarely consists of a single fill. Instead it incorporates a number of fill components produced by differences in source supply and dispersal subsystems: e.g. bay-head delta, washover fan and storm surge platforms, tidal flood delta, basins, marshes and intertidal flats.

5.

Lagoons generally are net sediment sinks. Over the long term the type and rate of sediment accumulated is determined by the degree to which the rate of accretion adjusts to long term submergence. Energy dissipation of waves and currents probably is significant in

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208

modulating the minimum depth, or base level, to which a n openwater lagoon can accrete. Most lagoons attain a near balance between the rate of accumulation and rate of submergence. Lagoon evolution should be viewed as a resultant of accretion and RSL rates rather than a sink destined to be filled in a few millennia. 6.

Fine suspended particles are modified during transport by continuous aggregation, break-up, and reforming. These processes are encouraged by shallowness, wind wave resuspension of bed sediment, and high organic activity. The gross effect is to increase settling velocity and retain fine sediment within lagoons.

7.

Benthic organisms modify bed sediments through mucal binding, secretions and algal cover resulting in greater stabilization and increasing erosion resistance. In contrast, feeding activities and bioturbation destabilize the sediment, change the physical properties, destroy layering, and can exchange particles or pore water chemicals across the sediment-water interface.

8.

Climatic influence on lagoons is most pronounced in intertidal sediments and in source material from watersheds. Key processes are: (1)freeze-thaw cycles in high latitudes; (2) subaerial aeolian transport and chemical-biogenic precipitation in low-latitude arid zones, and (3) high sediment availability and fluvial yield in low-latitude humid zones.

Future Research Needs This review reveals our knowledge of sediment transport is imperfect and processes need increased study and scrutiny. Future research areas deserving priority effort are: (1)the basic nature of cohesive sediment erosion and deposition in response to turbulence, shear stress, biogenic activity and combined wave and current action, (2) the effects of meteorologically induced transport in cycling and exchange of sediment within a lagoon. Of particular importance are episodic events and their effect on residence time of sediment and the function of a lagoon as source or sink. (3) The mechanism by which the accretion surface adjusts to varying rates of relative sea-level rise and development of a n equilibrium base level. (4) The climatic imprint on lagoon sediments as a function of latitude with a focus on intertidal zones and stretches of coast near climatic boundaries. Although the sediment dynamics of coastal lagoons are now being modelled in some very innovative ways, it is difficult to find comprehensive data sets that test the predictive accuracy of these models or verify that they address all of the major processes influencing sediment transport at appro-

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priate temporal and spatial scales. In particular, to categorize the long-term behavior of tidally-induced transport in entrance channels often characterized as “lowpass filters”, one must draw on a knowledge of interactive processes that is still incomplete and rather poorly elucidate by short-term field measurements. Where long-term measurements are available (mainly water level fluctuations at tide stations), they reveal significant temporal (daily to monthly) variations in the quarter-diurnal tide due to multiple constituent interaction (Boon, 1988). These variations strongly modulate the Md/MZ non-linearity index and therefore imply strong modulation of the residual sediment transport as well. Lagoons offer excellent opportunities for study because of their great diversity and accessibility. Much remains to be done and there is a great deal of scope and challenge. Acknowledgements

The authors are indebted to numerous investigators whose data have served as input and a framework for concepts and interpretations. The review benefitted from discussions with colleagues, including George Allen, David Aubrey, Keith Dyer, K.O.Emery, and John Wells. Kaye Stubblefield drafted the figures, William Jenkins photographed the figures, and Beth Marshall typed the manuscript. This is Virginia Institute of Marine Sciences contribution number 1637. References Allen, G.P., Bouchet, J.M., Cabronnel, P., Castaing, P., Gayet, J.,Gonthier, E., Jouanneau, J.M., Klingebiel, A., Latouche, C., Legigan, P., Ogeron, C., Pujos, M., Tesson, M. and Vernette, G. 1973 Environments and sedimentary processes of the north Aquitaine Coast. Guidebook, Geologic Institute Bassin CAquitaine, 183 pp. Allen, J.R.L. 1965 Late Quaternary Niger Delta and adjacent areas: Sedimentary environments and lithofacies. Bulletin American Association of Petroleum Geologists 49,547-600. Aller, R.C. 1980 Diagenetic processes near the sediment-water interface of Long Island Sound, 11. Fe and Mn. In Estuarine Physics and Chemistry: Studies in Long Island Sound (Saltzman, B., ed.). Advances in Geophysics 22, Academic Press, New York, pp. 351-415. American Society of Civil Engineers (ASCE) 1963 Friction factors in open channels, progress report of the Task Force on friction factors in open channels ofthe Committee on Hydromechanics of the Hydraulics Division. (Silberman, E., ed.). Journal of the Hydraulics Division 89,97-143. Arakel, A.V. 1982 A study of geochemistry and hydrodynamics in Hutt Lagoon (Western Australia). Oceanologica Acta 4,9-19.

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Chapter 8

Biogeochemistry of Heavy Metals in Coastal Lagoons Luiz Drude De Lacerda Departamento de Geoquimica, Universidade Federal Fluminense, CEP-24.210 Niterdi, RJ, Brazil

Coastal lagoons are temporary sinks for most trace metals with accumulation primarily controlled by sedimentation rates and metal loads. Sabkha environments, however, can be enriched with certain metals both actively and differentially through rapid sulfide formation mediated by stromatolite-forming cyanobacteria. Typical brackish water coastal lagoons can export metals, in particular, those soluble under anoxic conditions, such as manganese (Mn) and arsenic (As),and those forming stable chloro-complexes, such as cadmium (Cd). Organic compounds resulting from partial degradation of organic matter under anoxic conditions, as well as organic substances released by phytoplankton, can also form complexes and eventually export certain metals to coastal waters. Water chemistry of most trace metals in coastal lagoons is highly dependent on biological processes occurring in the water column that effect pH, dissolved oxygen, and quantity and quality of dissolved organic substances.

Introduction

Coastal lagoons occupy transitional littoral zones between the sea and the mainland, having developed during the rise of sea level between 5,000 and 8,000 years BP. In general, they are highly productive and provide spawning and nursery grounds for migratory species. These environmental characteristics have made most coastal lagoons ideal sites for fisheries, mariculture, tourism, and accelerated urbanization, which inevitably leads to environmental conflicts. Many coastal lagoons have a restricted connection to the sea, which acts as an intermediate step in the transport of water and substances from land to sea. Such lagoons are depositional environments and function as filters, by retaining materials supplied by rivers, the atmosphere, and the ocean for long time periods (Kjerfve and Magill, 1989). Salinity in restricted coastal lagoons can vary from fresh water to hypersaline.

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Biogeochemistry of Heavy Metals in Coastal Lagoons

Some coastal lagoons have no direct connection with the ocean. These are referred to as choked lagoons (Kjerfve and Magill, 19891, and normally form in hot, dry climates. They may be highly alkaline environments due to an increase in dissolved carbonates. Sabkha environments are a form of choked coastal lagoon, and may have played an important role in the formation of pre-Cambrian stromatolite-like ore deposits (Renfro, 1974). Most coastal lagoons are affected by human activities other than traditional fisheries and tourism. They receive a n increasing load of contaminants from urban, industrial, and agricultural sources, which can accumulate to dangerous levels in coastal lagoons because they are depositional environments. However, far from being static sinks to such toxic substances, lagoon biogeochemical processes do affect the physio-chemical and environmental properties of most contaminants, altering their bioavailability, and consequently, their associated environmental risks (Krumbein, 1981; Postma, 1981). Among the many anthropogenic contaminants reaching coastal lagoons, heavy metals are of greatest environmental concern. Some, like copper, are highly phytotoxic, affecting phytoplankton and consequently, primary production (Fowler, 1982; Bryan, 1984). Others, such as lead (Pb) and cadmium (Cd), produce a generalized toxicity to marine biota, leading to changes in natural community structure by causing local extinctions of more sensitive species (Bowen, 1979;Moriarty, 1978).Tin (Sn) and mercury (Hg) may form complexes with organic ligands, allowing them to accumulate progressively through the food chain to very high concentrations, eventually reaching humans and causing serious and sometimes fatal health problems (Mitra, 1986; Harris, 1971). I n general, all heavy metals tend to concentrate in marine biota, are virtually non-degradable, and thus produce long-lasting effects upon the environment. Heavy metals may continue to cause environmental health problems even after their major sources are eliminated or controlled. Despite the potential risks associated with metal contamination in coastal lagoons, few comprehensive studies have been completed on this subject. More knowledge and understanding of the effects of heavy metals in coastal lagoons are needed to enable development of predictive models for rational management of these resources. This chapter reviews the existing knowledge of the biogeochemical behavior of heavy metals in coastal lagoon ecosystems. The majority of the discussion is based on data obtained in brackish water lagoons. However, a comparison of data obtained in hypersaline lagoons and the so-called ‘sabkha’environments is also presented. Sources of Heavy Metal Input to Coastal Lagoons

Most heavy metals are input to coastal lagoons through fluvial processes, direct effluent discharge, the atmosphere, and in situ activities producing metal containing effluents, such as shipping and other harbor activities. In sabkha environments, metals can reach the lagoon by percolation through

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TABLE 8.1 Major anthropogenic sources and transport pathways of heavy metals t o coastal lagoons. Most important major sources and pathways appears first in columns, as well as most toxic metals. Metal

Major source

Input pathway

Toxicity

Hg,As

Chlor-alkali plants (HG), fungicides, gold-mining, fuel combustion Electronics, pyrometallurgical, fertilizer production plastic industry Fuel-combusion, pyrometallurgical Pyrometallurgical, anti-fouling paints, sewage sludge, sugar cane processing Pyrometallurgical, sewage sludge

atmosphere, rivers, direct effluents

high to humans and high trophic level species

Cd

Pb Cu

Zn,Ni, Cr

rivers, direct effluents, high to fish and man, atmosphere low t o other biota atmosphere, rivers, direct effluents rivers, direct effluents rivers, direct effluents

high to man and most biota high t o phytoplankton and benthic algae, low to other biota low to all biota

sediments. These inputs can vary from lo3 kg per year in the case of large river flows, to a few micrograms in the case of percolation through metalpoor sediments (Salomons, 1983; Lacerda et al., 1987; Lyons et al., 1983; Barcellos et al., 1988). Table 8.1 summarizes the principal heavy metal pollutants and their major sources to coastal lagoons. In general, anthropogenic sources are the most important source of metals to coastal lagoons (Donazollo et al., 1981; Lyons and Fitzgerald, 1980; SCOPE, 1985; Nriagu, 1979). For example, in the Jacarepagua lagoon, SE Brazil, anthropogenic inputs can reach 87%, 83%, 71% and 53% of the total inputs of nickel (Ni), Copper (Cu), Pb, and Mn, respectively (Barcellos et al., 1988). The major anthropogenic sources of toxic heavy metals to coastal lagoons are: (1)fuel combustion, mostly of Pb, Hg, and As;(2) non-ferrous metals smelters and iron and steel plants for production of Cd, Ni, Pb, zinc (Zn), and Cu; and (3) dumping of sewage sludge, for chromium (Cr), Cu, Ni, and Zn (Nriagu and Pacyna, 1988; Nriagu, 1979; Brown 1988). Major access pathways of heavy metals to coastal lagoons are fluvial inputs. As an example, Sepetiba Bay is a leaky lagoon South of Rio de Janeiro, Brazil, with a heavy metallurgic industry in the watershed. Yearly inputs from one of its major rivers totalled: 362,928,38,955, and 44 lo3 kg of Zn, Mn, iron (Fe), and Cu, respectively (Pedlovsky et al., 1990). Fluvial inputs of the same order of magnitude have been also reported for other lagoons, such as the Ijselmeer, The Netherlands (Salomons, 1983), and the Jacarepagua Lagoon in Rio de Janeiro, Brazil (Barcellos et al., 1988).

224

Biogeochemistry of Heavy Metals in Coastal Lagoons

Atmospheric inputs are generally much smaller than fluvial inputs. In Sepetiba Bay atmospheric Zn inputs are very high, reaching circa 26.8 103 kg y e a r ' for the total lagoon area. However, this is only 7.4%of the fluvial input of 362 lo3kg for Fe and Mn. Atmospheric inputs of 61 and 6.5 103 kg year', respectively, are only 0.2%and 0.7%of the respective yearly inputs (Pedlovsky et al., 1990). These inputs, however, can be the major source of certain elements, for example, boron (Bo), germanium (Ge), and vanadium (V), which are typically enriched in atmospheric ash from thermal plants. I n Lake Macquarie, New South Wales, atmospheric ash from local power plants is a n important source of Bo, Ge, V, Zn, and Cu to lagoon surface sediments, and at localized sites can reach up to 40% of the total input of such elements (Crawford et al., 1976). Atmospheric inputs can otherwise affect lagoons far from direct pollutant sources (Botello and Menelewicz, 1988). Patchineelan et al. (1988) found a six-fold excess of Pb in surface sediments of the Guarapina lagoon, located about 70 km east from the nearest industrial area in Rio de Janeiro. Atmospheric sources of Pb are particularly high in the Rio de Janeiro metropolitan area (Trindade et al., 1981). At Lover's Lake, Bermuda, atmospheric inputs account for the majority of metal fluxes to the lagoon sediments and can vary from 3 to 14 pg cm-' year-' for Cd, Cu, Pb, and Zn and 43 pg cm-l year' for Cr (Lyons et al., 1983). Direct effluents released into coastal lagoons may also be important and are mostly due to metals associated with urban wastes, shipping, and other harbor activities. As most lagoon watersheds are subject to intense tourism activities, such inputs are frequently seasonal (Barroso, 1988). Langston et al. (1988) found TBT (tributhyl-tin) concentrations in waters of Poole Harbour, England, 60%higher in summer than winter due to increasing boating activities. Boating can also be the sole contamination source in lagoons located far off continents. Georges (1988) related elevated Pb and Cd concentrations in atoll lagoon waters off the India's coast in the Arabian Sea, due to such activity. Sewage, depending on treatment, may be very enriched in heavy metals, in particular Pb, Zn, Cr, and Cu. Emission rates for these metals can vary from ng/l in treated sludge to mg/l in untreated sludge (Nriagu and Pacyna, 1988; Salomons and Forstner, 1984). Along tropical coasts for instance, untreated urban sewage is frequently the principal source of heavy metals to coastal lagoons (Garcia, 1989). Because sewage is normally associated with high biological oxygen demand (BOD), reducing conditions are favored, which increase heavy metal remobilization and solubility in the water column, resulting in a synergistic interaction between the two types of contamination. This augments heavy metal toxicity to the entire ecosystem (Souza et al., 1986). In sabkha environments and closed hypersaline lagoons with no riverine input, metal inputs are very small. They generally enter the system through

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solubilization of oxidized conglomeratic sandstone by hot Na-K-Ca-C1 brines at the interface with the overlying reducing sediments of euxenic lagoonal environments. Dissolved metals react with autogenic sulfides and may then be concentrated (Eugster, 1985). Lopes et al. (1986) estimated that 2,000 years would be necessary to reach concentrations of circa 1.0 ppm of Cu through such processes in a hypersaline lake, Lagoa Vermelha, along the coast of Rio de Janeiro. Therefore, the potential for metal toxicity in such environments is likely to be very small or nonexistent. These observations suggest that coastal lagoons are far from pristine environments with regard to heavy metal contamination along most of the world’s coastline. Even where metal inputs are very low, coastal lagoons may contain higher concentrations of metals than pristine marine or fluvial environments.

Fate of Heavy Metals in Coastal Lagoons Heavy metals reach coastal lagoons mostly by fluvial inputs. Suspended particles are by far the most important metal carrier from rivers to coastal lagoons (Barcellos et aZ., 1988; Lacerda et al., 1987; 1988). When entering the brackish lagoon environment, deposition of fine particles due to a decrease in current velocity, flocculation of negatively charged clay particles, and general decrease of metal species solubility occurs. This leads to a gradual accumulation of heavy metals in sediments, in a similar manner to most estuaries (Salomons and Forstner, 1984). The accumulation process varies in magnitude, depending on the specific adsorption constant between the metal and particle and the solubility constant of the new metal complexes formed with the species present in the lagoonal waters. For example, the high specificity of Cu for carboxyl and polyphenolic functional groups (Whitfield and Turner, 1986) and the high stability constant of Cd chloro-complexes (Comans and van Dijk, 1988), will favor the permanence of these metals in solution relative to metals like Fe and Cr with high affinities for inorganic particles with fast settling velocities (Santschi, 1988).Therefore, different accumulation rates of metals will depend not only on the input loads, but also on the chemical characteristics of each metal (Salomons, 1983; Souza et al., 1986). Deposited metals will be affected strongly by sediment characteristics. One of the most important parameters is grain size (Lucas et aZ., 1986; Horowitz and Elrick, 1987). Fine sediments with high surface area scavenge much higher amounts of metals than sands. Therefore, metal accumulation in coastal lagoon bottom sediments will also be a function of fine sediment distribution and movement (Osuma et al., 1986).Because coarse sediments have a faster settling rate than fine sediments, they are deposited in the inlets where current velocities are maximum. The finest grained sediments

226

Biogeochemistry of Heavy Metals in Coastal Lagoons

are, on the other hand, deposited in the innermost reaches where current velocity approaches zero. A gradient is thus normally observed in many lagoons, with the highest metal concentrations found in the fine sediments deposited in the innermost reaches of coastal lagoons. But under situations where high seasonality of fluvial inputs occurs, significant changes in sediment grain size distribution (and thus heavy metal concentration) can occur, changing most gradients formed during low fluvial fluxes. Carmo (1990) has shown a four-fold decrease in Zn concentration in river deltas in a highly seasonal lagoon along the Rio de Janeiro coast. In summer, when tropical downpours fall over the area, fine sediments deposited at the mouth of the freshwater inlets are washed into the open lagoon, leaving metal-poor sands behind, which gradually enrich in Zn content throughout the year. It is obvious that this process is highly dependent on the amount and frequency of summer rains, which may vary widely from year to year and from lagoons that experience different climates. In the Ijsselmeer, The Netherlands, deposition of fine sediments due to flocculation and decreasing river velocity at the innermost reaches of the lagoon are responsible for the accumulation in bottom sediments of circa 47.3 lo3 kg of Cu per year, which amounts to 52% of the annual Cu inputs. For other metals such as Cd (8.1 t year'), Cr (67.6 t year') and Zn (472 t year'), this accumulation can reach 80%, 60% and 69%, respectively, of total annual fluvial inputs for these metal into the Ijsselmeer (Salomons, 1983). Lacerda et al., (1987) have also found high (75%)retention rates of heavy metals in Sepetiba Bay Lagoon, Brazil. This behavior, however, is highly dependent on the physio-chemical properties of each metal and lagoon. The Ijsselmeer exports a net rate of Mn and As to the adjacent sea of the order of 400 t y e a r ' and 1.8 t year', respectively, due to remobilization from lagoon sediments by various biogeochemical process (Salomons, 1983). Some aspects of metal biogeochemistry in coastal lagoons, which eventually result in high retention rates of heavy metals, will be discussed later. Although very few balance studies exist either for lagoons or estuaries, it seems that in general coastal lagoons accumulate heavy metals at a similar rate as coastal plain estuaries. In the Gironde estuary, France, retention rates of Zn, Cu, Pb, and Ni were reported as 17%, 3%, 13%, and 17%, respectively, of the total input (Jouaneau, 1982). In the Scheldt estuary, The Netherlands, retention rates were reported much higher, reaching 59% for Ni and 77%to 90%for Zn (Kerdijk and Salomons, 1982; Wollast and Peters, 1978). I n Narragansett Bay, USA, retention rates were more variable, from only 15%to 30% for Cd, to 70%to 90%for Cu, and 80%to 100%for Pb (Nixon et al., 1987).Those estimates are comparable to the few existing for coastal lagoons (Salomons, 1983; Lacerda et al., 1987) discussed above. It seems, however, that coastal lagoons always have high retention rates, whereas estuaries have more variable rates.

L. Drude De Lucerda

"1

227

F

HE

LE

L

LS

Fig. 8.la. Patterns of heavy metal distribution in sediments of coastal lagoons receiving metal loads from a major fluvial source. Data are based on Lacerda and Abrao (1987) and Baish et al. (1988).

A typical distribution pattern of heavy metals in lagoon sediments along the river-lagoon-sea gradient is presented in Fig. &la, based on data from Lacerda and Abriio (1987) and Patchineelan (19861,for the Guarapina Lagoon, Rio de Janeiro, Brazil. There exists a decrease in metal concentrations from fluvial sediments towards the river-lagoon interface. This is probably due to dilution of fluvial metal-rich sediments in brackish metalpoor lagoons with higher current velocity resulting in the 'washing' of fine sediments from river bed to the lagoon (Lacerda and Abrfio, 19871, veryu similar to the situation in many estuaries (Salomons and Forstner, 1984). A similar pattern is also found for suspended sediments, although remo-

Biogeochemistry of Heavy Metals in Coastal Lagoons

228 1.b

RG- City urban arwr ond O f f ~ ~ 6 I -lndwtrialopoond.Mwnts -peak-

Fig. 8.lb. Patterns of heavy metal distributionin sediments of coastal lagoons receiving metal loads from different industrial and urban direct effluents. Data are based on Lacerda and Abrao (1987)and Baish et al. (1988).

bilization of such metals as Cd, As,and Hg, due t o changing environmental physio-chemical conditions is also responsible for the decrease in suspended particle metal concentrations. In general, however, changes in the physiochemical parameters, mainly of pH, Eh, and salinity, do affect metals adsorbed onto suspended particles much faster than those bound to bottom sediments (Ahlf, 1983; Salomons, 1980; Schoer et al., 1983; Salomons and Forstner, 1984). Inside the lagoon, an enrichment of all metals occurs, with concentrations even higher than the metal-rich fluvial sediments, decreasing again at the sea-lagoon interface, suggesting an active accumulation process in the inner section of the lagoon (Lacerda and Abriio, 1987). This distribution pattern seems to occur in any lagoon receiving metals associated with fluvial effluents. Hall et al. (1987a,b) found that the Aveiro Lagoon, Portugal, which has already received circa 80 tonnes of Hg, has a steep gradient in Hg concentration in sediments. Sediments close to the fluvial source contain 184 pg/g

L. Drude De Lacerda

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Hg, while those close to the lagoon-sea interface have Hg concentrations of 0.183 pg/g, decreasing further to about 1000 pg/g per km towards the sea. In Lake Macquarie, Australia, high Pb and Zn concentrations close t o major incoming rivers were found downward to a depth of 50 cm in sediment cores, while close to the sea entrance, the total concentrations of these same metals fell away at around 20 cm only, clearly showing the strong accumulation of metals close to their sources (Ray and Crawford, 1984). In general, the accumulation of heavy metals in the inner portion of coastal lagoons also corresponds to a n increase in fine, pelitic particles (Krumbein, 1982), organic matter, and cation exchange capacity of the lagoon sediments in relation to fluvial and marine ones (Lacerda and Abriio, 1987). The situation described above is typical of lagoons where major trace metal sources are fluvial. Where direct effluents exist inside the lagoon itself, a different scenario may result. In the Patos Lagoon, South Brazil, metal distribution in sediments is determined by the specific direct effluent present in different parts of the lagoon, and spatial separation among peaks of metal concentrations are very clear. The peaks of Cd, Cr, and Pb derived from industrial effluents are separated from those of Cu and Zn derived mostly from urban sources (Fig. 8.lb) (Baish et al., 1988). In hot, dry climates with large evaporation deposits, coastal hypersaline lagoons develop. Along such shorelines where fluvial inputs are scarce or nonexisting, concentration of heavy metals in lagoon sediments also occurs, but through a different mechanism. The ground water table, lying at or very close to land surface, is the major source of metals. Groundwater solutes are deposited at the sub-areal or sub-aqueous evaporites at the bottom of such hypersaline lagoons (Hohn et al., 1986). High productivity due to a resupply of nutrients lost by evaporation are typical of hypersaline environments. Beneath the algae-covered interface of the lagoon sediments, organic ooze forms, saturated with sulphate-rich water. This is a n ideal substrate for sulphate reducing anaerobic bacteria. The anaerobic sulfide-rich facies act as efficient reducing agents for metals. The algae mats are a source of both reduced sulfur compounds as well as metals, since cyanobacteria can have concentration factors of the order of lo3 to lo2. Metals are probably fured by the cyanobacteria, and, through anoxic decomposition of organic matter, are transferred into the sulfide sediment reservoir or to autogenic carbonate minerals (Gaudette and Lyons, 1984). I n laboratory experiments using sediment columns from a hypersaline lagoon, Maddock and Lopes (1988) found that 98%to 99% of added Cu was absorbed in only 14 days in the sediments. Weakly bound forms of Cu were the main sink; 50% of added Cu was bound to organic compounds and sulfides and 15%in carbonates. However, even considering the short time period of the experiment, about 27%of the added Cu was strongly bound to very recalcitrant organic-metal compounds. The distribution of metals in

Biogeochemistry of Heavy Metals in Coastal Lagoons

230

hypersaline lagoon sediments is therefore a function of the magnitude of microbiological processes at the sedimentiwater interface rather than the location of external metal sources. Diagrams of the two processes of metal accumulation in bottom sediments of coastal lagoons are presented in Fig. 8.2, showing the different mechanisms acting upon metals entering the lagoon from river inputs (Fig. 8.2a) and metals entering from leaching of oxidized sandstone by hot saline solutions in sabkha-like lagoons (Fig. 8.2b). These two types of accumula-

. . . . . . . . . . . . . .

P

2. b

F*viol Input

Coogubtion

setting pmcipitotion water

orparic rid,

. . . . . .

. . . . . . . . . . . .

Fig. 8.2. Patterns of heavy metal accumulation in two distinct lagoonar environments. a, ‘sabkha’,hypersaline lagoons. b, brackish water lagoon with metals entering from rivers andor other freshwater inputs. (-1 represents major metal pathways. The ‘sabkha’ model is more efficient for metals forming highly stable sulfide compounds such as Hg and Cd, and those forming refractory stable organic complexes, such as Cu. The brackish water lagoon model is more efficient for particle-bound metals such as Fe, Cr, Zn, Th and Co.

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tions can transform lagoon bottom sediments into long-term sinks of heavy metals and eventually become important sources of these substances after the lagoons themselves have disappeared. Renfro (1974) proposed that ‘sabkha processes’ may be a clue to pre-Cambrian formation of metalliferous stratiform black-shales such as ‘Kupferschiefer’ type deposits of the Zambian and Creta copperbelts. Lacerda and Cunha (1988) found that old lagoon sediments presently under sand bar deposits are important sources of Mn, Fe, Cu, and Zn for coastal freshwater lakes in the sand dunes of Southeastern Brazil, through percolation of the present sandy sediments during the seasonal vertical movements of ground water. Heavy Metal Cycling in Coastal Lagoons

Although accumulation of heavy metals in bottom sediments of coastal lagoons seems to be a general rule, such systems are far from being static sinks for these substances. The biogeochemical processes occurring at the water-sediment interface will keep heavy metals constantly cycling among the many lagoon compartments, resulting in comparatively higher concentrations of heavy metals in lagoon waters when compared to adjacent sea or river waters. The accumulation of metals in lagoon bottom sediments (Fig. 8.1) will cause a large increase in the dissolved pore water concentrations of those metals that can be orders of magnitude greater than the concentration of overlying waters (Postma, 1982). The enrichment of pore water metal concentrations occurs due to solubilization of substances and metal complexes subjected to redox processes; in particular, (1)oxidized compounds of Fe, Mn, and As; (2) desorption processes as a consequence of the lower P h typical of pore waters; and (3) the partial mineralization of organic matter and subsequent solubilization of metals complexed with dissolved organic species, which readily react with free amino acids, amino sugars, and polyphenols of low to medium molecular weight (Salomons and Forstner, 1984; Lacerda and Rezende, 1987; Henrichs and Farringtonm, 1979; Mopper et al., 1980; Lindberg and Harris, 1974; Barcelona, 1980). In those lagoons where residence times of pore waters and their associated heavy metals are short and variable, forcing by wind, tides, biota, fluvial inputs, and anthropogenic activities such as dredging induce a patchiness to the sediment environment. This results in high metal remobilization to the water column (Postma, 1982; Camacho-Ibar and Alvarez Borrego, 1988; Hall et al., 198713). Hall et al., (1987b) showed that low salinity waters could leach Hg from contaminated sediments of the Aveiro Lagoon, Portugal, and disperse the Hg by seasonal climatic changes and forcing by tides and fluvial input. Nevissi and Schell(1975) studied the fate of plutonium (Pu) and americium (Am) in Bikini Atoll lagoon and found that 16 years after the last nuclear

232

Biogeochemistty of Heavy Metals in Coastal Lagoons

test on the atoll, these radionuclides were neither completely buried in the lagoon sediments nor completely discharged to the ocean. They concluded that remobilization at the water-sediment interface by currents, winds, and storms would enhance the complexing of the released ions with suspended particles. Coagulation with organic detritus and incorporation within the plankton would cause metal-containing suspended particles to increase in size, allowing settling, remineralization, and thus, recycling within the atoll ecosystem. Although physical factors can be of great importance, biological processes at the sediment-water interface are probably the major force affecting metal release from sediments to the overlying water column in coastal lagoons. Lacerda and Rezende (1993) proposed that diurnal variations of Zn concentration in pore waters inside a mangrove forest along Sepetiba Bay was due to dissolution of Zn-polysulfides by the oxygen of algal photosynthesis. Algal photosynthesis is also reported to be the main cause for the observed seasonal variation of Zn dissolved concentration in the Ijsellmeer, The Netherlands. During summer, high photosynthetic rates increase water Ph up to 9.0, leading to the absorption of Zn onto suspended particles and consequently decreasing soluble Zn species. In winter when algal photosynthesis is low and respiration rates are high, the lowering of water Ph to neutral or slightly acid causes Zn desorption from suspended particles with consequent increase in Zn dissolved concentrations (Salomons, 1977; Salomons and Forstner, 1984). In some tropical coastal lagoons, mangrove forests have also been reported to strongly affect water pH. In Taqueriga Lagoon, Venezuela, Cauvet (1988) found a reduction in water pH from 9.2 to 7.3 due to mangrove organic matter partial degradation. Although no metal data exists for the system, the observed variability in water pH would likely cause the same variability in dissolved metal species, as found for the Ijsellmeer. Aquatic macrophytes may play a significant role in metal remobilization in lagoon sediments by selectively accumulating and releasing metals according to their physiological necessities and/or by changing the sediment and water physio-chemical conditions. The uptake of heavy metals by lagoon seagrasses is dependent on the specific metal involved. Metals such as Zn and Mn, which are involved in the enzymatic metabolism of growth and photosynthesis, are preferentially accumulated in leaves, therefore being easily released or remineralized after leaf senescence. Other metals such as Pb and Cu, however, are preferentially accumulated in roots and rhizomes, being more easily buried and eventually accumulated in sediments (Lacerda and Rezende, 1987b). Seagrass and other lagoon macrophytes may alter physio-chemical parameters of sediments such as granulometry, cation exchange capacity, organic matter content and redox potentials (Pullich, 1982). This can cause spatial variability in pore water composition in the range of centimeters to

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meters (Carnacho-Ibar and Alvarez-Borrego, 1988). Lacerda and Rezende (1985) found a two-fold increase in Cu and Zn lagoon sediments under the seagrass Halodule wrightii Arschers, due to a ten-fold increase in fine particles and organic matter content in comparison with sediments outside the seagrass beds. This macrophyte has also been reported to increase Mn dissolved concentration in selected areas of Corpus Christi Bay, Texas. When the macrophyte colonizes bare areas, the redox potential of sediments decreases, which causes an increase in microbial activity upon deposited organic debris (Pulich, 1982). I n lagoons with very low metal inputs, such as atoll lagoons, seagrass processes may be the singlemost important source of heavy metals to the water column. Seagrasses actively pump metals from pore waters and release them through leaves or during leave senescence and decomposition. George (1988) found that a n increase in the concentration of non-labile, organically bound Cd, Pb, and Cu in atoll lagoon waters originated from decaying processes in local seagrass beds. Pu and Am may undergo a similar cycle in Bikini Atoll lagoon (Nevissi and Schell, 1975). The above examples indicate that liberation of metals from pore waters can be regarded as the major internal source of metals to coastal lagoons (Salomons, 19831,keeping them in a permanent cycle from sediment to pore water to water column and again to sediments with quite a high efficiency. The metal remobilization processes occurring at the sedimentiwater interface may also control the cycling of important nutrients to the lagoon metabolism. In tropical coastal lagoons, most of the phosphorus is strongly bound to Fe oxides deposited at the oxidized boundary layer at the sediment surface. As this layer becomes reduced due to sediment accretion and oxygen consumption during organic matter decaying processes, dissolution of Fe oxides occurs, precipitating Fe sulfides and releasing phosphorus to the overlying waters (Tolentino et al., 1986). In fact, in some tropical lagoons this may be the major controlling process of phosphorus cycling (Esteves et al., 1984; Scwarzbold et al., 1986). Even after total infilling of a coastal lagoon, pore waters can still be a n important source of trace metals. Lacerda and Cunha (1987) found that appreciable amounts of Fe, Mn, Zn, and Cu are released to freshwater marshes growing over old lagoon sediments (Fig. 8.3). Buried, reduced lagoon sediments are seasonally flushed by ground water. Metals percolate and are transported up to the rhizosphere of the marsh where they may be absorbed by plants or deposited at the sediment-water interface, which is rich in peat-like material from decaying marsh plants. The peaks in metal concentration at the organic-rich horizons clearly illustrate this process (Fig. 8.3). These examples show that although metals show a general trend in being buried and accumulating in coastal lagoon bottom sediments, a n important fraction of the deposited metals is constantly being remobilized to the water column through pore water processes.

Biogeochemistry of Heavy Metals in Coastal Lagoons

234

.

. . .

Fig. 8.3. Distribution profiles of total soluble Mn and Fe through a fresh water dune marsh sediment column at Barra de Marica, Brazil. Metals diffuse up through the pore waters and precipitate at the oxic water-peat interface. Data are based on Lacerda et a l . (1986) and Lacerda and Cunha (1988).

Fate of Heavy Metals in the Water Column

Some physical and chemical parameters of coastal lagoons have a direct relationship with the concentration and variability of dissolved metal species in the water column (Santschi, 1988) (Table 8.2). A closed hydrography and limited exchange with the sea results in long residence times of waters, frequently on the order of weeks to months (Kjerfve and Magill, 1989). The shallow depth of coastal lagoons facilitates resuspension of bottom sediments by wind and tidal forcing, which promotes the release of dissolved metals from pore waters to the overlying water by diffusion transport. It also prevents development of stratified thermal layers and thus oxygenation of the entire water column. Continuous decomposition of organic matter results, and consequently, the release of incorporated metals (Esteves et al., 1984; Cossu et al., 1987). As a result, metal inputs tend to be concentrated in lagoon waters that experience long residence times and almost continuous resuspension of sediments. The permanence of most metals in solution depends on the potential for complex formation with organic and inorganic ligands. Coastal lagoon waters are characteristically enriched in such substances. This is mainly because autogenic organic production is higher in coastal lagoons than in most areas of the continental platform due to continual phytoplanktonic growth fed by land derived nutrients and those resuspended with bottom

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TABLE 8.2 Major biogeochemical factors affecting selected trace metals in the water column of coastal lagoons, based on Santschi (1988) Factor

Process

Effect

Salinity

chloro-complexation desorption

Cd2++ CdC12 Pb-part. + Pb2+ Cu-part. + Cu(OHI2

Plankton synthesis

bio-incorporation of soluble forms of nutrient-like metals, mostly Ni,As,Cu, and Zn

organic-bound metals

Dissolved organic carbon

complexation and coagulation

mainly dissolved forms of Hg, Pb, Cu and Fe to metal-organic complexes

Particle settling

scavenging

mostly Cr, Pb, Sn, Th,Co, Mn and Fe

Bacterial degradation solubilization of organic matter organification changing redox-potential Sediment resuspension oxidation of reduced forms [Mn, Fe(OH),l

incorporated nutrient-like metals mainly Co as vitamin BIZ,and Hg as alkil-Hg mostly Mn and Fe dissolution of oxidehydroxidebound metals

sediments. Also, the shallow depth and presence of large flat margins around coastal lagoons facilitate colonization by macrophytes such as seagrasses, saltmarsh grasses, and mangroves, which supply organic matter to lagoons. Frequently, dissolved organic carbon (DOC) concentrations can be orders of magnitude higher than in the open sea. DOC is generally refractory to further degradation (Cauvet, 1988) and metals complexed to such substances will be kept in solution almost permanently (Lindberg and Harris, 1974). Dissolved organic matter from different origins, however, presents different metal complexing capacity. Mackey and Szymezak (1988) found that Cu complexing capacity of waters in Port Hac King, Australia, was not correlated with terrestrial organic matter export to the lagoon, but increased to a factor of 10 due to a phytoplankton bloom that resulted from intrusion of nutrient rich waters. Jones and Thomas (1988)and Jones et al.,

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Biogeochemisty of Heavy Metals in Coastal Lagoons

(1982) showed that when ‘red tide’ algae approaches low salinity waters, it releases humic substances that exude from filaments. These humics can chelate Fe from water to a level of 5%, and actively influence the cycling of most metals in coastal lagoons. Humic substances, in particular, monocarboxylic acids, would facilitate any reaction with metal ions adsorbed onto particulate material by promoting a dissolution process whereby colloidallybound metal ions are converted to labile metal ions. Complexation and coagulation may also affect other metals such as Pb, Cu, and Hg and regulate their concentrations on a time scale of hours to days (Santschi, 1988). Dissolved complexed metal concentrations will therefore increase in the presence of marine humic substances. Because euthophycation or at least periodic algal blooms are characteristic of the majority of coastal lagoons, and the sediments are a continuous internal source of metals to the water column, high concentrations of dissolved complexed metal species are expected to be present in almost any lagoon. These complexed metal-organic species generally decrease metal availability to lagoon biota. However, most organic-metal complexes can easily release their adsorbed metals when other more abundant ions, such as Ca, out-compete them for the organic binding sites (Campbell and Evans, 1987). This causes frequent, temporary situations of concentration peaks of metals in the water column, which increases the potential for toxicity to the biota. Apart from DOC, dissolved metal species will also be highly dependent on the variability of some physio-chemical parameters, most importantly, redox potential and salinity. In most coastal lagoons, salinity and redox potential are highly variable. Redox potential easily reaches negative values (up to -500 mV) due to consumption of dissolved oxygen during decaying of algal blooms or after strong resuspension of reduced sediments. Metals such as Fe, Mn, As, and others complexed with Fe and Mn oxi-hydroxides will have increased dissolved concentrations during such occasions. Salinity also varies both spatially and temporally from values <1%. to seawater concentration. Metals forming stable chloro-complexes such as Cd and Hg are highly soluble at intermediate salinities (Table 8.2). It is therefore expected that the concentration of dissolved metal species can frequently reach high values in coastal lagoon waters. These concentration peaks may be important sources of metals to lagoon organisms, and eventually become a toxicological threat to some of them. In summary, because of the biogeochemical nature of coastal lagoons, they tend to accumulate high concentrations of heavy metals. The increasing anthropogenic pressure upon these ecosystems may thus cause a rapid increase in the environmental toxicity of heavy metals, leading to decreased water quality, endangerment to local flora and fauna, and destruction of their natural resources.

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Acknowledgements

This chapter is the result of a long-term research project on the coastal lagoons of Rio de Janeiro, Brazil, with continuous support from Financiadora de Estudos e Projetos (FINEP), Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico (CNPq) and Fundacso de Amparo a Pesquisa do Estado do Rio de Janeiro. Drs. B.A. Knoppers, B. Kjerfve, L. Liang, and W.T.Lawrence kindly commented on earlier drafts of the manuscript. I would also like to thank the many colleagues and students who kindly provided me with original data, in particular Drs. Sambasiva R. Patchineelan, John E.L. Maddock, and Alvaro R.C. Ovalle. Prof. Horst M. Fernandez kindly provided the data on Lagoa de Jacarbpagua. References Ahlf, W. 1983 The River Elbe: behavior of Cd and Zn during estuarine mixing. Environmental Technology Letters 4,405-410. Azevedo, H.L., Monken, H.R. and Melo, V.P. 1988 Study of heavy metal pollution in the tributary rivers of Jacarepagua Lagoon, Rio de Janeiro State, Brazil, through sediment analysis. In Metals in Coastal Environments of Latin America (Seeliger, U., Lacerda, L.D. and Patchineelan, S.R., eds.). Springer-Verlag, Berlin, pp. 21-29. Baish, P.R.N., Niencheski, L.F.H. and Lacerda, L.D. 1988 Trace metal distribution in sediments of the Patos Lagoon estuary, Brazil. In Metals in Coastal Environments of Latin America (Seeliger, U., Lacerda, L.D. and Patchineelan, S.R.,eds.). SpringerVerlag, Berlin, pp. 59-64. Barcellos, C.C., Fernandes, H.M. and Azevedo, H.L.P. 1988 The role ofthe Arroio Pavuna river in the transport of particulate heavy metals to Jacarepagua lagoon, Brazil. Science of the Total Environment 75, 211-223. Barcelona 1980 Dissolved organic carbon and volatile fatty acids in marine sediment pore waters. Geochimica y Cosmochimica Acta 44,1977-1984. Bmoso, L.V. 1987 Diagnostic0 ambiental da Lagoa de Araruama, RJ. B. FBCN 22, 30-65. Batley, G.E. 1987 Heavy metal speciation in waters, sediments and biota from Lake Macquarie, New South Wales. Australian Journal of Marine and Freshwater Research 38,591-606. Botello, A.V. and Mendelewicz, M. 1988 Pollution and pollutants in the Tenninos Lagoon region. In Ecology of Coastal Ecosystems in the Southern Gulf ofMexico: The Terminos Lagoon Region (Yanez Arancibia, A. and J.W. Day, Jr., eds.). Universidad Nacional Autonoma de Mexico, Mexico, DF, pp. 415430. Bowen, H.J.M. 1979Environmental Chemistry of the Elements. Academic Press, London, 333 pp. Brown, B.B. 1988 Fate of metals in biota and biological interactions in the tropical coastal zone. In Metals in Coastal Environments ofLatin America (Seeliger, U., Lacerda, L.D. and Patchineelan, S.R., eds.). Springer-Verlag, Berlin, pp. 102-121. Bryan, G.M. 1984 Pollution due to heavy metals and their compounds. Marine Ecology Progress Series 5 , 1289-1431.

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Camacho-Ibar, V.F. and Alvarez-Borrego, S. 1988 Nutrient concentrations in pore waters of intertidal sediments in a coastal lagoon: patchiness and temporal variations. Science of the Total Environment 75,325-339. Campbell, J.H. and Evans, R.D. 1987 Inorganic and organic ligand binding of lead and cadmium and resultant implications for bioavailability. Science ofthe Total Enuironment 62,219-227. Carmo, M.A.A. 1989 Geochimie de metaux lourds a Lagune de Guarapina, Rio de Janeiro, Bresil. PhD Thesis. University of Bordeaux (in press), France. Comans, R.N.J. and van Dijk, C.P.J. 1988 Role of complexation processes in cadmium mobilization during estuarine mixing. Nature 336, 151-154. Cossu, R., Fraja Frangipane, E., Degobbis, D., Orio, A.A. and Andreottola, G. 1987 Pollution and eutrophication in the Venice Lagoon. Water Science Technology 19, 813-822. Crawford, E.A., Roy, P.S., Brooks, K., Zamberlain, A., Scott, T., MacKay, N.J. and Chrojka, R. 1976 Heavy metals in bottom sediments and fish from Lake Macquarie. Report, Geological Survey of NSW, GS 1976/283. Donazzollo, R., Merlin, O.H., Vitturi, L.M., Orio, A.A., Pavoli, B., Perrin, G. and Rabitti, S. 1981 Heavy metal concentration in surface sediments from the Gulf of Venice, Italy. Marine Pollution Bulletin 12,417425. Esteves, F.A., Ishii, I.H. and Camargo, A.F.M. 1984 Pesquisas limnologicas em 14 lagoas do litoral do Estado do Rio de Janeiro. In Restingas: Origem, Estrutura, Processos (Lacerda, L.D., Araujo, D.S.D., Cerqueira, R. and Turcq, B., eds.). EDUFF, Niter&, pp. 443-454. Eugster, H.P. 1985 Oil shales, evaporites and ore deposits. Geochimica and Cosmochimica Acta 46,619-635. Fowler, S.W. 1982 Biological transfer and transport processes. In Pollutant transfer and transport in the sea (Kullenberg, G., ed.). CRC Press, Florida, pp. 1-65. Garcia, H.G. 1989 Estudio de la contaminacion marina por metales pesados en algunas areas cubanas. PhD Thesis, Centro Nacional de Investigaciones Cientificas, Cuba, 28 PP. Gaudette, H.E. and Lyons, Wm.B. 1984 Trace metal concentrations in modern marine sabka sediments. In Microbial Mats: Stromatolites (Cohen, Y., Castenholz, R.W. and Halunson, H.O., eds.). MBL Lectures in Biology vol. 3, Alan R. Liss, Tnc., New York, pp. 411424. George, M.D. 1988 Distribution of labile and non-labile forms of Cd, Pb and Cu in Lakshadweep Lagoon Waters. Indian Journal of Marine Science 17, 111-113. Hall, A., Duarte, A.C., Caldeira, M.T.M. and Lucas, M.F.B. 1987a Sources and sinks of mercury in the coastal lagoon of Aveiro, Portugal. Science of the Total Environment 64,7547. Hall, A., Duarte, A.C., Oliveira, J.P. and Pereira, M.E. 1987b Release of mercury from polluted sediments under a salinity gradient. Sixth International Conference on Heavy Metals in the Environment 2,224-226. Harris, R.G. 1971 Ecological implications of mercury pollution in aquatic systems. Biological Conservation 3,279-283. Henrichs, S.M. and Famngtonm, J.W. 1979 Amino acids in interstitial waters of marine sediments. Nature 279,319422. Hohn, A., Tobshall, H.J. and Maddock, J.E.L. 1986 Biogeochemistry of a hypersaline lagoon east of Rio de Janeiro, Brazil. Science of the Total Environment 58, 175-186.

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Horowitz, A.N. and Elrick, K.A. 1987 Sediment trace element chemistry: Interrelation with surface area, grain size and geochemical substrate. Sixth Znternational Conference on Heavy Metals in the Environment 2,227-229. Jones, G.B. and Thomas, F.G. 1988 Effect of terrestrial and marine humics on copper speciation in a n estuary in the Great Barrier Reef Lagoon. Australian Journal of Marine and Freshwater Research 39,19-31. Jones, G.B., Burdon-Jones, C. and Thomas, F.G. 1982 Influence of Trichodesmium red tides on trace metal cycling at a coastal station in the Great Barrier Reef lagoon. Oceanologica Acta, 3 19-326. Jouaneau, J.M. 1982 Matieres en suspension et oligo elements metalliques duns le systeme estuarien Girondin: Comportement et flux. T. Dout., University of Bordeaux. Kerdijk, H.N. and Salomons, W. 1981 Heavy metal cycling in the Scheldt estuary. Delft Hydraulics Report M164-M1640/M1736. Kjerfve, B. and Magill, K.E. 1989 Geographic and hydrodynamic characteristics of shallow coastal lagoons. Marine Geology 88,187-199. Krumbein, W.E. 1982 Biogeochemistry and geomicrobiology of lagoons and lagoonary environments. Coastal Lagoon Research, Present and Future. UNESCO Technical Papers in Marine Science 33,97-110. Lacerda, L.D. and Rezende, C.E. 1985 The effect of Halodule wrightii Archers on the geochemical partitioning of Mn, Zn, and Cu in coastal sediments. Fifth Znternational Conference on Heavy Metals in the Environment 2,292-294. Lacerda, L.D. and Rezende, C.E. 1986 Metals in the seagrass Halodule wrightii Archers during one growing season. Revista Brasileira Botanica 9,87-90. Lacerda, L.D., Pfeiffer, W.C. and Fiszman, M. 1987 Heavy metal distribution, availability and fate in the Sepetiba Bay, SE Brazil. Science of the Total Environment 65,163-173. Lacerda, L.D. and Abriio, J.J. 1987 Nickel geochemistry in sediments of a naturally eutrophicated coastal lagoon. Ciencia e Cultura 39, 186-188. Lacerda, L.D., Martinelli, L.A., Rezende, C.E., Mozeto, A.A., Ovalle, A.R.C., Victoria, R.C., Silva, C.A.R. and Nogueira, F.B. 1988 The fate of trace metals in suspended matter in a mangrove creek during a tidal cycle. Science of the Total Environment 75,169-180. Lacerda, L.D. and Cunha, C.T. 1988 Iron and Manganese distribution in plant communities of a dune marsh. Acta Limnologica Brasiliensa 2,803-812. Lacerda, L.D. and Rezende, C.E. 1987a Metal geochemistry in mangrove sediments.Ann. Symp. Ecossistemas Costa Sul-sudeste do B r a d ,960Paulo: Acad. Cienc. 3,123-131. Lacerda, L.D. and Rezende, C.E. 1993 Heavy metal biogeochemistry in mangrove ecosystems. In The cycling of Hg, As, Pb, and Cd in the Environment (Hutchinson, T., Meema, P. and Gordon, C., eds.). Willey Eastern, New Dehli, 302 pp. Langstron, W.J., Burt, G.R. and Mingjiang, Z. 1987 Tin and organotin in water, sediments, and benthic organisms of Poole Harbour. Marine Pollution Bulletin 18, 634-639. Lindberg, S.E. and Harris, R.C. 1974 Hg-organic matter associations in estuarine sediments and interstitial water. Environmental Science Technology 8,459-462. Lopes, C.E.A., Teixeira, A.C.D., Maddock, J.E.L., Tobschall, H.J. and Hohn, A. 1986 Absorption of metals by benthic microbial mats and sediments of Lagoa Vermelha, Brazil. Science of the Total Environment S8,55-62. Lucas, M.F., Caldeira, M.T., Hall, A., Duarte, A.C. and Lima, C. 1986 Distribution of mercury in the sediments and fishes of the Lagoon of Aveiro, Portugal. Water Science Technology 18,141-148.

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Lyons, W.B. and Fitzgerald, W.F. 1980 Trace metals fluxes to nearshore Long Island Sound sediments. Marine Pollution Bulletin 11, 157-161. Lyons, W.B., Armstrong, P.B. and Gaudette, H.E. 1983 Trace metal concentrations and fluxes in Bermuda sediments. Marine Pollution Bulletin 14,65-68. Mackey, D.J. and Szymczak, R. 1988 Seasonal variations in the copper complexity capacity of Port Hacking. Australian Journal of Marine and Freshwater Research 39, 125-132. Maddock, J.E.L. and Lopes, C.E.A. 1988 Behaviour of pollutant metals in aquatic Lacerda, sediments. In Metals in coastal environments of Latin America (Seeliger, U., L.D. and Patchineelan, S.R., eds.). Springer-Verlag, Berlin, pp. 100-105. Mitra, S. 1986 Mercury in the environment. Transactions of Technical Publications, Switzerland, 327 pp. Mopper, K., Dawson, R., Leibezelt, G. and Ittekkot, V. 1980 The monosaccaride spectra of natural waters. Marine Chemistry 1 0 , 5 5 4 6 . Moriarty, F. 1975 Pollutants and anima1s:Afactualperspective. George Allen and Unwin Ltd., London, 140 pp. Nevissi, A. and Schell, W.R. 1975 Distribution of plutonium and americium in Bikini Atoll Lagoon. Health Physiology 28,539-547. Nixon, S.W., Hunt, C.D. and Nowicki, B.L. 1987 The retention of nutients (C,N,P), heavy metals (Mn,Cd,Pb,Cu), and petroleum hydrocarbons in Narragansett Bay. In Biogeochemical processes at the land-sea boundary (Lanerre, P. and Martin, J.M., eds.). Elsevier Publishing Co., The Netherlands, pp. 99-122. Nriagu, J.O. 1979 Global inventory of natural and anthropogenic emmissions of trace metals to the atmosphere. Nature 279,409-411. Nriagu, J.O. and Pacyna, J.M. 1988 Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 333, 134-139. Osuma, F.P., Botello, A.V. and Villanueva, J. 1986 Heavy metals in Coatzacoalcos estuary and Ostion lagoon, Mexico. Marine Pollution Bulletin 17, 516-519. Patchineelan, S.R., Leitao Filho, C.M., Kristotakis, K. and Tobschall, H.J. 1988 Atmospheric lead deposition into Guarapina Lagoon, Rio de Janeiro State, Brazil. In Metals in Coastal Environments of Latin America (Seeliger, U., Lacerda, L.D. and Patchineelan, S.R., eds.). Springer-Verlag, Berlin, pp. 65-76. Patchineelan, S.R. 1986 Estudos dos sistemas de drenagem das bacias d a s lagoas de Marica e Guarapina. Projeto FINEP-UFF. Departamento de Geoquimica, Universidad Federal Fluminense, Niteroi, 135 pp. Pedlovski, M., Lacerda, L.D., Ovalle, A.R.C., Silva, E. and Watts, P.P. 1990 Entradas atmosfericas de metais traco para a Baia de Sepetiba. Ciencia e Cultura (in press). Phleger, F.B. 1982 A review of some general features of coastal lagoons. Coastal Lagoons Research, Present and Future. UNESCO Technical Papers in Marine Science, Paris 33, 7-12. Postma, H. 1982 Process in the sediment and in the water-sediment interface. Coastal Lagoon Research, Present and Future. UNESCO Technical Papers in Marine Science, Paris 33, 111-118. Pullich, W.M. 1982 Edaphic factors related to shoalgrass (Halodule wrightii Aschers.) production. Botanica Marina 25,467-475. Ray, P.S. and Crawford, E.A. 1984 Heavy metals in a contaminated Australian estuary - dispersion and accumulation trend. Estuarine, Coastal and Shelf Science 19, 341-358.

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Renfro, A.R. 1974 Genesis of evaporite-associated stratiform metalliferous deposits -A sabkha process. Economic Geology 6 9 , 3 3 4 5 . Salomons, W. and Forstner, U. 1988 Metals in the Hydrocycle. Springer-Verlag, Berlin, 349 pp. Salomons, W. and Forstner, U. 1984 Metals in the Hydrocycle. Springer Verlag, Berlin, 297 pp. Salomons, W. 1983 Trace metal cycling in a polluted lake (Ijsselmeer, The Netherlands). Delft Hydraulics Laboratory Report S 357/EEC Contract No. 199-7-1 ENV N. 50 pp. Salomons, W. 1980 Adsorption processes and hydrodynamic conditions in estuaries. Environmental Technology Letters 1,356-365. Santschi, P.H. 1988 Factors controlling the biogeochemical cycles of trace metals in fresh and coastal marine waters as revealed by artificial radioisotopes. Limnology and Oceanography 33,848-866. Scharzbold, A., Fonseca, O.J.M. and Guerra, T. 1986 kspectos limnologicos do Sac0 de Tapes. Acta Limnologica Brasiliensa 1,89-112. Schoer, J., Hong, Y.T. and Forstner, U. 1983 Variations of chemical forms of iron, manganese and zinc in suspended sediments from the Elbe and Weser rivers during estuarine mixing. Environmental Technology Letters 4,277-282. SCOPE 1985 I Intern. SCOPE Metals Cycling Workshop. Rapporteur’s Reports on Scientific Community Problems of the Environment. Toronto, 16 pp. Souza, C.M.M., Pestana, M.H.D. and Lacerda, L.D. 1986 Geochemical partitioning of heavy metals in sediments of three estuaries along the coast of Rio de Janeiro (Brazil). Science of the Total Environment 5463-72. Tolentino, M., Esteves, F.A., Roland, F. and Thomaz, S.M. 1986 Composiclo quimica do sediment0 de 14 lagoas do litoral fluminense e sua utilizacao na tipologia desses ecossistemas. Acta Limnologica Brasiliensa 1,431-447. Trindade, H., Pfeiffer, W.C., Fiszman, M. and Oliveira, C.E. 1981 Concentracoes atmosfericas de total de particulas em suspenslo e metais na cidade do Rio de Janeiro. Ciencias Culturas 33,996-1005. Whitfield, M. and Turner, D.R. 1986 Chemical speciation in tropical waters - A cautionary tale. Science of the Total Environment 88,9-36. Wollast, R. and Peters, J.J. Biogeochemical properties of an estuarine system: The River Scheldt. In Biogeochemistry of Estuarine Sediments. UNESCO, Paris, pp. 279-293.

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Chapter 9

Aquatic Primary Production in Coastal Lagoons Bastiaan Knoppers

Departmento de Geoquimica, Universidade Federal Fluminense, CEP-24.210Niter&, RJ, Brazil

Measurements of hourly, daily and annual rates of aquatic primary production in coastal lagoons are now numerous, but the mechanisms that regulate primary production are still difficult to assess. Short term dynamics in functioning and diversity in geomorphological configuration, hydrodynamics, nutrient supply, and autotrophic populations, as well as the usage of various methods in primary production measurements, are among some reasons. Mesotrophic to eutrophic choked lagoons with depths exceeding about two meters and light limitation at the bottom are commonly phytoplankton based. In shallower choked to restricted lagoons with high nutrient loading perennial macroalgae appear in succession with phytoplankton. Restricted to leaky lagoons harbor more macrophytes and species predominance is selected by a combination of depth, tidal flushing, and nutrient loading. Nutrient budgets for coastal lagoons are still lacking, but external inorganic nitrogen loading seems to supply 5-20% and sediment release 530% of primary production demand. Regenerated production is thus high and its largest fraction is maintained by nutrient recycling in the water column. In comparison to primary production yield, approximately 10-25% of carbon is accumulated in lagoon sediments. Many tropical and sub-tropical lagoons particularly those with typical dry/wet seasons are marked by seasonal shifts between autotrophic and heterotrophic metaboli m Be olding exceptions, annual primary production ranges from 200 to 500 g C m-fyear-qand is highest in tropical choked phytoplankton based lagoons and those gover ed by macr ph es. Global coastal lagoon primary production is f l is similar to the contribution by upwelling areas estimated at 10 kg C year- Pand and by a factor of four less than estuaries where primary production per unit area is similar to that in lagoons.

A

Introduction

The fixation of carbon measured as the rate of photosynthesis is called primary production and its assessment has manifold implications. It is the major source of organic matter production for the marine food web and t h u s represents the building block for mass balance studies in aquatic systems. It is of relevance because of the debate on the role of oceanic carbon fixation as

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a sink to atmospheric build-up of carbon dioxide (Sarmiento and Toggweiler, 1984; Peterson and Melillo, 1985; Platt et al., 19891, and it partially serves to assess the trophic state, the potential eutrophication, and also to some extend to predict fisheries yields of shelf waters, estuaries, and lagoons (Nixon, 1982; Welsh et al., 1982). In the last two decades, a considerable amount of information on primary production has become available for coastal lagoons. At first sight this is not apparent because in many publications lagoons are often denominated as either estuaries, bays, and or embayments. Various reviews on nutrient behavior, primary production, and fisheries yields, and also first approaches to relate primary productivity to general physical characteristics, have been presented for tropical and temperate coastal lagoons (Qasim et al., 1969; Vannucci, 1969; Qasim, 1979; Gilmartin and Revelante, 1978; Boynton et al., 1982; Nixon, 1982; Welsh et al., 1982; Vaulot and Frisoni, 1986). However, for the majority of coastal lagoons, it is still problematic to estimate total autotrophic production because many studies have focused on single populations of primary producers. Primary production in lagoons may be dominated by either phytoplankton, benthic micro- and macroalgae, macrophytes, and in specific cases algal mats, or a combination of these populations. This imposes a n array of specialists, analytical methods, and experimental designs with measurements at numerous different spatial and temporal scales. Furthermore, the lack of ordinance of productivity measurements to relevant temporal scales of governing physical and chemical processes, poses major difficulties for the comparison of data sets of the highly diverse functioning coastal lagoons. Similar problems are encountered for neritic systems (Platt and Rao, 1975; Harris, 1980; Harris, 1986; Platt et al., 1989). Nixon (1982) came to the conclusion, that apart from some exceptions, the annual range of total primary production for coastal lagoons lies within 200 and 400 g C m-2 year', a range comparable to the one for estuaries and medium productive coastal upwelling regions (Lieth and Whitaker, 1975; Platt and Rao, 1975). In lagoons where phytoplankton production is reduced, production is supplemented by benthic algae and/or seagrasses. The mechanisms involved in the selection and succession of primary producers and those that control the magnitude of total primary production have as yet to be described by an integrated ecological concept (Yaiiez-Arancibia et al., 1985; Yaiiez-Arancibia, 1987) and nutrient budgets are still lacking for coastal lagoons.

The Measurement of Primary Production in Coastal Lagoons

Numerous reviews have widely covered the experimental design and analytical methods, including their advantages and drawbacks, for productivity measurements in aquatic systems (Berman and Eppley, 1974; Wil-

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liams et al., 1979; Harris, 1980; Morris, 1980b; Gieskes and Kraay, 1984; Harris, 1986; Platt et al., 1989).However, some specific problems may arise when studying primary production in coastal lagoons. These systems may be either dominated by phytoplankton, benthic macro- and microalgae, macrophytes, or even algal mats, and, in many cases, a combination of these primary producers. Each population differs in their ecophysiological and ecological response, and as such, whatever method is applied refers to a specificspatial and temporal scale and delivers a different index in primary production (Harris, 1986; Platt et al., 1989). Henceforth, in lagoons with a high diversity in primary producers a wide array of hardly comparable techniques with an enormous effort in research has to be applied (Reyes and Merino, 1991). In such cases, the most suitable alternative for measurements of total production is the ‘Diurnal Curve’ method which simultaneously monitors dissolved oxygen and carbon dioxide evolution in situ during 24 hour cycles. The method is of particular interest, as it enables an estimation of whole system metabolism. In spite of its early introduction in the mid 50s (Odum and Hoskin, 1958; Odum et al., 1959; Odum and Wilson, 1962; Carmouze et al., 1991; Reyes and Merino, 19911, the method has not been a popular tool. Its lack of application is probably related to the large time expenditure needed in the field and modelling of correction factors for gas exchange at the water-atmosphere interface. In mesotrophic to eutrophic phytoplankton based coastal lagoons, standard lightidark bottle incubation techniques with measurements of the evolution of dissolved oxygen or the incorporation of isotopic carbon (W)have been most common, and additional measurements of carbon dioxide evolution have been restricted to some coastal and calcifying Atoll lagoons (Park et al., 1958; Teal and Kannwisher, 1966;Sournia, 1977; Sournia et al., 1981; Carmouze, 1985; Carmouze et al., 1991).The drawbacks of these standard incubation techniques are aggravated by the dynamic functioning of coastal lagoons. Many coastal lagoons respond in a rapid manner to short term changes in meteorological forcing, riverine cycles, tides, and temperature changes, with a marked variability in vertical mixing and lateral advection of water masses, horizontal and vertical turbidity gradients, frequent resuspension of surficial sedimentary matter, and thus also short-term changes in light regime (Okuda, 1981;Kjerfve, 1986;Lara-Lara et al., 1981;AlvarezBorrego and Alvarez-Borrego, 1982; Knoppers and Moreira, 1988). Interpretation of results obtained by the incubation techniques should thus include the previous history of physical events and preferentially be grouped and weighted according to their frequency of impact. The quality of results of the ‘4c bottle technique varies in accordance to the growth rates and diversity of phytoplankton and as such also to the trophic state of the system (Harris, 1986). For example, in oligotrophic systems where phytoplankton attains maximum growth rates and diver-

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sity, gross photosynthesis is given, and in eutrophic systems with lower growth rates and diversity, net photosynthesis is measured. As many coastal lagoons are characterized by seasonal shifts from mesotrophic to eutrophic conditions and from autotrophic to heterotrophic metabolism (Sournia et al., 1981; Nixon, 1982;Vaulot and Frisoni, 1986; Machado and Knoppers, 1988; Sandoval-Rojo et al., 1988), the quality of results by this technique shifts accordingly to the seasonal cycle of events. In mesotrophic to eutrophic lagoons, the use of the dissolved oxygen incubation technique may also pose some problems. Large diurnal temperature changes and primary production rates are often encountered in these lagoons and bring about marked variability in dissolved oxygen levels with in many cases super saturation of up to 180%.These conditions result in rapid bubble formation in the incubation bottles which might be lost upon fixation. Results thus become invalid and this process may only be counteracted by extremely short incubation times, the use of micro titration techniques, or concomitant measurements of dissolved oxygen and carbon dioxide (Parket al., 1958;Teal and Kannwisher, 1966;Sournia, 1977;Sournia et al., 1981; Carmouze, 1985). Comparisons between lagoons by ecophysiological indices based upon the biomass index chlorophyll a, such as the assimilation ratio (PB) and the light harvesting efficiency, hold some caveats. Apart from the fact that these indices constantly change in accordance with species composition and physiological state of populations (Knoppers, 1982; Hecky and Kilham, 1988), they are biased in coastal lagoons by the frequent resuspension of surficial sedimentary matter that contain pigment degradation products. Studies have shown that functional chlorophyll a estimates by trichromatic equations may, in comparison to high pressure liquid chromatography (HPLC), bring about a substantial overestimation (Jacobsen, 1978 and 1980; Fielding et al., 1988). In contrast, the often criticized monochromatic method by Lorenzen (1967) appears to provide a more suitable estimate for sedimentary chlorophyll (Fielding et al., 1988). The HPLC method with its wide spectrum of pigment analysis is undoubtedly most suitable for shallow coastal lagoons that present frequent resuspension of sedimentary matter, but i t remains a costly alternative to standard methods. Primary production of macrophytobenthos may be assessed by a variety of methods, including in situ estimates of biomass increment and dissolved oxygen and carbon dioxide evolution by the ‘Diurnal Curve’ method, and lightldark incubations with a larger volume than those applied for phytoplankton. Measurements of microphytobenthos and benthic algal mat productivity necessitate some special methodological adaptations. In situ incubations with small light and dark acrylic domes with both the inclusion and exclusion of the sediment surface is one alternative, and controlled in uztro experiments with intact surface sediment samples obtained by box or piston corers, is the other (Zeitzschel and Davies, 1978; Asmus, 1985; Fielding et

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al., 1988). Monitoring of parameter evolution during incubations may be conducted manually or via the use of continuous recording electrodes. However, care should be taken to avoid resuspension of matter by excessive stirring of the incubated overlying water which promotes excessive nutrient release from the bottom, the stirring rates in general, and prolonged incubation times which may alter redox conditions of the sedimenhwater interface. These methodological problems are aggravated by the presence of sedimentary organic rich flocculent layers, commonly found in coastal lagoons, and are similar to those encountered in studies of sediment oxygen consumption and nutrient release rates (Aller, 1980; H u m p and Martens, 1981; Nixon and Lee, 1981; Kemp et al., 1982; Balzer, 1984; Machado and Knoppers, 1988). An elegant experimental design has been introduced by Revsbeck and Ward (1984) with the application of micro-electrodes for the measurement of the vertical structure of physical and chemical parameters of surficial sediments. Primary productivity of both micro-phytobenthos and algal mats is assessed by continuous profiling over several diurnal cycles and with a vertical resolution ranging from microns to centimeters. Physical Setting and the Primary Producers

Coastal lagoons may be classified based on their geomorphological and hydrological criteria into choked, restricted, and leaky systems (Kjerfve, 1986).Accordingly, the main feature which controls their physical functioning is the number and nature of entrance channels and as such tidal exchange. Examples of various types of coastal lagoons are presented in Fig. 9.1. All types of autotrophic populations proliferate in coastal lagoons. According to dominance of autotrophic populations, three types of lagoons may be distinguished. At one end of the spectrum lie the phytoplankton based systems with some seasonal contribution of micro- and macro phytobenthos in shallower depths and with or without anaerobic autotrophic bacteria depending upon stratification of the water column and stagnation and anoxia of bottom waters. The intermediate position holds the benthic macroalgal and macrophyte based systems, and at the other end of the spectrum the algal mat systems of hypersaline lagoons and lagoons with intertidal zones that are subject to prolonged exposure during dry seasons. Table 9.1 presents a compilation of some physiographic data, annual primary production, and corresponding dominance of primary producers, for a variety of choked, restricted, and leaky coastal lagoons. Several approaches attempted to link the magnitude of primary production to simple physiographic and hydrological characteristics of lagoons and also estuaries. Included are studies on the dependance of phytoplankton doubling rates upon the tidal exchange ratio of estuaries (Ketchum, 1954),

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on nutrient loading and primary production versus the dynamics of river run-off, tides, and winds, and estuarindagoon morphometry (Jaworski, 1981; Boynton et al., 19821, and also on the tidal and biogenic matter exchange with the adjacent sea (Nixon and Lee, 1981; Nixon, 1988). The overall concepts are still conflicting and there seems to be no simple link between a single physical factor and the magnitude of primary production and autotrophic biomass. Some advances were made by Welsh et al. (1982), who compared simple parameters such as size, the area to volume ratio, and tidal characteristics with total primary production for four estuaries and five choked to restricted lagoons of the southern New England coast, USA. Total production, benthic production, and the benthic to pelagic production ratio were only strongly correlated to the area to volume ratio, and the tidal exchange ratio was of less significance. However, the physical characteristics between the systems were highly diverse and they were pooled 42?42'w

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86 4 7 ' W N18"401

N18' 20'

Fig. 9.1. Some typical lagoons of the world's continental coastline. Above, from left to right am top to bottom: choked Lagoa de Guarapina (Brazil), choked Lagune d'Ebri6 (Ivory Coast), restricted Laguna de TBnninos (Mexico),choked Laguna de Nichupte (NLS,MBxico). Opposite page, from left to right and top to bottom: restricted to leaky Biscayne Bay (USA), choked to restricted Bahia Falsa-San Quentin (MBxico),leaky Wadden Sea with its western (WWS)and eastern sections (EWS) and Yssel Meer and Ems-Dollard (Netherlands), choked Potter Pond (USA), and choked Peel-Harvey Estuary (Australia).

B. Knoppers

249

8lo50'W

-50'

N

30O24' 25O 30"

-

0 2 4

6 810

10

PE

TABLE 9.1 Physiographic data, mean annual primary production a n d dominant plant communities in coastal lagoons. CHK= Choked, Restricted, LKY= Leaky Lagoon

Type

Area (km')

Tropical and Subtropical Cochin Backwaters, India Ebrie, Ivory Coast

LKY CHK

500 556

Mukwe, Ghana Langebaana, S. Africa

Mean depth (m)

ResidencePrimary time production (weeks) (g c m-2 yr-'1

Dominant plant community

References

Qasim et al. (1969) Pages et al. (1981);Dufour (1984) Kwei (1977) Fielding et al. (1988)

3.5 4.5

14

124 235

phytoplankton phytoplankton

CHK RES

0.4 0.5 35 3.0

-

416 653

Guarapina, Brazil

CHK

6.5 1.0

phytoplankton macrophytes 54% microphytobenthos 26% phytoplankton phytoplankton

Patos, South Section, Brazil Cananeia, Brazil Saquarema, Brazil Cianaga Grande, Colombia Barra Navidad, Mexico Bojorguez, Mexico

CHK

-

CHK CHK CHK CHK

1.0 2.5 23 1.0 423 1.6 5 1.5 2.5 1.7

Chautengo, Mexico CHK El-Verde, Mexico CHK Huizache Caimanero, Mexico CHK

36 1.0 0.5 1.0 128 0.8

LKY

Mitla, Mexico Nichupte, Mexico

CHK CHK

36 50

2.0 2.2

Terminos, Mexico Urias, Mexico

RES CHK

2500 11

3.5 0.6

3 -

2 4 -

1-2 200 4-6 -

150 -

66 2 -

324 and 412 50 125 468 1555 242 489 248 521 453 551 183 228 620

RES=

Machado and Knoppers (1988);Moreira and Knoppars (1990) Proena (1990) Tundisi (1969) Carmouze et al. (1991) Hernandez and Gocke (1990) Sandoval-Rojo et al. (1988) Reyes and Merino (1991)

phytoplankton phytoplankton phytoplankton phytoplankton phytoplankton macrophytes, macroalgae and others phytoplankton Mee (1978);Mandelli (1981) phytoplankton, macrophytes Flores-Verdugo e t al. (1988) phytoplankton Arenas (1979);Moore and Slinn (1984) phytoplankton Mee (1978) macrophytes, macroalgae Merino et al. (1990) Reyes and Merino (1991) phytoplankton Day et al. (1982,1988) Phytoplankton RoblesJarero (1985)

Apalichola Bay, USA

LKY

Lower Barataria Bay, USA

RES

Biscayne Bay, USA

RES

Forleague Bay, USA Harrington Sound, Bermuda

LKY CHK

Temperate Beaufort, USA Pamlico River, USA

LKY RES

4.1 230

Newport River, USA South River, USA North River, USA Cape Lookout Bight, USA Alewife Cove, USA

RES RES RES LKY CHK

Bissel Cove, USA

-

Chimnteague Bay, USA

RES

Flax Pond, USA

CHK

Great South Bay, USA

RES

Hempstead Bay, USA

RES

688

576

phytoplankton macrophytes macrophytes 54% phytoplankton 46% phytoplankton phytoplankton macrophytobenthos

2.0

52

221-302

3-12 20

196439 304

-

-

40

27 25

1.0 2

1-2 10

-

1.1

0.2

1.2

1

0.7

4.0

1

310

46.5

360

4

4.2

240

phytoplankton

2.0

93 1.5 4.8 14.6

0.1

370

115 200-500

phytoplankton phytoplankton

119 288 70 50 348

820

phytoplankton phytoplankton phytoplankton phytoplankton phytoplankton 47% macrophytes 40% microphytobenthos 13% macrophytes 87% microphytobenthos 6% phytoplankton 7% -

180 1.6

1

419

1.6

7

455

0.5

1-2

281

macrophytes 73% phytoplankton 13% microphytobenthos 13% phytoplankton 85% macrophytobenthos 15% phytoplankton 63% macrophytes 15% microphytobenthos 22%

Eastabrook (1973);cited in Nixon and Pilson (1983) Day et al.(1973) Madden et al. (1988) Roman et al.(1983)

P

9

s

2

a

Randall and Day, 1987 Bodungen et al.(1982) Thayer (1991,1974) Day et al. (1978);Kuenzler et al.(1978) Fisher et al. (1982) Fisher et al. (1982) Thayer (1971) Fisher et al. (1982) Welsh et al. (1982)

Welsh et al. (1982)

Boynton (1974)cited in Nixon (1982) Moll (1974)cited i n Nixon (1982) Lively et al. (1983) Udell et al. (1969) Welsh et al. (1982)

(continued)

6: +

N

(continuatwn) Lagoon

VI

lu

Type

Area

(km')

Mean depth (m)

2000

12

Dominant Residence Primary time production plant (weeks) ( g ~ m - ~ y r - ' )community

Long Island Sound, USA Peconic Bay, USA Potters Pond, USA

LKY RES CHK

1.4

0.7

2-3

209 162-213 320

Charlestown Pond, USA

CHK

7.9

0.9

2-3

316

Wadden Sea, Netherlands

LKY

770

3.6

1-2

100-200

Ems-Dollard, Netherlands

LKY

480

3.0

2-4

130-400

-

-

2

-

phytoplankton phytoplankton macrophytes 63% phytoplankton 37% macrophytes 74% phytoplankton 13% macrophytobenthos 13% phytoplankton microphytobenthos phytoplankton

References

Riley (1956) Bruno et ul. (1982) Nowicki and Nixon (1985) Thome-Miller and Harlin (1984) Cade and Hegeman (1974) Cad& (1980) Cad& (1986a)and Colijn et al. (1983) b

Mediterraneau Biguglia, France Diana,France Maguio, France Thau, France Urbino, France Edku, Egypt Mariut, Egypt

9

E

CHK CHK CHK CHK CHK CHK RES CHK RES

289 183 225 204

24 6.9 14.6 13.4 61

-

1.5 6.0 1.0 4.5 5.0 1

-

475

-

1.0

-

1847

14.5 5.7 31.6 75 7.9

297

phytoplankton phytoplankton phytoplankton phytoplankton phytoplankton phytoplankton macrophytes macrophytes phytoplankton

Vaulot and Frisoni (1986) Vaulot and Frisoni (1986) Vaulot and Frisoni (1986) Vaulot and Frisoni (1986) Vaulot and Frisoni (1986) Samaan (1974) Aleem and Samaan (1969)

$.

2.3

a

z

6g. J

5-

B. Knoppers

253

together irrespective of differences in the degree of predominance of autotrophic populations. I n contrast, a strong link between primary production and the flushing rate was evidenced by Vaulot and Frisoni (1986)for five mediterranean lagoons, and between the trophic state and flushing rate by Knoppers et al. (1991)for six lagoons of Brazil. All lagoons were choked, similar in morphometry and hydrodynamics, and phytoplankton based. In both cases, the flushing rate was obtained from computations of the hydrological balance. From comparative studies in several Rhode Island lagoons, Thorne-Miller et al. (1983)suggested, that the distribution and predominance of macrophytes and macroalgae is related to the degree of enclosure and as such tidal flushing. Poorly flushed choked lagoons were characterized by the macrophytes Potamogeton sp. and Ruppia sp. and filamentous green algae, and well flushed restricted lagoons by the macrophyte Zosteru sp. and unattached green and red algae. I n these cases, salinity represents a selection factor which is ultimately related to tidal exchange. Figure 9.2shows a rough attempt to relate mean annual primary production to mean residence times of water and Figs. 9.3 and 9.4 primary production to depth for phytoplankton and macrophyte based lagoons, respectively. The data are compiled in Table 9.1, and information on residence times of water were estimated, considering the tidal prism and the fresh water inflow. It is obvious that comparisons should be conducted from computations of the hydrological balance, but for some exceptions, these are still lacking for coastal lagoons. Primary production of phytoplankton based

PP a20

t so0

x FT-200 FTA,l50

5

@FT-52 0 FTw61

Rrsidrna tlmr of wahr (wrrkr)

Fig. 9.2. Primary production and the residence time of water for phytoplankton based lagoons (dots) and macrophyte based lagoons (crosses). References and data sets are found in text (Table 9.1).FT = Residence time of water in weeks.

Aquatic Primary Production in Coastal Lagoons

254

c

t4 300B 200t

I

\.

1 0

: b,

:100-

:

. . --.- -.- .

Zm 14.5

0

.

an42

- 0 ,

d

.*.

E -

0

0 0

1

2 3 4 Mean depth ( f m

1

6

Fig. 9.3. Primary production and mean depth for phytoplankton based lagoons. References and data sets are found in the text (Table 9.1). PP = Primary production.

lagoons tend towards a dampened increase with the residence time of water, whereas macrophyte based lagoons tend towards high primary production with lower residence times and a linear increase with depth in lagoons without light limitation at the bottom. The large scatter of the data probably reflect the rudimentary estimations of the residence times of water, incomplete descriptions of the physical setting of the lagoons in works on primary production, measurements of primary production by different methods and sampling frequencies, and differences in the ecophysiological and ecological response of the populations. Similar comparisons for lagoons dominated by macroalgae are as yet unfeasible, because estimates of their annual primary production have been scant. However, the occurrence of macroalgae is consistent in shallow choked to restricted lagoons with substantial cultural nutrient loading, such as Venice Lagoon, Italy (Sfriso et al., 19871, the NichuptB Lagoon System, Cancun, MBxico (Merino et al., 1990; Reyes and Merino, 19911, Piratininga Lagoon, Brazil (Knoppers et al., 19911, and the Peel-Harvey, Australia (Hodgkin and Birch, 1982). Algal mats prevail in hypersaline lagoons, or in those characterized by large intertidal flats subject to prolonged exposure, such as Shark Bay and Spencer Gulf, Australia (Bauld et al., 1980; Bauld, 1984), Guerrero Negro and Ojo de Liebre lagoons, Mexico (Javor and Catenholz, 19841, and the Long Island Sound flats, USA (Burkholder et al., 19651.

255

B. Knoppers

(Q PPl847) I200

yean

dep(h(Zm)

Fig. 9.4. Primary production and mean depth for macrophyte based lagoons. References and data sets are found in text (Table 9.1).PP = Primary production.

In many tropical and sub-tropical choked lagoons subject to marked dry/wet seasons, closure and rupture of the tidal inlet, resulting from changes in river flow, is a major factor regulating levels of primary production and succession of autotrophic populations. Yaiiez-Arancibia et al. (1985) summarized the sequence of physical+cological events for these types of lagoons (Fig. 9.51, which are often encountered along the Pacific coast of MBxico and western Australia (Mee, 1978; Hodgkin and Birch, 1981; Mandelli, 1981; Flores-Verdugo et al., 1988; Sandoval-Rojo et al., 1988). Conditions may vary between mesohaline to hypersaline, mesotrophic to eutrophic, and nutrient and light limitation. River flow may, depending on its time scale of impact, either enhance photosynthesis by nutrient input or suppress by light limitation due to a n increase in turbidity (Randall and Day, 1987). In choked multiple basin lagoons with a single access to the sea at its exterior basin (Lankford, 1976; Kjerfve, 1986), spatial variability in functioning is a marked feature. Depending on the geomorphological configuration of the basins, the width of interconnecting channels and depth of sills between individual basins, differences in salinity, residence times of water, structure of the water column (i.e homogeneous or stratified), light and nutrient limitation, and primary production levels and species assemblages, are encountered (Mee, 1978; Nixon, 1982; Thorne-Miller et al., 1983; Dufour, 1984; Knoppers et al., 1984; Carmouze and Caumette, 1985; Odebrecht, 1988; Reyes and Merino, 1991).

256

Aquatic Primary Production in Coastal Lagoons

i I

i

y.i !

2.4.

2.5-

>

S 35 PPM E> P*R+F

S I5 A 34PPM

S 4 I I PPM E+F< P * R R

A

S

0

N

D

E

F

M

A

M

J

J

A

m Fig. 9.5. A conceptualmodel of seasonal stages ofthe hydrological cycle, nutnents, and primary production for tropical to sub-tropical lagoons with typical drybet seasons of the Pacific Coast of Mexico. S = Salinity, P = precipitation, E = evaporation, R = river M o w , F = flux of water by percolation through the sand barrier, T = tidal exchange, F" = orthophosphate, N = total inorganic nitrogen, Prim.Prod. = primary production. Adapted from YAiiez-Arancibia et al. (1985).

Nutrient Sources and Sinks

Information on the short temporal and seasonal behavior of nutrients has been reviewed by Nixon (1982), estimations of external nutrient supply, internal recycling, and accumulation are relatively scant, and complete nutrient budgets are non-existent for coastal lagoons. Nutrients are imported to coastal lagoons via the atmosphere, rivers, direct land run-off, groundwater seepage, and the sea, and exported via tidal exchange, sediment accumulation, and denitrification. An additional source is nitrogen fixation by certain species of cyanobacteria, which can counteract some of the loss by denitrification. Internal sources are pelagic and benthic nutrient regeneration. The relative importance of a nutrient source varies considerably between lagoons. Choked coastal lagoons with well developed drainage basins in the humid tropics and temperate regions receive their major external nutrient supply from rivers, the remaining outside sources may play a secondary role (Jaworski, 1981; Mandelli, 1981; Boynton et al., 1982). Examples of such lagoons are the Peel-Harvey estuary, Western Australia (McComb et al., 19811, Guarapina Lagoon, Brazil (Moreira and Knoppers, 19901, Ebrie Lagoon, Ivory Coast (Dufour, 1984), and the Backwaters of Cochin, India (Qasim, 1979). In restricted to leaky lagoons such as along some sections of

B. Knoppers

257

the eastern coast of USA (Thayer, 1974; Fisher et al., 1982; Roman et al., 1983; Nixon and Pilson, 1983) and the northern European coast (Cadee, 1986a; Baretta and Ruardij, 1988),riverine and marine sources are both of importance. In contrast, choked, restricted, and leaky lagoons of arid regions as found along the Pacific coast of MBxico (Gilmartin and Relevante, 1978), and southwestern Australia (Hodgkin and Lenanton, 19811, the major fraction of new nutrients is supplied by the sea and the atmosphere. Of particular interest are those lagoons that lie in regions of coastal upwelling. A well studied example is San Quentin Bay, MBxico, (Lara-Lara et al., 1980; Farfan and Alvarez-Borrego, 1983),which receives new nutrients during intrusion of upwelled water. Nutrient Supply by Atmosphere and Groundwater

In general, little is known on the supply of nutrients by the atmosphere and groundwater seepage to coastal lagoons. Direct estimates of atmospheric input have been made for some estuaries and lagoons, but input is so variable making generalizations unfeasible. Nitrogen input from the atmosphere compared to total nitrogen supply from other sources to the Rhode Island Ponds and also Buttermilk Bay, USA, range between 4 and 9% (Nixon and Pilson, 1983; Lee and Olsen, 1985;Valiella and Costa, 1988). In regions subject to acid rain deposition, nutrient input is expected to be higher. Lee (1977) studied groundwater seepage velocities at Beaufort, North Carolina, and observed a decrease with distance from shore and an inverse trend with water elevation. Changes in pore water conductivity and hydraulic pressure have been shown to be responsible for variability in seepage velocities in Great South Bay, New York (Bokuniewicz, 1980). In some lagoons such as TBrminos lagoon, Mdxico (Stevenson et al., 1988), some of the Rhode Island ponds, Massachusetts (Lee and Olsen, 1985), and Buttermilk Bay (Valiela and Costa, 19881, water seepage supplies a considerable fraction of nutrients, and may surpass stream and direct run-off input. However, this large contribution is also due to groundwater contamination by domestic discharge. In Potters Pond, Rhode Island, low inorganic phosphate and high N/P ratios in the water column are influenced by a high input of nitrate and low phosphate from groundwater (Nowicki and Nixon, 1985). Riverine Input

Some examples of total areal inorganic nitrogen loading and its percentage contribution to the demand of primary production are presented in Table 9.2. In cases where information is available, the contribution by the atmosphere, groundwater seepage, and tidal exchange are included in the

TABLE 9.2 Dissolved inorganic nitrogen (DIN) load, nitrogen demand by primary production a n d percentage supply to demand of primary production in coastal lagoons. Major external DIN sources a r e given as: R = river, S = sewage, P = precipitation, M = marine, GW = ground water. Asterisk (*) refers t o a mean value obtained from a given range in the literature. DINload (mmol N m-' yr

Demand by primary Demand production supplied (mol N m-' year-') (%)

Harrington Sound, Bermuda Charlestown Pond, USA

136 561

3.86 3.12

4 18

Ninigret Pond, USA

340

2.98

11

Potter Pond, USA

710

3.18

22

Pamlico Sound, USA Long Island Sound, USA Apalichola Bay, USA Barataria Bay, USA Laguna de Terminos, Mexico Lagoa Guarapina, Brazil

860 400 560 570 20 313

4.41* 2.58 4.53 4.53 2.87 5.18

20 15 12 12 1 6

Lagoa Urussanga, Brazil Lagoa Fora, Brazil

26 156

5.89 5.73

<1 3

Lagune Mauguio, France Lagune Thau, France Ems-Dollard, Netherlands Lagune d'Ebri, Ivory Coast

291 582 414 410

2.57 2.84 3.77* 2.97

11 20 11 14

Lagoons

Main sources

~

References

Bodungen et al. (1982) Nowicki and Nixon (1985);Lee and Olsen (1985) Nixon and Pilson (1985);Thorne-Miller et al. (1983) Lee and Olsen (1985);Thorne-Miller et al. (1983) Davies et al. (1978);Nixon and Pilson (1983) Nixon and Pilson (1983);Riley (1959) Nixon and Pilson (1983) Day et al. (1978);Nixon and Pilson (1983) Day et al. (1988);Stevenson et al. (1988) Moreira and Knoppers (1990);Knoppers et al. (1990) Costa-Moreira (1989);Carmouze et al. (1991) Carmouze et al. (1991);Knoppers et al. (1991) Vaulot and Frisoni (1986) Vaulot and Frisoni (1986;) Baretta and Ruardij (1988);Cad& (1980) Dufour and Slephoukha (1981);Dufour (1984)

B. Knoppers

259

overall computations of nitrogen areal loading. The data show, that apart from few exceptions where cultural eutrophication is marked, fresh water nitrogen supplies between 5 and 20% of the areal primary production demand. Considering that most of the examples are from humid regions, the overall areal nitrogen loading in relation to primary production demand is lower than expected. However, dissolved and particulate organic matter, after its oxidation, may in comparison, also be a substantial source of new nutrients (Smith and Mackenzie, 1987), but its contribution to coastal lagoons demands more studies. It follows, that the major nutrient fraction which sustains primary production in coastal lagoons comes from the autochthonous decomposition of organic matter. Autochthonous Supply

Lagoon studies on sediment nutrient release to the water column are not always complemented with studies on primary production. A compilation of sediment release rates of total inorganic nitrogen and orthophosphate and their percentage contribution to the demand of primary production is given in Table 9.3. Although data are scarce, total inorganic nitrogen supply seems to sustain 10-30% of primary production demand and orthophosphate 3-30%. Beholding some exceptions, such as Forleague Bay, Colorado Lagoon and Guarapina Lagoon, N:P release ratios agree fairly well with the demand according to the Redfield ratio of phytoplankton (Table 9.3). The contribution by the dissolved organic fraction has been noted to be of significant importance in shallow water sediments (Nixon et al., 1976; Teague et al., 1988). Reports on pelagic nutrient remineralization and its contribution for the sustenance of regenerated production are almost absent for coastal lagoons. Only indirect estimates can be computed. Considering that 5-20% of primary production is supported by external inorganic nitrogen sources (Table 9.2) and 10-30% by sediment release (Table 9.31, the largest fraction must be supplied by remineralization in the water column or supplemented in some cases by nitrogen fixation and it seems that this is a major feature characterizing coastal lagoons. Nitrogen Fixation

The only lagoon for which both data on planktonic nitrogen fixation and freshwater nitrogen input is available, is the Peel-Harvey, Australia. Nitrogen fixation by the Cyanobacterium Nodularia spumigena amounted to 17%of total nitrogen input to the lagoon (Huber, 1986).Although eutrophic lagoons often have cyanobacteria, few measurements on nitrogen fixation

N

TABLE 9.3

m

0

Sediment release rates of dissolved inorganic phosphate (DIP) and nitrogen (DIN) and their contribution to the demand by primary production in some lagoons. Values of Forleague Bay include the dissolved organic fraction.

Lagoon

DIP (pmol m-' d-')

DIN (mmol m-' d-')

Primary production demand supplied in %O by

DIP Potter Pond, USA Long Island Sound, USA South River, USA Neuse River, USA Forleague Bay, USA (upper)

408

Forleague Bay, USA (lower)

720

Colorado Lagoon, USA Hamngton Sound, Bermuda Guarapina, Brazil

548 88 175

Kanehoe Bay, USA Maribago Reef, Phillipines Bowling Green Bay, Aust.

12 23-63 -23-28

16.7 38-120 150 340

1.23 0.46-1.50 2.71 5.38 -13.3

4 8-27 24 31 156

DIN 14 7-2 1 27 31

106

120

4.66 2.11 0.98

13 25

20 9

1.18 -

3 -

21

13.0

4.15-0.9

References

-

-

Nowicki and Nixon (1985) Aller (1980b) Fisher et al. (1982b) Fisher et al. (1982b) Randall and Day (1987);Teague et al. (1988) Randall and Day (1987);Teague et al. (1988) Murphy and Kremer (1985) Bodungen et al. (1982) Machado,l989; Machado and Knoppers (1988) Smith (1979) Balzer et al. (1985) Ullman and Sandstroem (1987)

b

9

8c. T

2. 4

z

Eg. 3

5.

B. Knoppers

261

and observations on species having heterocysts have been reported. The fact that nitrogenase activity has been detected in Oscillatoria species lacking heterocysts (Mee et al., 1984) and for other picoplanktonic cyanobacteria, suggest significant phytoplankton nitrogen fixation in coastal lagoons. A review by Howarth et al. (1988) demonstrates, that nitrogen fixation in seagrass beds and phytoplankton and algal mat based lagoons (Bautista and Pearl, 1985)attain significant rates rangingfrom 0.1 to 5 mol m-2year1. Nitrogen fixation by heterotrophic bacteria is considerable at seagrass rhizospheres and in organic rich sediments with or without macrophytes and by autotrophic heterocyst cyanobacteria under more stable physical conditions with low turbulence and mixing, stratified water columns, and on surficial sediments (Carr and Whitton, 1982). There is evidence, that nitrogen fixation by epiphytes may supplement nitrogen to seagrasses during inorganic nitrogen depletion in El Verde Lagoon, M6xico (FloresVerdugo et al., 1988). Nutrient Loss by Denitrification

Denitrification rates of surficial sediments in estuarine and coastal waters may attain 50-250 pmol N m-2 h-l and molecular nitrogen fluxes may yield 15-70% of total nitrogen flux. The loss of nitrogen via denitrification may exceed input via nitrogen fixation in various systems and seems to be highest in sediments of eutrophic waters (Seitzinger, 1988). The major source is nitrate from nitrification in sediments but nitrate may also be made available from ground water seepage. Denitrification is a major process responsible for nitrogen limitation in estuaries and coastal waters, and as such probably also in coastal lagoons. In lagoons with considerable nitrogen fixation, nitrogen loss by denitrification could to a certain extend be counteracted. Nutrient Loss by Accumulation in Sediments

The process of accumulation of matter i n coastal lagoons deserves some special attention as it not only represents a considerable loss of biogenic elements from the pelagic benthic cycle but also the mechanism that describes the role of coastal lagoons as efficient traps in the transfer of matter between land and sea. A variety of factors make a comparison of accumulation rates between coastal lagoons problematic. The computation is conducted by quantification of changes in the sedimentary record of bulk properties as a function of the time of impact of natural or catastrophic events a n d o r the sedimentary record of various radioisotopes such as 137Cs, 21Tb,and W, and necessitates information on sediment composition, density, and porosity (Lerman, 1988). Large spatial variations in sedimentation rates and sediment composition, and sediment resuspension by tidal ex-

Aquatic Primary Production in Coastal Lagoons

262

change and winds, and bioturbation, bring about many caveats in computations (Benninger et al., 1979;Aller, 1980; Sharma et al., 1986). Furthermore, in cases where computations of accumulation rates are feasible, complementary data on primary production are usually lacking. The data compiled in Table 9.4 have to be considered as a tentative approach to elucidate the relationship between primary production and accumulation of carbon in some coastal lagoons. The element carbon has been chosen instead of nitrogen as a reference, because it is the more accessible in the literature. Data on accumulation rates in Table 9.4 were either taken directly from the literature or from own computations. For Venice Lagoon, references on total annual primary production are not available, except for the work on phytoplankton production by Vatova (1969) and on the seasonal dry weight cycle of macroalgae by Sfriso et al. (1987). In contrast, Barataria Bay estimates of both accumulation rates and primary production represent a rough mean because of the large spatial variation of both processes. It seems that carbon accumulation rates attain TABLE 9.4 Sedimentation, carbon accumulation, and primary production rates in some lagoons. References are for data on sedimentation rates a n d in some cases carbon accumulation rates. References for primary production see Table 9.1.Range of data demonstrate the spatial variation in the systems. Lagoon

Sedimentation Accumulation Primary rate rate production (cm yr-I) (g C m-' yr-') (g C m-2 yr-')

Barataria Bay, USA Long Island Sound, USA Guarapina, Brazil

0.7-1.3

mean 170

360

0.47

0.3 0.2-0.6 0.1-0.3

mean 26

209

0.12

30-90

324

0.09-0.27

Venice, Italy

0.3-0.7

8-47

147

0.05-0.32

34

150

0.22

0.9-1.1

80-100

300

0.26-0.33

0.1-0.3

7-55

158

0.04-0.34

0.15

6-14

310

0.02-0.05

Marsdiep, Wadden Sea Ems, Wadden Sea Kiel Bight, Germany Narragansett Bay, USA

Accumulation to References primary production ratio Hatton et al. (1982) Berner (1978); in Aller (1980) Patchineelam et al. (1988); Barros (1986) Donazollo et al. (1982);Pavoni et al. (1987) Cadee (1978, 1980) Cade6 (1980, 1986) Balzer et al. (1986) Nixon and Pilson (1983)

B. Knoppers

263

a range between 10 and 25% of primary production and burial rates tend to increase with primary production. Autochthonous produced matter represents in many cases the principle source for accumulation as reflected by some examples on the total organic carbon isotopic (l2P3Cratio) composition of surface sediments for some lagoons (Table 9.5). TABLE 9.5 Total organic carbon isotopic ('%/13C)composition in surficial sediments of some lagoons. PPL = Phytoplankton; MAP = Macrophytobenthos. Lagoon

S'3C%~of Sediment Source Material References

Alvarado, Mexico -21.8 Carmen y Machona, Mexico -23.0 to -29.2 Madre, Mexico

-21.1 to-23.9

PPL MAP Sewage

PPL

Botello and Macko (1982) Botello and Macko (1982) Botello and Macko (1982)

Mangrove Tamiahua, Mexico -20.5 Terminos, Mexico -19.0 to -22.5 Lower Barataria Bay, USA -22.7 Barataria Bay, USA Narragansett Bay, USA Guarapina, Brazil

-13.3 to -26.3 -21.7 -19.1 to 20.3

PPL PPL PPL MAP C3 ta C4 plants

PPL PPL

Botello and Macko (1982) Botello and Macko (1982) DeLaune and Lindau (1987) DeLaune and Lindau (1987) Gearing et al. (1984) Erlenkeuser and Knoppers (unpublished data)

New Versus Regenerated Production

New nutrients introduced to the system from external sources sustain the fraction of new primary production (PneW) of total primary production (PJ and nutrients supplied by internal cycling maintain the fraction of regenerated production, i.e Preg (Dugdale and Goering, 1969, Eppley and Peterson, 1979; Platt et al., 1989).The contribution of new production to total production is described by the f ratio which attains values of 0.14 for oceans, 0.2 for upwelling areas, and 0.17 for coastal zones in general. Platt et al. (19891, indicated that the f ratio for the latter two systems is still too low. Within the general context of the coastal zone, organic matter after its oxidation, may be the major source supplying new nutrients (Smith and Mackenzie, 1987). If this is the case, the fresh water inorganic nitrogen supply which satisfies 5-20% of areal primary production in coastal lagoons (Table 9.21, is too low a range to estimate an f ratio for coastal lagoons. More information on the contribution by nitrogen fmation, precipitation, groundwater seepage, etc., is needed. Assuming that the range for carbon accumulation is within

264

Aquatic Primary Production in Coastal Lagoons

10-25% of primary production for some lagoons and primarily originates from autochthonous production (Tables 9.4 and 9.5) a n f ratio could be suggested. A further alternative to estimate P,,, could be based upon the external phosphorus supply instead of nitrogen, as suggested by Smith (1988) for open ocean atolls and reef lagoons. However, many coastal lagoons are subject to cultural eutrophication with excessive phosphorous loading in relation to nitrogen, and as such are being driven towards nitrogen limitation. Autotrophic Versus Heterotrophic Metabolism Smith (1988) detected a trend towards a balance between autotrophism and heterotrophism for tropical reef and open ocean atoll lagoons. A similar trend for coastal lagoons in general has as yet not been reported, although lagoons are either balanced or tend towards slight heterotrophism. Reyes and Merino (1991) presented a compilation of ten lagoons studied by the ‘Diurnal Curve’ method containing primary producers ranging from phytoplankton, benthic macroalgae, to macrophytes. With the exception of some lagoons dominated by Thalassia spp. flats, the majority were, on a n annual basis, balanced to slightly autotrophic. Other observations were obtained for various lagoons of the Mexican coast such as Chautengo (Mee, 19781, T6rminos (Day et al., 19821, El Verde (Flores-Verdugo, 19851, Estero de Urias (Robles-Jarero, 19851, and Barra de Navidad (Sandoval-Rojo et al., 1988), in Colombia for La Cienaga Grande de Santa Marta (Hernandez and Gocke, 19901, and of the southeastern Brazilian coast for Saquarema (Carmouze et d . ,1991). Some presented a balanced metabolism others slight autotrophism or heterotrophism. It is the seasonal shift between autotrophic and heterotrophic metabolism that characterizes many coastal lagoons. This becomes particularly evident in choked lagoons of the tropical and sub-tropical zones. High peaks of respiration have been reported during the dry seasons and attributed to the accumulation of organic material during the wet season (Sandoval-Rojo et al., 1988; Hernandez and Gocke, 1990; Carmouze et al., 1991), and dystrophy accompanied by mass mortality of algae and also fish kills may, at some stage of the seasonal cycle, be a characteristic interannual reoccurring event (Bouti6re et al., 1982; Hodgkin and Birch, 1986). Case Studies on Primary Production

It was the intention in the preceding sections to present some mechanisms responsible for primary production control. Some links have been established and it has become obvious t h a t although budgetary assertions are useful as a subsidy to establish a n overall ecological concept for lagoons, they are at present limited by information. The drive to maintain a holistic

B. Knoppers

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approach to coastal lagoons is often confronted by reductionism because each lagoon is, in spite of regional commonalities, rather specific in its functioning and human impact. In order to demonstrate their high diversity, some exemplary case studies are presented in accordance to geographical location, geomorphological configuration, predominance of autotrophic populations, and particularly the history of human impact.

Phytoplankton Based Lagoons Typical examples of phytoplankton based lagoons are numerous (Table 9.1).They may harbor other primary producers during some stage of the annual cycle, but their contribution to total areal production is insignificant in comparison to phytoplankton. Tropical and subtropical choked lagoons with a dampened seasonality in precipitation usually present a unimodel pattern in primary production with highest rates in late summer (Nixon, 1982), others with marked dry/wet seasons a uni- to bimodal pattern (Yaiiez-Arancibia et al., 1985, Fig. 9.5),and temperate choked to leaky lagoons a bimodal pattern with light limited production in winter (Nixon, 1982,Cade6 and Hegeman, 1977).

A Choked Lagoon: Lagoa de Guarapina, Brazil Guarapina lagoon is an example of a typical small sub-tropical shallow choked mesohaline eutrophic phytoplankton based lagoon (Fig. 9.1,Table 9.1)with a humid climate (Kjerfve et al., 1990).It lies approximately 50 km east of the city of Rio de Janeiro, and eutrophication is quasi-natural (Knoppers et al., 1990). Annual phytoplankton primary production (W technique) amounts to 412 g C m-2 y e a r ' and net primary production (oxygen technique) to 324 g C m-2 year', and follows a typical unimodal pattern with highest rates in summer and autumn and lowest in winter and spring (Fig. 9.6,Machado and Knoppers, 1988;Moreira and Knoppers, 1990).Phytoplankton succession is a marked feature with dinoflagellates (Gyrnnodiurn sp. and Prorocentrum minimum) during winter and cyanobacteria (Oscillatoria minima and Synecochoccus sp.) in summer. Minor banks of the macroalga Cladophora uagabunda proliferate sporadically between winter and the end of summer. Pelagic remineralization accounts for the largest nutrient source and probably also nitrogen fixation for the sustenance of the summer biomass increment by cyanophytes. The Tripton carbon to phytoplankton carbon ratio (TC:PPC ratio) is 3:l during winter-spring and reverses in summer-autumn suggesting that suspended detritus is a n important stock for autochthonous nutrient remineralization in early summer (Knoppers and Moreira, 1990).An elevation of the trophic state of the lagoon is being counteracted by control of tidal exchange through periodic dredging of the

Aquatic Primary Production in Coastal Lagoons

266

I60

4 200. \

O

A,

'4 \ A'S'O'N'D 'J 'F'Y'A'M'J'J 1988 1988

Fig. 9.6.Annual cycle of primary production (dots)and chlorophyll a (crosses)in the subtropical choked Lagoa de Guarapina (Brazil).Modified from Moreira and Knoppers (1990).

tidal inlet, open since 1951. Sediment profiles on the composition of isotopic carbon ('2C/l3c) suggest, that prior to the permanent breach of the tidal inlet, a higher fraction of land derived plant material accumulated in the lagoon.

A Choked Multiple Basin Lagoon: Ebrib, Ivory Coast, Africa Ebri6 Lagoon is a typical example of a n elongated mesotrophic to eutrophic multiple basin choked phytoplankton based system, with a single outlet to the sea (Fig. 9.1).The lagoon is marked by a large regional gradient in precipitation, freshwater and nutrient inflow, salinity, residence times of water, nitrogen to phosphorous limitation, primary production, phytoplankton species assemblages, and anthropogenic impact. Nutrient loading via domestic discharge from Abidjan is extensive (Dufour and Slepoukha, 1981; Dufour, 1982). P limitation occurs in the areas bordered by forests, minor N limitation in the Savannah bordered areas, and clear N limitation in the estuarine-oceanic part. The mean net phytoplanktonic production amounts to 235 g C m-2 y e a r ' (Dufour, personal communication) and highest primary production rates and phytoplankton biomass were observed during the wetter periods (Fig. 9.7). Diatoms and Cyanophytes are the main populations. As the major section of the lagoon is mesotrophic to eutrophic, primary production is often light limited in the central deeper areas which are characterized by sporadic stratification and anoxia (Pages et al., 1981; Carmouze and Caumette, 1985). Cyanophytes may proliferate upon the halocline and filter out nutrients supplied from below. Species composition and the structure of the food web exhibit marked spatial variability, with 80% of the phytoplankton biomass being consumed by herbivores in the interior sections, and 25% in the exterior parts (Dufour, personal communication). The exterior section is subject to severe cultural eutrophication, but dilution effects by tidal exchange have managed to prolong a n elevation of the trophic state. The lagoon is slightly autotrophic.

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" J F M A M J d t M S O N D

Fig. 9.7. Annual cycle of primary production in the tropical choked Lagune d'Ebrie (Ivory Coast). Polluted area is situated close to Abidjan City and the rural area refers to a central section of the lagoon. Adapted from Pages et al. (1981) and Dufour and Slephoukha (1982).

A Restricted Lagoon: Laguna de Te'rminos,Mbxico Over the last two decades Laguna de TBrminos has been subject to intensive ecological research (Yaiiez-Arancibia and Day, 1988). It is the largest restricted Mexican lagoon and it is connected to the Gulf of MBxico by two tidal inlets (Fig. 9.1). The system is polyhaline, its tropical humid climate is characterized by typical dry/wet seasons, and turbidity due to resuspension of matter is relatively high. Mangrove swamps fringe the lagoon and the north and southeastern sections of the littoral zone harbor seagrass beds dominated by Thalassia testudinum, and also Syringodium filiforme and Halodule haubertii. Productivity of the Thalassia beds is highest during the spring dry season (Moore and Wetzel, 1988). Their growth is light limited below approximately 1 m depth, which explains its lack of further proliferation towards the deeper sections of the lagoon (Kemp et al., 1988). The primary production on a per unit area basis is approximately 4% of phytoplankton primary production which attains 219 g C m-2 y e a r ' (Day et al., 1982; Moore and Wetzel, 1988). Phytoplankton primary production rates are higher in nearshore waters close to seagrass beds, mangroves, and river mouths, as a consequence of stimulatory effects by higher nutrient levels and humic substances (Day et al., 1982;Hopkinson et al., 988). Seasonal variability in primary production is inversely related to nutrient N:Pratios, with highest rates during the wet season and lowest in the dry season (Botello and Mandelli, 1975; Day et al., 1988). The lagoon seems to tend towards slight heterotrophism in shallow areas, but in all, presents a balanced metabolism. Cultural eutrophication is at its onset, but is minimized by tidal exchange and the large volume of the lagoon.

Aquatic Primary Production in Coastal Lagoons

268

A Leaky Lagoon: The Wadden Sea, Netherlands The Dutch Wadden Sea forms part of the European Wadden Sea which extends over approximately 600 km along the Dutch, German, and Danish coasts. It is a n example of a temperate shallow leaky tidal flat lagoon consisting of a series of tidal basins protected by a string of barrier islands from the North Sea (Fig. 9.1). The most impacted areas are the Marsdiep in the west, separated from the Ijsselmeer by a large dam in 1932, and the Ems-Dollard in the east, subject to cultural eutrophication by nutrient loading from agricultural and domestic run-off via the Ems river. Monitoring of phytoplankton and benthic microflora production, carbon dynamics and ecological modelling has been extensive (CadBe and Hegeman, 1974 and 1977; Cadee, l980,1984,1986a, 198613,1986~;Dankers et al., 1984; Postma, 1985; Vosjan, 1987; Baretta and Ruardij, 1988). The water column of the lagoon is constantly mixed, shallow sediments are frequently resuspended by mesoscale tidal action, and primary production is limited by light due to turbidity in deeper sections and during winter. The mean annual range of phytoplankton primary production, established over a period of twelve years in Marsdiep and the central section, varies from 145 to 200 g C m-2 y e a r ' and in the Ems-Dollard from 200 to 300 g C

-2 -I 1.8- 0C.m.d ID-

Q80

b

J F M A Y J J A S O N D

Fig. 9.8. Annual cycles of primary production of (a) phytoplankton and (b)microphytobenthos in the temperate leaky Western Dutch Wadden Sea. On an areal basis, phytoplanktonprimary production is the main component of total production. Adapted from Cadee and Hegeman (1977) and Cadee (1980).

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m-2year-l. In the inner mud flat regions microphytobenthos accounts for a n interannual range between 58 and 177 g C m-2 yea+ (Cadbe, 1980). The contribution of macrophytes by Zostera spp. is negligibly small with 5-26 g C m-2 year' (Goedheer, 1977; in Cadbe, 1980). The seasonal cycle of phytoplankton primary production differs considerably from microphytobenthos (Fig. 9.8). The former tends towards a bimodal pattern with a spring and autumn bloom and several intermediate minor blooms during summer, whereas the latter follows a unimodal pattern with its peak in summer. In spring diatoms bloom, followed by predominance of the flagellate Phaeocystis pouchetii, and flagellates and diatoms alternate towards winter. Interannual seasonal patterns and spring bloom species composition vary to a great extend as a function of changes in the spring onset of favorable light conditions and water column stability (Cadke, 1986). The menace of cultural eutrophication in both the Wadden sea and the North Sea is now of large debate by recent occurrences of the Haptophycea species Phaeocystis sp. and Chrysochromulina sp. with mass blooms causing nuisance effects for the tourist and mussel industry. Macrophyte Based Lagoons Macrophyte based lagoons have been well studied (Table 9.1). In many cases, macroalgae are entangled with macrophytes during a period of the seasonal cycle, posing problems in estimating total primary production. Epiphytic communities often thrive upon the macrophytes and may be a n important component of primary production. Choked Lagoons: The Rhode Island Salt Ponds, USA

The Rhode Island salt ponds, northeastern USA (Fig. 9.1, Table 9.11, include Charlestown-Ninigret-Green Hill, Trustom, Potter, and Point Judith Ponds and are examples of temperate shallow choked to restricted polyhaline lagoons (Lee and Olsen, 1985). The dominating freshwater nutrient source is given by groundwater seepage to all ponds. The larger ponds exhibit an average annual primary production of around 300 g C m-2 y e a r ' with seagrasses, mainly Zostera spp., accounting for a n average of 50%, macro- and microalgae 40%, and phytoplankton 10% of total production. The ponds marked by larger tidal exchange, such as Ninigret and Point Judith, are predominated by Zostera sp. banks and entangled macroalgae and the more closed ponds including Green Hill and Trustom, by Ruppia sp. and Potamogeton spp. and various filamentous macroalgal banks. The seagrasses follow a typical unimodal seasonal pattern in biomass development in contrast to the more dampened mode of the macroalgae, as reflected by an example for Ninigret Pond (Fig. 9.9). In this pond, a

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Aquatic Primary Production in Coastal Lagoons

Fig. 9.9. Annual biomass cycle of the macrophyte Zosteru marina and macroalgae in the temperate choked Ninigret Pond (USA).Adapted from Thorne-Milleret al. (1983).

sequence of changes occurred since its permanent opening in the fifties. Previously, macrophyte species of Potamogeton sp. and Ruppia sp. were predominant thereafter, gradual replacement by Zostera sp. occurred. It is postulated that Zostera sp. growth was favored by light availability from a n increase in transparency, by higher salinity, and enhanced nutrient exchange from intensive lateral advection of water (Thorne-Miller et al., 1983; Thorne-Miller and Harlin, 1984).A shift from Chlorophytes to Rhodophytes was also observed as marine influence augmented. Cultural eutrophication seems to have promoted macroalgae, but the impact by breachway opening, as manifested in changes of residence times of water and salinity, remains the more obvious reason for species composition changes and maintenance of macrophytes (Lee and Olsen, 1985).

A Choked Lagoon: Nichupte-Bojorguez Lagoon, Cancun, Mkxico Nichupte-Bojorguez lagoon (Fig. 9.11, Cancun, Yucatan Peninsula, Mexico, is a multiple basin choked system with a sub-humid climate (Merino et al., 1990). In its natural state, it was dominated by a Thalassia testudinum community and oligotrophic waters (J6rdan et al., 1978) but, over the last two decades, cultural eutrophication has particularly affected the subsystem of Bojorguez. Spatial variation in sewage discharge has promoted community diversity and gradual substitution of the Thalassia sp. beds. Ruppia maritima-Halodule wrightii epiphyte communities thrive close to sewage discharge areas, Lyngbia sp. and Oscillatoria sp. cyanophytic algae are distributed in deeper areas over organic rich sediments, and Thalassia testudinum is found in less affected regions (Reyes and Merino, 1991). Macroalgae of the genus Chaetomorpha and Acetabularia also proliferate in

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the lagoon. Despite cultural eutrophication, annual mean autotrophic and heterotrophic metabolism is still balanced for the entire lagoon (Reyes and Merino, 1991). The Ruppia-Halodule community is predominantly autotrophic, and the benthic algae and seagrass beds shift seasonally between autotrophic and heterotrophic metabolism. Macroalgal-based Lagoons Macroalgal-based lagoons have received little attention. Some well studied examples are the Peel-Harvey, western Australia (Hodgkin and Lenanton, 1981), Venice Lagoon, Italy (Sfriso et al., 19871, and also Lakes Edku, Mariut, and Menzalah, Egypt (Aleem and Samaan, 19691, which also harbor macrophytes. However, reports on lagoons with macroalgae as the sole primary producer are absent, because the majority of dominant species are perennial. The functioning of algal banks resembles phytoplankton based systems with stratified water columns and algal mats, the only difference being the magnitude of vertical scale. Algal banks are characterized by marked vertical gradients of physical and chemical properties at a decimeter to centimeter scale. The top layer is oxic and represents the site of primary production and consecutive lower layers become more oxygen depleted and are sites of remineralization of matter produced at the top. The vertical density structure is usually induced by temperature as a consequence of light attenuation by the dense algal top layer. Primary production is to a large extend driven by internal recycling of nutrients which are transported to the surface by diffusion, sporadic lateral advection, and activity of micro- and macrofauna.

A Choked Lagoon: The Peel-Harvey, Western Australia The Peel-Harvey system consists of two interconnected basins and a single tidal channel to the sea (Fig. 9.1). It is subject to a subtropical dry/wet climate (Hodgkin and Birch, 1982). Since the sixties, severe cultural eutrophication, in the form of excessive nutrient loading (especially phosphorus) from fertilizer usage in the drainage basins, induced a sequence of changes of the local flora and fauna. Initially, this was manifested by massive nuisance growth of the benthic macroalgae Cladophoru spp. which dominated the picture for several years. Thereafter, faster decomposing species such as Chaetomorpha spp., Enteromorpha spp., and Ulua spp. emerged, and lately continuous excessive blooms of phytoplankton diatom and dinoflagellate populations and of the nitrogen fixing cyanophyte Nodularia spumigena are common (Hodgkin and Birch, 1986). Succession between these diverse autotrophic populations is related to the seasonal shift between allochthonous and autochthonous phosphorus

Aquatic Primary Production in Coastal Lagoons

Fig. 9.10. The successional pattern between phytoplankton (pelagic chlorophyll) and the macroalga Cladophora sp. (biomass in dry weight) in the subtropical choked Peel-Harvey Estuary (Australia).Adapted from Birch and Gabrielson (1984).

supply, residence times of water, and salinity (Hodgkin and Birch, 1982, 1986; Birch and Gabrielson, 1984). Phytoplankton diatom blooms thrive on new nutrients from riverine input during the wet season, the Nodularia blooms take over after collapse and decomposition of the diatom blooms and are enhanced by nutrient supply from the bottom at the onset of the dry season. After decomposition of the cyanophyte blooms, phytoplankton and benthic macroalgae are sustained by nutrients from pelagic and benthic remineralization, until the wet season begins (Gordon et al., 1981b; McComb et al., 1981; Hodgkin and Birch, 1982; Birch et al., 1983; Hodgkin and Birch, 1986; McComb and Lukatelich, 1986). The characteristic pattern of succession between pelagic and benthic primary producers is given in Fig. 9.10. The effects of mass blooms are nuisance odor during decomposition, deoxygenation of the sediments, and fish kills, especially after collapse of the cyanophyte populations. Management options for the system have been forwarded, the most obvious are diminishment of the allochthonous nutrient load, especially phosphorus, and augmentation of tidal exchange (Hodgkin and Birch, 1982,1986).

A Restricted Lagoon: Venice, Italy Venice Lagoon is a shallow restricted Mediterranean lagoon with ample tidal exchange. Human impact is given by constant dredging of accessory channels and substantial effluent discharge from the City of Venice (Pavoni et al., 1987; Sfriso et al., 1987). Vatova (1961) established an annual phytoplankton primary production of 147 g C m-2 year', but lately, massive proliferations of benthic macroalgae and succession by phytoplankton has become a common interannual reoccurring feature (Sfriso et al., 1987). After collapse of preceding macroalgae, the sediment is enriched with biogenic

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273

I

W A M J J A S O N D J F W A Y

1985

we6

Fig. 9.11.Annual cycle of macroalgal biomass (a), pelagic chlorophj I),and surface sediment content of total nitrogen and phosphorus (c) in the mediterranean leaky Laguna di Venezia (Italy). Adapted from Sfriso et al. (1987).

matter, and subsequent decomposition supplies regenerated nutrients for the sustenance of a phytoplankton bloom (Sfriso et al.,1987; Fig. 9.11). Although the successional pattern resembles the one described for the Peel-Harvey system, phytoplankton succession with the absence of cyanophyte blooms is less dynamic, probably due to larger tidal exchange.

Arid Choked to Leaky Lagoons: Algal Mat Based Systems Algal mats are benthic microbial communities which colonize a variety of habitats ranging from inland salt lakes, coastal lagoons with marked dry/ wet seasons, and subtidal, intertidal, and supratidal zones of arid to semiarid eury- to hypersaline regions (Krumbein, 1981; Bauld, 1984).They are often referred to as organo-sedimentary structures built by microbial activity and sediment deposition. The microbial communities usually consist of cyanobacteria and photosynthetic green and purple bacteria, and may also harbor diatoms. Depending on population composition, they may be morphologically distinguished into smooth, tufted, pustular, gelatinous mats (Bauld, 1984). The different populations thrive in a vertically laminated

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Aquatic Primary Production in Coastal Lagoons

sequence of microlayers with diatoms and cyanobacteria at the surface followed by photosynthetic and heterotrophic bacteria respectively, and with or without intermittent inorganic microlayer deposits. As a consequence of the different physiological features of the populations and lamination, the mats reflect extreme vertical stratification of physicochemical properties. Their study is of particular interest to elucidate the processes involved in stromatolite formation, and as such, the evolution of photosynthesis on Earth and probable formation of associated sulfide mineral deposits, and also paleoecological reconstruction of environments (Bauld, 1984). Exemplary studies on microbial mat primary production have been conducted in Solar Lake, Gulf of Aqaba (Cohen et al., 1977a,b,c, and 1980), a small hypersaline closed lagoon fed by seawater seepage through a barrier which separates it from the sea, in Shark Bay and Spencer Gulf, Australia, within secluded intertidal zones (Bauld et al., 1980; Bauld, 19841, Long Island Sound intertidal flats (Burkholder et al., 19651, and Laguna Guerrero Negro and Ojo de Liebre, M6xico (Javor and Castenholz, 1984). Annual estimates of primary production are absent but daily areal rates of algal mats are substantial and comparable or may surpass phytoplankton, macroalgal, and macrophyte communities of other lagoons. Estimates for Shark Bay ranged from 0.09 to 0.48 g C m-2 d-’ and for Spencer Gulf 0.2 to 3.1 g C m-2 d-1. Extreme values were recorded in Solar Lake with 5.12 g C m-2 d-1 and in Long Island Sound with 4.45-8.1 g C m-2 d-l. This suggests the presence of high accretion rates of organic matter in these lagoons. However, the largest fraction of the photosynthetically produced organic matter is rapidly remineralized at the expense of sulfate reduction by sulfate-reducing bacteria, which usually occurs immediately below the surface. Both processes seem to be closely correlated and sulfate reduction is supposed to be the dominant terminal microbiological process in many mats (Skyrring and Chambers, 1980). Light is the main factor regulating the vertical sequence of primary producers. Each microbial layer selectively absorbs certain wavelengths of light until the onset of limitation in deeper layers (Javor and Castenholz, 1986). Salinity and desiccation are also governing features. Algal mat based lagoons of arid and semi-arid regions have been less subject to human impact. Global Lagoon Primary Production

The contribution of coastal lagoon primary production to global primary production is unknown and may, at present, only be induced from a tentative estimate. Barrier coasts usually backed by lagoons amount to 13.1% or 32,068 km of the world’s combined continental coastline (Leont’yev and Nikiforov, 1965; Cromwell, 1971). Nixon (1982) established a median area of 78 km2 for 97 lagoons, which corresponds to a median width of approxi-

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mately 10 km. The total coastal lagoon area should thus amount to approximately 320,000 km2. Considering a mean annual primary production of 300 g C m-2as being a plausible estimate for coastal lagoons, total coastal lagoon production amounts to 10" kg C year'. Global coastal lagoon production attains levels close to those estimated for upwelling regions and by a factor of four less than for estuaries where primary production per unit area is similar to that in lagoons (Table 9.6). In comparison, total coastal lagoon production does not seem impressive, but their world coverage, enclosure, and recycling and accumulation potential of matter, indicates that they play a substantial role as efficient filters and modificators in the transfer of matter between land and sea, and as such within the terrestrial-marine link of the global carbon cycle. TABLE 9.6 Net areal primary production and total production of coastal lagoons in comparison t o other marine systems of the world. System data were compiled from Lieth and Whitaker (1975), Platt and Rao (1975)and Platt et al. (1989).The lagoon estimate is from this study. System

Ocean Upwelling Shelf Estuaries Lagoons

Area (lo4 km2)

Net production (g c m-2 year-')

Total production

125

41.5 0.2

1.4

500 183 300

0.3

300

332

0.4 33

kg C year-')

4.1 0.4 0.1

References Aleem, A.A. and Samaan, A.A. 1969 Productivity of Lake Mariut, Egypt. Part 11. Primary production. Znternationales Revue der gesamten Hydrobiologie 64,491-527. Aller, R.C. 1980 Diagenetic processes near the sedimenkwater interface of Long Island Sound.I.Decomposition and element biogeochemistry (S,N,P). Advances in Geophysics 22,237350. Alvarez-Borrego, J. and Alvarez-Borrego, S. 1982 Temporal and spatial variability of temperature in two coastal lagoons. CALCOFZ Reports XXIZI, 188-197. Arenas, V. 1979 Balance anual del carbono organico, nitrogeno, y fosforo en el sistema lagunar Huizache-Caimanero, Mexico. PhD Dissertation, Universidad Nacional Aut.6noma de MBxico. Asmus,R. 1985 Nutrient flux in short term enclosures of intertidal sand communities. In Nutrient Cycling, Processes in Marine Sediments (MUUS,K., ed). Proceedings 20th European Marine Biology Symposium, Olsen and Olsen Press, Hirtshals, Denmark, 1-17.

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Balzer, W. 1984 Organic matter degradation and biogenic element cycling in a nearshore sediment ( E e l Bight). Limnology and Oceanography 29, 1231-1246. Balzer, W., Bodungen, B.von and Pollehne 1985 Benthic degradation of organic matter and regeneration of nutrients in shallow water sediments off Mactan, Philippines. The Philippine Scientist 2 2 , 3 0 4 1 . Baretta, J. and Ruardij, P. 1988 Tidal Flat Estuaries. Simulation and analysis of the Ems-Estuary. Ecological Studies 71. Springer Verlag, Berlin, 353. Bauld, J. 1981 Geobiological role of cyanobacterial mats in sedimentary environments: production and preservation of organic matter. Journal of Australian Geology and Geophysics 6, 307-317. Bauld, J. 1984 Microbial mats in marginal marine environments: Shark Bay, western Australia, and Spencer Gulf, south Australia. In Microbial mats:Stromatolites (Cohen, y., Castenholz, R.W. and Halvorsen, H.O., eds). Allan R. Liss, New York, 39-58. Bauld, J., Burne, R.V., Chambers, L.A., Ferguson, J. and Skyrring, G.W. 1980 Sedimentological and geological studies of intertidal cyanobacterial mats in northern Spencer Gulf, south Australia. In Biogeochemistry of ancient and modern environments (Truedinger, P.A., Walter, M.R. and Ralph, B.J., eds). Australian Academy of Science, Canberra, 157-166. Bautista, M.F. and Pearl, H.W. 1985 Die1 Nz fixation in a n intertidal marine cyanobacterial mat community. Marine Chemistry 16,369477. Benninger, L.K., Aller, R.C., Cochran, J.K. and Turekian, K.K. 1979 Effects of biological sediment mixing on the 210-Pb chronology and trace metal distribution in a Long Island sediment core. Earth Planetary Science Letters 43,241. Berman, T. and Eppley, R.W. 1974 The measurement of phytoplankton parameters in nature. Scientific Programming 61,219-239. Birch, P.B. and Gabrielson, J.O. 1984 Cladophora growth in the Peel-Harvey estuarine system following blooms of the cyanobacterium Nodularia spumigena. Birch, P.B., Gabrielson, J.O. and Hamel, K.S. 1983 Decomposition of Cladophora. I. Field studies in the Peel-Harvey estuarine system, Western Australia. Botanica Marina 26,165-171. Bodungen, B.V. 1982 The Bermuda Marine Environment 111. Bermuda Biological Station Special Publication 18, 1-123. Bokuniewicz, H. 1980 Groundwater seepage into Great South Bay, N.Y. Estuarine, Coastal and Shelf Science 10, 437-444. Botello, A. and Mandelli, E. 1975 A study related to water quality of Terminos lagoon and adjacent coastal areas, Campeche, Mexico. Final Report, project Gu-853. Universidad Nacional Autenoma de Mexico, Mexico. Botello, A. and Macko, S.A. 1982 Oil pollution and the carbon isotope ratio in organisms and recent sediments of coastal lagoons in the Gulf of Mexico. Oceanologica Acta 5, 55-61. Boutiere, H., Bovee, F.de, Delille, D., Fiala, M., Gros, C., Jacques, G., Knoepffler, M., Labat, J.P., Panouse, M. and Soyer, J. 1982 Effet d'une crise dystrophique dans l'etang de Salses-Leucate. Oceanologica Acta 4 2 3 1-242. Boynton, W.R., Kemp, W.M. and Keefe, C.W. 1982 A comparative analysis of nutrients and other factors influencing estuarine phytoplankton production. In Estuarine Comparisons (Kennedy, V.S., ed). Academic Press, New York, 69-90. Bowman, M.J. 1975 Pollution prediction model of Long Island Sound. Proceedings Special Conference on Civil Engineering of Oceans, American Society of Civil Engi-

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varying rates of sea level rise. In Estuarine variability (Wolfe, D.A., ed). Academic Press, New York, 241-259. Stevenson, J.C., Madden, C.J. and Hopkinson, C.S., Jr. 1988 Sources of new nitrogen in a tropical seagrass system, TBrminos lagoon, with special reference to N-fixation. In Ecology of Coastal Ecosystems in the southern Gulf of Me'xico - The Te'rminos lagoon region (YAfiez-Arancibia,A. and Day, J.W., Jr.,eds). Universidad Nacional Aut6noma de Mexixo, Mexico, 159-170. Teague, K.G., Madden, C.J. and Day, J.W., Jr. 1988 Sedimentrwater oxygen and nutrient fluxes in a river-dominated estuary. Estuaries 11, 1-9. Teal, J.M. and Kanwisher, J. 1966 The use of pC02 for the calculation of biological production, with examples from waters off massachusetts. Journal of Marine Research 2 4 , 6 1 4 . Thayer, G.W. 1971 Phytoplankton production and the distribution of nutrients in a shallow unstratified estuarine system near Beaufort, N.C. Chesapeake Science 12, 240-253. Thayer, G.W. 1974 Identity and regulation of nutrients limiting phytoplankton production in shallow estuaries near Beaufort, N.C. Oecologia 14,75-92. Thorne-Miller, B. and Harlin, M.M. 1984 The production ofZosteru marina L. and other submerged Macrophytes in a coastal lagoon of Rhode Island. Botanica Marina 27, 539-546. Thorne-Miller, B., Harlin, M.M., Thursby, G.B., Brady-Campbell, M.M. and Dworetzky, B.A. 1983 Variations in the distribution and biomass of submerged macrophytes in five coastal lagoons in Rhode Island, USA. Botanica Marina 26,231-242. Tundisi, J. 1969 Plankton studies in a mangrove environment. It's biology and primary production. In Lagunas Costeiras, Un Simpdsio. Memdrias Simpdsio Znternacional de Lagunas Costeiras. Universidad Nacional AuMnoma de MBxico-UNESCO,MBxico, 485494. Udell, H.F., Zarudsky, J., Doheny, T.E. and Burkholder, P.R. 1969 Productivity and nutrient values of plants growing in the salt marshes of the town of Hempstead, Long Island. Bulletin Torrey Botany Club 96,42-51. Ullman, W.J. and Sandstroem, M.W. 1987 Dissolved nutrient fluxes from nearshore sediments of Bowling Green bay, central Great Barrier Reef lagoon (Australia). Estuarine, Coastal and Shelf Science 24,289403. Valiela, I. and Costa, J.E. 1988 Eutrophication of Buttermilk Bay, a Cape Cod coastal embayment: Concentrations of nutrients and watershed nutrient budgts. Enuironmental Management 12,539-553. Vannucci, M. 1969 What is known about production potential of coastal lagoons. In Lagunas Costeiras, Un Simpdsio (Castaiiares, A.A. and Phleger, F.B., ed). UNAMUNESCO, Universidad Nacional AuMnoma de Mexico,457477. Vatova, A. 1961 Primary production in the high Venice lagoon. Jornul Conseil Znterantionale pour la Exploration du Mer 26, 148-155. Vaulot, D. and Frisoni, G.F. 1986 Phytoplanktonic productivity and nutrients in five Mediterranean lagoons. Oceanologica Acta 9 , 5 7 4 3 . Vosjan, J.H. 1987 A sketchy outline of the fate of organic matter in the Dutch Wadden Sea. Hydrobiological Bulletin 21, 127-132. Welsh, B.L., Whitlatch, R.B. and Bohlen, W.F. 1982 Relationship between physical characteristics and organic carbon sources as a basis for comparing estuaries in southern New England. In Estuarine Comparisons (Kennedy, V.S., ed). Academic

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Aquatic Primary Production in Coastal Lagoons

Press, New York, 53-67. Williams, P.J.LeB, Raine, R.C.T. and Bryan, J.R. 1979 Agreement between 14-C and oxygen methods for measuring phytoplankton production: reassessment of the photosynthetic quotient. Oceanologica Acta 2 , 4 1 1 4 1 6 . Yaiiez-Arancibia, A. 1987 Lagunas costeiras y estuarios: cronologia, critBrios, y conceptos para una classificaci6n ecol6gica de sistemas costeiros. Revista de la Sociedad Mexicans de Histdria Natural 3 9 , 3 5 5 4 . Yaiiez-Arancibia, A. and Day, J.W., Jr. 1988 Ecology of coastal ecosystems in the southern Gulf of Me'xico - The Te'rminos lagoon region. Universidad Nacional Autonoma de MBxico, 637pp. Yaiiez-Arancibia, A., Lara-Dominguez, A.L. and Alvarez-GuillBn, H. 1985 Fish community ecology and dynamics in estuarine inlets. In Fish community ecology in estuaries and coastal lagoons: towards an ecosystem integration (Yaiiez-Arancibia, A., ed). Universidad Nacional Autonoma de MBxico, Editorial Universitaria, MBxico, 127168. Zeitzschel, B. and Davies, J.M. 1978 Benthic growth chambers. Rapport P.-v. Re'union du Conseil Znternationale pour la Exploration d u Mer 1 7 3 , 3 1 4 2 .

Coastal Lagoon Processes edited by B. Kjerfve (Elsevier OceanographySeries, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

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C h a p t e r 10

Secondary Productivity in Coastal Lagoons Salil Alvarez-Borrego Division de Oceanologia, Centro de Investigacion Cientifica y de Education Superior de Ensenada, B.C. Ensenada, Baja California, Mdxico

Secondary productivity is the net production of consumers above respiration and excretion. A major difference between secondary and primary productivity is that no single measurement can be used to characterize the secondary productivity of a community. We still know very little about the secondary productivity of coastal lagoons. With scarce available data, it seems that the zoobenthos are the most productiv animals, specially the suspension feeders (up to more than 200 g AFDW m-2 year-’ for the whole macrobenthic assemblage but more commonly with values around 20). The zooplankton have a productivity as high as half that of the zoobenthos. The nekton has a productivity between about 10% of and similar to that of the zooplankton. Relatively much more is known of the secondary productivity of temperate lagoons than that of tropical lagoons; and much needs to be done to have a good set of cases for comparisons. In the case of leaky lagoons, with short turnover times, a detailed description of their hydrodynamics and that of the adjacent oceanic areas is a prerequisite to work on secondary productivity. This is specially important in the case of estimating secondary productivity of zooplankton. In the case of nekton, crustaceans, and mollusks, we are moving towards a situation with very few coastal lagoons left for analysis of natural secondary production. Most cases will be in the realm of fishery and aquaculture sciences.

Introduction The definition of secondary productivity adopted by the International Biological Programme is the net production of consumers above respiration and excretion. I n general, the study of secondary production can be taken to concern all those processes involved in the consumption, transformation and utilization of food by t h e animals (Edmondson and Winberg, 1971). Winberg (1971) indicates that the term ‘secondary production’ originates from Thienemann’s (193 1) definition: The production of a species-population for a known period is considered to be the sum total of growth increments of all the individuals existing at the start of the investigated period

288

Secondary Productivity in Coastal Lagoons

and remaining to the end, as well as the growth of newly born individuals and those which, for various reasons (due to being consumed, dying or other causes), do not survive to form part of the final population biomass existing at the end of the period. Winberg (1971) indicated that among Russian authors the word ‘productivity’ means the characteristic property of a particular population, community or water body which is responsible for the large or small level of production; and that we should recognize the need for distinguishing between the terms ‘productivity’ and ‘production’. Nevertheless, Edmondson and Winberg (1971) used the two terms indistinctly, and I will do the same throughout this chapter, without the Russian connotation for productivity. A major difference between secondary and primary productivity is that no single measurement can be used to characterize the secondary productivity of a community. To estimate secondary productivity we depend largely on values calculated indirectly from population size and physiological data, or measured indirectly as respiration and excretion. Two general kinds of approaches to secondary productivity can be recognized (Edmondson and Winberg, 1971): the approach through population dynamics and the physiological approach. In the first, one is concerned with such properties as rates of growth and death, and the structure of populations in terms of age and size. In the second, one is concerned with rates of feeding, assimilation, respiration, excretion and a variety of metabolic processes within organisms. I n both approaches, due consideration has to be given to rates of activity and how the rates are changed by environmental conditions. Also, turnover rate or replacement time for the various components have to be appropriately estimated. Conclusions are based on measurements in natural populations of changes that take place over a period, and in the laboratory of activities and composition of the animals. Often, studies are directed towards estimating the production of individual species-populations; and this raises the question of how to relate this production to that of the ecosystem as a whole; summing the productions of species-populations belonging to different trophic levels is incorrect. To make an extensive review of the methodologies used to estimate secondary productivity is beyond the scope of this chapter. There are excellent manuals - like those by Winberg (1971), Edmondson and Winberg (1971), and Crisp (1971) - and theoretical considerations have been elaborated by different authors (i.e. Leblond and Parsons, 1977 and 1978; Ricker, 1978). The purpose of this chapter is to review what has been done in coastal lagoons. Unfortunately, much too little has been done. No contribution on secondary productivity was published in the memoirs of a n International Symposium on Coastal Lagoons (Ayala-Castaiiares and Phleger, 1969) carried on at Mexico City in November, 1967. A Coastal Lagoons UNESCO-IABO seminar was carried out in August 1978 at Duke University Marine Laboratory (UNESCO, 1981), and a UNESCO-IABO-SCOR

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symposium took place at the University of Bordeaux I in September 1981 (Lasserre and Postma, 1982); the proceedings of neither event explicitly mentions secondary productivity. The situation is not much better for estuaries than for coastal lagoons. Ketchum (1983) edited a book on estuaries and enclosed seas, and there were only two short reviews on secondary productivity: one by Miller (1983) on production of copepods, which discussed mainly the case of a very small coastal lagoon; and one by Wolff (1983) on the production of benthic animals. Zooplankton Productivity

As mentioned by Miller (1983), one of the most sophisticated attempts to determine the production of copepods was the study by Landry (1978) of the population of Acartia clausi in Jakle's Lagoon. Jackle's Lagoon is a small, almost closed impoundment adjacent to Griffin Bay in Puget Sound, Washington, U.S.A.It is roughly circular, about 200 m across, with sides sloping smoothly to a central basin of 125 m diameter that is 3.5 m deep. There were several samplings at different locations of the lagoon, and a general representative ratio was estimated between the overall copepod density in the lagoon and that at a central location where a time series was generated. This series spanned from March of 1973 to September of 1974. Sampling was twice-weekly in 1973, but only once-weekly in 1974. The lagoon was initially isolated from the bay, but in the middle of the first year of study a dam at the connecting creek was broken. After that time there was some exchange, especially at night, when tides were highest and older life-cycle stages were near the surface. This introduced some uncertainty into the calculations. Nevertheless, the study area was more confined and stable than those in other studies estimating marine zooplankton productivity. Landry (1978) showed that the population of A. clausi was very strongly concentrated near the bottom during the day, which made sampling with a simple coring tube a suitable method. The tube, which was closed at the sediment surface by a rotating flap, was dropped from a boat and the enclosed water drained out through a mesh net. Landry (1978) used Ricker's (1946) approach to calculate production. I f B represents the biomass of the population, G represents the instantaneous rate of growth, and M the instantaneous rate of mortality, then db --(G-M)B dt

B = Bo exp[(G - M ) t ] where BOis the biomass a t time t = 0. The average biomass, (B),or a unit of time is

290

Secondary Productivity in Coastal Lagoons 1

@>= J BOexp [(G - m)tI dt 0

- Bo[exp(G- M) -

11

(G - M )

and the secondary productivity for that unit of time is

P = G(B) =

G Bdexp(G - M) - 11 (G - M )

which is the expression given by Clarke et al. (1946). To obtain the total production for the species, the production of all age-groups must be summed (Ricker, 1946). From Ricker (1978), we have a clear definition of G and M , given the number (N) and the average weight (w)of the animals in a cohort or life-cycle stage, at times to and tl (where tl - t o = 1)

In

[i;) [::) -

F:)

=In - +In -

and thus

): [

G = In

-M = In

[%)

Landry (1978) estimated the growth rate for each life-cycle stage, Gi, directly from the time required for shift of the abundance mode of a cohort from one stage to the next, and measurements of the change in length between stages. The length measures were converted to weight by application of a general length divided by dry weight relationship. Growth was nearly exponential, with varying rates for each cohort. Time series data (Fig. 10.1)were also used to obtain an approximate estimate of the mortality of each stage within a cohort. First, the area under the curve of numerical abundance against time for a stage in a given cohort was determined (AJ. This value was divided by the duration (Di, in days) of the stage, producing a n estimate somewhere between the number of copepods which initially

S. Aluarez-Borrego

291

Fig. 10.1. Seasonal cycles of abundance during two years of individual developmental stages of A C U F ~cluusi ~ U in Jackle’s Lagoon, Washington, U.S.A.(Redrawn by Miller, 1983, from Landry, 1978).

entered the stage and the number which successfully molted to the next developmental stage. This was taken to be the number of copepods which survived to the midpoint of the stage duration. Second, it was assumed that there is simple exponential mortality during the developmental time of the stage. Thus, the mortality rate (Mi) for the stage is 1 M i= - In Si

Di

s,=A, D ~ + I Al+1Dl

where S; is the fractional survival and Di is the duration from the midpoint of the ith to the midpoint of the (i + 1)th stage. These mortality rates were

292

Secondary Productivity in Coastal Lagoons

used to estimate the stage-specific average biomass on days without sampling. If N,,o is the abundance and wl,o is the dry-weight, of the ith stage, on the sampling date, then

where is the average biomass of the ith stage for the day after the sampling date. To calculate daily productivity, Ricker’s (1946) equation was applied adult

i=l

and the resulting daily productivity estimates and cumulative production for the whole lagoon are shown in Fig. 10.2, converted t o organic carbon

Fig. 10.2. Seasonal cycle of secondary productivity and cumulative secondary production of Acartia clausi during two growing seasons in Jakle’s Lagoon, Washington, U.S.A. (Redrawn by Miller, 1983, from Landry, 1978).

S.A1 varez-Borrego

293

using the measured relationship: body carbon = 37% of dry weight. The annual production was 5.0 g C m-2 yea+ from April 1973 through March 1974. The growing season rate was 14 mg C m-2 d-I as an average, with much higher rates for the deeper part of the lagoon (up to 70 mg C m-2 d-l). These numbers were essentially the overall zooplankton production of the lagoon, because Acartia clausi was by far the dominant species during the study period. No primary productivity estimates were made for Jakle's Lagoon with which to compare the secondary productivity values. Miller (1983) compared Landry's (1978) values for Jakle's Lagoon with those obtained by Heinle (1969) for the Patuxent River sub-estuary of the Chesapeake system (U.S.A.). Values for the Patuxent River are on the same order of magnitude as those in Jakle's Lagoon. Heinle (1969) compared his values with the primary productivity estimates of Stross and Stottlemeyer (1965) for the Patuxent River sub-estuary, and something less than 10%of the primary productivity seems to appear as secondary productivity in the holoplankton. From these two cases, Miller (1983) concluded that zooplankton production in temperate estuaries is on the order of 5 to 10 g C m-2 year', mostly concentrated in about one-third of the year. He emphasized that no tropical studies were available for comparison. The situation has not changed; no new studies have been reported in the literature on this subject. Nekton Productivity

There is a general consensus among different authors that estuary and coastal lagoon fish communities are much more productive per unit area than those of the open ocean (i.e.: Saila, 1975;Nixon, 1982;Day et al., 1989). It is very arduous to generate true fish secondary production data for coastal lagoons and estuaries. The complex migration patterns make it difficult to use many of the standard methods (Day et al., 1989). Because of this, data on catch per unit effort or standing crop are frequently used as a n index of fish productivity. Table 10.1 has catch data condensed by Saila (1975) and Day et al. (1989) for estuaries and coastal lagoons; the range is 6-146 g wet wt m-2 year', with most values falling below 50. Some of the ecosystems listed in Table 10.1 have produced fish annual yields comparable to culture ponds and lagoons without intensive management (Saila, 1975). Caution must be exercised when dealing with catch per unit effort data as a n index of secondary production of fish, because catch per unit effort depends on factors such as age of the fishery, ability to catch, the migratory patterns, the management regulations or absence of them, and even the effort itself. Often, dividing by effort does not normalize the catch data for comparison. Besides, it is not known how much of this secondary production is imported from the adjacent ocean. A coastal lagoon with a young fishery

294

Secondary Productivity in Coastal Lagoons

TABLE 10.1 Annual yields of fish (g wet wt m-2year-') from lagoons and estuaries without artificial feeding (after Saila, 1975; and Day et al. 1989).All author references are found in Saila's and Day et al.'s contributions. Location

Italy

Etang de Biguglia, Morocco Chesapeake Bay, U.S.A. Flax Pond, NY,U.S.A. Texas coastal lagoons Barataria Bay, LA, U.S.A. Mexican Pacific lagoons Terminos Lagoon, Gulf of Mexico, Mexico Cuban lagoons Hawaii Taiwan Phillipines

Java Singapore India

Fish production 6-10 9-17 15 13 9 11-15 12-58 35-73 25-67 20 22-28 20 94-186 47-50 50-100 50 15-63 167 11-17 6-124

Reference

Beadle (1946) de Angelis (1960) d'Ancona (1954) Belloc (1938) Mansueti (1961) Hall and Woodwell (unpublished data) Jones et al. (1963);Hellier (1962) Wagner (1973);Day Jr. et al. (1973) Yaiiez-Arancibia (1978) Yaiiez-Arancibia and Lara Dominguez (1983) Holcik (1970) Cobb (1901) Lin (1968) Rabanal(1961) Frey (1947) Tang (1967) Schuster (1952) Le Mare (1950) Pillay (1954) Pakrasi et al. (1964)

and low effort may have a catch per unit effort much larger than another lagoon with a relatively old fishery with high effort, although they might have had similar initial natural conditions. Citing Oglesby (19771, Nixon (1982) mentioned that limnologists have been surprisingly successful in finding a relatively simple relationship between primary production and the fisheries yields of various lakes around the world. Nixon (1982) found a similar relationship for marine systems, except that the fisheries yield of marine waters at a given level of primary production is some 10-20 times greater. Unfortunately, there are few lagoons for which both primary production and fisheries data are available, and Nixon (1982) found that those for which a n assessment can seem to be made to fall roughly within the range we might expect for productive marine

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systems. He concluded that the total yield per unit area of unpolluted lagoons can appreciably exceed that of such well-known fishing areas as the Peru upwelling system, Georges Bank, and the North Sea. According to Day et al. (19891, one reason estuaries and coastal lagoons produce so much fish is that the individual species occupying them have high production rates. In part this is because many of the species are in estuaries and coastal lagoons during their juvenile period, a n age at which productivity is usually highest. These latter authors mentioned that it is difficult to make comparisons of total fish community secondary productivity because there are few studies and differences in methods used often obscure general trends. It was first assumed, based on fisheries yield, that tropical estuaries should be more productive than temperate estuaries. Tropical waters carry many more fish species than comparable waters in the temperate zone (Wagner, 1973; Moore, 19781, but biomass and fish production may not be higher (Haedrich and Hall, 1976). However, in tropical lagoons with ephemeral inlets, fishery productivity can be high for a short period (Yaiiez-Arancibia, 1978). Much more work is needed before we can make good comparisons of productivity rates. We may not be able to study the 'natural' secondary productivity of fish in many coastal lagoons at all, because of the presence of either a fishery or a n aquaculture project. With the presence of a fishery, the original natural abundance and productivity of fish might have been totally changed, especially in cases with an old fishery with large effort, and with a spatial extent including the adjacent oceanic area. Aquaculture projects tend to modify the environment and manipulate the species so that production is enhanced. There is a report covering the period 1979-81 of a n aquaculture project in the Esteros de las Salinas de San Fernando, in Cadiz, Spain (Arias et al., 1984). During that period, conditions were very close to what could be considered as an experiment to estimate natural secondary productivity of fish in a coastal lagoon. The esteros are located in a zone with a total area of about 6 8 ~ 1 m2. 0 ~ There were two types of esteros: those dedicated mainly to production of salt, with some fish rearing as a secondary activity, and those exclusively dedicated to fish production. The main elements of those dedicated to fish production were: a channel for water exchange, with floodgates to control the water flux; the estero itself, with a n area between 2x104 and 35x104 m2, where the fish growth took place (maximum depths were 3 4 m), and a hibernation reservoir, with a n area of 5-10x103 m2, where those fish that did not reach commercial size were kept through winter. In general terms, water temperature oscillated in the esteros between 14 and 27"C, salinity between 36 and 88%0,and dissolved oxygen between 9 and 0 ml 1-l, during winter and summer, respectively (Arias and Establier, 1979). The culture cycle in the esteros lasted a year, with three main phases: juveniles intake, growth, and fishing. Naturally produced larvae and juve-

296

Secondary Productivity in Coastal Lagoons

niles of 37 species entered the esteros with the tides. Due to overlapping of spawning season of different species, there was a continuous recruitment of juveniles throughout the whole year. However, impounding ofjuveniles was greatest from the end of fall through the middle of spring, because the majority of species spawn during this period. Only 10 of these species had a significant abundance for commercial purposes. The least abundant of these 10 species had a 2% abundance. Growth lasted about seven months. It started during spring with the intake of water at high spring tides. During growth the fishes fed exclusively on natural food found in the esteros. Food was plenty and diverse due to high primary and secondary productivities (no data on these were given in the report). This, and the increase of temperature from May through September, allowed for a rapid growth of fishes. Drake (1983)reported that there was practically no competition for food. Throughout the growth phase, the water was partially renewed with the tides, with a net at the floodgate to avoid the escape of fishes (and, from the experimental point of view, the entrance of fishes). During this phase some problems were encountered. During neap tides the esteros may have 15 to 20 consecutive days without water renewal, and this was the main danger because oxygen may be depleted causing large mortalities. The only solution to this problem was to pump water during hours, but frequently it was not enough. Another problem was furtive fishing and the escape of fishes due to breaking of dikes by storm waves during high tides. No estimates are given in the report on the effect of these problems on fish production. Fishing, the last phase, was carried out from October through January. Usually fishing in a single estero lasted one day, but in large esteros it lasted up to four days. Previous to fishing, the estero was almost totally emptied. Fishing was carried out with nets, and the last catches were done in the mud to extract bottom fishes. Fish larger than about 23 cm were considered of commercial size, and the smaller ones were taken to the hibernation reservoir and kept there until spring, and then released back to the estero for growth. Production varied between esteros (both totally and by species) and these variations were attributed to a diversity of factors such as physical, physicochemical, climatological, biological and human, acting throughout the culture. In general terms, production varied between 4 and 43 g wet wt m-2 year', with a n average yield for all esteros of 24 g wet wt m-2 year'. These figures are very much within the majority of the data in Table 10.1.Arias et al. (1984)reported that alterations were being made to the esteros to increase their productivity; mainly by increasing the percentage of area suitable for fish growth, and by introducing juveniles produced in the laboratory. As manipulation increases, the Esteros de S an Fernando are moved farther away from their natural productivity levels, and become more a case for aquacultural analysis.

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Benthic Secondary Productivity

Benthic production studies for estuaries and coastal lagoons are more abundant than those for fishes and other groups. This is possibly due to the lesser motility of benthic species. Most reported data are on the productivity of mollusks. According to Wolff (1983), the secondary production of estuarine benthic animals varies widely from place to place, but it is generally high compared to those of other ecosystems. This is due to a high level and easy availability of food and a preponderance of opportunistic benthic species characterized by high growth rates and a rapid turnover. Wolff (1977) considered that some of the factors causing high benthic secondary productivity in estuaries are: shallow depths of only up to few meters, where sinking phytoplankton may form a large source of food for the benthos; tidal currents, making the phytoplankton production much more readily available through both the vertical turbulent diffusion and the almost continuous horizontal transport of plankton-loaded water; and the possible preponderance of r-selected species. I n the case of leaky coastal lagoons, strongly affected by tidal action and with short turnover times (Kjerfve and Magill, 1989), the same factors may also cause high benthic secondary productivities. In general, suspension-feeding animals have the highest secondary productivities, and aquaculture techniques can greatly increase their yield, like those of mussels and oysters. Ceccherelli and Borgato (1985)studied the life cycle and secondary production of the meiobenthic harpacticoid Canuellaperplexa in a lagoon of the Po River Delta. A population of the species was sampled from the muddy bottom of the lagoon (at 2.5 m depth) nearly fortnightly during a year. Three 19.6 cm2 sediment cores were taken at each sampling date. Different copepodite stages and adults were sorted, classified and counted separately from the first 5 cm of each core. Specimens of different stages and sexes were measured from sub-samples. The trends of the following biological variables were investigated: densities, both of total individuals and of adults and copepodites separately, sex ratio, percentage of ovigerous females, egg number per female, and rate of population increase. Dry weights of adults of C.perplexa were derived from the literature. Those of copepodites were estimated according to the assumption that there is a correlation between weight variations of copepodite stages and their respective body volumes. Total biomass was about 0.37 g C m-2. Production was estimated by means of the indirect PIB ratio method of Banse and Mosher (1980) log - = a +log M

where M is the mean body mass of adults upon reaching maturity, and it is used for scaling the annual PIB. Ceccherelli and Borgato (1985) estimated

Secondary Productivity in Coastal Lagoons

298

new parameters, a and b, for meiobenthic organisms, from production and biomass data from the literature. Annual production of C. perplexa was 2.2 g C m-z, which corresponded to a n annual PIB ratio of 5.9. This production was mainly sustained by the mass development of population during spring and early summer. During late summer, production tended to decrease. Winter population was almost exclusively represented by the diapause stages of fifth copepodites and adults. Boysen-Jensen (1919) proposed a method to estimate the production of benthic animals, such as bivalves and polychaete worms. His original calculations for animals of the Limfjord (Denmark) were underestimations because he based them on only two series of observations; he sampled every year only in spring and in autumn. But his basic principles have received wide recognition and have been used and further developed by a number of investigators (Winberg, 1971). Boysen-Jensen (1919) determined the biomass at the beginning and end of the year, as well as the new recruitment of young, directly from the bottom samples. The total loss by predation and by other causes of death, Be, was taken as

where N1 and N Zare the numbers of individuals in the population at the beginning and end of the year, and B1 and BZ are the biomass at the beginning and end of the year. The production of the older generation, excluding the newly born individuals is

P = Be + Bz- B1 = B e + AB Boysen-Jensen's (1919) annual production, that is the production of the whole population, is the sum of the increments for individuals of the older year-classes present at the beginning of the year and the increments of the newly born individuals. Bachelet (1982) estimated the production of two bivalves: Macoma balthica and Scrobicularia plana. Both in the Gironde estuary, and the second also in the Arcachon Bay (Atlantic coast, France). He compared the estimates of somatic production (Pg)using two methods: integration of Allen's curve, which is based on growth,

n

G, Aw,

Pg = i=l

299

S. Aluarez-Borrego

where i represents the age classes; and the removal-summation method, which is essentially Boysen-Jensen's (1919) method, based on mortality, Pg = AB + Be n

n

Be = C t n ANiwi i=l

Bachelet (1982) also estimated the gonadal production, by determination of the weight lost during the reproduction period; and he found that it represents a n important percentage of total production (from 8 to 35% for M. balthica; 48% for S. plana). Bachelet's (1982) somatic production estimates by the two methods gave results from identical to a difference of less than a factor of two. We can see that algebraically both methods are the same. 1

Pg,=-(Ni+Nz) 2

1

~i)=-(N1~2-Niw + iN z w z - N 2 ~ 1 ) 2

( ~ 2 -

1 2

- 2~+ 2 Niwi) + ( N Z W-ZNiwi) = - ( N I w ~- N 2 ~ 1 N = Be,+ ABL

Thus, differences are possibly due to data manipulation. Wolff and de Wolf (1977) used these two expressions for somatic production (NiAwi and AB + B e ) indistinctly for their estimates of production of zoobenthos in the Grevelingen Estuary (The Netherlands) (Table 10.21, and they obtained nearly identical results. Bachelet (1982) also found that for several populations of a given species, annual production and PIB ratios may cover a large range of values according to habitat, number of age classes, and -above all -the relative importance of each cohort. He expressed that the different values obtained for total production, somatic production, and PIB cannot be interpreted without a sound knowledge of life histories, and this implies a prudent use of PIB ratios for predicting Pg and total production values. Bachelet's (1982) total production for M. balthica amounted to 0.7-3.2 g ash-free dry weight (AFDW) m-2 year' (PIB = 0.60-1.951, and for S. plana it was 0.6-25.2 g AFDW m-2 y e a r ' (PIB = 0.76-5.69).

Secondary Productivity in Coastal Lagoons

300

TABLE 10.2 Production values for some macrobenthic species (g AFDW m-' year-'). Authors marked with a n asterisk are cited by Wolff (1983)."he two asterisks mean this value is in g dw m-'year-'. Location

Reference

Species

Production

Long Island Sound, U.S.A. (subtidal)

*Sanders (1956)

Nephtys incisa Cistenoides gouldii Pandora gouldiana Yoldia limatula

9.3 1.7 6.1 3.2

Conway Bay, Wales (intertidal) *Hughes (1970)

Scrobicularia plana

3.13

Petpeswich Inlet, Canada (intertidal)

*Burke and Mann (1974)

Mya arenaria Macoma balthica Littorina saxatilis

11.6 1.9 3.3

Southampton Water, England (intertidal)

*Hibbert (1976)

Cerastoderma edule Mercenaria mercenaria

29.71 4.14

Grevelingen Estuary, Netherlands (intertidal) (and subtidal)

*Wolff and de Wolf Cerastoderma edule (1977) Macoma balthica Arenicola marina Hydrobia alvae Littorina littorea

10-120 0.3-7.9 3.3-6.3 7.2-12.8 6.1

Gironde Estuary and Arcachon Bachelet (1982) Bay, France (intertidal)

Macoma balthica Srobicularia plana

0.7-3.2 0.6-25.2

Prevost lagoon, France (subtidal)

Guelorget and Mazoyer-Mayere (1985)

Cerastoderma glaucum

9.2-32.5

Lagoon in Po River Delta, Italy (subtidal) (meiobenthic copepod)

Ceccherelli and Borgato (1985)

Canuella perplexa

LO**

Baie d'Aouatif, Banc d'Arguin, Mauritania (intertidal)

Wolff et al. (1987)

Anadara senilis

2.28

~~

301

S. Alvarez-Borrego

.0

---

P.--

Station X Station 3 Station ii

,

O

N

73

D

J

F

M

A

M

J

J

A

S

O

N

D

J

F

M

I

A

74

Fig. 10.3. Secondary productivityof Cerustodermuglaucum in Prevost lagoon, Mediterranean coast of France (after Guelorget and Mazoyer-Mayere, 1985).

Guelorget and Mazoyer-Mayere (1985) reported the secondary production of the bivalve Cerastoderma glaucum in the Prevost lagoon (Mediterranean coast, France). They used the Mass6 (1968) modification of Boysen-Jensen's method. C. glauccum, as the other mollusks in Prevost lagoon, has a n abundant fall recruitment and a heavy death rate in winter and mainly in summer. This is a typically marsh species and presents a maximum development in the most confined areas. Its somatic production was 10.9-38.2 g dry weight m-2 year' (PIB = 2.5-4.8). According to Birger (1961) (cited by Winberg, 1971) the ash content of estuarine mollusks is about 15% of dry weight. Thus, this production is 9.3-32.5 g AFDW m-2 year'. With the exception of two high values, one in January and one in April, Guelorget and Mazoyer-Mayere's (1985) data show that somatic production of C. glaucum in the Prevost lagoon was similar throughout the year (Fig. 10.3).As a comparison, Figure 10.4 shows that somatic production reported by Wolff and de Wolf (1977) for the zoobenthos of the Grevelingen Estuary was, in general, highest during spring and summer and lowest during fall and winter. Previous to Wolff and de Wolfs study, the fresh-water input from the rivers Rhine and Meuse was cut off, and the Grevelingen was converted into a salt-water coastal body. Wolff et al. (1987) studied the West African bloody cockle, Anadara senilis. They estimated the annual production of this bivalve to be 2.28 AFDW m-2, in the Baie d'Aouatif (Banc d'Arguin), Mauritania. Their esti-

Secondary Productivity in Coastal Lagoons

302

B

10

0

30N

E

-

2200-

-

I10O-

0.-

-

-

D

O Cardium EZZd Arenicolo

-

Mocorno

Hy4rob10 nllmm L i l l a r i n a

\

rn rn

-

n

U

n

-I 01

1

E

10.

Fig. 10.4. Seasonal cycle of secondary productivity of zoobenthos at different locations in the Grevelingen Estuary, the Netherlands. (C,D and E are locations near the entrance;A and B are located in the middle, about 10 km from the entrance)(after Wolff and de Wolf, 1977).

mates were based on the empirically obtained relationship between age and shell height and that between shell height and AFDW. Table 10.2 shows production values summarized by Wolff (1983), plus those mentioned above. If we compare these values with those of Table 10.1 for fishes, taking into consideration that in the transformation from wet weight to ash-free dry weigh, t, the fish numbers are reduced to about lo%, we can see that total benthic productions are in general one order of magnitude greater than fish production.

5'. Alvarez-Borrego

303

The Pelagic-Benthic Coupling There is very little literature on the coupling between pelagic and benthic communities of coastal lagoons. Few reports give numbers on quantities of primary productivity by phytoplankton and other autotrophs consumed by the zoobenthos, or quantities of benthic organisms taken by carnivores, such as some of the benthic species (crabs, starfishes, etc.), fishes or birds. Describing the place of zoobenthos in the estuarine food web, Wolff (1983) mentioned that the ecological efficiency of the Grevelingen benthos would be between at least 6.8 and 9.6% if detritus is available as food; and these figures would be 16.2-19.5 if detritus is not available as food. These values were obtained by neglecting any consumption by zooplankton, meiobenthos or mullets. The production attained by the detritivores relative to primary production is very similar to that achieved in a typical plant-herbivore community. This suggests that the consumers utilize plant debris directly as a food resource before it has been mineralized through the decomposer food chain (Newell, 1982). Living phytoplankton cells and long-lasting particulate debris may thus form the dominant food resource for the consumer community of estuaries, lagoons and nearshore waters. Wolff (1983) also mentioned that all available evidence suggests that land or salt-marsh-derived detritus has a less important role as a food source for estuarine benthos than was assumed earlier. Instead, primary production in situ and the import of organic matter from the sea seem to be the main food sources. The crab Carcinus maenas consumes minimally 1.5 g AFDW m-2 year', which constitutes about 5% of the annual production of the benthic macrofauna (Klein Breteler, 1976). The starfish Asterias rubens maximally consumed some 0.3-1.8 g AFDW m-2y e a r ' of mussels Mytilus edulis and other mollusks in the Grevelingen, after damming (Van der Schoot, 1974). The same species took 2 g AFDW m-2year' or more in Kiel Bay (western Baltic) (Anger et al., 1977). According to Day et al. (1989), the question of how fish predation alters the size, structure, and population abundance of freshwater and marine zooplankton in estuaries and coastal lagoons unfortunately has not been studied. The first and most obvious way that nekton affect their environment is by cropping and often depleting food organisms. Experiments excluding fish from benthic areas have shown large changes in the numbers and biomass of the benthos, supporting the concept that fish cropping is important in determining community structure and biomass (Virnstein, 1977, 1978, Holland et al., 1980). The plaice, Pleuronectes platessa, consumed between 1.7 and 5.0 g AFDW m-2year' of benthic animals on the Balgzand tidal flats in the Dutch Waden Sea, in four consecutive years (Kuipers, 1977). Gobiid fishes (Pomatoschistus minutus and P. microps) may consume some 1.4 g AFDW m-2 y e a r ' in the same area (Van Beek, 1976).

304

Secondary Productivity in Coastal Lagoons

Birds cannot be neglected as predators of estuarine benthos (Wolff, 1983). Wolff et al. (1976) estimated that birds ate about 3.4 g AFDW m-2 year’ of benthic animals in the Grevelingen, and t h a t quantity was about 6% of the total production of the macrobenthos. Concluding Remarks

We still know very little of the secondary productivity of coastal lagoons. With the scarce available data, it seems that the zoobenthos are the most productive animals, specially the suspension feeders, followed by the zooplankton with a productivity as high as about half that of the zoobenthos. The nekton has a productivity between about 10% of and similar to that of the zooplankton. Relatively much more is known of the secondary productivity of temperate lagoons than that of tropical lagoons; and much has to be done to have a good set of cases for comparisons. Parker (1981) emphasized the need for a detailed description of the hydrodynamics of a study area as a prerequisite to work on biological problems. Often, a description of the internal hydrodynamics of a coastal lagoon is not enough; we have to know the behavior of the adjacent oceanic area. Events like changing coastal currents, upwelling, and even El Niiio, may have a n impact inside the lagoon (Lara-Lara et al., 1980; Milldn-Nuiiez et al., 1982; Farfan and Alvarez-Borrego, 1983; Silva-Cota and Alvarez-Borrego, 1988). Kjerfve (1986) proposed a classification of coastal lagoons with basically three types: choked lagoons, with a narrow entrance channel, which at times may be completely closed off and lack significant tides; intermediate restricted lagoons; and the leaky lagoons, usually strongly affected by tidal action, with oceanic salinity, and persistent tidal circulation patterns. Leaky systems have the shortest turnover time, restricted lagoons have a longer turnover time, and chocked lagoons have the longest turnover time (Kjerfve and Magill, 1989). The greatest immediate effect of tidal currents is, of course, on the zooplankton. Generation of time series, like Landry’s (1976) for Jackle’s Lagoon, has no proper meaning in the case of lagoons with a significant water exchange with the adjacent coastal area. A sampling program to study the zooplankton secondary productivity in a leaky lagoon would have to consider the possibility of great semidiurnal and diurnal variabilities due to the transport effect of tidal currents (i.e. Fig. 10.5).A preliminary survey to have a n idea of the variability would be necessary before the design of the sampling program. After this, a n attempt could be made to obtain representative weekly zooplankton samples by taking not only one, but sampling all day, every two hours, once each week. Then, a n integrated sample could be prepared by mixing proper fractions of these twelve samples. Fractions could be weighted by the criterion of current speed at the sampling time, or

S. Aluarez-Borrego

305

J u n e , 1979

July , 1979

Fig. 10.5.Zooplankton carbon times series, with two hours sampling interval, for a location at the entrance of San Quintin Bay, Baja California, northwestern Mexico (aRer Farfh and Nvarez-Borrego, 1983).

something else related to space coverage (i.e. tidal height). Then, a n expression similar to Huntley's (1980)could be used to estimate production,

where Ni is the number of animals in weight &ss i; j is the number of weight classes of animals in the community; Wi is the geometric mean weight of an animal in the ith weight classiu is the assimilation efficiency (i.e. 0.7);fis the feeding constant (i.e. 4.85);Vsis the geometric mean volume of a particle in size class s; Vo is the volume of the smallest filterable particle (approx. 25 pm3); h is the number of size classes of particles which are fed upon; k is the respiration constant (i.e. k = 0.349e0.05w,where T is temperature in "C); m is the respiration exponent (i.e. m = 0.92e4.013; and C,is the carbon concentration of particles in size category s (pg C ml-9. With this kind of approach it might be possible to obtain a secondary production estimate from a single plankton sample, a carbon sample, a water sample for evaluation of particle size and concentration, and a temperature reading. All these samples would have to be integrated in the same manner. However, there remains the problem of characterizing constants like a,f,k and m for the zooplankton communities of different lagoons. In the case of nekton, crustaceans, and mollusks, we are moving towards a situation with very few coastal lagoons left for analysis of natural secondary production. Most cases will be in the realm of fishery and aquaculture sciences. 'Production' will depend on socioeconomic criteria, political situations, and techniques to manipulate the animals and their environment.

306

Secondary Productivity in Coastal Lagoons

Acknowledgments

I thank C.B. Miller and W.J. Wolff for their valuable criticisms. Their comments were very helpful in revising the manuscript. I also thank M.J. De-La-Cerda for typing the first version of the manuscript. References Anger, K., Rogal, U., Schriever, G. and Valentin, C. 1977 In-situ investigations on the echinoderm Asterias rubens as a predator of soft-bottom communities in the western Baltic sea. Helgolander WissenschaftlicheMeeresuntersuc hungen 29,439-459. Arias, A.M. and Establier, R. 1979 Datos sobre las condiciones fisicoquimicas y el fitoplancton en 10sesteros de Cadiz. Instituto de Investigaciones Pesqueras, Informe Tkcnico 70,40 pp. Arias, A.M., Drake, P. and Rodriguez, R.B. 1984 Los esteros de las salinas de San Fernando (Cadiz, Espaiia) y el cultivo extensivo de peces marinos. In L’Aquaculture d u Bar et des Sparides (Barnabe, G. and Billard, R., eds). INRA Publ., Pan’s, 447-463. Ayala-Castaiiares, A. and Phleger, F.B. 1969 Coastal Lagoons: a Symposium. Universidad Nacional Autonoma de Mexico Press, Mexico, 688 pp. Bachelet, G. 1982 Quelques problemes lies a l’estimation de la production secondaire. Cas des bivalves Macoma balthica et Scrobicularia plana Oceanologica Acta 5, 421-431. Banse, K. and Mosher, S. 1980 Adult body mass and annual productionhiomass relationships of field populations. Ecological Monographs 50,355479. Boysen-Jensen, P. 1919 Valuation of the Limfjord. I. Studies on the fish food in the Limfjord 1909-1917. Rep. Dan. Biol. Stri. 26, 1-44. Ceccherelli, V. U. and Borgato, L. 1985 Ciclo biologico e produzione secondaria di Canuella perplexa (Copepoda, Harpacticoida). Oebalia 11,825-827. Clarke, G.L., Edmondson, W.T. and Ricker, W.E. 1946 Mathematical formulation of biological productivity. Ecological Monographs 16, 336-338. Crisp, D.J. 1971Energy flow measurements. In Methods for the Study ofMarine Benthos (Holme, N.A. and McIntyre, D., eds). International Biological Programme Handbook 16, Blackwell Scientific Publications, Oxford and Edinburgh. Chapter 12,197-279. Day Jr., J.W., Hall, C.A.S., Kemp, W.M. and Yaiiez-Arancibia, A. 1989Estuarine Ecology, Wiley-Interscience, New York, 422-424. Drake, P. 1983 Biologia de 10s Mugflidos en 10s esteros de las salinas de San Fernando (Cadiz). Niveles de la produccion extensiva, crecimiento y relaciones tr6ficas. Tesis Doctoral, Universidad de Sevilla, Sevilla, Espaiia, 120 pp. Edmondson, W.T. and Winberg, G.G. 1971 A manual on methods for the assessment of secondary productivity in fresh waters. International Biological Programme Handbook 17, Blackwell Scientific Publications, Oxford and Edinburgh, 358 pp. Farfan, B.C. and Alvarez-Borrego, S. 1983 Variability and fluxes of nitrogen and organic carbon at the mouth of a coastal lagoon. Estuarine, Coastal and Shelf Science 17, 599-6 12. Guelorget, 0. and Mazoyer-Mayere, C. 1985 Croissance, biomasse et production de Cerastoderma gluucum dans une lagune mediterraneenne: l’etang du Prevost (Herault, France). Vie Marine 7 , 15-27.

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Haedrich, R.L. and Hall, C.A.S. 1976 Fishes and estuaries. Oceanus 19,55-63. Heinle, D.R. 1969 Temperature and zooplankton. Chesapeake Science 10,186-209. Holland, A.F., Mountford, N.K., Hiegel, M.H., Kaumeyer, K.R. and Mihursky, J.A. 1980 Influence of predation on infaunal abundance in upper Chesapeake Bay. U.S.A. Marine Biology 57, 221-236. Huntley, M.E. 1980 Development and testing of a new method for estimating the production of marine zooplankton. Doctoral Thesis, Dalhousie University, Halifax, Canada, 120 pp. Ketchum, B.H. 1983 Estuaries and Enclosed Seas. Ecosystems of the World 26,Elsevier, Amsterdam, 500 pp. Klein Breteler, W.C.M. 1976 Settlement, growth and production of the shore crab, Carcinus maenas, on tidal flats in the Dutch Wadden Sea. Netherlands Journal of Sea Research 10,354-376. Kjerfve, B. 1986 Comparative Oceanography of Coastal Lagoons. In Estuarine Variability (Wolfe, D., ed), Academic Press, New York, 63-82. Kjerfve, B. and Magill, K.E. 1989 Geographic and hydrodynamic characteristics of shallow coastal lagoons. Marine Geology 88,187-199. Kuipers, B.R. 1977 On the ecology of juvenile plaice on a tidal flat in the Wadden Sea. Netherlands Journal of Sea Research 11,56-91. Landry, M.R. 1978 Population dynamics and production of a planktonic marine copepod, Acartia clausi, in a small temperate lagoon on San Juan Island, Washington. Int. Rev. Ges. Hydrobiol. 63,77-119. Lara-Lara, J.R., Alvarez-Borrego, S. and Small, L.F. 1980Variability and tidal exchange of ecological properties in a coastal lagoon. Estuarine and Coastal Marine Science 11, 613-637. Lasserre, P. and Postma, H. 1982 Coastal Lagoons. Proceedings of a n UNESCO-IABOSCOR International Symposium. OceanologicaActa 5,461pp. LeBlond, P.H. and Parsons, T.R. 1977 A simplified expression for calculating cohort production. Limnology and Oceanography 22, 156-157. LeBlond, P.H. and Parsons, T.R. 1978 Reply to comment by W. E. Ricker. Limnology and Oceanography 23,380382. Mas&, H. 1968 Sur la productivitk des peuplements marins benthiques. Cahiers de Biologie Marine 9,36-372. MillPn-Nuiiez, R., Alvarez-Borrego, S. and Nelson, D.M. 1982 Effects of physical phenomena on the distribution of nutrients and phytoplankton productivity in a coastal lagoon. Estuarine, Coastal and Shelf Science 15,317437. Miller, C.B. 1983 The zooplankton of estuaries. In Estuaries and Enclosed Seas, Ecosystems of the World 26 (Ketchum, B. H., ed), Elsevier, Amsterdam, 103-149. Moore, R. 1978 Variations in the diversity of summer estuarine fish populations in Aransas Bay, Texas, 1966-1973.Estuarine and Coastal Marine Science 6,495-501. Newell, R.C. 1982The energetics of detritus utilization in coastal lagoons and nearshore waters. In Proceedings International Symposium on Coastal Lagoons, SCOR I IABO I UNESCO (Lasserre, P. and Postma, H., eds). Oceanologica Acta 5,347455. Nixon, S.W. 1982Nutrient dynamics, primary production and fisheries yields of lagoons. I n Proceedings International Symposium on Coastal Lagoons, SCORI IABOI UNESCO (Lasserre, P. and Postma, H., eds). Oceanologica Acta 5,357471. Oglesby, R.T. 1977 Relationships of fish yield to lake hytoplankton standing crop, production and morphoedaphic factors. Journal of Fisheries Research Board of

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Canada 34,2271-2279. Parker, R.R. 1981 Guidelines ofecosystem research in coastal lagoons. In Coastal Lagoon Research, Present and Future, Proceedings of a Seminar, UNESCOIIABO, UNESCO Technical Papers in Marine Science 33,305414. Ricker, W.E. 1946 Production and utilization of fish populations. Ecological Monographs 16,373-391. Ricker, W.E. 1978 On computing production. Limnology and Oceanography 23,379-380. Saila, S.B. 1975 Some aspects of fish production and cropping in estuarine systems. In Estuarine Research, I. Chemistry, Biology,and the Estuarine System (Cronin, L.E., ed). Academic Press, New York, 473493. Silva-Cota, S. and Alvarez-Borrego, S. 1988 The “El Niiio” effect on the phytoplankton of a northwestern Baja California coastal lagoon. Estuarine, Coastal and Shelf Science 27, 109-115. Stross, R.G. and Stottlemeyer, J.R. 1965 Primary production in the Patuxent River. Chesapeake Science 6,125-140. Thienemann, A. 1931 Productionsbegrift in der Biologie. Arch. Hydrobiol. 22,616-621. UNESCO. 1981 Coastal Lagoon Research, Present and future. Proceedings of a n UNESCO I LAB0 Seminar, UNESCO Technical Papers in Marine Science 33,348 pp. Van Beek, F.A. 1976 Aantallen, groei, produktie e n voedselopname van de zandgrondel (P. minutus) en de wadgrondel (P. microps) op het Balgzand. Intern NlOZ Rapp., 1976-1979. Van der Schoot, E. 1974 Onderzcek naar het aandeel van Asterias rubens in de koolstofkringloop van het Grevelingen-bekken. Delta Instituut v m r Hydrobiologisch Onderzoek Yerseke, Doctoraalverslag 05-1974. Virnstein, R. 1977 The importance of predation by crabs and fishes on benthic infauna in Chesapeake Bay. Ecology 58,1199-1217. Virnstein, R. 1978 Predator caging experiments in soft sediments: Caution advised. In Estuarine Interactions (Wiley, M.,ed). Academic Press, New York, 261-273. Wagner, P. 1973 Seasonal biomass, abundance, and distribution of estuarine dependent fishes in the Caminada Bay system of Louisiana, Ph. D. Thesis, Louisiana State University, Baton Rouge, 193 pp. Winberg, G.G. 1971 Methods for the estimation ofproduction of aquatic animals. Academic Press, London, 175 pp. Wolff, W.J. 1977 A benthic food budget for the Grevelingen Estuary, the Netherlands, and a consideration of the mechanisms causing high benthic secondary production in estuaries. In Ecology ofMarine Benthos (Coull, B.C., ed). Belle W. Baruch L ibrary in Marine Science, University of South Carolina Press, Columbia, S.C., 267-280. Wolff, W.J. 1983 Estuarine Benthos. In Estuaries and Enclosed Seas, Ecosystems of the World 26 (Ketchum, B.H. ed). Elsevier, Amsterdam, 151-182. Wolff, W.J., van Haperen, A.M.M., Sandee, A.J.J., Baptist, H.J.M. and Saeijs, H.L.F. 1976 The Trophic role of birds in the Grevelingen estuary, the Netherlands, as compared to their role in the saline Lake Grevelingen. In Proceedings 10th European Symposium on Marine Biology, 2, Population Dynamics (Persoone, G. and Jaspers, W., eds). Universa Press, Wetteren, 673-689. Wolff, W.J. and De Wolf, L. 1977 Biomass and production of zoobenthos in the Grevelingen Estuary, the Netherlands. Estuarine, Coastal and Shelf Science 5, 1-24. Wolff, W.J., Abou Gueye, A.M., Piersma,T. and Mamadou Alassane Sall. 1987 Distribution, biomass, recruitment and productivity of Anadara senilis (L.) (Mollusca: Bi-

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valvia) on the Banc d’Arguin, Mauritania. Netherlands Journal of Sea Research 21,

243-253. Yaiiez-Arancibia,A. 1978 Taxonomy,ecology and structure of fish communitiesin coastal lagoons with ephemeral inlets on the Pacific Coast of Mexico. Instituto de Ciencias del Mar y Limnologta, Uniuersidad Nacional Autdnoma de Mbxico, Special Publication 2, 306 pp.

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Coastal Lagoon Processes edited by B. Kjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

311

Chapter 11

Macrofaunal Community Structure and Life Histories in Coastal Lagoons R.S.K. Barnes Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

The nature of the animals inhabiting coastal lagoons is analyzed and it is concluded that the characterization of faunas as 'lagoonal' is more an expression of the physiography of the habitat they happen to occupy than a statement of their biological identity. Also reviewed are the processes likely to structure the benthic animal communities of lagoons -both those determining the species present in any given lagoon and those responsible for the precise combination of species inhabiting any specific region within a system - with emphasis on studies conducted since 1979. Processes examined include: patterns of recruitment and colonization;predation (by infaunal species; small epibenthic crustaceans and juvenile fish; adult fish and birds); disturbance; adult/larval interactions; interference and exploitative competition; and environmental adversity. It is considered that lagoonal populations are often kept below the carrying capacity of their habitat, but as yet no study has assessed the relative contribution of each of the potential structuring forces on the long-term dynamics of any system. What we gained in the 1980s is a greater understanding of how some of the agents of mortality operate; we are no nearer to putting them in rank order of importance. In any event, most of the relevant work has been carried out in non-lagoonal environments: many more specificallylagoonal studies are a priority. Finally, the life-histories of lagoonal species are compared with those of their marine relatives. Few typically lagoonal adaptations are known, although there are some tantalizing glimpses of possible differences in the literature; again, more systematic comparisons of lagoonal and non-lagoonal animals are urgently needed.

Nature of Lagoonal Organisms and Communities

With a few exceptions, lagoonal organisms a n d the communities they comprise are t h e same as those that occur in other brackish-water or coastal marine, soft-sediment habitats (Remane, 1971; Barnes, 1980). Fringing communities are reed- or sedge-beds, salt-marshes or mangrove-swamps, dependent on salinity and/or latitude, and submerged ones are those of (a) unvegetated sands or silts, (b) t h e sea-grass or pond-weed meadows domi-

312

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

nated by Zostera, Halodule, Ruppia, Potamogeton or other angiosperms, and (c) beds of green algae, including charophytes. The most obvious macroscopic invertebrates are grazing, deposit-feeding, suspension-feeding and predatory annelids, molluscs and crustaceans (Table l l . l ) , whilst migratory decapod crustaceans and fish enter lagoons seasonally to feed and/or breed in both high and low latitudes (Muus, 1967; Villalobos et al., 1969; Barnes et al., 1979;Yaiiez-Arancibia et al., 1982; Paul, 1982; Snelson et al., 1984; Stoner, 1988; etc.). Most lagoonal species are essentially marine in character, although in the more dilute lagoonal waters, basically freshwater forms, such as corixid bugs and a wide variety of beetles, can be important, at times dominating, elements of the fauna (Barnes et al., 1971; Barnes and Jones, 1972). Even in the more saline systems, dipteran larvae can be a significant faunal component, as, for example, in the Varna Liman, Bulgaria (Caspers, 1951) TABLE 11.1 Trophic guilds of consumers in lagoons (partly after Reise, 1985) Guild

Food

Major taxa

interstitial consumers

bacteria, flagellates, diatoms, juvenile macrofauna, and each other

ciliates; gastrotrichs; nematodes; ostracods; harpacticoids; flatworms

grazerddeposit-feeders

diatoms, detritus and associated small organisms (including juvenile macrofauna)

gastropods; annelids; sipunculans; amphipod, isopod, tanaid and decapod crustaceans; insects; echinoderms

benthic suspension-feeders

bacteria, detritus, phyto- and zooplankton

sponges; bivalves; hydroids; bryozoans; polychaetes; cirripede and decapod crustaceans

benthic carnivores

other macrofauna

anthozoans; nemertines; polychaetes; gastropod and opisthobranch molluscs; decapods; insects; fish; birds

pelagic suspension-feeders

detritus, phyto- and zooplankton

rotifers; copepod, mysid and decapod crustaceans

pelagic carnivores

other macrofauna

scyphozoans; decapods; fish; birds

herbivores

sea-grasses; algae

tetrapods

R.S.K.Barnes

313

TABLE 11.2 Specialistlagoonal speciesof the northeasternAtlantic (specieslagoonal only during the larval phase omitted)(after Barnes, 1989b) Charophycae Cnidaria Polychaeta Insecta

Crustacea Mollusca Bryozoa

Chara canescens, Chara baltica, Chara connivens, Lampmthumnium papulosum, Laomedea loveni, Edwardsia ivelli, Nematostella vectensis Armandia cirrhosa, Alkmria mmijni S i a r a selecta, S i a r a stagnalis, Sigara concinnu,Agabus conspersus, Berosus spinosus, Coelambusparallelogrammus, Dytiscus circumfixus, Enochrubicolor,Enochrus melanocephalus, Enochrus halophilus, Haliplus apicalis, Ochthebius marinus, Ochthebiuspunctatus, Paracymus aeneus Sphaeromu hwkeri, Idotea chelipes, Gammarus chevreuxi, Gammurus insensibilis, Corophium insidiosum, Palaemonetes varians Hydrobia ventmsa, Hydrobia neglecta, Conopeum seurati, Victorellapavida

and the Grado-Marano, Lungo and Monaci lagoons, Italy (Vatova, 1979; Gravina et al., 1989). Nevertheless, at least in some areas, there appears to be a number of species that are more characteristic of lagoons than of other coastal brackish habitats, albeit that they are closely related to marindestuarine or freshwater species (Table 11.2). Although even here it is likely that these specialist lagoonal forms are not lagoonal per se but, in respect of those of marine affinity for example, are really restricted to non-tidal conditions (Barnes, 1989a). Their apparent lagoonal distribution pattern simply reflects the fact that along many coastlines, lagoons are the only areas of non-tidal sea water. Such species also occur, however, in the land-locked brackish seas of Eurasia (the Black, Baltic, Caspian, etc.) (Zenkevitch, 1963; Remane, 1971; Landin, 1972; Marty et al., 1974) and they are especially characteristic of geographical regions in which such intracontinental bodies of salt water have occurred (c.f. Wolff, 1973). At the most gross level, it would seem from the above that salinity regime is an important determinant of faunal composition (D’Ancona, 1959; BenTuvia, 1983),and a number of studies have identified a ‘freshwatercommunity’ near the point of freshwater inflow, a ‘marine’ one adjacent to the sea-water influx zone, and a ‘brackish’ or ‘lagoonal’ assemblage over the remainder of the system, as in the Gippsland Lakes, Australia (Poore, 1982), the Lagoon of Venice, Italy (Sconfietti, 1988) and Italian lagoons generally (Ardizzone, 19851, for example. Significant effects of salinity have also been shown in between-lagoon comparisons (Gravina et al., 1988; Barnes, 1988a; 198913).Whether there is any real salinity-based difference

3 14

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

between the marine and brackish faunal components is debatable, however (Barnes, 1989a). Guelorget and Perthuisot (1982, 1983) argue that it is nearness to the point of contact with the adjacent sea that is the real variable responsible for within-lagoonal faunistic changes, not salinity regime (see also Amanieu et al., 1981). Thus in the Aetoliko/Messologhi/ Klissova lagoon system in Greece, the fauna shows no effect of salinity on distribution (over the range 40-160% sea water), although two species associations comparable to ‘lagoonal’ and ‘marine’ ones nevertheless occur (Nicolaidou et al., 1988); and Guelorget, Perthuisot and their followers have obtained comparable results in other Greek systems, e.g. the Rhodia, Tsoukalio and Logarou lagoons (Guelorget et al., 1986), in the Pontine lagoons, Italy (Fresi et al., 1985; Gravina et al., 1989) and in the Lagune de Nador, Morocco (Guelorget et al., 1987; Clanzig, 1989). The characterization of faunas as lagoonal is thus more a n expression of the physiographic nature of the environment in which they happen to be living than a statement of much biological relevance.

Processes Structuring the Macrofaunal Communities Within the broad confines of geographical region and general habitat type including the nature of the sediments (see, for example, Quintino et al., 1987, on the Lagoon of Albufeira, Portugal, and Parker, 1969, on the Rockport Bay and Hadley Harbor lagoons, U.S.A.), what species are present in any semi-isolated system like a lagoon and in what relative abundances are highly variable, dynamic properties that can be modelled by the same dynamic equation used as the basis of population biology. The species occurring, granted the available pool of potential colonizers, will be a product of colonization and extinction rates, whilst the abundances of the component species will reflect increments of recruitment and decrements of mortality. At the species composition level, of course, recruitment is the same as colonizatiodrecolonization, and decrements of mortality can proceed to extinction. These processes can be considered to operate at two distinct, but nested levels: (a) on the organisms present in a given lagoon; and (b) since few if any organisms will be distributed evenly within even the smallest and most homogeneous system, on those present within any local patch of substratum or whatever. Local abundance can reflect decrements of emigration as well as of mortality, and stochastic events and some biological interactions are more likely to result in patchy distributions than in global effects within a lagoon. In fact, very few studies of the processes structuring lagoonal communities have been carried out, and the discussion below will have to rely heavily on work conducted in other habitat types that are considered by this author to be potentially similar to lagoons. In practice, I have restricted my nonlagoonal coverage to such other shallow-water, soft-sediment systems as

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share numerous species with genuine lagoons. Most of the available literature derives from studies of the intertidal zone, and the extent to which this can be extended to the more pond-like lagoonal systems is not known. There are certainly marked differences in the forces structuring even intertidal populations at different heights above low water. Sanchez-Salazar et ul. (19871, for example, found that high up the shore, cockles (Cerustodermu) suffered high adult but low juvenile mortality, whereas lower down, adult mortality was low but juvenile losses were high. The crab Curcinus was the major structuring agent of low tidal populations and shorebirds, especially the oystercatcher Huematopus, that of high level ones. Growth rates also vary with intertidal location, not always predictably on the basis of potential feeding time (Peterson and Black, 1987; 1988a). All things being equal, one might imagine that lagoonal populations would approximate to the lowest of intertidal ones, rather than to those subject to considerable periods of aerial exposure, but as we shall see this has been questioned. This review is also largely restricted to work carried out since the seminal paper of Peterson (1979a). Virtually all studies of the processes structuring aquatic communities have used a caging or enclosure technique to exclude or include suspected interacting organisms of interest, and this basic tool is not without its critics. Caging artifacts (especially in respect of changed patterns of sedimentation and the dependent biological effects), experimental and statistical design faults (particularly lack of proper controls and inappropriate durations of experimental treatments), and other problems have all been held to negate the conclusions reached by various studies (see Virnstein, 1978; Hulbert and Oliver, 1980; Underwood, 1981; 1988; Bradley, 1983; Connell, 1983; Gallagher et ul., 1983; Hurlbert, 1984; Kneib, 1988). Nevertheless, as Valiela (1984) has pointed out, there are not many alternatives available to study biological interactions and the potentially powerful caging approach cannot be abandoned. Instead, more attention should be paid to experimental design than has always been the case in the past (see Hurlbert, 1984; Kneib, 1988; and Underwood, 1988 for particularly helpful comments). In the 1970s and earlier, competition was ‘the central ecological dogma’, although as Sih et ul. (1985) noted in their review, by the early 1980s, largely as a result of pioneering work on rocky shores, predation had begun to usurp this role of the main force thought to structure ecological systems, or at least had come to share centre stage. Thus “competition and predation are fundamental in controlling benthic community structure, although the nature of the sedimentary environment has made it very difficult to conduct manipulative experiments” (Lopez, 1988) and “physical processes determine overall habitat conditions and productivity cycles whereas biological processes such as predation and competition define specific community relationships” (Livingston, 1984).By the mid 1980s, however, several more

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

processes had begun to assume equal status in the pantheon. Considering agents of mortality, various studies have highlighted the importance not only of conventional competition and predation, but disturbance (for example bioturbation), adulfflarval interactions, interference, indiscriminate consumption, and environmentally-induced mortality; and the unimportance of all agents of mortality as structuring forces has been gaining popularity in cases where abundances are limited by the supply of recruits. The current ‘state of the art’ has not progressed all that far beyond the stage at which each author is free t o argue the merits of his or her favoured combination of factors to the detriment of the others. Menge and Sutherland (1987), however, have recently presented a general integrative model that may have a wider applicability than the rocky shores that acted as its midwife. Below I assess these various potential structuring forces insofar as lagoons are concerned. ‘Supply-sideEcology‘: Problems of Recruitment and Colonization Colonization is probably not a major problem in those lagoons that have a high degree of connectance with the sea. Many lagoons, however, have no inlevoutlet channel and receive sea-water influx only via percolation through relatively coarse isolating barriers of shingle or by the rare overtopping of their enclosing barrier during storms coincident with high tide: in some parts of the world such isolated systems are the norm (see, e.g., Baudin, 1980; Barnes, 1989a). These lagoons are characterized by impoverished faunas, especially of the component of marine affinity. Further, the species present may be a random subset of the general lagoonal species-pool in the geographical region concerned. In a study of 24 land-locked lagoons in East Anglia, U.K., Barnes (1988a) found no correlation of lagoonal faunas with such obvious habitat variables as area, sediment type or physiographic form, and that 80-95% (dependent on test employed) of possible betweenlagoon comparisons showed no significant faunal similarity whatever. Birds did not appear to be the agents of dispersal in that lagoons much frequented by wildfowl andor shorebirds did not possess more similar faunas than comparisons involving outgroups, and Barnes concluded that colonization was mainly effected during periods of coastal flooding (during the storm surges that affect the southern North Sea every 25 years or so; Sainty et al., 1938;Wagret, 1959; Banham, 1979); animals being dispersed odin floating mats of green algae. Coastal defense work specifically to prevent such coastal flooding has now limited the frequency and magnitude of marine transgressions, and most of the lagoons now contain relict faunas. Even some lagoons that are freely connected to much larger and more species-rich systems may be depauperate. Divanach et al. (1987), for example, consider that many aspects of the ecology of the lagoons of the Languedoc and Roussillon Mediterranean coasts of France are attributable to the absence of potential predators and competitors.

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Such apparent undersaturation of a number of lagoonal communities is also suggested by the consequences of invasion of some of these habitats by immigrant species. Reise (19851, for example, records that the arrival of Crepidula, Petricola, Ensis directus, and Tharyx in the European Wadden Sea from the other side of the Atlantic did not lead to any compensatory decline in diversity or density of the pre-existing native forms, even though organisms like Tharyx are now numerically dominant in some areas. Similarly, Fenchel(1975) pointed out that the faunas of Danish lagoons were not “natural units with a characteristic composition” but were dynamic and stochastic assemblages resulting from the interactions of colonization, local extinction brought about by unpredictably occurring, climatically induced catastrophes (see below), and biotic interactions (see also Siegismund and Hylleberg, 1987, and Chenill and James, 1987).For some fugitive species, a given lagoon could provide a temporary rehge until, maybe, competitively superior conspecifics finally succeeded in (re)colonizingit. At the scale of the individual species population, it is now accepted that some rocky shore species may be recruitment limited because of the vagaries of the supply of planktonic larvae (e.g. Underwood et al., 1983; Keough, 1984; Gaines and Roughgarden, 1985; Sutherland and Ortega, 1986; Grigg, 1988; Underwood and Fairweather, 1989). Recruitment limitation has also been postulated t o be of importance in structuring so&-sediment systems (Wildish, 1977; Mahoney and Livingston, 1982; Wilson, 1984a) but there have not been any critical studies to substantiate this, nor to differentiate between (a) failure of larvae to arrive in sufficient numbers (i.e. genuine recruitment limitation as discussed here) and (b) high benthic mortality of a larval supply that would otherwise be sufficient for attainment of the carrying capacity, other things being equal. Clearly, some lagoons are open-ended, non-equilibrium systems of the type discussed by Paine (19841, Wiens (1984) and Chesson and Case (19861, but we do not yet know how many of them fall into this category. Certain lagoons are undoubtedly at the other end of the spectrum, and the effects of immigrant molluscs are not always nearly so benign as in the Wadden Sea (see Mistri et al., 1987 on the lagoons of the Po Delta, for example). Agents of Mortality and their Effects

Predation In this section I will consider the effects of four different categories of predatory species on prey populations: (i) shorebirds; (ii)juvenile and small epibenthic fish, decapod crustaceans and other epifaunal invertebrates; (iii) infaunal polychaetes, nemertines, etc.; and (iv) adult and/or larger species of fish, crabs and other consumers. The reasons for distinguishing these different groups will be self-evident below.

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

Birds There is no question that individual shorebirds feeding on intertidal flats can consume huge numbers of soft-sediment macrobenthos per day. A medium-sized wader like a redshank (Tringa),weighing some 0.13 kg, eats in the order of 40,000 Corophium each day (Goss-Custard, 1969). A population of 10,000 redshank, such as is accommodated in the Dutch Wadden Sea in August for example (Smit and Wolff, 19831, therefore could eat about 400 million of this amphipod a day, equivalent t o 0.15 million a year. Similarly, the oystercatchers of the same area could eat 45 million Cerastoderrna each year, and the knot (Calidris) some 13 thousand million Macoma. The effect that this prodigious consumption has upon the prey populations, however, is almost certainly negligible (Muus, 1967; Raffaelli and Milne, 1987) in spite of the sometimes significant proportions of adult prey individuals taken (Evans et al., 1979; Puttick, 1980; Kent and Day, 1983; Quammen, 1984; Baird et al., 1985). This is for two main reasons. First, shorebirds feed on relatively large individuals of their preferred prey (see below) and large individuals are usually an insignificant proportion of the total prey population. Thus, in the Gironde Estuary, France, for example, Bachelet and Yacine-Kassab (1987) have shown that there has already been a mortality of 99% before Hydrobia ever achieve a size that would bring them within the sphere of interest of the numerous bird species that consume this gastropod. Only 13% of the post-metamorphic Gironde snails survive for more than 3 months, and less than 1%survive for 9 months. Using the largest known percentage mortalities inflicted on >1year old Hydrobia by birds (from Evans et al., 19791, bird predation could account for less than 0.1% of that to which even the postmetamorphic snails are subject. The second reason why bird predation is insignificant as a structuring force concerns the nature of the consumers. Birds are much larger than their prey (a redshank weighs some 100,000 times as much as its Corophium prey) and they are endothermal homeotherms. Their food requirements are therefore very large. If my redshank example eats 40,000 Corophium per day on average in winter, and if its prey have an intertidal distribution centred around the region of mean high water neap tide level, then prey will only be available to the predator for some 5 daylight hours a day, and the redshank will each have to obtain 1Corophium during each half second of this period in order to achieve their daily intake quota. And this is of an animal hidden from their sight, up to 4 cm below the sediment surface! Not surprisingly, shorebirds are highly selective of both foraging area (i.e. prey availability) and prey size: they forage as optimally as possible (Boates and Smith, 1979; Peer et al., 1986; Piersma, 1987; Zwarts and Esselink, 1989). This means that the effect of prey density on the birds is much greater than the effect of the birds on the invertebrates. Feeding must be concentrated in areas of greatest adult prey abundance and its effects are

R.S.K.Barnes

3 19

% 15

ri

1973

1 1

10

5

25

50

7 5 mm

%

19.30

25

5 0 mm

Mussel size

Fig. 11.1. The size range of benthic prey (Mytilus)taken by shorebirds (Haemtopus) (dashed line) in relation to the size range available to them (solid line) in the European Wadden Sea. Data for August-November 1973, and November, 1980.(After Zwarts and Drent).

necessarily patchy; areas of even moderate prey density may have to be ignored if they cannot supply sufficient acceptable items per unit time. The smaller size ranges of the prey, be they Hydrobia or the mussel Mytilus (Fig. l l . l ) , are too small to be taken and, in any event, would not be energetically worth the effort. If shorebirds cannot limit their intertidal invertebrate prey populations, it is inconceivable that they could structure the less accessible subtidal populations that characterize lagoons. Small Epibenthic Predators

Many authors, however, have shown that excluding small fish the size of gobies, the juvenile stages of larger fish species, and small crabs, shrimps, prawns and similarly sized epibenthic predators can have a dramatic effect on the density of infaunal species, and often on the numbers of such species present, relative to control areas (e.g. Brock, 1980; Holland et al., 1980; Wiltse, 1980; Dauer et al., 1982; Peterson, 1982a; Federle et al., 1983;

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

TABLE 11.3 Abundance of macrofauna (retained on a 0.25 mm mesh sieve) in muddy substrata in the European Wadden Sea compared with that in areas enclosed, to exclude epibenthicpredators, within 1 mm mesh cages for four months during late summer/autumn (data in Reise, 1978). Numbers are those per tenth of a square metre Uncaged mud Hydrobia MYa Cerastodermu Spisula Tubificoides Pygospio Spio Polydora Malacoceros Thnryx Capitella Heteromastus Nephtys Eteone Corophium + 14 other species

Total

Caged mud

0

10 87 1282 185 3055 350 50 532 405 5322 140 222 5 117 490 87

1189

12339

2 0 7 0 820 17 2 0 0

7 92 240

2 0 0

Sheridan and Livingston, 1983; Botton, 1984; Summerson and Peterson, 1984; Wiltse et al., 1984; Hoffman et ul., 1984; Jensen and Jensen, 1985; Kneib, 1985; Reise, 1985; Schubert and Reise, 1986; etc.). Table 11.3 will serve as an example of the effect of excluding epibenthic predators; the same results have also been obtained by enclosing such predators within caged areas (e.g. Nelson, 1981;Fitzhugh and Fleeger, 1985;Ronnet al., 1988).The Table also shows two further features characteristic of such experimental manipulations: the adults of deep-burrowing species, such as Heteromastus, are least or completely unaffected (Holland et al., 1980; Shaffer, 1983);and opportunistic species like Cupitella may be unaffected or may actually invade cages because of sedimentological changes induced (Gray, 1981). Effects are not restricted to the infaunal elements. Federle et al. (19831, for example, describe changes in the types of bacteria dominating the substratum and in overall microbial abundance resulting from predator exclusion, presumably via effects on deposit feeders.

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In contrast, other authors (e.g. Berge and Hesthagen, 1981;Shaffer, 1983; Berge and Valderhaug, 1983; Thrush, 1986; Raffaelli et al., 1989; Jaquet and Raffaelli, 1989) have failed to find any infaunal increase under comparable experimental treatments. Apart from the obvious inference that predators do not play a part in structuring those systems, it is clear that the caging technique does not always produce the ‘expected’ results. Sih et al. (1985) in a survey of 60 marine caging studies found that 37% of them produced “unexpected” or “counterintuitive” changes in abundance of the supposed prey species. In some cases, it appeared that certain small epibenthic predators had managed to pass through the mesh of the ‘exclusion cages’ and had even accumulated within them (as occurred in the Indian River Lagoon study of Young et al., 1976; Young and Young, 1977), and in other studies the timing and/or duration of caging experiments has been such as to restrict the possible effects of the predators to adult or relatively large prey individuals. We saw above that the most significant benthic mortality can occur at the immediately postmetamorphic stage and hence, to detect the real impact of epibenthic consumers, it may be necessary to exclude (or include) them specifically at the times of recruitment to the sediments. Gee et al. (19851, for example, recorded negligible predatory effects of crabs and gobies on large prey individuals but significant reduction in the smallest size classes of prey. Of even greater potential significance as complicating factors are second order effects such as those relating to other (infaunal) predators forming part of the ecological system enclosed within the cages (see below). There are also some lagoonal subsystems in which predators do not appear t o exert such profound effects and which serve as refuges for susceptible prey species or life-history stages. The presence of surface material in the form of shell debris or protruding worm tubes reduces the foraging success of mobile epibenthic consumers: exclude the consumers and infaunal abundance in and around such material may decline to ambient levels (Woodin, 1981); add artificial worm tubes and infaunal density increases significantly (Dauer, Tourtellote and Ewing, 1982). An even more important refuge system is provided by beds of submerged macrophytes both for the prey (Peterson, 1982a; Nelson et al., 1982; etc.) and for the epibenthic predators themselves (Rozas and Odum, 1988) which may hide in the sea-grasses, etc. during the day and roam out to forage over nearby unvegetated zones, where their foraging efficiency is higher and their freedom of movement less restricted, during the night (Summerson and Peterson, 1984). The effect of predator exclusion on the infauna of submerged aquatic plant beds is therefore usually minor (Heckand Thoman, 1981; Brenchley, 1982; Summerson and Peterson, 19841, although some authors favour predation as one of the main structuring forces even here (Sheridan and Livingston, 1983; Orth et al., 1984; Summerson and Peterson, 1984).

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

The refuge effect, together with the increased food supply for grazers, increased spatial heterogeneity, etc., result in there being a general positive relationship between density and diversity of macrofauna and macrophyte abundance both in lagoons (e.g. the Indian River Lagoon; Gore et al., 1981; and the Mazoma Lagoon, Greece; Nicolaidou and Karakiri, 1989) and elsewhere (Heck and Wetstone, 1977; Homziak et al., 1982; Peterson, 1982a), irrespective of latitude (Virnstein et al., 1984). Dieback of the submerged vegetation can then cause a marked reduction in local invertebrate biomass, as has been recorded in the Swartvlei Lagoon, South Africa, for example (Davies, 1982; Whitfield, 1984).

Infaunal Predators If the effect of excluding epibenthic predators from areas of sediment is to enhance survival of their infaunal prey species, then either a pool of unused prey individuals may be created which can be exploited by yet other predatory species living within the same substratum (if the prey of the epibenthic species are not these infaunal predators) or else, if the fish, etc. above the surface feed preferentially on the predatory infauna, there will be knock-on effects on their prey species. In either event, the effect of excluding the epibenthic predators will not necessarily be any change in overall infaunal abundance or diversity but may be an alteration in the relative abundances of different members of the fauna. Equally, the effect of adding predatory species (whether epi- or infaunal) t o enclosures can, paradoxically, be to increase the densities of many of the ‘prey’species. Thus Kneib (1988) found that the anemone Nematostella increased in the presence of the killfish Fundulus, and Commito (Cornmito, 1982; Commito and Shrader, 1985) has demonstrated that increasing the density of the polychaete Nereis increased the numbers of various other infaunal species. In both cases (see also Frid and James, 1988), the counterintuitive results were obtained because of interactions within the complex food web of the infaunal compartment. The killifish were preying on the prawn Palaemonetes which itself was a consumer ofNematostella (in turn a predator on Hydrobia);whereas the Nereis consumed Corophium (Cornmito, 1982) and probably Nephtys (Cornmito and Shrader, 1985) whose densities were negatively related to those of various infaunal annelids (see also Beukema, 1987).Here Nereis was acting as the top infaunal carnivore, but it too may have infaunal predators. Ambrose (1984a) describes a GlyceralNerezslother-infaunasystem in which the densities of most infaunal species are highest in the presence of increased densities of the predatory Glycera (which consumes Nereis) and lowest in the presence of elevated numbers of Nereis (Fig. 11.2). In the light of the interaction between Palaemonetes and Nematostella above, it is noteworthy that in the lagoons of Britain, the anemone is only abundant at sites from which the prawn in absent (Barnes, unpublished).

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323 Bivhlves

Streblospio Mean number

scoloplos Polydora

Der 0.1 m1

all species Mean

4000 3000

number 2000

per 1000 O.lmz Qlycera added

a

Control

Nereis added

Fig. 11.2. The effect on various soft-sediment macrofaunal species of adding the predatory polychaetes Glycera or Nereis, compared to control areas without the addition of either predator. The results shown are those obtained 10 weeks after the addition of the worms to the inclusion cages. (After Ambrose, 1984a.)

Reviewing such complex interactions within the sediment, Ambrose (1984b) considered a three-level system to be operating (Fig. 11.31, with the predatory infauna occupying an intermediate position between the epibenthic (and indeed larger vertebrate) consumers and the grazing, deposit- or suspension-feeding invertebrates. There is indeed some evidence (Table 11.4) to suggest that the predatory infauna are the preferred prey of many of the epibenthic consumers (Ambrose, 1986a; and see Wilson, 19861, perhaps because many are active at or just below the sediment surface and are therefore more readily available to surface consumers and/or perhaps because of their greater body size on average. Nevertheless, the epibenthic consumers may also, for the very same reasons, concentrate on some of the

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

Fig. 11.3. The three-level system of predator-prey relations within soft sediments envisaged by Ambrose (1984b).The magnitude of the arrows is proportional to the suggested importance of the relationship.

TABLE 11.4 Changes in the ratio of predatory t o non-predatory animals within the sediment after caging to exclude epibenthic predators (from data compiled by Ambrose, 1984b) Numbers of experiments in which ratio in

muddy-sand sea-grass beds

caged > than in control areas

caged < than in control areas

caged = that in control areas (within 10%)

18 11

8

4 3

5 ~

.

non-predatory prey (data in Reise, 1978;1981;1985).Cages installed immediately after spatfall of young Cerustoderma that excluded juvenile crabs, flatfish and shrimp, all known consumers of the spat, produced a twentyfold increase in cockle survival compared to ambient and control areas (Fig. 11.4).Referring back to the effect of submerged macrophytes on community

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100

Numbers per

0.01 mz 50

A

B

Fig. 11.4.Numbers ofjuvenile cockles (Cerastoderm)in the European Wadden Sea: A, in the ambient sediment and beneath 20 mm mesh exclusioncages; and B, beneath 5-0.5 mm mesh cages. Cages were in place from April to July, 1975. (After Reise, 1981.)

Fig. 11.5. The complexity of predator-prey relations within the benthic and epibenthic macrofauna of most soft-sedimenthabitats (cf. Fig. 11.3).

structure mentioned above, Ambrose (198413) also notes that, correlated with the decreased role of epibenthic predators there, the predatory component of the infauna is especially numerous and diverse.

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Macrofaunal Community Structure and Life Histories in Coastal Lagoons

The three-level system is, of course, a minimum estimate (Cornmito and Ambrose, 1985; Ambrose, 1986a) since, as in the Fundulus- PalaemonetesNematostella-Hydrobia example of Kmib (19881, more than one species of consumer may be interpolated between epibenthic fish and detritus-feeding worm or gastropod (Fig. 11.5). Many of the predatory infaunal polychaetes, nemertines, gastropods, etc. prey on other infaunal species that may themselves be carnivorous: in the Glycera,Nereis, andNephtys chain cited above, all three worms are predators (Ambrose,1984a; Schubert and Reise, 1986; Beukema, 1987).

Fish Although adult fish are known to exercise a structuring role in many aquatic habitats (Choat, 1982), their effect on the faunas of soft-sediment areas is poorly understood. Such evidence as is available suggests an importance, as consumers, of the same order of magnitude as that of shorebirds (Wolff and Wolf, 1977; Nelson, 1981; Baird and Milne, 1981; Mahoney and Livingston, 1982; Kent and Day, 1983; Quammen, 1984; Raffaelli and Milne, 1987; Mattila and Bonsdorff, 1989), although they may limit the low tide distribution of some susceptible intertidal species (Posey, 1986). Adult crabs (e.g. Callinectes or Carcinus) may be important seasonally (Quammen, 1984) and in relatively deep water, as at 23 m depth in Lough Hyne, Eire (Thrush, 1986); their effect in lagoonal sea-grass beds, however, has been considered to be very minor (Nelson, 1981). Perhaps of more importance than their ability to cause significant infaunal mortality is the cropping of parts of prey individuals that are located near the sediment surface; for example the tail ends of lugworms (Arenicola and Abarenicola) and Heteromastus, the tentacular crowns of sabellid worms (Was, 1979a and b) and siphons of clams (Peterson and Quammen, 1982). The cropped individuals can regenerate the lost portions, but in order to do so considerable diversion of resources away from growth and/or reproduction must take place with concomitant effects upon productivity and recruitment. Although data are scarce, it has been considered that the small epibenthic predators are themselves controlled by the larger predatory fish and crabs (Evans, 1983; Reise, 19851, if not by interspecific competition for their infaunal prey (Evans and Tallmark, 1985; Pihl, 1985; Moller et al., 1985), and that juvenile fish and crustaceans are using the lower half of the intertidal zone as a refuge from this predation (Fig. 11.6).

The Role of Predators and Parasites As noted above, most of the work presented here has been conducted in the relatively easily investigated littoral zone, and its relevance to the greater part of lagoons might therefore be questioned because of known

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327

t Small epibenthic juveniles

m

Magnitude of potential effect On

prey populations

High Water Mark

intertidal zone

Water Mark

Subtidal zone

Fig. 11.6.The differential effects of large and small epibenthic predators on prey populations in relation to depth of water, as hypothesized by Reise (1985).

differences between intertidal and subtidal ecosystems (e.g. Knott et al., 1983) and because of conclusions such as those of Reise (1985) that epibenthic predation is a structuring force only over the lower half of the shore (Fig. 11.6). Several lagoons, however, lack the larger fish predators that might control epibenthic species in marine environments: many lagoons are equivalent, at least in terms of their water depth, to intertidal zones experiencing continual high-tidal water cover. Their use as nursery grounds by marine decapods and fish (see Hewe and BruslB, 1980; Roblin and Bruslk, 1984; Yaiiez-Arancibia and Pauly, this volume, and references cited above) can plausibly be interpreted as much as a means of avoiding larger fish predators (see, e.g., Kneib, 19871, as of exploiting rich supplies of benthic food. Dauer et al. (19821, investigating the 1.5m depth zone in the lagoon-like Broad Bay, U S A . , certainly found that epibenthic predator exclusion there had the same effects as those recorded for intertidal locations. Although, therefore, the roles of birds and large nekton may be minor (Fig. 11.71, it seems likely that those of the smaller epibenthic prawns, young crabs and fish are a significant source of mortality on benthic invertebrates, as also is that of the infaunal predatory component. It is known that lagoonal populations of Hydrobia are more heavily infested with digenean parasites than are adjacent marine ones (Barnes, 1990 and unpublished), and if this is generally true, parasitism may be as important a cause of mortality as predation, as has been argued by Lauckner (1980; 1986). As yet, however, we know almost nothing of the interaction of parasites with the ecology of their lagoonal hosts (although see Sousa, 1983, on digenean parasitization of the gastropod Cerithidea in

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

328

i f small epibenthic

Number of individuals

I\ Resulting curve if large epibenthic

individual

age

Fig. 11.7. A survivorship curve of a hypotheticalbenthic prey population and the hypothesized effect of excluding (a) small epibenthic predators, such as juvenile crabs, prawns and fish,and (b)large predatory species, such as birds and large fish, that preferentiallyconsume large prey individuals (after Reise, 1985).

Bolinas Lagoon, U.S.A.).It is possible that the mortality that they induce is only felt by large, post-reproductive individuals (Lauckner, 1980). Interference

A number of biological interactions that are capable of depressing the numbers of the participants and, potentially, of excluding certain species from areas inhabited by others do not fall within the compass of the words ‘predation’ and ‘competition’ as they are usually understood. These include the effects of alterations to the sedimentary environment resulting, for example, from locomotory or feeding activities, of ‘accidental’ destruction of minute larvae or other early life-history stages by larger animals, and of negative interactions between organisms where there is no evidence that any shared resource is in fact limiting their joint abundance. Such effects can all be embraced by the term ‘interference’. Disturbance The action of the predators considered in the previous sections is not confined to the consumption of prey individuals, since in order to find food in soft substrata, in which most organisms live below the surface, it may be necessary to pick over the surface layers, to burrow, to excavate pits, to suck up mouthfuls of particles, and so on. These all create a bioturbatory disturbance that may uncover animals from concealment, destroy their burrow systems and generally destabilize the uppermost few centimetres of the

R.S.K.Barnes

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substratum (Thistle, 1981; Probert, 1984). Juvenile fish may turn over every square metre of the sediment surface, down to a depth of 2 mm, whilst they are in residence in their nursery area, bioturbating 1% of the available surface per day (Billheimer and Coull, 1988);crabs can have a similar effect (Gee et al., 1985). In areas subject to strong water movement, these effects, although apparently severe, are dwarfed by physically-induced changes to the sediment distribution (Grant, 1983).Although 30% of the surface sediment of the North Inlet Estuary, U.S.A.,was covered by the feeding pits of rays, Grant calculated that ray bioturbation was less than 1%of that caused by tidal migration of sand ripples. Indeed, only one animal species moved sediment at a rate of more than 1% of that achieved physically, and since that one bound the moved sand with mucus, its effect was a stabilizing one. Nevertheless, rays have been shown to have significant effects on sediment-inhabiting crustaceans (Reidenauer and Thistle, 1981;Van Blaricom, 19821,and in many lagoons the activities of predators and other sediment movers (see below) can be predicted to be the dominant component because lagoons, almost by definition, are regions of relatively weak water movement. Only the zone adjacent to the marine inleiloutlet channel(s) is likely to be physically disturbed to the extent documented by Grant (1983).Knock-on effects, however, may mean that the effect of predators is magnified many times beyond its original impact. Rays can cause considerable damage to beds of sea-grasses, leading to the production of bare areas (Orth, 1975) with all the consequences discussed earlier that this may have on macrofaunal abundance. Almost all information on predator-induced disturbance and its effects on macrobenthic species is anecdotal, not least because of the difficulties inherent in separating the predatory and disturbance elements. Kneib (1985),however, was able to distinguish the two in the prawn Pulaemonetes by surgically removing the dactyls from their chelae. This prevented the prawns from grasping prey items as they picked over the sediment, and any effects of the treated individuals on the benthos could then be ascribed to disturbance. His results showed that although Pulaemonetes with functional chelae reduced the densities of the smaller macrofauna, only Hydrobza declined as an apparent consequence of prawn disturbance. Other sediment movers will be considered below under the heading Interference ‘competition’.

Adult ILarval Interactions

It is clear that the nature and abundance of the organisms inhabiting a given patch of sediment can have a marked bearing on the success with which settling larvae or other juvenile stages can establish themselves there, although the effects are sometimes subtle and counterintuitive (Am-

330

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

brose, 1 9 8 4 ~ )Juvenile . stages of the macrofauna may face overt predation from meiofaunal organisms, especially turbellarians (Watzin, 1983; 1986) (although syllid larvae may thrive in the face of turbellarian abundance because they feed on various flatworms; Watzin, 19831.A more covert form of predation is incidental destruction of larvae by pre-existing adults of the same or of different species during their feeding or locomotory activities. The microscopic young are not necessarily consumed; they may simply be filtered from suspension in the near-bottom water and then deposited onto the substratum bound up with inert material in a mucous pseudofecal pellet, or be buried by sediment movements induced by larger organisms (e.g. Wiltse, 1980; Elmgren et al., 19861. That passive destruction of juveniles is a significant force in structuring soft-sediment communities was pioneered by Woodin (19761, and since then negative interactions have been demonstrated between successful larval settlement and resident gastropods (Hunt et al., 19871, clams (Williams, 1980; Peterson, 1982b1, spionid polychaetes (Levin, 1981; Wilson, 1981; Whitlatch and Zajac, 1985; Tamaki, 1985; Luckenbach, 1987;Wilson, 1988), nereid polychaetes (Kent and Day, 1983; Luckenbach, 19871, capitellid, cirratulid, nephtyid and phyllodocid polychaetes (Wilson, 19881, tanaids (Brenchley, 1981; Highsmith, 1982), ophiuroids (Crowe et al., 1987), and amphipods in the classic PontoporeialMacoma interactive system in the Baltic (Elmgren et al., 19861. Not all larvae are equally affected: some may have chemical defences deterring capture (Cowden et al., 1984). Once again, however, quantitative data are rare and the magnitude of the larval losses incurred is open to debate. Woodin (e.g. 19761, for example, interprets the age-class effects that are seen in filter-feeding bivalve populations in terms of destruction of all settling larvae for several years by one successful year class of established animals until, on the death of that cohort, the area is released again for colonization by larvae that in turn may grow into organisms that occupy the substratum exclusively for some time. Commit0 (1987), on the other hand, found no such effect in the mussel Mytilus, neither did Maurer (1983) in Mercenaria nor Olafsson (1989) in Mucoma; and other authors ascribe the age-class phenomenon to the action of environmentally induced mortality (Gray, 1981) and rare escapes from predatory control (Reise, 1985). Neither are all adult-larval interactions necessarily negative. Highsmith (1982), for example, records that recruitment of young Dendraster is greatest in sediments already containing large numbers of adults, for not only do the adults exclude the tanaid Leptochelia which would otherwise prey on the settling larvae, but the larval sand dollars are attracted by a chemical cue released by the adults. Similarly, Crowe et al. (1987) found that moderate densities of the bivalve Abra appeared to facilitate larval settlement in the Gullmar Fjord, Sweden, and moreover Gallagher et al. (1983) advocate a larval facilitation model rather than the more popular inhibition one.

R.S.K. Barnes

331

.-m X c

r

m

m

I Hobsonia Pseudopoiydora Tanais Macoma

I

+

0

lo I

0

I+

Artificial tubes

+

+

7

0

0

+

+

0

--

0

0

+

0

0

0

+

0

0

-

+

0

-

Il

-

Facilitation inhibition 0 no significant effect

Fig. 11.8. Prevalence of adulfflarval facilitationand inhibitionin an intertidal deposit-feeding community in Puget Sound, U S A . (after Gallagher et al., 1983).

They suggest that larvae actively select settlement sites near adult tubedwelling deposit feeders in order to explain the higher recruitment of oligochaetes, Munuyunkiu, etc. around the tubes of spionid and ampharetid polychaetes (Fig. 11.81, and they argue that this is not a refuge-from-predation effect of the type discussed above. We are clearly still far from having assembled a suite of experimentally-tested ‘rules’ explaining larval settlement (Woodin, 1986).

Interference ‘Competition’ Interference ‘competition’ is a category of competition-like effects occurring in circumstances in which there is little or no reason to believe that any shared resource is really insufficient for the requirements of all concerned. It merges into competition for space, since patterns of utilization of the sedimentary space underlie much interference ‘competition’; it differs, however, in that intolerance of the presence of others or of the habitat changes that they induce are the operative agencies rather than any direct effects of preemption of actual physical space. The responses elicited in the ‘competitively inferior’ are therefore avoidance, emigration, etc. and not necessarily direct mortality or decreased growth or reproductive rates. In 1970, Rhoads and Young proposed the ‘trophic-group amensalism

332

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

hypothesis' in which they suggested that deposit feeders rendered the habitat unsuitable for suspension feeders by making the surface layers of sediment fluid, faecal-pellet rich and easily resuspended. Such soft muddy material would (a) clog the filtration systems of the suspension feeders, (b) bury their newly settled larvae, and (c) cover any surfaces to which sessile species could otherwise attach. In the later 1970s, further ecologicallyequivalent groups of species were added to the hypothesis and its base was widened from being purely a matter of feeding type to one embracing general life style as well: it became the 'functional-group amensalism hypothesis'. Thus Woodin (1976) distinguished deposit feeders, suspension feeders (which could consume the larvae of members of other functional groups and prevent their establishment; see above), and tube builders, which restricted the amount of available space and freedom of movement of mobile species; and Woodin and Jackson (1979) added yet more categories and abandoned feeding type altogether in their classification. They proposed five functional groups: (i) mobile burrowers that destabilize the sediment (including by their feeding activity); (ii)sedentary organisms that cause the sediment to be more easily resuspended; (iii) sedentary organisms that do not inhabit tubes but that still straddle the sedimenvwater interface and modify the local hydrography such as to reduce sediment resuspension and, by virtue of buried parts, bind the subsurface particles together; (iv) tube builders that stabilize the sediment by incorporating it, often in mucus-bound form, into their tubes; and (v) neutral species having no impact on sediment deposition or resuspension. This increase in the number of groups recognized by amensalism hypotheses perhaps reflected a n increasing awareness of the artificiality of many of the categories proposed earlier. Brenchley (1981), for example, described a system in which the addition of mobile burrowers did lead to a decrease in the numbers of tube dwellers, but where both the bioturbating-burrower and the stabilizing tube-builder categories were largely comprised by the same trophic group, deposit feeders; and Hulbert and Oliver (1980) obtained results of the type earlier considered to result from amensalism in the complete absence of one of the protagonist groups. One of the major problems of amensalism hypotheses is that they only describe the predicted results of interactions within dense assemblages, and populations of the required level of abundance are highly unusual in many regions (see Gray, 1981; Reise, 1985; and below). Those working on only moderately dense communities have failed to detect the presence of exclusion effects; Wilson (1984a), for example, found that none of the functional groups present was able to alter the environment sufficiently to render it unsuitable for the other co-occurring groups. A second category of difficulty with the hypotheses is the occurrence of notable exceptions, even within particularly dense aggregations (Maurer, 1983). Commito (1987) and Commito and Boncavage (19891, for example, document a positive interaction

R.S.K.Barnes

333

between the density of suspension-feeding mussels (Mytilus)and the abundance of the free-living, deposit-feeding oligochaete Tubificoides. Size is a further cause of asymmetry in amensal relationships, large animals more easily excluding smaller ones than vice versa (Posey, 1987). Others have described interactions of considerable complexity that cut right across the boundaries of accepted trophic or functional groups. Studying two tubedwelling and deposit-feeding polychaetes, Clymenella and Polydora, and a filter-feeding clam, Gemma, in Little Narragansett Bay, U.S.A.,Weinberg (1984) showed that the two polychaetes interacted differently with the mollusc. Polydora could affect recruitment of Gemma by consuming its larvae, and could influence its adult survival by interfering with the clam’s siphons and thereby reduce its available feeding time. Clymenella, however, did not have any effect on Gemma except perhaps a positive one, since it negatively influenced the numbers of Polydora. Weinberg (1984) concluded that the accepted functional groups in amensalism hypotheses were “too general to predict and explain the species interactions” and that although interactions between species were indeed widespread, each system had to be analyzed on an individual basis. In spite of the absence of a small number of set rules, categories of interaction can certainly still be recognized. Thus burrowing species, especially those ingesting sediment at depth and voiding it onto the surface or carrying out burrow construction, are responsible for considerable movement of sediment and consequent disturbance in a manner exactly analogous to that induced by large predators and considered above. The Callianassa populations in Mugu Lagoon, U.S.A.,transport a 75 cm thick layer of mud to the surface each year, and rates of deposition of up to 400 ml of wet mud per individual per year have been recorded for deposit feeders (Rhoads, 1974). Suspension feeders may have an equivalent or even greater effect, each individual depositing over 600 mg wet-weight of pseudofaecal and faecal material per day (Rhoads, 1974);in a two year period Mytilus in the European Wadden Sea has been found to deposit a layer of mud 60 cm thick (Ehlers, 1988). This translocation can have marked repercussions. Wilson (19811, for example, recorded reduction in the abundance of Pygospio through mortality brought about by lugworms (Abarenicola), although other members of the fauna (Pseudopolydora,Corophium, etc.) were not affected by the surface deposition; conversely, the faecal mounds of holothurians may provide the only safe refuge from consumption by sediment eaters for annelids and crustaceans (Rhoads and Young, 1971) and the activities of deep-living worms can oxygenate and loosen subsurface sediments and render them inhabitable by other macrofauna (Flint and Kalke, 1986a).Peterson (1984) and Posey (1986) record the burrowing of Callianassa as having a similar detrimental effect to that described by Wilson (1981); Wiltse (1980) has shown that the disturbance created by the bull-dozing snail Polinices de-

334

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

creases the abundance of small spionids (though not of the larger sipunculans); and Gaston and Nasci (1988) regard the disturbance caused in the lagoonal Calcasieu Lake, U.S.A.,by the dominant subsurface deposit-feeding polychaetes as a n important community-structuring force. Yet further examples abound in the literature (Thayer, 1983), with, most commonly, as above, the smaller infauna being more severely affected, as they are, for example, by the activities of the sentinel crab Macrophthalmus in Otago Harbor, New Zealand (Thrush, 19881, of the mud snail Ilyanassa in the Bay of Fundy, Canada (Wilson, 19881, and of cockles, Cerastoderma, in the European Wadden Sea (Jensen, 1985). That it is not only the redeposited sediment that can create the critical disturbance can be illustrated by Murphy’s (1985) study of the Colorado Lagoon, U.S.A.Here it may well be the turbidity produced by the burrowing activities of Callianassa that negatively affects growth and survival of the clam Mercenaria by imposing excess feeding costs, as in the original Rhoads and Young model: the bivalve is abundant in the lagoon, from which the prawn is absent, but is rare in the adjacent marine environment that is dominated by Callianassa. Similar incompatibility between Callianassa and a clam (in this case Sanguinolaria) has been noted in Tijuana Slough, U.S.A.,by Peterson (1977). The reaction of the disturbed species is often attempted emigration. Pygospio, for example, migrates away from areas subject to lugworm bioturbation, although lugworm populations can be extensive and there may be few refuges available for the smaller worm (Wilson, 1981); the mechanical disturbance engendered by the ragworm Nereis increases the emigration rate of Corophium (Ronn et al., 1988); and the disturbance caused by the mud snail Ilyanassa, though not that of the smaller Hydrobza, induces emigration in the amphipod Microdeutopus (DeWitt and Levinton, 1985). Predators and conspecifics produce a similar response in Rhepoxynius (Ambrose, 1984d; 1986b).Once again, however, levels of complexity that are not understood are present. The spionids Pygospio and Pseudopolydora have abundances that may be negatively correlated in the field, but field experiments (Wilson, 1983) indicate that although survivorship is influenced by intraspecific effects within each species, there are generally no such interspecific interactions. Nevertheless in the laboratory, at least, Pygospio migrates away from concentrations of Pseudopolydora though not from those of its own species. An apparent mismatch between the effects of Pseudopolydora on other small polychaetes and their small-scale distribution patterns in the field has also been recorded by Levin (1982). Infaunal species are also capable of detecting more subtle indications of the unsuitability of a given patch of sediment. Thus Pseudopolydora larvae avoid sediment in which Abarenicola has been living previously (Woodin, 1985), and the polychaete Protodriloides will not enter substrata that have contained the gastrotrich Turbanella (Gray, 1981): in each case a chemical

R.S.K. Barnes

335

‘signature’ is presumed to be involved. In like manner, the mud snail Hydrobia can detect sediments that have previously been occupied either by itself or by Corophium (although Corophium cannot distinguish between Hydrobia-grazed and control sediment notwithstanding that the first of the two contains lower amounts of microalgal food) (Morrisey, 1988a,b). Other interspecific effects are brought about by direct aggression, including forms of within-group cannibalism (Pajunen and Ukkonen, 1987). The growth rate and survival of juvenile lugworms (Arenicola), for example, are negatively related to the abundance of ragworms (Nereis).Nereis takes u p residence within the burrow systems of the lugworms and not only interferes with their respiratory and defaecatory movements, but bites the tail ends of the Arenicola, attacking them at frequent intervals and even killing them (Witte and Wilde, 1979). Ragworms will leave their own burrows to harass Arenicola living up to 15 cm away. The pieces bitten from the lugworms’ rear ends are not normally ingested, indicating the aggressive rather than predatory nature of the behaviour. The end result is that the young lugworms migrate away from centres of ragworm interference. Kristensen (1988) records similar aggressive behaviour within guilds of Nereis species, whilst the ragworm also interacts with Corophium by a combination of aggressive predation, interference and disturbance, although there is some evidence that Corophium may have a reciprocal effect on Nereis recruitment (Olafsson and Persson, 1986; Ronn et ul., 1988).Other types of organism may also sustain injury from interspecific interactions although in ways that are little understood. The bivalve Mucoma can suffer direct physical harm following contact with the amphipod Pontoporeia, for example (Elmgren et al., 1986).

The Role of Interference Although, as elsewhere, most of the results illustrated above have been obtained from work in the intertidal zone, there is no reason to believe that the same phenomena are not also found in the permanently submerged zones of habitats like lagoons. Indeed, their importance is likely to be greater subtidally than littorally (Woodin, 1974). Few, if any, of the effects described above, however, are likely to be lagoon-wide in their occurrence, and then only in the smallest systems. That is to say that they are likely to give rise to patchiness rather than set broad levels of possible species abundance and/or overall patterns of community composition. Dense beds of adults can prevent settlement of larvae only during their own lifetime (Woodin, 1976); sediments relocated by bioturbators become invaded by animals, at both the adult and larval stages (Bonsdorff‘, 19801, often quite rapidly (Davis and Lee, 1983) dependent on season and life-history strategy of the available colonists (Zajac and Whitlatch, 1982a and b; Levin, 1984); interspecific effects may be reciprocal (Olafsson

336

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

and Persson, 1986); refuges from disturbance are likely to occur (Woodin, 1978; Dauer, 1984); and so on. Woodin (19781, studying part of the lagoonal Chincoteague Bay, U.S.A., identified five types of potential refuge from the effects of interference: biologically-created refuges (e.g. beds of sea-grasses or worm tubes that decrease the rate of sediment disturbance howsoever caused; see, e.g., Peterson et al., 1984); natural substratum or other physical heterogeneity; the presence of zones outside the tolerated ranges of the interferers; the unpredictable or incomplete areal coverage of the disturbance (fish may forage only over a small percentage of the available sediment per unit time, bioturbating some 1% of the surface per day; Billheimer and Coull, 1988; and see Dauer, 1984); and the presence of temporal periods when the interference is absent. A classic example of the latter is when autumn storms roll up the carpet ofAmpelisca tubes in Barnstable Harbor, U.S.A., and free the sediment for occupation by Ilyanassa (Mills, 1969). The same refuges apply equally to the effects of epibenthic and other predators. There are no compelling reasons to assume that (a)the intensity of any of the sources of interference and (b) the occurrence of mitigating refuges, interactions, etc. vary latitudinally or in any other geographical manner, and so whilst interference may be responsible for generating within-habitat patchiness, it is unlikely to be responsible for between-habitat differences, except in so far as chance absences of certain faunal elements from landlocked lagoons are concerned. Competition

Levinton (1972; 1979) argued that deposit feeders ought as a rule to be food-limited and that competition between them should be expected to be intense. It has often been observed that the soft-sediment infauna appear to have divided up various resource axes (sediment type, burrow depth, feeding style, and so on) between the available species, as in the Corpus Christi Bay (Fig. 11.9)and Mugu Lagoons (Fig. l l . l O ) , U.S.A.,for example. Levinton interpreted this resource subdivision as evidence of a legacy of competition past. (Other forms of separation of deposit feeders in space, as for example differential distribution of congeneric species along the salinity gradient, may be related more to the maintenance of reproductive isolation than to any effects of competition; see the work of Fenchel and Kolding (1979), Kolding and Fenchel(1979) and Kolding (1986) on the Gummarus of the Limfjord, Denmark.) Suspension feeders, on the other hand, he considered unlikely to compete for food, although being sessile or sedentary and presumably not being food-limited, they could well be expected to compete for space. This hypothesis of the differential reaction of deposit and suspension feeders to food quantity has been tested by Olafsson (1986). The

R.S.K. Barnes

337

N C

?

Owenia

0

c 3

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.-

c

%

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m

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Mediomastus

0

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Streblospio Heteromastus

Discriminant function 1

Fig. 11.9.Niche separation of the benthic polychaete species in the Corpus Christi Bay lagoon, U.S.A.,as shown by multivariate discriminant analysis (after Flint and Kake, 1986b). The data are presented as 95% confidence ellipses for the individual discriminant scores of the various species. Discriminant function 1 was related chiefly to sediment particle size and organic content, whilst discriminant function 2 was based on temperature and productivity of the overlying water and on depth below the sediment surface of anoxic conditions.

MUDDY SAND

0

,

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cm devth

Fig. 11.10.Partitioning betweenvarious infaunal species oftheresource axis provided by depth at which the body is located within the substratum in two sediment types within Mugu Lagoon, U.S.A.(after Peterson, 1977).

bivalve Macoma can feed in both modes, as can Scrobicularia, simply by varying the position in space of its inhalant siphon: it deposit feeds in summer and when in muddy sediments, and is predominantly a suspension feeder in winter and in sandy substrata (Olafsson, 1986; Zwarts and Wanink, 1989). Olafsson did indeed find evidence of strong density-dependent competition in Macoma populations (1000 per square metre) in muddy sediments, leading to reduction in individual growth rate, but no such effects in the sand-inhabiting clams.

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

338

Nevertheless, although Levinton’s arguments seem inherently reasonable (Barnes, 1984), most studies have failed to find any evidence of competition in the field, either interspecific (Young et al., 1976; Brock, 1980; Holland et al., 1980; Dauer et al., 1982; Peterson, 1982b; Wilson, 1983; 1984a; 1988; Wiltse et al., 1984; Reise, 1985; Kouwenberg et al., 1987; Morrisey, 1987; 1988a and b; Black and Peterson, 1988) or intraspecific (Young et al., 1976; Holland et al., 1980; Dauer et al., 1982; Kent and Day, 1983; Wiltse et al., 1984; Reise, 1985). Dauer et al. (1982) and Wiltse et al. (19841, for example, increased the available food supply but failed to find any responses in either overall macrofaunal density or in any of the component species. Further, the interspecific competition demonstrated by Woodin (1974) and Dauer, Tourtellote and Ewing (1982) was more akin to the interference type discussed above. The best evidence for the occurrence of interspecific competition for resource in short supply stems from the work of Peterson in Mugu Lagoon. Peterson and Andre (1980) (and see Peterson, 1977) carried out transplantation experiments on clams in Mugu. They found that the deep-burrowing Sanguinolaria showed reduced growth rates when confined with other deep-burrowing species but not when in company with the shallow-living Protothaca (Fig. 11.11);further, it actively emigrated from regions with high densities of the other deep-burrowers. This was consonant with its field distribution in relation to the other Mugu clams, it being scarce where other deep-burrowers occurred but abundant elsewhere in the lagoon. Tellingly, Sanguinolaria was also negatively influenced by the presence of dead ........s + p ........................ 1975

O 5 I

S+P 1976

....................................

Fig. 11.11. The effect on the growth of the bivalve Sanguinolaria (S)of the presence of Protothuca (P), live (T) or dead (dT) Tresus, and live (X)or dead (dX)Saxidomus in various combinations.The growth rate of Sanguinolaria in the absence of other bivalves is set at unity. (From data in Peterson AND Andre, 1980.)

R.S.K. Barnes

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Surface

-m

1

2

3

4

5

8

-

foragers

7

8

8

L

m

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Subsurface- foragers

2

4

8

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10 12

14 16

Rank order of species abundance

Fig. 11.12. The relationship between the average resource overlap a deposit-feedingannelid has with its annelid competitors and the rank order of its abundance in Barnstable Harbor, U.S.A. (after Whitlatch, 1980).

shells of the deep-burrowing Saxidomus and Tresus, indicating genuine shortage of space rather than any interspecific interaction as the cause. Evidence for competition between clams has also been obtained by Levinton (1977) from Quisset Harbor, U.S.A., in which Yoldia exerted an impact on Solemya by disturbing its burrows. Although this is an interference-type interaction, it can reasonably be interpreted as a form of competition for burrowing space. Finally, Whitlatch (19801, working on infaunal deposit-feeding annelids in Barnstable Harbor, estimated interspecific overlap in resource usage (e.g. depth location within the sediment and particle size spectrum) and noted that when food was apparently abundant, species that were similar in terms of their resource requirements co-existed and species richness was high, whereas when potential food sources were scarce, such species no longer co-existed, the excluded species being those suffering the greatest overlap in their resource requirements, and species richness was low. Such patterns (Fig. 11.12) are highly suggestive of interspecific competition for food.

340

Macrofaunal Community Structure and Life Histories in Coastal Lagoons

Experimental evidence of intraspecific competition is available from a rather limited number of studies. It has been recorded within spionids by Wilson (1983); in Hydrobia by Morrisey (1987) and Cherrill and James (1987), the latter authors working in the Broad Water lagoon, U.K., and reviewing the evidence for competition in this genus; and in the clams Protothaca and Chione in Mugu Lagoon (Peterson, 198213)and Katelysia in Princess Royal Harbour, Australia (Peterson and Black, 1988b) (see also Olafsson, 1986, cited above). Territorial aggression and fighting between tube-holders and tube-seekers is known in nereids (Roe, 1975) and spionids (Levin, 1981). Otherwise evidence of competition in lagoonal environments or animals has relied on correlations between population density and emigration rate (Ambrose, 1986b) and between animal growth rates or abundance and nutritional state of the presumed food (Levinton and Lopez, 1977; Levinton and Bianchi, 1981; Graf et al., 1983) or times of maximum/ minimum food availability (Grassle et al., 1985; Rudnick et al., 1985).Competition for space therefore certainly occurs in some clams, and that for food is highly likely, but the frequency with which either has been observed to follow transplant or caging studies, even those continued over long periods of time, is much less than would be expected on the basis of Levinton’s (1972; 1979) arguments or on general ecological dogma. The inference can only be that in the majority of areas and at most times the species concerned are being maintained below the carrying capacity of their habitat, in respect of both food and space, by other sources of mortality, and it is only when one or more populations evade these limitations that competition occurs (see below). Environmental Adversity

Lagoonal organisms appear able to tolerate quite considerable degrees of normal environmental variability without change in their community composition (Peterson, 1975; Gore et al., 1981; Rainer, 1981; Flint and Younk, 1983; Dauer, 1984; etc.) although sometimes with concomitant population oscillations (Moller, 1986; Conde and Diaz, 1989) or changes in individual growth rates (Matthiessen, 1960) and life spans (Maslin and Pattee, 1989). Nevertheless, their habitat can occasionally be extremely hostile. Peterson (1982b; 1985)and Quammen (19841, for example, describe the consequences of a once-in-a-hundred-years rainstorm that mobilized vast quantities of silt and clay and caused the deposition of a 10 cm layer of mud in Mugu Lagoon. Locally, the clams Protothaca and Chione suffered 100% mortality; and among the bivalve molluscs generally, the suspension feeders, those with short siphons, large individuals and dense populations all experienced substantial mortality. Cyclones affecting the Lake St Lucia lagoon, South Africa, not only redistributed sediment but lowered salinities for many

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months thereafter (Cyrus, 1988), and storms in north temperate zones can have similarly drastic results (Casabianca, 1972-3; Rees et ul., 1977). Periods of hypoxia have been recorded as inducing mass mortality in the Etang de Berre, France (Stora and Arnoux, 19831, Cabbage Tree Basin, Australia (Rainer, 19811, Old Tampa Bay, U.S.A. (Dauer, 1984), and, together with catastrophically lowered salinities and reduced water levels, in Danish lagoons (Fenchel, 19751, amongst others; whilst “dystrophic crises” have a similar effect in the Etangs palavasiens and de Salses-Leucate, France (Amanieu et al., 1978-79; Boutiere et al., 1982). In the European Wadden Sea, “severe winters with prolonged frost [and ice] capable of killing all cockles” occur on average once every ten years (Reise, 1985). The Cerustoderma year-classes of 1976 and 1978 were both killed by severe winter conditions in early 1979, for example. Clams are not the only organisms to suffer during these Wadden Sea ‘ice winters’, amongst other populations dramatically reduced are those of the epibenthic predators and it is notable that recruitment of cockles is unusually high in the summers following widespread mortality of the consumers of their spat (Jensen and Jensen, 1985; Reise, 1985). Equivalent effects of ice have been demonstrated in other high-latitude areas as well (e.g. Wilson, 1988). The few examples above should suffice to make the point that climatic effects, from the ice winters of the north temperate zone through to tropical storms and dry seasons, can influence biological communities. What is not yet known, largely because of the sporadic nature of climatically-induced mortality and the difficulty of controlled experimentation under these circumstances, is the significance of environmental adversity in relation to the other sources of mortality in the long-term structuring of lagoonal systems. Conclusions

Not surprisingly, different workers favour different sources of mortality as being the major structuring forces in lagoon-like environments, and all potential sources have their advocates: predation (Reise, 19851, competition and interference (Woodin, 1974), inadequate recruitment and physical effects (Wilson, 1984b), juvenile mortality (Peterson, 1979b), environmental stress (Sacchi, 1973; Amanieu and Lasserre, 1982) and so on. A fundamental question to be answered is whether lagoonal populations are generally at the carrying capacity of their habitat or whether other factors keep their numbers below this level, and if so how frequently? As we have seen, there is little available evidence to suggest that density-dependent processes have a critical role in lagoons (or at least in those studied), although in at least some areas ‘competitive crunches’ (Wiens, 1978) do seem to occur. Gray (1981) and Reise (1985) have both pointed out that the importance of competition and competition-like interference is mainly argued by those working along the Pacific coast of the U.S.A.,whilst studies carried out on

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Atlantic shores stress processes keeping populations well below carrying capacity. Population densities may well be higher in the Pacific than in the Atlantic (Woodin, 1983) and it does appear that shallow-water Pacific communities are generally the richer in species (Valentine and Moores, 1974). The dense, single age-class dominated benthic assemblages clearly demarcated from other such assemblages by sharp boundaries described by Woodin (1976) do not seem to have counterparts in European seas (Gray, 1981);and whereas these Pacific communities would not appear to be highly susceptible to predation, several of those in the Atlantic are dominated by such species (Dauer et al., 1982). Unfortunately, we do not have nearly enough information to be able to say whether this Atlantiflacific distinction is a real one (see also Vermeij, 1987) or is only an artifact of the very limited number of sites investigated. Neither is there significant evidence yet available from outside the northern-hemisphere waters of these two oceans. There is little to be gained by endeavouring to argue the merits of any of the various sources of mortality (including problems of lack of available recruitsholonists as a notional source of mortality) as the factor controlling lagoonal systems at levels below the carrying capacity. All those reviewed above are doubtless important at some times in some localities: no study has yet assessed the relative contribution of each to the long-term dynamics of any system, lagoonal or not; and little such work has been carried out specifically in lagoons. In any event, lagoons clearly vary. The small landlocked ones investigated by Barnes (1988a) are non-saturated, non-equilibrium systems subject to unpredictable changes of the type discussed by Chesson and Warner (1981). The Indian River Lagoon studied by Young et al. (1976), Nelson (1981), Gore et al. (1981), Snelson et al. (1984), Virnstein and Howard (1987a; b) and others is, on the other hand, a benign environment in the sense of Menge and Sutherland (1987). There is no reason to imagine that just because both are lagoons they should be structured by the same processes. In general, the conclusions of Peterson (1979a) still hold; what we have gained in the last ten years is a greater understanding of how some of the agents of mortality operate; we are no nearer putting them in rank order. The greatest need in the future is for more work of the type reviewed above to be carried out in genuine lagoons: the smaller lagoons are self-contained, simple, and often with limited species diversity in workable depths of water; they are therefore ideal systems in which to analyze the processes structuring communities. Life Histories of Lagoonal Animals

Larvae and other life-history stages have already figured several times in the pages above, but the purpose of this final section is to question whether lagoonal animals have any reproductive or developmental traits that distinguish them from their marine and/or freshwater relatives. Very few studies

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indeed have been carried out into this field, and most of those few, like that of Karakiri and Nicolaidou (1987) and Nicolaidou and Karakiri (1989) on the amphipods of Mazoma Lagoon, Greece, and those of Kroer (1986; 1989) and Kouwenberg and co-workers (1984; 1985; 1987) on the isopods of the Limfjord, Denmark, and Etang de Bages-Sigean, France, respectively, have not disclosed any features that could be considered to be specifically lagoonal. Neither has comparison of nearby lagoonal and marine populations of the same species generally provided any evidence of apparent lagoonal adaptation (see, for example, Pantoustier and Prunus, 1977, on the isopod Jaera in the Mediterranean and in the Lacs de Bizerte and Tunis, Tunisia, and Bynum, 1978, on the amphipod Caprella within and outside Bogue Sound, U.S.A.). There are, however, a few tantalizing glimpses of differences in the literature. Diaz and Bevilacqua (1986; 1987), for example, compared the development of the crab Aratus within the Tacarigua Lagoon, Venezuela, and in the adjacent marine environment, and found differences in the effect of salinity on its first zoeal larval stage; and Muus (1967) records that the lagoonal phase of the winkle Littorina saxatilzs (which may, or may not, be a separate species L. tenebrosa; Graham, 1988) produces only a small fraction of the young carried by the marine form and matures at a smaller size. A number of gastropods occur in two reproductive forms (or perhaps as pairs of sibling species differing in life-history characteristics, as argued by Hoagland and Robertson, 1988, and Bouchet, 19891, one in brackish, and often lagoonal, waters and the other in the sea; e.g. Rissostomia membranacea and Brachystomia rissoides. In all such cases known, the brackish populations produce relatively few, relatively large eggs per capsule that undergo direct development, whilst the marine ones lay more, smaller eggs that hatch into planktonic larvae (e.g. Rasmussen, 1944; 1951; Rehfeldt, 1968). In Tenellia pallida an identical brackishlmarine dichotomy is found in Danish waters (Rasmussen, 19441, although the reproductive variation in this species has no salinity control in American populations (Eyster, 1979). It is tempting to regard these cases as equivalent to the rather better known system in the lagoonal versus marine species of the mud snail Hydrobia, in which the essentially intertidal marine H. (Peringia) uluae produces many small eggs per capsule, veliger larvae, etc. whereas the characteristically lagoonal H. (Hydrobia) ventrosa and H. (HJ neglecta lay capsules each usually containing only a single large egg which hatches into a young snail (Lassen, 1979; Fish and Fish, 1981; Barnes, 198813).It would be only too easy to discuss the production of large directly-developing eggs in terms of decreased predation on the young, increased competitive ability, or any other supposed adaptation to the lagoonal habitat, but, in Hydrobia at least, whether these features are in any sense ‘lagoonal’ is open to question. The life-history characteristics of H. ventrosa and H. neglecta are

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those typical of the almost entirely freshwater family Hydrobiidae as a whole, and thus these species may simply have brought their ancestral reproductive features with them when they invaded lagoons, just as H. ulvae brought its marine ones with it when it did the same (Barnes, 198813). In only two cases has there been any attempt at a comparative investigation of the life-history differences in lagoonal versus marine populations of the one or of a pair of sibling species, both of them involving molluscs. Although being fundamentally estuarindmarine, H. ulvae also occurs in a number of lagoons, and the population in the isolated Holkham Salts Hole lagoon, U.K., has been compared with that a few hundred metres away in the North Sea intertidal zone by Barnes (1988b; 1990).Although numbers of eggs per capsule, size of egg, and development type (the characters varying in the lagoonal versus marine comparisons above) are identical in the two, the numbers of eggs produced per expected life time and the pattern of seasonal egg production are markedly different (Table 11.5). These differences could plausibly be related to the contrasting processes likely to structure the two populations: the lagoonal one would be expected to suffer heavy juvenile mortality from the abundant small gobies and Palaemonetes but be relatively free from predation on the a.dults; the intertidal one, on the TABLE 11.5 Certain reproductive and life-history characteristics of adjacent lagoonal and intertidal-marine populations of the gastropod Hydrobia uluae in Norfolk,U.K. in 1988 and 1989 (from data in Barnes, 1988b; 1990) Intertidal marine 21.7 Mean no. eggs per capsule 100 p Mean zygote diameter Mean shell length on hatching 152 pm Length of free-swimming phase up to 3 weeks Maximum mean expectation of reproductive life 12 weeks Maximum mean numbers of eggs produced per 446 reproductive lifetime Mean expected % of net adult resources devoted to 99 egg production (AF'DW eggs/AFDW eggs + growth increment) 2927 Potential mean annual output of eggs per female Maximum mean output of eggs per female per week 337 Maximum length of adult life (attained by minimum of 1.2 years 5%of adults that had reached maturity) % Adults surviving for 1 year after reaching maturity 10 39,250 Maximum numbers of adults per square metre

Lagoonal 21.4 98 Fm 154 Fm up to 3 weeks 21.5 weeks 175 79

412 68 2.5 years 28 5,160

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other hand, is likely to experience the converse, with lower juvenile but higher adult mortality (Barnes, 1990). The second investigation is that into the sibling cockles Cerastoderma glaucum and C. edule, of which the first is lagoonal and the second is estuarine and marine (Rygg, 1970). In this pair, their breeding biology does not differ to any great extent (Kingston, 1974; Yankson, 1986), although C. glaucum may mature at a smaller size and have shorter lived larvae (see review of Barnes, 1980). The main difference between the two concerns the biology of the postmetamorphic stages (Muus, 1967). Young lagoonal cockles inhabit the submerged macrophytes and (especially) filamentous algae, and only descend to the substratum when some millimetres in length. C. edule, in contrast, like ‘normal’ soft-sediment bivalves settles onto the substratum at metamorphosis and burrows into it. This tendency to inhabit submerged or floating lagoonal vegetation when juvenile, presumably to decrease predation rates (see above), is widespread amongst lagoonal species (Verhoeven, 1980), even in those like the cockles that are usually benthic. In Holkham Salts Hole, for example, Hydrobia and Littorina begin their life in the Ruppia and Chaetomorpha, and it may be no coincidence that L. tenebrosa, which is almost entirely confined to this vegetation throughout its life, bears all the hallmarks of a paedomorphic origin. As yet, however, the only information that we have on features that may turn out to be specifically lagoonal are these “tantalizing glimpses’’ above; more systematic comparisons are urgently needed. Summary

Lagoonal communities of organisms comprise suites of essentially marine or freshwater species, dependent on ambient salinity, together with a characteristically lagoonal component, particularly in the more enclosed, land-locked systems. These latter often possess impoverished, under-saturated faunas, largely as a result of the difficulties of colonization. Potential forces structuring the macrofaunal benthic communities of lagoons are reviewed, as, briefly, are the techniques used to provide the relevant data. Birds are considered to have no significant structuring effect. There is, however, a large body of information suggesting that small epibenthic and epifaunal fish and invertebrates may maintain prey populations below the carrying capacity of their habitat, except amongst submerged macrophytes. Infaunal invertebrate predators, periods of environmental adversity, disturbance, interference competition and aduWjuvenile interactions may all have a similar effect, the last three especially acting to produce patchy distributions within otherwise homogeneous substrata. Release from indiscriminant consumption, interference or destruction, perhaps through the agency of higher-order predators such as large fish,

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can result in the attainment of sufficiently high population densities as to cause competition for lagoonal resources; although judging from the available evidence this may be a relatively rare event. Inter- and intraspecific competition for space, however, has been well documented in lagoonal bivalves and in a few other animals. The life histories of specialist lagoonal species appear to be essentially the same as those of their marine or freshwater relatives: no traits can be postulated as adaptations to the lagoonal environment. Nevertheless a few comparisons of lagoonal and adjacent marine populations of the same species have disclosed consistent reproductive or developmental differences that may reflect the contrasting forces structuring subtidal lagoonal and intertidal marine systems. Small lagoons are ecologically simple, self-contained environments, often with limited species diversity and easily workable depths of water. Hence they form ideal systems in which to analyze the processes structuring communities. I t is therefore all the more surprising that so little such work has as yet been carried out. References Amanieu, M., Guelorget, 0. and Michel, P. 1978-79 Evolution structurale des peuplements de la macrofaune benthique des e tangs palavasiens. Vie Milieu 28-29, 531-547. Amanieu, M., Guelorget, 0. and Nouquier-Soule, J. 1981 Analyse de la diversitd de la macrofaune benthique d'une lagune littorale Mdditerranhenne. Vie Milieu 31, 303312. Amanieu, M. and Lasserre, G. 1982 Organisation et evolution des peuplements lagunaires. Oceanologica Acta 4 (Supplement), 201-213. Ambrose, W.G. 1984a Influences of predatory polychaetes and epibenthic predators on the structure of a soft-bottom community in a Maine estuary. Journal ofExperimenta1 Marine Biology and Ecology 81, 115-145. Ambrose, W.G. 1984b Role of predatory infauna in structuring marine soft-bottom communities. Marine Ecology Progress Series 17, 109-115. Ambrose, W.G. 1984c Influence of residents on the development of a marine soft-bottom community. Journal of Marine Research 42,6334354. Ambrose, W.G. 1984d Increased emigration of the amphipod Rhepoxynius abronius (Barnard) and the polychaete Nephtys caeca (Fabricius) in the presence of invertebrate predators. Journal of Experimental Marine Biology and Ecology 80,67-75. Ambrose, W.G. 1986a Importance of predatory infauna in marine soft-bottom communities: reply to Wilson. Marine Ecology Progress Series 3 2 , 4 1 4 5 . Ambrose, W.G. 1986b Experimental analysis of density dependent emigration of the amphipod Rhepoxynius abronius. Marine Behavior and Physiology 12,209-216. Ardizzone, G.D. 1985 Progetto Laghi Costieri, 1982-1984. Relazione Finale Gruppo di Zoologia. Latina: Amministrazione Provinciale di Latina. Bachelet, G. and Yacine-Kassab, M. 1987 Intkgration de la phase post-recrutde dans la dynamique des populations du gastkropode intertidal Hydrobia ulvae (Pennant).

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Journal of Experimental Marine Biology and Ecology 111,37-60. Baird, D., Evans, P.R., Milne, H. and Pienkowski, M.W. 1985 Utilization by shorebirds of benthic invertebrate production in intertidal area. Oceanography and Marine Biology Annual Review 23,573-597. Baird, D. and Milne, H. 1981Energy flow in the Ythan Estuary, Aberdeenshire, Scotland. Estuarine, Coastal and Shelf Science 13,455472, Banham, P.R. 1979 Some effects of the sea flood at Wells-next-the-Sea, 11th January 1978. Transactions of the Norfolk and Norwich Naturalists’ Society 25,263-266. Barnes, R.S.K. 1980 Coastal Lagoons, Cambridge University Press, Cambridge. Barnes, R.S.K. 1984 Estuarine Biology 2nd Edition, London: Arnold. Barnes, R.S.K. 1988a The faunas of land-locked lagoons: chance differences and the problems of dispersal. Estuarine, Coastal and Shelf Science 26,309-318. Barnes, R.S.K. 1988b On reproductive strategies in adjacent lagoonal and intertidal-marine populations of the gastropod Hydrobia ulvae. Journal of the Marine Biological Association of the U.K. 68, 365-375. Barnes, R.S.K. 1989a What, if anything, is a brackish-water fauna? Transactions of the Royal Society of Edinburgh, Earth Sciences 80,235-240. Barnes, R.S.K. 1989b The coastal lagoons of Britain: an overview and conservation appraisal. Biological Conservation 49, 295-3 13. Barnes, R.S.K. 1990 Reproductive strategies in contrasting populations of the coastal gastropod Hydrobia ulvae. 11. Longevity and life-time egg production. Journal of Experimental Marine Biology and Ecology 138,183-200. Barnes, R.S.K., Dorey, A.E. and Little, C. 1971 An ecological study of a pool subject to varying salinity (Swanpool, Falmouth). An introductory account of the topography, fauna and flora. Journal of Animal Ecology 40,709-734. Barnes, R.S.K. and Jones, J.M. 1972 Observations on the fauna and flora of reclaimed land a t Calshot, Hampshire. Proceedings of the Hampshire Field Club and Archaeological Society 29, 81-91. Barnes, R.S.K., Williams, A., Little, C. and Dorey, A.E. 1979 An ecological study of the Swanpool, Falmouth. I V Population fluctuations of some dominant macrofauna. In Ecological Processes in Coastal Environments (Davy, A.J. and Jefferies, R.L., eds). Blackwell Scientific, Oxford, 177-197. Baudin, J-P. 1980 Contribution 81’6tude Bcologique des milieux saumhtres MBditerranBens. 11. Le peuplement de 1’6tang de Citis (B.-D.-R.). Vie Milieu 30, 3034308. Ben-Tuvia, A. 1983 The Mediterranean Sea, B. Biological aspects. In Ecosystems of the World 26 Estuaries and Enclosed Seas (Ketchum, B.H. ed). Elsevier, Amsterdam, 239-251. Berge, J.A. and Hesthagen, I.H. 1981Effects of epibenthic macropredators on community structure in an eutrophicated shallow water area, with special reference to food consumption by the common goby, Pomatoschistus microps. Kieler Meeresforschungen Sonderhest 5,462-470. Berge, J.A. and Valderhaug, V.A. 1983 Effect of epibenthic macropredators on community structure in subtidal organically enriched sediments in the inner Oslofjord. Marine Ecology Progress Series 11, 15-22. Beukema, J.J. 1987 Influence of the predatory polychaete Nephtys hombergi on the abundance of other polychaetes. Marine Ecology Progress Series 40,95101. Billheimer, L.E. and Coull, B.C. 1988Bioturbation and recolonization of meiobenthos in juvenile spot (Pisces) feeding pits. Estuarine, Coastal and Shelf Science 27,335340.

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Black, R. and Peterson, C.H. 1988 Absence of preemption and interference competition for space between large suspension-feeding bivalves and smaller infaunal macroinvertebrates. Journal of Experimental Marine Biology and Ecology 120,183-198. Boates, J.S. and Smith, P.C. 1979 Length-weight relationships, energy content and the effects of predation on Corophium volutator (Pallas) (Crustacea: Amphipoda). Proceedings of the Nova Scotian Institute of Sciences 29,489499. Bonsdorff, E. 1980 Macrozoobenthic recolonization of a dredged brackish water bay in SW Finland. Ophelia 1, 145-155. Botton, M.L. 1984The importance of predation by horseshoe crabs, Limuluspolyphemus, to an intertidal sand flat community. Journal of Marine Research 42,139-161. Bouchet, P. 1989 A review of poecilogony in gastropods. Journal of Molluscan Studies 55,67-78. Boutiere, H., Bovee, F. de Delille, D., Fiala, M., Gros, C., Jacques, G., Knoepffler, M., Labat, J.P., Panouse, M. and Soyer, J., 1982 Effet d‘une crise dystrophique dans 1’Etang de Salses-Leucate. Oceanologica Acta 4 (Supplement), 231-242. Bradley, R.A. 1983 Complex food webs and manipulative experiments in ecology. Oikos 41,150-152. Brenchley, G.A. 1981 Disturbance and community structure: a n experimental study of bioturbation in marine soft-bottom environments. Journal of Marine Research 39, 767-790. Brenchley, G.A. 1982 Mechanisms of spatial competition in marine soft-bottom communities. Journal of Experimental Marine Biology and Ecology 60, 17-33. Brock, V. 1980 Notes on relations between density, settling, and growth of two sympatric cockles, Cardium edule (L.) and C. glaucum (Bruguibre). Ophelia 1,241-248. Bynum, K.H. 1978 Reproductive biology of Caprella penantis Leach, 1814 (Amphipoda: Caprellidae) in North Carolina, U.S.A. Estuarine, Coastal and Marine Science 7 , 473-485. Casabianca, M-L. de 1972-3 Influence des apports d’eau douce sup la dynamique des populations de Crustaces constructeurs de l’gtang de Biguglia, Corse (Corophium insidiosum C., Tanais cavolini M.E., Erichthonius brasiliensis D.). Vie et Milieu 23C, 45-63. Caspers, H. 1951 Quantitative Untersuchungen iiber die Bodentienvelt des Schwarzen Meeres im bulgarischen Kustenbereich. Archiv fur Hydrobiologie 45,1-192. Chenill, A.J. and James, R. 1987 Evidence for competition between mudsnails (Hydrobiidae): a field experiment. Hydrobiologia 150,25-31. Chesson, P.L. and Case, T.J. 1986Nonequilibrium community theories: chance, variability, history, and coexistence. In Community Ecology (Diamond, J. and Case, T.J., eds). Harper and Row, New York, 229-239. Chesson, P.L. and Warner, R.R. 1981 Environmental variability promotescoexistence in lottery competitive systems. American Naturalist 117,923-943. Choat, J.H. 1982 Fish feeding and the structure of benthic communities in temperate waters, Annual Review of Ecology and Systematics 13,423449. Clanzig, S . 1989 Evolution des peuplements malacologiques de la lagune mediterraneenne de Nador (Maroc). Vie Milieu 39,71-76. Commito, J.A. 1982 The importance of predation by infaunal polychaetes in controlling the structure of a soft-bottom community in Maine, U.S.A. Marine Biology 68,7741. Commito, J.A. 1987 Adult-larval interactions: predictions, mussels and cocoons. Estuarine, Coastal and Shelf Science 25,599-606.

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Commito, J.A. and Ambrose, W.G. 1985 Multiple trophic levels in soft-bottom communities. Marine Ecology Progress Series 26,289-293. Commito, J.A. and Boncavage, E.M. 1989 Suspension-feeders and coexisting infauna: an enhancement counterexample. Journal of Experimental Marine Biology and Ecology 125,3342. Commito, J.A. and Shrader, P.B. 1985 Benthic community response to experimental additions of the polychaete Nereis virens. Marine Biology 86, 101-107. Conde, J.E. and Diaz, H. 1989 The mangrove tree crab Aratus pisonii in a tropical estuarine coastal lagoon. Estuarine, Coastal and Shelf Science 28,639-650. Connell, J.H. 1983 Interpreting the results of field experiments: effects of indirect interactions. Oikos 41,290. Cowden, C., Young, C.M. and Chia, F.S. 1984 Differential predation on marine invertebrate larvae by two benthic predators. Marine Ecology Progress Series 14,145-149. Crowe, W.A., Josefson, A.B. and Svane, I. 1987Influence of adult density on recruitment into soft sediments: a short-term in situ sublittoral experiment. Marine Ecology Progress Series 41,61-69. Cyrus, D.P. 1988 Episodic events and estuaries: effects of cyclonic flushing on the benthic fauna and diet of Solea bleekeri (Teleostei) in Lake St Lucia on the south-eastern coast of Africa. Journal of Fish Biology 33 (Supplement A), 1-7. D’Ancona, U. 1959 The classification of brackish waters with reference to the North Adriatic lagoons. Archivio di Oceanografia e Limnologia 11 (Supplement), 93-109. Dauer, D.M. 1984 High resilience to disturbance o f an estuarine polychaete community. Bulletin of Marine Science 34,170-174. Dauer, D.M., Ewing, R.M., Tourtellote, G.H., Harlan, W.T., Sourbeer, J.W. and Barker, H.R. 1982 Predation, resource limitation and the structure of benthic infaunal communities of the lower Chesapeake Bay. Znternationale Revue der Gesamten Hydrobiologie 67,477-489. Dauer, D.M., Tourtellotte, G.H. and Ewing, R.M. 1982 Oyster shells and artificial worm tubes: the roles of refuges in structuring benthic communities of the lower Chesapeake Bay. Znternationale Revue der Gesamten Hydrobiologie 67,661-677. Davies, B.R. 1982Studies on the zoobenthos ofsome southern Cape coastal lakes. Spatial and temporal changes in the benthos of Swartvlei, South Africa, in relation to changes in the submerged littoral macrophyte community. Journal of the Limnological Society of Southern Africa 8,3345. Davis, H.W. and Lee, H. 1983 Recolonization of sediment-associated microalgae and effects of estuarine infauna on microalgal production. Marine Ecology Progress Series 11,227-232. DeWitt, T.H. and Levinton, J.S. 1985 Disturbance, emigration, and refugia: how the mud snail Zlyanassa obsoleta (Say) affects the habitat distribution of a n epifaunal amphipod, Microdeutopus gryllotalpa (Costa). Journal of Experimental Marine Biology and Ecology 92,97-113. Diaz, H. and Bevilacqua, M. 1986 Larval development ofAratuspisonii (Milne Edwards) (Brachyura, Grapsidae) from marine and estuarine environments reared under different salinity conditions. Journal of Coastal Research 2,4349. Diaz, H. and Bevilacqua, M. 1987 Early developmental sequences ofAratuspisonii (H. Milne Edwards) (Brachyura, Grapsidae) under laboratory conditions. Journal of Coastal Research 3,63-70. Divanach, P., Kentouri, M. and Cavalier, F. 1987 Organisation du reseau trophique

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Note: This chapter was accepted for publication by the Editor on 11January 1990.

Coastal Lagoon Processes edited by B. Kjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V. All rights reserved

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Chapter 12

Coastal Lagoons as Fish Habitats Alejandro Ytiiiez-Arancibiaa,Ana Laura Lara Domingueza and Daniel Paulyb aPrograma de Ecologia, Pesquerias y Oceanografia del Golf0 de Mdxico (EPOMEX), Universidad Autbnoma de Campeche, Apartado Postal 520, Campeche 24000, Campeche, Mdxico; bInternational Center for Living Aquatic Resources Management (ICLARM), M.C. P.O. Box 2631, 0718 Makati, Manila, The Philippines

Many fish species use coastal lagoons as habitats during at least some part of their life cycles. We discuss these characteristics with emphasis on latitudinal differences. Higher biomass of coastal fishes will occur when lagoon-estuarine habitat space is available and accessible. The concept of seasonal programming describes how juveniles use that lagoon habitat space. We also consider the lagoon-estuarine dependence of subtropical and tropical coastal fishes.

Introduction Coastal lagoons are defined in geomorphological terms (Phleger, 1969; Lankford, 1977; Kjerfve, 1986; Yiiiiez-Arancibia, 1987). I n this, they differ geologically from estuaries, which are defined in terms of the hydrological conditions and gradients of salinity and temperature (density) (Cameron and Pritchard, 1963; Pritchard, 1967; Fairbridge, 1980; Y6iiez-Arancibia, 1987; Kjerfve and Magill, 1989). Kjerfve (1989) proposed a new classification for “inland waters which are connected to the sea”, based on six classes (estuaries, coastal lagoons, fjords, bays, tidal rivers and straits),defined by various combinations of physical and geomorphological processes. Kjerfve’s assignment of adjacent classes to estuaries and coastal lagoons is due to their sharing numerous characteristics. This is also the reason why Day and Yiiiiez-Arancibia (1982) had earlier proposed the (ecological) concept of “lagoon-estuarine environments”, i.e. shallow semi-enclosed water bodies of variable volume, connected to the sea in a permanent or ephemeral manner, with variable temperature and salinities, permanent muddy bottoms, high turbidity, irregular topographic characteristics, and biotic elements.

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In this chapter we present the fish fauna occurring in various lagoonestuarine environments, with emphasis on those features of lagoons that are actually ‘used’by the fish populations, and on some of the mechanisms involved therein. We concentrate on fishes because they generally constitute the overwhelming bulk (up to 99%)of the lagoon-estuarine nekton, both in terms of numbers and biomass (McHugh, 1967).The few nonpiscine members of the lagoon-estuarine nekton are squids, portunid crabs, penaeid shrimps, reptiles, and mammals such as dolphins. Much information is now available on the life cycles of fishes in relation to lagoon-estuarine environment (McCleave et a1., 1984; Yaiiez-Arancibia, 1985; Dadswell et al., 1987; Weinstein, 1988; Henderson and Margetts, 1988), and various schemes have been proposed to link these cycles with environmental parameters and the observed high abundances of fishes in lagoons and estuaries (Day et al., 1981; Pauly, 1982; Weinstein, 1982; Yaiiez-Arancibia, 1986). Two important points have emerged from these studies: - the utilization of the lagoon-estuarine environment is an integral part of the life cycle of numerous fishes, particularly so in the Neotropics and the Indo-Pacific (Longhurst and Pauly, 1987); - the lagoon-estuarine environment is mainly utilized by juveniles and young adults. We shall here consider: (1)the fishes that ‘use’coastal lagoons; and (2)the features of the lagoon habitat(s) that are actually being used by these fishes. Which Fishes Use Coastal Lagoons? Although the exact species and dominance ranks change between sites, the dominant fishes of lagoon-estuarine habitats usually belong to only a few taxonomic groups. High latitude estuaries have simple ichthyofaunas, dominated in the northern hemisphere, by Salmonidae (salmon and trout), Osmeridae (smelt and capelin), Gasterosteidae (sticklebacks),Ammodytidae (sandlance), and Cottidae (sculpins) and their relatives (Dadswell et a1., 1987; Day et al., 1989). In the southern hemisphere the Salmonidae are replaced by their ecological analogs, the Galaxioidae (McDowall and Eldon, 1980; McDowall and Frankenberg, 1981). Important families for temperate systems are the Anguillidae (freshwater eels), Clupeidae (herring-like fish), Engraulidae (anchovies),Ariidae (marine catfish), Cyprinodontidae (killifish), Gadidae (cod), Gasterosteidae (stickleback), Serranidae (basses), Sciaenidue (sea trout, drums, croakers), Sparidae (sea-breams), and Pleuronectidae (flounders) (Haedrich, 1983; McCleave et al., 1984). In the tropics and subtropics, the important families are the Clupeidae, Engraulidae, Ariidae, Sciaenidae, Poecillidae (guppy), Chanidae (milkfish),

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Synodontidue (lizardfish),Belonidae (silver gars), Mugilidae (mullets), Polynemidue (threadfins), Gobiidae (gobies),Cichlidae (tilapia and related fishes), Dasyatidae (rays), Tetraodontidae (puffers), Gerreidue (mojarras), Leiognathidae (slipmouth, ponyfish), Pomadasyidue (grunts), and various families of flatfishes, such as the Bothidae, Soleidae and Cynoglossidue (Pauly, 1982; Yfiiiez-Arancibia, 1985, 1986; Lowe-McConnell, 1987). Several authors have pointed out that there are a greater number of fish species in tropical and subtropical lagoon-estuarine ecosystems than in comparable temperate or boreal systems (Yaiiez-Arancibia, 1985; LoweMcConnell, 1987; Yafiez-Arancibia and Sanchez-Gil, 1988; and Fig. 12.1). However, some large high-latitude estuaries, such as Chesapeake Bay, Narraganset Bay and Delaware Bay, in the northeast USA, also have a large number of fish species (Day et al., 1989). The theory of island biogeography (MacArthur and Wilson, 1967), which relates species number to the size of habitats could possibly provide a framework for these apparent exceptions.

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Latitude ( O N or S) Fig. 12.1. Latitudinal trends averaged by 5" latitude (after Gross et al. 1988). (A)Number of anadromous and catadromous species occurring in coastal lagoons. (B)Primary production of limnic and marine habitats.

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Coastal Lagoons as Fish Habitats

As for the general pattern, i.e. that low latitude lagoons have more species than those in high latitude, it could well be explained in terms of the cost of osmotic regulation relative to total metabolism, which is much lower at high temperatures. Various classifications have been proposed to analyze the fish communities in lagoon-estuarine environments. As they involve some form of diadromous migration, we briefly define: Diadromous: fishes migrate between fresh and marine waters. Anadromous: fishes spend most of their lives in the sea and migrate to freshwater for breeding (e.g. salmon and shad). Semi-anadromous: fishes do not travel all the way to freshwater, but spawn in low-salinity lagoon-estuarine waters. Cutadrornous: fishes spend most of their lives in freshwater and migrate to the sea for breeding. A well-known example are eels of the genus Anguilla. Amphidromous: fishes migrate from freshwater to the sea, or vice versa, not for the purpose of breeding, but regularly at other stages in their life cycles, often for feeding. McHugh (1967) identified the following groups: (1) Freshwater fishes that occasionally enter brackish-water. (2) Truly estuarine species that spend their entire lives in estuaries. (3) Estuarine-marine species that use the estuaries and lagoons primarily as a nursery ground, generally spawning and spending much of their adult life at sea, but often returning seasonally to estuaries and lagoons. (4) Marine species that pay regular seasonal visit to estuaries and lagoons, usually as adults looking for food. ( 5 ) Anadromous and catadromous species in transit. (6) Occasional visitors, which appear irregularly. Variants of this scheme exist (Pauly, 1975; Y6iiez-Arancibia and Nugent, 1977) and they tend to emphasize the role of migration and other movements. Thus, Miller and Merriner (1985), based on Cronin and Mansueti (19711, pointed out that marine (‘sport’) fishes exhibit, five distinct life histories or patterns of migration for habitat utilization: (1)inshore anadromy, referring to species which migrate to spawn in estuarine habitats and move toward coastal waters as developing juveniles or adults; (2) inshore catadromy, referring to species which migrate to spawn in offshore habitats and toward inshore habitats as developingjuveniles; (3) inshore nonmigratory, referring to species which utilize estuarine or nearshore coastal habitats at all stages; (4) offshore nonmigratory, referring to species which utilize ocean habitats at all stages; and (5) anadromous, referring to oceanic species which migrate to freshwater to spawn while juveniles migrate to the ocean as smolts. It is understood that some of the nonmigratory inshore and offshore species or stages may actually undertake substantial seasonal migrations or movements along the coastline. Fig. 12.2 presents a conceptual model of the stimuli involved.

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Coastal Lagoons as Fish Habitats

Recently, Zijlstra (1988) based on Louis and Lasserre (1982), presented a classification consisting of six types, pertaining to different regions ranging from temperate to tropical climates and which may therefore have a worldwide application: (1) anadromous and catadromous species; (2) resident species spending their entire lives in the coastal areas; (3) marine species, spawning in the coastal region and using i t as nursery ground; (4) marine species, which use the coastal area as a nursery area, but which spawn and spend most of their adult life in the offshore region; ( 5 ) marine species, which pay regular seasonal visits to the coastal zone, usually as adult; and (6) adventitious visitors, which appear irregularly and have no apparent estuarine or coastal requirements. Real fishes, however, will not adhere strictly to any of these classifications. Rather, a number of populations will belong to several types, independent of how well-conceived the types are. More interesting are classifications that are quantitative, even if this implies that they become site-specific in the process. Deegan and Thompson (1985) provided such classification for the Mississippi delta. Similarly, Yafiez-Arancibia et al. (1988b) proposed a classification scheme for Terminos Lagoon, Southern Gulf of MBxico, which provides salinity ranges for the various groupings, i.e.:

- freshwater spawners, occurring in waters < 10%0. - brackish water groups, limited to 10-34%0. - marine spawners, occurring in waters 2 35%0. Classifications of this type can be constructed, more or less rigorously, for any desired level of resolution, using clustering-type techniques (i.e. using station species lists us. observed environmental parameters; Fig. 12.3) and/or the results of physiological experiments, e.g. on salinity tolerance. Fish production in lagoon-estuarine systems is generally high (Day et al., 19891, and this generally leads to high fishery catches. One reason for this high production is that many of t h e fish species of importance in lagoonestuarine environments are either: (1) r-selected sensu (Pianka, 1978), i.e. relatively small, fast-growing animals with high productionhiomass (P/B) ratio; or (2) K-selected juveniles, i.e. fast-growing, high production stage fishes whose adult form, however, typically occurs outside the lagoon. The high P/B ratios of the constituent species are not the only explanation for the generally high production of lagoon-estuarine communities. The high biomasses themselves, i.e., the high carrying capacities of these ecosystems in terms of food availability to fishes also play a crucial role.

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Fig. 12.3. Dendrogram of clustering of stations in Terminos Lagoon, Southern Gulfof Mexico, as derived using the matching coefficients index of similarity and the single linkage clustering method, based on 35,425 individual fish specimens, representing 134 species, sampled from July 1976 to April 1981 (after Yaiiez-Arancibia and Day, 1982).

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Coastal Lagoons as Fish Habitats

Which Features of Lagoon-Estuarine Habitats Do Fishes Actually Use? The two most important processes for living organisms are: (1) energy acquisition, i.e. feeding, as needed to maintain their structure, and to grow; (2) reproduction: only those organisms are still around whose ancestors bred and who endowed them with ability to do the same. Except for very few species, fishes do not use lagoons for reproduction proper, i.e. for late maturation, spawning/fertilization. It is tempting to relate this to the ephemeral nature and the strong environmental variability of these systems. Rather, it is as feeding grounds that lagoon-estuarine environments are important to fishes. The growthhatural mortality schedules of unexploited fish cohorts generally imply that biomasses are maximized near the mean age at first maturity of the individual fish, an adaptation obviously related to maximizing the output of reproductive products. However, small fishes always eat more per unit body weight than large fishes (Pauly, 1986). This results in the maximum of the absolute food consumption curve, of any stable fish population being shifted to the left, toward the juveniles (Fig. 12.4). Thus, it is the juveniles of fish populations, and not the adults, which exert the greatest pressure on the food supply. If a coastal population (B’) of adult fishes is to maintain itself, it must place its juveniles in a habitat with a carrying capacity >>B’. This point, apparently not made in the extensive literature on lagoon and estuaries, provides a n objective criterion for referring to these water bodies as critical habitats. However, for any coastal fish population, randomly spawning on the shelf adjacent to lagoon-estuary systems and letting the juveniles find their way into these systems, just does not happen. Rather, seasonal variations of abiotic parameters and of food supplies and competitors must be accommodated, and this is what leads to the phenomenon labeled here as seasonal programming, which refers to the temporal and spatial sequence of lagoon habitat uses by juvenile fishes (Yaiiez-Arancibia et al.,1988a). Seasonal programming of food resource utilization by various species in Terminos Lagoon is illustrated in Fig. 12.5. Various species use the lagoon at different times, thus reducing their competitive interactions. At the level of a given species, seasonal programming implies that its representatives (mainly juveniles, see above) feed, in the course of their ontogeny, on a succession of different food types, often gathered from different subareas within a given lagoon (Fig. 12.6). This sequential utilization of food type and feeding location may be highly repetitive, hence predictable, between seasons. In areas where lagoon systems have maintained themselves over long periods, evolutionary mechanisms may emerge which stabilize and refine such seasonal programming, making the fish population in question gradu-

A. Yafiez-Arancibia, A.L. Lara Dominguez and D. Pauly

Iz=

371

1.5 y e b i l

I

1

I

C

c %BWD= Rorion-KX)/Wt

0

I

2

Impact on food base =

'

3

4

voBWD

I

Cohort age ( years)

Fig. 12.4. Schematic representation of the growth and decay of a cohort whose growth (A) can be depicted by the von Bertalanffl growth equation, and whose mortality by a negative exponential (B). In this example, cohort biomass (C) peaks at age t,,,=(l)=1year, which is also the age at which first maturity is achieved (Wm = 0.3 W-).Curve D depicts the decline of relative ration, commonly expressed as '% body weight per day'or %BWD. The product of curve D and curve C (El shows the predatory impact of the cohort, by age. As might by seen, tmax(2) < tmax(1)i.e,. it is the juveniles of a cohort which consume most of the food required by a cohort of fishes.

372

Coastal Lagoons as Fish Habitats

Fig. 12.5.Seasonalityof abundance of eight dominant fish species in Terminos Lagoon, MBxico. These fishes show a clear seasonal sequence of abundance and habitat utilization called seasonal programming. Note that the sum of the relative abundance of these eight species (lower graph) suggests an even utilization of Terminos Lagoon throughout the year (after Yiiiez-Arancibiaet al., 1988a).

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373

Fig. 12.6.Conceptual diagram illustrating the trophodynamic ecology of a hypothetical lagoon-estuarine fish species. The elements of time and time of the day, season and fish age (size), and space are shown. The spatial elements include transfers between ecosystem, e.g., from marine areas via lagoondestuarine t o riverine areas, as well as between habitats of the same system, e.g. from oyster reefs to seagrass beds within the lagoodestuarine system (after Yfiez-Arancibia et al., 1986).

ally more dependent on the lagoon system for the maintenance of high b'iomass. In other areas, where the lagoon systems do not persist or are not regularly open to juvenile immigrations, e.g. off West Africa, use of the lagoons are a matter of random movements along the coasts, and of an inshore movements, i.e. errunces. This implies (1)a lower conversion of primary and secondary production into fish flesh, and hence (2) lower biomasses of coastal fishes. The difficulty inherent in practical separation of random alongshore/ inshore movements, i.e. errunces, from evolutionarily fine-tuned, aimed movements towards and within lagoons, and the difficulties involved in precise field estimation of biomass and conversion efficiencies, make rigorous testing of these hypotheses difficult. This is the main reason for the debate on the degree of dependence of tropical coastal fishes on coastal lagoons. References Boehlert, G.W. and Mundy, B.C. 1988 Roles of behavioral and physical factors in larval and juvenile fish recruitment in estuarine nursery areas. In Larval fish and shellfish transport through inlets (Weinstein, M.P.,ed.). American Fisheries Society Symposium 3. Bethesda, Maryland, 5147.

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Coastal Lagoons as Fish Habitats

Cameron, W.M. and Pritchard, D.W. 1963 Estuaries. In The Sea 2. Wiley Interscience, London, 306-324. Cronin, L.E. and Mansueti, A.J. 1971 The biology of the estuary. In Symposium on the biological significance of estuaries (Douglas, P.A. and Stroud, R.H.A., eds). Sport Fishing Institute, Washington, DC, 1439. Dadswell, M.J., Klauda, R.J., Moffitt, C.M., Saunders, R.L., Rulifson, R.A. and Cooper, J.E. 1987 Common strategies of anudromous and catadromous fishes. American Fisheries Society Symposium 1,Bethesda, Maryland. Day, J.H., Blaber, S.J.M. and Wallace, J.H. 1981 Estuarine fishes. In Estuarine ecology with particular reference to Southern Africa (Day, J.H., ed.). A.A. Balkema, Rotterdam, 197-221. Day, Jr., J.W. and YBiiez-Arancibia, A. 1982 Coastal lagoons and estuaries: ecosystem approach. Ciencia Interam 22,12-26. Day, Jr., J.W., Hall, C., Kemp, M.W. and YBiiez-Arancibia, A. 1989 Nekton, the free swimming consumers. In Estuarine ecology. Wiley-Interscience, John Wiley and Sons, New York, 377-437. Deegan, L.A. and Thompson, B. 1985 The ecology of fish communities in the Mississippi River deltaic plain. In Fish community ecology in estuaries and coastal lagoons: towards an ecosystem integration (YBiiez-Arancibia, A., ed.). UNAM Press, MBxico, DF, 35-56. Fairbridge, R.W. 1980 The estuary: its definition and geodynamics cycle. I n Chemistry and biochemistry of estuaries (Olausson, E. and Cato, I., eds). Wiley-Interscience, John Wiley and Sons, Inc., New York, 1-36. Gross, M.R., Coleman, R.M. and McDowall, R.M. 1988 Aquatic productivity and the evolution of diadromous fish migration. Science 239,1291-1293. Haedrich, R.L. 1983 Estuarine fishes. In Ecosystem of the world. 26. Estuaries and enclosed seas (Ketchum, B.H., ed.). Elsevier Scientific Publishing Company, Amsterdam, 182-207. Henderson, P.A. and Margetts, A.R. 1988 Fish in estuaries. J o u m l of Fish Biology 33, 254 PP. Kjerfve, B. 1986 Comparative oceanography of coastal lagoons. In Estuarine variability (Wolfe, D., ed.). Academic Press, Inc., New York, 63-82. Kjerfve, B. 1989 Estuarine geomorphology and physical oceanography. In Estuarine ecology (Day, Jr., J.W., Hall, C.A.S., Kemp, M.W. and YBiiez-Arancibia, A., eds). Wiley-Interscience, John Wiley and Sons, New York, 47-78. Kjerfve, B. and Magill, K. 1989 Geographic and hydrodynamic characteristics of shallow coastal lagoons. Marine Geology 88, 187-199. Lankford, R.R. 1977 Coastal lagoons of MBxico: their origin and classification. In Estuarine processes (Wiley, M., ed.). Academic Press, Inc., New York, 182-215. Longhurst, A.R. and Pauly, D. 1987Ecology of tropical oceans. Academic Press Inc., San Diego, California. Louis, M. and Lasserre, G. 1982 Etude des peuplements des poissons dans les lagunes des mangroves de Guadeloupe (Antilles franqaises). In Coastal lagoons (Lasserre, P. and Postma, H., eds). Oceanol. Acta 5,333338. Lowe-McConnell, R.H. 1987 Ecological studies in tropical fish communities. Cambridge University Press, Cambridge, England. MacArthur, R.H. and Wilson, E.D. 1967 The theory of island biogeography. Princeton University Press, Princeton, New Jersey.

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McCleave, J.D., Arnold, G.P., Dodson, J.J. and Neill, W.N. 1984 Mechanisms of migrations in fishes. Plenum Press, New York. McDowall, R.M. and Eldon, G.A. 1980 The ecology of whitebait migrations (Galaxiidae: Galaxias spp). Fisheries Resource Bulletin 20, 172 pp. McDowall, R.M. and Frankenberg, R.S. 1981The Galaxiid fishes ofAustralia. Rec. Austr. Mus. 33,443-605. McHugh, J.L. 1967 Estuarine nekton. In Estuaries (Lauff, G.H., ed.). Special Publication 83. American Association for the Advancement of Science, Washington, DC, 581-619. Miller, J.M. and Memner, J.V. 1985 Determinants of habitats dependency in marine sport fishes. In Proceedings of First World Angling Conference, France Cap-d’Agde, 12-18 September 1984.IGFA, Fort Lauderdale, Florida, 119-130. Pauly, D. 1975 On the ecology of a small West-Africa lagoon. Ber. dt. wiss. Kommn. Meeresforsch 2446-62. Pauly, D. 1982 The fishes and their ecology. In Small-scale fisheries of Sun Miguel Bay, Philippines: biology and stock assessment (Pauly, D. and Mines, A.N., eds.). ICLARM Technical Reports 7, 124 p. International Center for Living Aquatic Resources Management, Manila Philippines, 15-33. Pauly, D. 1986 A simple method for estimating the food consumption of fish populations from growth data and food conversion experiments. Fisheries Bulletin 84, 827839. Phleger, F.B. 1969 Some general features of coastal lagoons. In Coastal lagoons, a symposium (Ayala-Castaiiares, A. and Phleger, F.B., eds). Symposium International, Coastal Lagoons UNAM-UNESCO, MBxico, DF, 5-26. Pianka, E.R. 1978 Evolutionary ecology. Second Editon. Harper and Row Publishers, New York. Pritchard, D.W. 1967 Observation on circulation in coastal plain estuaries. In Estuaries (Lauff, G.H., ed.). Special Publication 83. American Association for the Advancement of Science, 37-44. Weinstein, M.P. 1982 Commentary: a need for more experimental work in estuarine fisheries ecology. Northeast Gulf Science 6,59-64. Weinstein, M.P. 1988 Larval fish and shellfish transport through inlets. American Fisheries Society Symposium 3. Bethesda, Maryland. Yaiiez-Arancibia, A. 1985 Fish community ecology in estuaries and coastal lagoons: towards a n ecosystem integration. UNAM Press, MBxico, DF. Yaiiez-Arancibia, A. 1986 Ecologia de comunidades de peces en sistemas costeros tropicales. In Ecologta de la zona costera: andlisis de siete tdpicos. AGT Editor, MBxico, DF, 127-154. Yaiiez-Arancibia, A. 1987 Lagunas costeras y estuarios: cronologia criterios y conceptos para una clasificacion ecologica de sistemas costeros. Publicaci6n ACIESP 54, 1-36. Yaiiez-Arancibia, A. and Day, Jr., J.W. 1982 Ecological characterization of Terminos Lagoon, a tropical lagoon-stuarine system in the Southern Gulf of MBxico. In Coastal lagoons (Lasserre, P. and Postma, H., eds.). Oceanol. Acta 6 431-440. Yaiiez-Arancibia, A. and Nugent, R. 1977 El papel ecologico de 10s peces en estuarios y lagunas costeras. An. Cento. Cienc. del M a r y Limnol. Univ. Nac. Auton. Mkxico 4, 107-1 14. Yaiiez-Arancibia, A. and Sanchez-Gil, P. 1988 Ecologta de 10s recursos demersales marinos: fundamentos en costas tropicales. AGT Editor, MBxico, DF. Yaiiez-Arancibia, A., Lara-Dominguez, A.L., Aguirre, A. and Diaz, S. 1986 Feeding

3 76

Coastal Lugmns as Fish Habitats

ecology of tropical estuarine fishes in relation to recruitment processes. In ZOCIFAO Workshop on recruitment in tropical coastal demersal communities, workshop Report No. 44-Supplement (Yhi’iez-Arancibia,A. and Pauly, D., eds). United Nations Educational, Scientific and Cultural Organization, Pans, 73-94. Yaiiez-Arancibia, A., Lara-Dom’nguez, A.L., Ftojas Galaviz, J.L., Sanchez-Gil, P, Day, Jr., J.W. and Madden, C.J. 1988a Seasonal biomass and diversity of estuarine fishes coupled with tropical habitat heterogeneity (Southern Gulf of MBxico). Journal of Fish Biology 33, 191-200. Yaiiez-Arancibia, A., Lara-Dom’nguez, A.L., SBnchez-Gil, P., Rojas Galaviz, J.L., Alvarez, H., Soberon Chavez, G. and Day, Jr., J.W. 198813Dynamics ofcoastal nektonic communities in the Southern Gulf of MBxico. In Ecology of coastal ecosystems in the Southern Gulf of Mdxico: The Tenninos Lagoon region (Yaiiez-Arancibia, A. and Day, Jr., J.W., eds). Instituto de Ciencias del Mar y Limnologia Universidad Nacional Autonoma MBxico, Coastal Ecology Institute, Louisiana State University, MBxico, DF, 357-380. Zijlstra, J.J.1988Fish migrations between coastal and offshore areas. In Coastal ofshore ecosystem interactions: lecture notes on coastal and estuarine studies (Jansson, B.O., ed.). Springer-Verlag, Berlin, 257-272.

Coastal Lagoon Processes edited by B.Kjerfve (Elsevier Oceanography Series, 60) 8 1994 Elsevier Science Publishers B.V. All rights reserved

377

Chapter 13

Fisheries In Coastal Lagoons Daniel Paulya and Alejandro Y6iiez-Arancibiab aInternational Center for Living Aquatic Resources Management (ICLARM), MC P.O. Box 2631, 0718 Makati, Metro Manila, Philippines; bPrograma de Ecologia, Pesquerias y Oceanografia del Golf0 de Mdxico (EPOMEX), Universidad A u t h o m a de Campeche, Apartado Postal 520, Campeche 24000, Campeche, Mbxico

The ‘four factors’- recruitment, growth, natural and fishing mortalities -viewed in classical fish dynamics as needed for predicting future from present stock sizes, are reviewed in the context of lagoon fisheries, whose worldwide catches and management are briefly discussed. Emphasis is given herein to the bordigue, a form of tidal trap used in the western Mediterranean area to harvest fish previously recruited to a lagoon, and/or retain undersized fish for another growth cycle. This form of harvesvmanagement, which requires complex social arrangements, may, short of aquaculture, represent the optimum way of utilizing a lagoon to produce fish.

Introduction We shall briefly review the factors which determine fisheries yield in coastal lagoons, which have been defined by Ardizzone et al. (1988)as “bodies of waters (from brackish to hypersaline) partially separated from an adjacent sea by barriers of sand or other sediment, with openings through which seawater can flow” (see Kjerfke, this volume, for other definitions). ‘Fisheries’refers to harvesting of fish and aquatic invertebrates and hence, excludes aquaculture. The literature on coastal lagoons is immense; that on lagoon fisheries is also too large for a balanced review to fit into the space available for this chapter. We shall, therefore, concentrate on a few themes which we think are of special relevance to lagoon fisheries, yield prediction and management and to fishes only. Our examples will focus on one lagoon of the Gulf of Mexico, and West African and Mediterranean lagoons. We chose the first two locations because we worked there; and the Mediterranean cases,

378

Fisheries In Coastal Lagoons

TABLE 13.1 Fisheries yields of coastal lagoons (all groups included) as compared with the yields of other aquatic ecosystems' Systems

Coastal lagoons Continental shelves AfricdAsian reservoirs Coral reefs River floodplains Reservoirs (U.S.A.) Natural lakes

Yields (t km-2 year-') Median

Mean

n

5.1 4.8 4.2 4.1 3.2 1.3 0.5

11.3 5.9 7.5 4.9 4.0 2.4 2.8

107 20 41 15 33 148 43

because this is where the management scheme emerged to which we would like to point the readers' attention. Except for a few references, we shall thus not cover the important lagoon fisheries of the Indo-Pacific area. These lagoons are included, however, in the compilation of lagoon fisheries catches discussed below.

Fisheries Catches From Coastal Lagoons Kapetsky (1984) presented the largest data set on catches from coastal lagoons assembled to date, and compared these with catches from other exploited marine and freshwater ecosystems. As can be seen from Table 13.1,coastal lagoons are, overall, more productive than other ecosystems in terms of fisheries yield, whether one uses the mean or the median as a measure of central tendency. This can ultimately be accounted for by the generally high primary production in coastal lagoons (Nixon, 19821, for which three reasons may be suggested: - availability of organic matter inputs via rivers (Kapetsky, 1984; YaiiezArancibia and Aguirre-Leh, 1988); - shallowness, conducive to rapid remobilization of nutrients (Qasim, 1973a, 1973b; Jones, 1982; Nixon, 1982; Quignard, 1984); - velocity and volume of water exchanges between the sea and the lagoon (which also directly affects fish production via recruitment (cf. Bourquard and Quignard, 1984). However, coastal lagoons do not have uniformly high yields and indeed, the frequency distribution of Kapetsky's yield data is strongly skewed (Fig.

D. Pauly and A. Yariez-Amncibia

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13.1). Some factors leading to this nearly log-normal distribution are, with regard t o the numerous occurrences of unproductive lagoons: - extreme salinities and temperature fluctuations, turbidity, anoxic conditions or toxic discharges (Gunter, 1967); - very shallow sills, preventing sufficient recruitment; - excessive illumination or turbidity, of which either can lead to reduced primary productivity (Qasim, 1973a, 1973b); - lack of sufficient fishing effort (Quignard, 1984; Bailey, 1988).

05*’-

25

a

60

70

80

Annual c a t c h (t/km2)

Fig. 13.1. Frequency distribution of fishery catches from coastal lagoons (after Kapetsky, 1984). (A) Finfish, n = 106.(B)Penaeid shrimp, n = 51.

The few extremely productive lagoons in Fig. 13.1 may benefit, on the other hand, from a number of factors: - a coastal habitat supplying a large recruitment; - fertilization via agricultural runoff, or human sewage and through water exchanges with the sea; - a management regime which makes the best of the incoming recruitment. Fish Population Dynamics and Its Four Factors

Russel’s Axiom The definition of ‘fisheries’ in the introduction implies the need for management: common-property, open-access natural resources systems, given competing users, cannot produce high, sustained yields if left to themselves (Hardin, 19683.

Fisheries In Coastal Lagoons

380

1 1-

- - - - -(reproduction)----> .

I

Fig. 13.2. Schematic representation of the four key factors determining the future biomass of and catches from a stock: recruitmentwhere N = number of fish added to the stock, individual of the recruited fish, natural mortality and capture or fishing mortality. growth in weight (W) Feeding and reproduction, although necessaryfor stock build-upand maintenance,are usually not part of classical fish population dynamics (after Russel, 1931 and Ricker,1975).

Fishery science and especially its “operational” subdiscipline, fish population dynamics, are often articulated by means of Russel’s axiom, i.e.,

Bz = B,(R’ + G’)- (M’ + F’)

(13.1)

which states that a well-defined stock (= population) of fish of biomass (B1) will have, after an arbitrary period At,the biomass Bz as a result of positive processes (R’ = recruitment; G = growth) having added to the stock, while negative processes (M’ = natural mortality; F = fishery catches) have reduced it (Russel, 1931;Ricker, 1975). Equation 13.1 does not have a form allowing its direct use for assessment, and hence this equation is generally used only for defining a problem, as done here. On the other hand, the four processes included in the equation, plus some peripheral processes also considered by fishery biologists (Fig. 13.2)have been put in a mathematically tractable form (see Schaefer, 1957; Beverton and Holt, 1957;Ricker, 1975;Gulland, 1983;Pauly, 1984).Here, we shall present a few of the models that have resulted from these efforts, specifically those which have been used in dealing with coastal lagoons. The four factors shall be examined in the sequence: growth, natural mortality, fishing mortality and related factors (mainly catch/effort), with recruitment being last, because it is the most complex factor to investigate, t o model and to predict. Growth of Fishes in Coastal Lagoons Lagoon environments are highly seasonal, more so than the open marine environment to which they are connected. Thus, the food types (cf. AguirreLeon and Yaiiez-Arancibia, 1986;Chavance et al., 1984,and Fig. 13.3)and food consumption, and hence, the growth of lagoon fishes are bound to oscillate seasonally, whether the fish in question undertake seasonal migrations in and out of coastal lagoons or not. Various authors have modified the von BertalanfQ equation (von Bertalane, 19381,commonly used to express growth of fish, to accommodate seasonal growth oscillations (Longhurst and

D.Pauly and A. Yariez-Arancibia Bairdiella

381

chrysoura

0

c

Wet season

40r

n - 20

log 10 ( Stomach fullness index 1

Fig. 13.3. An example of seasonal changes in the stomach contents and hence, presumably the food consumption of a lagoon fish Bairdiella chrysoura, T6rminos Lagoon,Mexico (computed from Chavance et al., 1984).

Pauly, 1987;Hoenig and Chaudhury Hanumara, 1982;Somers, 1988;Soriano and Jarre, 1988). Figure 13.4presents a n application example pertaining to a lagoon fish, Eucinostomus gula (Gerreidae) from TBrminos Lagoon, Mexico (AguirreLe6n et al., 1989). The growth models presented by these authors cannot accommodate long periods of zero growth. Therefore, we present here a new model, which can accommodate a (winter) period of growth stagnation called no-growth time (NGT).To fit the curve, the time axis is divided into one growth and one nogrowth time over each period of one year. Then, during growth time, we have L, = L , [l - exp( - w)]

(13.2)

in which L, is the length at age t , and where

K ~~

w = K(t’ - to) +

2x

(t’ - t,) - sin

3

2n ( t o - tJ (13.3) 1 - NGT

where t’ is obtained by subtracting from the age t the total amount of NGT the fish experienced, since t = 0.

Fisheries In Coastal Lagoons

382

J

F

M

A

M

J

J

A

S

O

N

D

Months

Fig. 13.4. Seasonal growth of Eucinostomus gula (Gerreidae)from TBrminos Lagoon, Mexico as estimated from length-frequencydata using the ELEFAN I program of Pauly and David (1981). Adapted from Aguirre-Le6n et al. (1989). 40

-5 r

-

.' "

35 -

#*--

4'

30-

---..-e-

/'

25 -

4-

;2 0 -

a -

L,

0 c

0

I-

Dicentrarchus lobrox

15 -

'0

-

=75 cm. (set value)

K ' = 0.31 (0.73year)-' NGT = 0 . 2 7 (Jon- M a r )

5-

o-LJ

F M

A M J

J

A

S O N

D

J F M A M J J

A

S O

N D J

F M A

M J J

A S

O N

D

The other parameters are L, the asymptotic length, i.e., the average length the fish would attain if they lived indefinitely; K' a parameter expressing how fast L, is approached, to the theoretical 'age' at length zero if the fish always grew according to the equation; and t, is a parameter adjusting a seasonal cycle to start at t = 0. Note that the seasonal growth itself (outside of NGT) is described by a sine wave curve with period 1NGT, and that the unit of Ic is (year - NGT)-l instead of year1. An application example for this model is given in Fig. 13.5. As might be seen, the model predicts a no-growth time of about 3 months (January to March) for Dicentrarchus Zabrux in l'lhang d'Or, France, a feature which earlier growth models could not have picked up. When growth is not seasonal, i.e., when one deals with data points that are one year apart or when working with otolith microstructures of fish not

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383

Age ( y e a r )

Fig. 13.6. Growth of Archosargus rhomboidah (Sparidae) in T&minos Lagoon, Mexico as inferred from counts of otolith microstructures (recomputedfrom Chavance et al., 1986). The new parameter estimates derived here differ from those originally presented because a non-linear routine was used to fit the data; the new estimate of L, = 22 cm is closer to the maximum length in TBrminos Lagoon (23 cm) than the original estimate of L, = 20 an (6. Chavance et al., 1986).

belonging to a given cohort, the standard von Bertalanffy curve can be used. It has the form:

where L , and t o have the same definitions as in Eqs. 13.2 and 13.3,where K has the dimension of time-l (e.g., year’). An application example is presented in Fig. 13.6. The growth of fishes within coastal lagoons relative to that of conspecifics growing in other habitat appears to be a function of (a)the type of lagoon and/or of habitats being compared, (b) the species of fish, and (c) the life stage of the fish species. Thus, Chauvet (1988)suggests that shallow, eutrophic lagoons lead to improved growth compared with deep lagoons, strongly influenced by the marine regime. He also noted, with regard to Mediterranean species such as Sparus auratus (Fam. Sparidae) and Dzcentrurchus Zubrux (Fam. Moronidae) that the lagoon habitat appears to lead to higher growth rates among the juveniles and young adults (Fig. 13.71,while among the older specimens of these two species, growth within lagoons is at best equivalent to, and generally less than, that in the marine environment (Chauvet, 1988).

Fisheries In Coastal Lagoons

384

0 Dicentrarchus labrax

A Sparus auratus

i

0

Age (years)

I

2

3

Age (years)

Fig. 13.7. Growth differences among juveniles of two species of fishes important to Mediterranean lagoon fisheries. Note improved growth in the lagoon habitat (after Chauvet, 1984, 1988).

The West African coast offers further and clearer indications of at least small coastal lagoons being habitats leading, independently of fishing effects, to reduced final (maximum) sizes among resident fishes: (a) the maximum observed size of Sarothemdon melanotheron (Cichlidae) in the small poikilohaline Sakumo lagoon was 19.5 cm (Pauly, 1976)vs. 25 cm in the much larger, estuarine Lagos lagoon (Fagade, 1974), and (b) two forms of the Ethmlosa fimbriuta (Clupeidae)appear to occur in West Africa, one occurring along the coast, and in estuaries and large ‘open’coastal lagoons, and reaching sizes up to 30 cm, the other limited to length of about 15 cm and occurring and reproducing only within closed lagoons (Longhurst and Pauly, 1987). Improved growth of juveniles combined with limited maximum sizes of adults are not necessarily contradictory. Indeed, rapid juvenile growth due to availability of abundant food and high habitat temperatures generally implies a reduction of maximum adult sizes. Longhurst and Pauly (1987) give reasons why this should be so.

Natural Mortality of Fishes in Coastal Lagoons The natural mortality of a fish population is straightforward and can be modelled using:

N 2= N 1e-MAt

(13.5)

where N1 and N Z are the numbers of fish at the beginning and end, respectively, and M is the instantaneous rate of natural mortality during a period At. Instantaneous rates such as in Eq. 13.5 e.g., natural (M)and

D.Pauly and A. Yariez-Arancibia

385

fishing (F)mortality can be added to yield total mortality (2) and the latter used in equations analogous to Eq. 13.5. Beverton and Holt (1959)were the first to demonstrate rigorously that the growth performance of fishes is strongly correlated with their natural mortality, i.e., that the growth parameter K of fishes with similar L, values (Eq. 13.4) generally represents a constant proportion of M. Their findings were generalized by Pauly (1980) based on data from 175 fish stocks to yield the empirical equation: (13.6)

where L, = asymptotic length (total length, in cm), K = year1 and T = temperature in “C. Hence, lagoon fishes, which tend to have higher K and lower L, values than their conspecifics in open waters, can be expected to have generally higher natural mortalities than their open-water counterparts. This explains the observation of Chauvet (19881, who noted that “strangely, it is in the lagoon habitats that are most favorable for growth that the mortality rates are highest ...”. It is important, however, to distinguish between the relatively low natural mortality rates affecting late juveniles and adults, which are fairly constant and somehow predictable, from those affecting larval and early juveniles, which are high and largely unpredictable. Moreover, an important distinction with regard to lagoons is that between natural mortality rates as discussed above and catastrophic mortalities, as caused e.g., by dystrophic crises (Chauvet, 1988). The former may be seen, at least as far as stocks rather than individuals are concerned, as a gradual process. Thus, natural mortality, as influenced or determined by predation, will be more or less continuous over a certain period,and can be compensated for by population growth, leading to the observed narrow range ofMIKvalues, otherwise, stock could not maintain themselves in evolutionary times. Catastrophic mortalities, on the other hand, which are quite frequent in lagoons, are episodic events, usually connected with sudden changes of water characteristics such as dissolved 02,H a content, temperature, which can induce large-scale death among resident stocks and sometimes their total annihilation. Many natural or anthropogenic factors may cause catastrophic mortalities in coastal lagoons, including: - eutrophication, leading to nighttime depletion of oxygen and/or benthic production of H2S which can be released into the water column by storms; - cold or hot spells, particularly effective in shallow lagoons (Gunter, 1952; 1957; 1967); - terrigenous pollution e.g., from agricultural pesticides.

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Catastrophic mortalities are difficult to incorporate into standard population dynamics model and have indeed not generally been considered explicitly in fisheries management. Their probability of occurrence and their prevention are, however, important aspects of management schemes involving bordigues and aquaculture in coastal lagoons.

Fishing Mortality and Related Statistics Fishing mortality defined above as F = 2-M can also be defined as:

F = -C B

(13.7)

where C is the fisheries catch (in weight) during a given period and B is the mean biomass during that same period. Equation 13.6 provides the rationale for attempts to estimate the biomass of fishes in coastal lagoons using e.g., tagging (see Lam Hoai and Lasserre, 1984).Another important definition involving F is C=q.f*F

(13.8)

and hence, C/f = q-f, where C is as defined above, f represents the fishing effort and q is the catchability coefficient of the gear represented by f. These definitions imply that C/f (the catch per unit of effort or CPUE of earlier authors) is, given a constant q , proportional to biomass and hence can be used to monitor the status of a stock whose absolute biomass may be unknown (Fig. 13.8A). Lam Hoai and Lasserre (1984)reviewed the applicability of these and derived models, notably yield per recruit (YIR) and cohort analysis, to lagoon fisheries, and the methods, e.g., surveys or tagging studies, were used to estimate their parameters. They concluded that: - the assumption of equilibrium implicit in most standard models for analyzing the population dynamics of fish (notably Y/R analysis) render these models questionable when applied to data from lagoon fisheries; - the scattered and small-scale nature of fisheries operation in coastal lagoons generally makes the routine collection of C/f data too costly. Hence, such data are lacking for most lagoons (Kapetsky, 1984)or are largely unreliable (Bailey, 1988); - the methods that may be most appropriate for lagoon fisheries, i.e. virtual population analysis or cohort analysis (Gulland, 1965;Pope, 19721,which have the advantage of not requiring estimates of fishing effort, do require catch-at-age data, which can be obtained in a cost-effective fashion only for the most important species in major lagoon fisheries;

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387

A

_-_-

b

\

*

.

*

t

.

-\

c v

1

, I 1 1 1 1 1 1

0.2

0.5

I

I

2

I

I 1 1 1 1 1 1 1

5

10

I

20

Illlld

100

50

No. of fishers . kni2 ( log scale) 2

N

20 15

.-

‘E Y

5

p

O

*

0

b

*

Floodplains

o

0

c

0

0

0.1 0.5 I

1

I

I

I

l

l

5

10

15

20

25

30

No. of fishers. k m 2 (square root scale)

Fig. 13.8. Examples of the relationshipsbetween fishing effort per area, i.e. fishing intensity, and dependent variables. (A) Relationship between C/f and fishing intensity in 42 coastal lagoons (Kapetsky, 1984).(B) Relationship between yield, C, and fishingintensity in 13 coastal lagoons (plus 15 river floodplains, which appear to have similar yields) (Bailey, 1988).

it may be most appropriate to manage lagoon fisheries as a black-box system, using a form of adaptive management in which the catches resulting from initial interventions are monitored and used to refine a next set of interventions. Bailey (1988), working on various tropical small-scale fisheries data sets, including coastal lagoons, noted that the inclusion of fishing effort considerably added to the precision of empirical, i.e., black-box models in predicting yields in such systems. He derived, for lagoons (n= 13) and floodplains (n = 15), the joint model (Fig. 13.8B): -

Fisheries In Coastal Lagoons

388

log,(yield + 1)= -0.19

+ 1.44 f“.5

- 0.19f

(13.9)

for an annual yield expressed in t km-2, and where f i s the fishing effort per area (here, fishers/km2 of lagoon or floodplain area). This model has an intercept unequal to zero and hence, predicts a non-zero yield where f = 0. It should not be used with small values off(Bailey, 1988). While pleased with the predictive power of his model given the heterogeneity of the available data set (Kapetsky, 19841, Bailey (1988) also noted that external approaches such as his may have reached a dead end, not because of their empirical nature, but because of the lack of acceptable data “in terms of number of systems and quantity of variables measured consistently” and that the relevant paradigm may thus “have reached a n impasse with respect to prediction of future size of stocks or yield.” While based on different premises, this conclusion echoes those of Lam Hoai and Lasserre (1984). The application of classical methods of fish population dynamics and fishing management to coastal lagoon fisheries appears inappropriate, even if isolated elements of lagoon resource systems can be described by these classical methods.

Recruitment to (Fish Stocks) in Coastal Lagoons Providing reliable predictions of the recruitment of a fish stock for use in fishery management is something that has largely eluded fishery biologists, despite massive effort since the very beginning of fishery science as a discipline of its own near the end of the last century (Pauly, 1986). Classical population dynamics, as formulated in the text of Beverton and Holt (1957) or Ricker (19581, resolved this problem by combining derivative of equations such as presented above into models for predicting yield per recruit as a function of control variable such as F and age at first capture (t,). An example of such model is: (13.10) where r1= tc- t o and r2 = tc- t,, 2 and K are as defined above, W,is the weight corresponding to L,, and recruits of age t, are defined as fully metamorphosed young fish whose growth is described adequately by an equation also describing the growth of the adults, whose instantaneous rate of natural mortality is similar to that of the adults, and who occur in or swim into some fishing ground (Pauly, 1984). Following Ricker (19541, numerous fishery biologists have attempted to develop models linking parent stock and subsequent recruitment. The various curves are not reviewed here. Their lack of usefulness has been sufficiently demonstrated (Sharp and Csirke, 1983).

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Yearround

389

Archosargus hombd&lis mopristis chtysoptera Haemulon aumlineawm

pstenia splendda Domsoma petenense Cichlasoma lenesmum Beom marinus

All habitats: EudmsrOmus gula (adults,dry season) Arius lelis (adults, wet season)

Fig. 13.9. Examples of fishes with characteristic migration patterns within, outside of, and into and out of TBnninos Lagoon, Southern Gulf of Mexico (redrawn from Ybaez-Araucibia et al., 1988).

Three groups of fishes occur in coastal lagoons: (i)sedentary species: those which spend their entire life cycles within coastal lagoons; (ii) seasonal migrants: those which enter the lagoon during a more or less well-defined season from either the marine or the freshwater side and leave it during another season (Fig. 13.9); and (iii) occasional visitors: those which enter and leave lagoon without a clear pattern within and among years. Group (i) is usually very limited, especially when species with planktonic stages are considered. Most lagoon fishes spawn outside lagoons. The tilapia Sarotherodon melanotheron, a male mouthbrooder, is an exception and its special ability to withstand large variations of temperature and salinity has enabled it to dominate completely the fauna of small closed

Fisheries In Coastal Lagoons

390

Sma II- sca le migrations

A

catadromous

eaa 8 lo;;ae

anadromous

mature adults

B

I

eaa 8

late luveniles 8 young adults

laiiae

mature adults

Large-scale (anadromous)migrations

I

Juveniles

/ - a

Rlvers

Lagoons /estuaries

Open sea

Fig. 13.10. Schematic representation of (A) small-scale migrations involving shelf, lagoodestuary and freshwater habitats and allowing certain tropical fish stocks to maintain higher abundances than if they utilized only one type of habitat, and (B) large-scale migrations. Note the marked difference of the scheme in A to the coldltemperate situation, characterized by large-scale anadromic migrations (after Ybliez-Arancibia and Sanchez-Gil, 1988).

lagoons along the West African coast (Pauly, 1976). Generally, however, it is members of Group (ii)which dominate the fauna of coastal lagoons, e.g. by contributing near 100% of the ichthyofauna of many Mediterranean lagoons (Quiguard, 1984). Group (iii) is represented in the Mediterranean by species such as Belone belone or Scomber scombrus (J.P. Quignard, pers. com.),by species such as Lutjanus spp or Caranx hippo in West African lagoons (Pauly, 19751, or species such as Bagre marinus, Cetengraulis edentulus,Archosargus rhomboidalis or Haemulon aurolineatum in lagoons of the southern Gulf of Mexico (Fig. 13.9).

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Different opinions exist with regard to the mechanisms of recruitment into lagoons of Group (ii)fishes. Quignard (1984) and Chauvet (1988), representing the Eastern Atlantic school, stress that the recruitment of Group (ii) fishes into coastal lagoons is not necessarily due to migrations (Harden-Jones, 1968). Rather, fishes of this group are either flushed into lagoons while still in the planktonic stage, or swim as early juveniles into lagoons against the outgoing current, either due to their effort to stay close inshore or due to coastal wanderings (errances in French texts) in search of food. The other school, which may be called Western Atlantic, stresses the important role that lagoons play in the life cycles of many coastal fishes. Here, lagoons are viewed as major elements of small-scale migrations, which, by involving lagoodestuarine habitats, allow a high standing stock to be maintained (Fig. 13.10). In either case, the relative level of recruitment into coastal lagoons will be determined between lagoons by the ease with which fish can penetrate into these, and between years by the overall number of potential recruits along the coast. This former point implies, as also emphasized by Chauvet (1988), that recruitment to coastal lagoons can be artificially increased by keeping the mouth of lagoons open during periods when juveniles of preferred species occur along the coast, or by deepening the sill of lagoons with very shallow mouths. Coping with between-year variability of recruitment to lagoons is more difficult, and one of the few practical approaches for dealing with this problem is management through bordigues, the gear and management tool to which the Food and Agriculture Organization of the United Nations (FAO) devoted its first manual on lagoon management (Chauvet, 1984). Management of Coastal Lagoon Fisheries Conventional Fisheries Management

As alluded to in the introduction, fisheries, at least in modern times, tend to quickly become ovefished unless they are managed, the result of an overwhelming harvesting capability applied to finite resources that belong to no one before they are caught (common property) and which everyone has the right to exploit (open access). Fisheries management, therefore, generally involves putting restrictions on either (a) access to the resources, (b) number of gears, (c) type of gear deployed, (d) timing of effort deployment, or (el some combination of these (Kapetsky, 1981). The models used to derive the numbers needed for such management are commonly (i) analytical models (Beverton and Holt, 19571, or (ii) surplus-production model (Schaefer, 1957;Munro, 19791,with either of these possibly containing some economic component, i.e. consideration of fishing costs and of gross returns.

392

Fisheries In Coastal Lagoons

One commonly-used analytic model, structured around the four factors discussed above, is the yield-per-recruit model (Eq. 13.91,which can be used to assess the optimum sizdage at first capture. The optimum mesh size in a given fishery with known fishing mortality identify the optimum fishing mortality, given a certain unique range of sizdage at which the fish of a given stock should be caught. Such computations can be made a bit more realistic by adding a size-value relation into the model, which leads to optimum monetary or non-monetary return per recruit being evaluated (Willman and Garcia, 1985; Die et al., 1988). Surplus-production models, on the other hand, can either be time- or space-structured. In the former, a time series of catch data from a given fishery is related to the corresponding time series of effort data such that the optimum level of effort, yielding the largest (hopefully) sustained catch (MSY) is identified (Schaefer, 1957; Ricker, 1975).The other class of surplus production model is based on the assumption that a number of units compared such as lakes, lagoons, and coastal stretches, had similar potential production before onset of a fishery, and that catch differences between them are due to different levels of effort (Munro, 1979).This leads to models such as Eq. 13.8, which allow rough estimation of potential or optimum yields for given levels of effort. This allows the assessment of whether a lagoon is under- or over-fished. Unfortunately, long time series of catch and effort data, needed for fitting time-structured surplus models, are generally lacking. Yield-per-recruit models, on the other hand, when thoughtlessly used to assess fishes or shrimp stocks within lagoons, tend to lead to the same results: wherever they are applied, they suggest that it is better to let the generally small fish and shrimp that occur inside lagoons escape, and be caught later by some offshore fishery. This reflects the fact that, until recently, fishery research viewed lagoons as one side of an antagonistic relationship, in which often poor, small-scale fishers catch large numbers of undersized fish and shrimp, which if they had escaped from the lagoons, would have contributed yields to more rational industrial fisheries outside the lagoons (Garcia and Le Reste, 1981). There exists, however, an approach by which the conflict between lagoon and coastal fisheries can be resolved, and this involves turning coastal lagoons from marginal sites for harvesting of undersized fish into production units making most of the natural recruitment they receive, just as any well-managed coastal fishery is supposed to do.

Using ‘Bordigues’ as a Tool for Fisheries Management The French word bordzgues (from the Provencal bourdigo) refers to a fishing gear widespread around the Mediterranean and representing the culmination of an evolution spanning over two millennia (Chauvet, 1988; McCann, 1988). It is essentially a trap. Its basic principle is simple: all young fish wanting to

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393

Fig. 13.11. Schematic representation of a ( k e s i a n ) bordigue, showing (A) how recruits are let through to the lagoon when the water flows out of the lagoon, and (B) how fish attempting to leave when the water flows into the lagoon are caught to be either marketed or returned to the lagoon for another growth cycle (after Chauvet, 1988).

get into the lagoon through a bordigue can; none of those who try to leave the lagoon can do so (Fig. 13.11). Bordigues, which are usually placed along the channel connecting a lagoon and the sea, thus differ in principle from gears such as traps, fish corrals, and gill nets that are deployed, ofien in great numbers, within the lagoon but which do not totally block the outlet, and hence let a fraction of the fish leave the lagoon to the sea. Here, the question might arise: how can a gear which retains all the fish attempting to leave a lagoon be useful to management? The point is that the bordigues are so constructed that the fish they retain are not killed, but rather graded according to size, and diverted into holding areas through adjustable panels of different mesh sizes. Only fish that have reached

Fisheries In Coastal Lagoons

394

Start of o bordigue's

B Ri

A

I I

Fish remaining inside IopOOfl Fish returning t o the sea F ' Fish captured by fishing M = Flsh lost to notural causes R ' Natural recruitment of m o l l fish into logoon Recruitment t o fishable stock of fish kept inside lagoon in previous y e a d s )

Fig. 13.12. Schematic representation of differences between (A) a lagoon exploited by gears deployed within the lagoon, and (B) a lagoon exploitedmanagedvia a bordigue. Note the build-up of fish biomass in case (B)within the lagoon, due to retention of undersized fishes, which would have left the lagoon in case (A) (after Chauvet, 1988).

market size are harvested. Others are recycled, i.e., returned to the lagoon to grow until the next harvest season along with fresh recruits that have just entered the lagoon. Figure 13.12 contrasts the operation of a bordigue over a period of several years after set up with the traditional exploitation scheme of a lagoon, as repeated every year. As might be seen in Fig. 13.12a, the harvest of a conventionally managed lagoon will be based only on the single recruitment preceding that harvest, there being no fish left in the lagoon from previous recruitment. Thus, catches will vary between years in proportion to recruitment levels, given consistent effort, and the fish caught will generally be undersized. Bordigues, on the other hand, lead to a harvest level that is a function of the average level of recruitment over several years, besides harvesting fishes that are always of optimum size. Thus, bordigues allow the kind of resource exploitation that is optimal in

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395

terms of yield-per-recruit analysis, just as in a well-managed trawl fishery. Fish that have not grown to sufficient size escape through the mesh, grow some more, and then may be caught, or again escape through the mesh of the trawl’s cod end, depending on whether they have grown to some size deemed as optimal in terms of weight or price. As in a trawl fishery, the size at harvest in a bordigue will thus depend on the ratio between growth and natural mortality. Therefore, it would make little sense for the small fish retained by a bordigue to be returned to a lagoon in which natural mortalities are such that they offset any gain in individual weight of the released fish. Thus, managing a lagoon by means of a bordigue also implies removing as many sources of natural mortality as possible, e.g., by filling in deep pools in which large predators can accumulate, as well as reducing the likelihood of catastrophic mortalities by preventing inflows of pollutants. Acadja-type structures within lagoon, which enhance the survival and the growth of young fish may be considered in this context. The technical aspects of the construction of bordigues are discussed in Chauvet (1988). It is worth mentioning that construction of bordigues requires sophisticated engineering and large investments. However daunting, the technical and financial aspects of the construction of bordigues, social factors are the key factors regarding their implementation. The use of bordigues as a fishing gear and management tool requires from those with access to or fishing rights in a lagoon an extremely high degree of cooperation and well-honed management skills. Bordigues must be staffed permanently to prevent them from becoming clogged with drifting seagrass, to change the retaining panels of different mesh size depending on which species are in the process of swimming to different retention chambers. Also, fishing within the lagoon must be restrained, as it could otherwise offset the gains obtained by returning to the lagoon undersized fish retained by the bordigue. Fishing in lagoons equipped with bordigues (Table 13.2) is not fitting for individualistic fishers. Bourquard and Quignard (1984)document the case of a bordigue which failed because of their non-cooperation. Discussion

We did not present the bordigues as a management tool for lagoon fisheries because we believe that this method of harvestinglmanaging is likely to become widely adopted outside its area of origin. We realize that, although it is technically applicable in principle in virtually any place in the world, the potential for its adoption outside the Western Mediterranean is rather unlikely because of the high degree of cooperation between fishermen that it required, and because of the high level of prerequisite management skills. Rather, we presented the bordigue as an important element in the continuum of lagoon fisheries, which range from totally unmanaged to partially

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396

TABLE 13.2 Comparative fishery yields of lagoons with and without systems to improve survival andor growth of recruits (Chauvet, 1988). LocatiodSystem

Annual mean yield (t km2)

Standard error (t km-2)

n

Africa Fishing only Acadja' and fishing

0.10 7.75

0.52 2.57

10 3

Mediterranean Without bordiguesb Non-permanent bordigues Permanent bordigues Permanent bordigues plus artificial stocking'

0.83 0.82 1.85 3.77

-

52 10 2 4

0.64 0.18 2.96

?See Kapetsky (1981)for a description of acadja-based fisheries. bComputed by subtracting bordigues yields from grand mean for Mediterranean. %s is a form of aquaculture (Ardizzoneet al., 1988).

and well-managed fisheries and beyond, to extensive, semi-intensive and intensive aquaculture operations. Indeed, the bordigue, although it is not an aquaculture operation is a key element of one of the most sophisticated capture fisheries regime one can imagine. It requires a level of cooperation and of management skill probably higher than that involved in extensive aquaculture, whose yield the bordigue can approximate (Table 13.2). Other worthwhile management regimes for lagoon fisheries do exist and have been discussed by Kapetsky and Lasserre (1984). References Aguirre-Lebn, A. and YBiiez-Arancibia, A. 1986 Las mojarras de la laguna de TBrminos: taxonomia, biologia, ecologia y dinamica tr6fica (Pisces: Gerreidae). An. Znst. Cienc. Mar Limnol. Univ. Nac. Aut6n. Mhxico 13,369-444. &uirre-h6n, A., Yaiiez-Arancibia, A. and Pauly, D. 1989 Recruitment model of Eucinostomus gula (Pisces: Gerreidae) in the southern Gulf of Mexico: life history with tropical estuary-shelf interrelationships. Presented at the XI11 Annual Meeting of the Early Life History Section, American Fisheries Society, 21-26 May 1989,Merida, Mexico. Abstract In El Programa y la Memoria. 57. Ardizzone, G.D., Cataudella, S. and Roissi, R. 1988 Management of coastal lagoon fisheries and aquaculture in Italy. FA0 Fish. Tech. Pap. 293. Rome, 103. Bailey, P.B. 1988 Accounting for effort when comparing tropical fisheries in lakes, river-floodplains and lagoons. Limnology and Oceanogmphy 33,963-972.

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Beauchot, M.L. 1987 Poissons osseux. In Fiches F A 0 d'zdentification des Especes pour les Besoins de la Piche. Mediterrane'e et Mer Noire. Zone de P2che 37. Rev. 1 Vo. ZZ Ve'rtebre's(Fischer, W., Schneider, M. and Beauchot, M.L., eds). Bertalane, L. von. 1938 A quantitative theory of organic growth (Inquiries in Growth Laws 11).Hum. Biol. 10, 181-213. Beverton, R.J.H. and Holt, S.J. 1957 On the dynamics ofexploited fish populations. U.K. Min. Agric. Fish., Fish. Invest. (Ser. 2) 19, 533 pp. Beverton, R.J.H. and Holt, S.J. 1959 A review of the lifespans and mortality rates of fishes in nature, and their relationships to growth and other physiological characteristics. CZBA Found. Colloq. Ageing 5, 142-180. Bourquard, C. and Quignard, J.P. 1984 Le complexe de peche de Salse-Leucate:bordigue et barrge de poissons. La Piche Maritime, 1272,3-11. Chauvet, C. 1984 Fisheries in the Lake of Tunis: fishery biology and increasing production by means other than regulation. In Management of coastal lagoon fisheries. F A 0 Stud. Rev. GFCM No. 61 1 (Kapetsky, J.M. and Lasserre, G., eds). 615-694. Chauvet, C. 1988 Manuel sur PamBnagement des @ches dans les lagunes c6tihres: la bordigue mediteranhenne. F A 0 Fish Tech. Pap. 290,77 pp. Chavance, P., Flores Hernandez, D., Yafiez-Arancibia, A. and Amezcua Linares, F. 1984 Ecologia, biologia y dinamica de 10s poblaciones de Bairdiella chrysouraen l a laguna de TBrminos, Sur del Golf0 de Mexico (Pisces: Sciaenidae).An. Znst. Cienc. del M a r y Limnol. Univ. Nal. Auton. Mexico 11, 123-162. Chavance, P., Yafiez-Arancibia, A., Flores-Hernandez, D., Lara-Dom'nguez, A.L. and Amezcua Linares, F. 1986 Ecology, biology and population dynamics of Archosargus rhomboidalis (Pisces, Sparidae) in a tropical coastal lagoon system, Southern Gulf of Mexico. An. Znst. Cienc. del M a r y Limnol. Univ. Nal. Anton. Mexico 13, 11-30. Die, D.J., Restrepo, V.R. and Hoenig, J.M. 1988 Utility-per-recruitment modelling: a neglected concept. Transactions of the American Fisheries Society 117,274-281. Fagade, S.O. 1974 Age determination in Tilapia melanotheron in the Lagos lagoon, Nigeria with a discussion of the environmental and physiological basis of growth marking in the tropics. In Ageing offish (Bagenal, T.B. ed), 71-77. Garcia, S. and Le Reste, L. 1981 Life cycles, dynamics, exploitation and management of coastal penaeid shrimp stocks. F A 0 Fisheries Technical Paper 203,215 pp. Gulland, J.A. 1965 Estimation of mortality rates. Annex to Rep. Arctic Fish. Working Group. Z.C.E.S. C.M. 1965 3 , 9 pp. Gulland, J.A. 1983 Fish stock assessment: a munual of basic methods. John Wiley and Sons, Chichester. Gunter, G. 1952 The impact of catastrophic mortalities for fisheries along the Texas coast. Journal of Wildlife Management, 63-69. Gunter, G. 1957 Temperature. In Treatise on marine ecology and paleoecology 1. Soc. Amer. Mem. 67 (Hedgpeth, J.W., ed). 159-184. Gunter, G. 1967 Some relationships of estuaries to the fisheries of the Gulf of Mexico. In Estuaries. Am. Assoc. Adv. Sci. Spec. Publ. 83 ( L a d , G.H., 4) 621-638. . Harden-Jones, F.R. 1968 Fish migration. Edward Arnold Ltd., London. Hardin, G. 1968 The tragedy of the commons. Science Wash.) 162,1243-1248. Hoenig, N. and Chaudhury Hanumara, R. 1982 A statistical study of seasonal growth model for fishes. Technical Report of the Department of Computer Science and Statistics, University of Rhode Island, Kingston, R.I. 91 pp. Jones, R. 1982 Ecosystems, food chains and fish yields. In Theory and management of

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tropical fisheries. ZCLARM Conference Proceedings 9,Manila, Philippines (Pauly, D. and Murphy, G.I., eds). 195-239. Kapetsky, J.M. 1981 Some considerations for the management of coastal lagoon and estuarine fisheries. F A 0 Fisheries Technical Papers 218,Rome, 47 pp. Kapetsky, J.M. 1984 Coastal lagoon fisheries around the world: some perspectives on fishery yields and other comparative fishery characteristics. In Management of coastal lagoon fisheries. F A 0 Stud. Rev. GFCM No. 61. Volume 1,Rome (Kapetsky, J.M. and Lasserre, G., eds). 97-139. Kapetsky, J.M. and Lasserre, G. (eds) 1984 Management of coastal lagoonfisheries. Gen. Fish. Counc. Medit. Stud. Rev. 61, Vol. 1: 1 4 3 8 ; Vol. 2: 439-776. FAO, Rome. Lam Hoai, T. and Lasserre, G. 1984 Stock assessment methods in coastal lagoon fisheries. In Management of coastal lagoon fisheries. F A 0 Stud. Rev. GFCM No. 61, Volume 1, Rome (Kapetsky, J.V. and Lasserre, G., eds). 438 pp. Longhurst, A.R. and Pauly, D. 1987 Ecology of tropical oceans. Academic Press Inc., San Diego, California. McCann, A.M. 1988 The Roman port of Cosa. Scientific American 258,84-91. Munro, J.L. 1979 Stock assessment models: applicability of utility in tropical small-scale fisheries. In Stock assessment for small-scale fisheries (Roedel, P.M. and Saila, S.B., eds). 35-47. International Center for Marine Resource Development, University of Rhode Island, Kingston, R.I. Nixon, S.W. 1982 Nutrient dynamics, primary production and fisheries yield of lagoons. In Actes Symposium International sur les Lagunes Cotieres, SCOR IZABO I UNESCO, Bordeaux, 8-14 Septembre 1981,Oceanol. Acta 6,357-371. Pauly, D. 1975 On the ecology of a small West-African lagoon. Ber. dt. Komm. Meeresforschung 24,46-62. Pauly, D. 1976 The biology, fishery and potential for aquaculture of Tilapia melanotheron in a small West African 1agoonAquaculture 7,33-49. Pauly, D. 1980 On the interrelationships between natural mortality, growth parameters and mean environmental temperature in 175 fish stocks. J . Cons. CZEM 39,175-192. Pauly, D. 1984 Fish population dynamics in tropical waters: a manual for use with programmable calculators. ZCLARM Studies and Reviews. Manila, Philippines. Pauly, D. 1986 Towards appropriate concepts and methodologies for the study of recruitment in tropical demersal communities. In Proceedings of the IREPI OSLR Workshop on the Recruitment of Tropical Coastal Demersal Communities, Campeche Mexico, 21-25 April 1986.ZOC (UNESCO) Workshop Rep. No. 44 (Yafiez-Arancibia, A. and Pauly, D., eds). 3-14. Pauly, D. and David, N. 1981 ELEFAN I, a BASIC program for the objective extraction of growth parameters from length-frequency data. Meeresforschung 28, 205-2 11. Pope, J.G. 1972 An investigation of the accuracy of virtual population analysis using cohort analysis. Znt. Comm. Northw. Atl. Fish. Res. Bull. 9, 65-74. Qasim, S.Z. 1973a Some problems related to the food chain in a tropical estuary. In Marine foodchains (Steele,J.H., ed).Oliver and Boyd, Edinburgh (Reprt. edition), 45-51. Qasim, S.Z.19731, Productivity ofbackwaters and estuaries. In The biology of the Indian Ocean (Zeitzschel, B., ed). Ecological Studies 3, Springer Verlag, Berlin, 143-154. Quignard, J.P. 1984 The biological and environmental characteristics of lagoons as the basis of fisheries management. In Management of coastal lagoon fisheries. F A 0 Stud. Rev. GFCM No. 61 (Kapetsky, J.M. and Lasserre, G., eds). 3-38. Ricker, W.E. 1954 Stocks and recruitment. Journal of the Fisheries Research Board

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Canada 11,559-623. Ricker, W.E. 1958 Handbook of computation for biological studies of fish populations. Bulletin of the Fisheries Research Board of Canada 119,300pp. Ricker, W.E. 1975 Computation and interpretation of biological statistics of fish populations. Bulletin of the Fisheries Research Board of Canada 191,382pp. Russel, F.S. 1931 Some theoretical considerations on the "overfishing" problem. J. Cons. CIEM 6,3-27. Schaefer, M.B. 1957 A study of the dynamics of the fishery for yellowfin tuna in the eastern tropical Pacific Ocean. Inter-Am. Trop. Tuna Comm. Bull. 2,247-268. Sharp, G.D. and Csirke, J. (eds) 1983 Proceedings of the Expert Consultation to Examine Changes in Abundance and Species Composition of Neritic Fish Resources. San Josk, Costa Rica, 18-29 April 1983.F A 0 Fisheries Report No.291. Somers, I.F. 1988 On a seasonally-oscillating growth function. Fishbyte 6,8-11. Soriano, M.L. and Jarre, A. 1988 On fitting Somers equation for seasonally-oscillating growth, with emphasis on T-subzero. Fishbyte 6,13-14. Willman, R. and Garcia, S. 1985 A bio-economic model for the analysis of sequential artisanal and industrial fisheries for tropical shrimp (with a case study of Suriname shrimp fisheries). F A 0 Fisheries Technical Paper 270,Rome. 49 pp. Yaiiez-Arancibia, A. and Aguirre-Le6n9A. 1988 Pesquerias en la region de la laguna de Terminos. In Ecologia de 10s Ecosistemas Costeros en el sur del Golf0 de Mkxico: L a Region de la Laguna de Tkrminos (YBiiez-Arancibia, A. and Day, J.W., Jr., eds). 431-452. Yaiiez-Arancibia, A.,Lara-Dominguez, A.L., Shchez-Gil, P., Rojas-Galaviz, J.L., Alvarez Guillbn, H., Soberh-Chavez, G. and Day, J.W. 1988 Dynamics of coastal nektonic communities in the Southern Gulf of Mexico. In Ecologta de 10s Ecosistemas Costeros en el sur del Golf0 de Mkxico: La Regibn de la Lagun de Tkrminos (YAiiezArancibia, A. and Day, J.W., eds). 357-380.

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Chapter 14

Aquaculture in Coastal Lagoons Donald J. Macintosh Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK

Lagoon fisheries production can be enhanced significantly through aquaculture, provided appropriate culture sites and species are selected. Salinity is the most important environmental variable in lagoons affecting the scope for aquaculture but, increasingly, habitat degradation, eutrophication and pollution are adding to the limitations on aquaculture in lagoon systems. Examples of culture methods operating in lagoons, including fish cages, pens for milkfish and shrimp, mollusc rearing on artificial substrates, and seaweed production are described,with emphasis on their environmental requirements and constraints. Some socio-economic problems involving aquaculture development in lagoons, compatibility with other uses of lagoons, and the special relationship between aquaculture and lagoon capture fisheries are discussed.

Introduction

History of'Aquaculture in Lagoons Lagoons have always been favoured sites of settlement for coastal populations and historically they have also served as prime habitats for gathering shellfish and for fishing. Shell mounds dating back 4000-6000 years have been found on the edge of some Pacific lagoons containing the remains of edible bivalves, fish bones, and the remnants of various fishing gears. This tradition continues today in many parts of the world, particularly among the island communities of the Pacific, where lagoon fish and shellfish remain a staple of the diet (Zann, 1983). Traditional forms of aquaculture in lagoons probably evolved gradually from early efforts to aggregate lagoon fish and molluscs in order to increase production. According to the trap and crop theory (&banal, 19881,early settlers learned t o place barriers in coastal lagoons, swamps and bays to

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confine fish and shrimp. At least ten centuries ago it is believed that some coastal inhabitants had already devised fencing and other structures to aggregate fish in lagoons. There were apparently independent developments towards aquaculture in various parts of the world, but particularly in the Mediterranean region where there are more than one million hectares of brackish water basins and lagoons. Oysters were probably the first farmed marine organisms, since artificial oyster beds in Lake Lucrinus near Naples are believed to date from around 95 BC (Stewart, 1982). It is known that the Romans cultivated oysters on ropes. The best documented evidence of fisheries management in lagoons also comes from Italy, since there is information on the use of fish barriers and other devices for controlling and catching fish dating t o the fourth and fifth centuries BC (Ardizzone et al., 1988). It is reported that some lagoons on the North Adriatic coast were fenced for fish rearing by the Romans (see Brown, 1977). Equivalent structures for aggregating and holding lagoon fishes have evolved in many other parts of the world. The coastal lagoons of Hawaii, for example, have been used from prehistoric times for rearing mullet and other popular species. It was common for 1-2 m wide ditches to be built to allow ingress of seawater and young fish. Wooden grilles placed in these ditches prevented the escape of large fish (Barnes, 1980). The famous fish-parks, or acadjus, in West African lagoons represent another simple form of aquaculture of long standing (Welcome, 1972). Although their function is simply to draw fish from the surrounding water by offering an attractive refuge, the yields reported of 5-8 ton ha-' fishing area compare favourably with those of many intensive forms of aquaculture in current operation. Operating on the same principle, but even more simply, are the brush pile fish aggregating structures made from mangrove branches used widely in South and Southeast Asia, for example in Negombo Lagoon, Sri Lanka (Samarakoon, 1986). In Asia, simple forms of coastal aquaculture may have started first in Indonesia, then spread to other Southeast Asian countries and later to the Indian subcontinent (Rabanal, 1988). In the Pacific, it has been traditional on some islands to transfer cockles (Anadara) for storage and on-growing to suitable sand beds at the edge of lagoons (Zann, 1983) and similarly to establish gardens for the giant clam (Tridacna)in suitable lagoon or reef sites (Maclean, 1978).

Scope of This Review In order to draw a distinction between aquaculture and the various other forms of fisheries management practised in lagoons, this review is concerned only with culture methods in which there is complete control over the movements of the stock for the purposes of routine husbandry and harvesting. This definition applies to fish and crustacean farming in ponds,

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cages, pens and tanks; to mollusc culture on artificial substrates, and to seaweed farming. Also implicit in the distinction between aquaculture, in senso stricto, and other forms of fisheries management in lagoons is that in the former there is (a) some degree of control or manipulation of the life-cycle of the organism, e.g. by collecting wild seed (fish fry, oyster spat, etc.) or through artificial breeding or nursing; and (b) use is made of feeds (or fertilizers) and/or artificial structures to increase productivity over that possible in the natural environment. The technical basis for aquaculture practices suitable for lagoons is, in most cases, similar to that for equivalent culture systems operating in other aquatic environments. While examples of typical lagoonal aquaculture systems are described, the main purpose of this review is to consider the special features of lagoon environments which create particular opportunities, limitations and potential conflicts for aquaculture development. Since many lagoons fall into the category of eutrophic coastal systems with high biological productivity, it is natural to regard aquaculture as a desirable form of exploitation of these environments. However the potential conflicts between aquaculture and other forms of resource utilization in coastal lagoons are considerable. In practice, even the relationship between capture fisheries and aquaculture has created many socio-economic problems, while all forms of fisheries, including fish culture, are severely threatened in many countries by environmental degradation and pollution of lagoon systems resulting from human interference in the name of coastal development. The Lagoon as an Environment for Aquaculture Lagoon Productivity In the general classification of aquatic ecosystems, lagoons have among the highest primary and secondary productivities known. The average natural fish production in lagoons is around 100 kg ha-l year1 (Kapetsky, 1984). This makes them about twice as productive per unit area as coastal seas and most fresh water systems. In addition to high primary productivity, Chauvet (1988) has shown that the growth and/or survival of important brackish water food fishes in the Mediterranean, such as mullets, seabream (Sparus) and seabass (Dicentrarchus)are higher in lagoons than in estuaries or littoral waters. This may be correlated, in part, with the higher nutrient levels and primary productivity in lagoons compared with coastal waters (Barnes, 1980). There are more than 100 coastal lagoons bordering the Mediterranean covering an area of about 850,000 ha. For 68 of these which have been studied, Chauvet (1988) gives their average production as 103kg ha-l year'

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in lagoons with permanent fish catching gates (bordigues)and 377 kg ha-' year1 if the fisheries management system includes artificial stocking of fingerlings. Increasingly, aquaculture will be expected to produce seed artificially in hatcheries for stocking lagoons to enhance natural fish production. The potential of culture-based capture fisheries in confined water bodies, such as lagoons, lakes and reservoirs, is already widely recognized by fishery scientists (e.g. Petr, 1985). Fish production is about double the above level in tropical West African lagoons, the average for 13 lagoons being 237 kg ha-' year', and this rises to 775 kg ha-', where acadjas (brush parks) are used. Similar yields would apply to tropical Asian lagoons, although many of these have been badly affected recently by over-fishing or pollution. For example, fish catches in Laguna de Bay (Philippines) used to average 433 kg ha-', based on traditional fishing methods, but the yields declined from the 1960s due to overfishing following the introduction of mechanized boats, monofilament nets and other non-selective forms of fishing (Davies et. al., 1987). Given the high average level of natural productivity in lagoons and the success of these simple forms of fisheries management in boosting fish yields significantly, it is obvious that the potential productivity from aquaculture in lagoon systems is also excellent. However other environmental variables in lagoons are critical in determining their suitability for particular culture methods or target species.

Environmental Considerations for Aquaculture Aquaculture is generally regarded as a site specific technology and this view certainly holds true for lagoons, each of which is different to all others. The general physical features which give each lagoon system its uniqueness: size, shape, tidal range, run-off tributary streams, climate, number and size of inlets, and kind and amount of sediment (Phleger, 1981)are all important in considering their potential for aquaculture. Of particular importance to aquaculture site selection are the effects of these general physical features on (a)water depth and quality; (b) currentltidal flow and water exchange; and (c) turbidity and sedimentation. Study of the hydrological conditions in lagoon systems is essential to any appraisal of their environmental suitability for aquaculture. The effects of any seasonality on these conditions is also of greatest importance. Seasonal climatic factors and physical changes to the lagoon-sea connection may greatly alter the scale of interaction between riverine water sources and tidal processes within the lagoon, causing dramatic seasonal changes in temperature, salinity and nutrient conditions and in sedimentation patterns and hence turbidity. Many of the cultivable fish, shellfish and seaweed species have a narrow tolerance range for one or more of these parameters.

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Within lagoon systems two distinct habitats are potentially available for aquaculture development, (1) the peripheral shallow-water zone around the lagoon’s edge, which normally consists of fine sediments supporting marshes or mangroves; (2) the central open-water zone, which according to the location and depth of channels, may vary from 1 to 4 m and feature coarser sediments such as sand, gravel or shell debris. Physical and chemical conditions for aquaculture may show significant differences between these two major habitats. While there is a greater degree of shelter around the lagoon margins, and often more convenient site access, there may be far greater influences from peripheral land development, including effluent discharge and high rates of sedimentation. Salinity From an aquaculture perspective, the dominant environmental characteristic of lagoons is their tendency to have widely fluctuating salinities. Although lagoons may be described as brackish, marine or hypersaline water bodies (Phleger, 19811, many lagoons show a marked spacial gradient in salinity from nearly marine saltwater conditions near the mouth of the system to areas under strong riverine or other freshwater influence which are very low in salinity. Depending on its size and shape, a lagoon may show

Freshwater

Decreasing salinity

Increasing salinity

7

_L

Barrier

channel

S

E

A

Fig. 14.1. Representation of a hypothetical lagoon illustrating the four possible salinity environments which may occur (many lagoons have only one or two of these salinity zones; based on Barnes, 1980).

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TABLE 14.1 Approximate salinity ranges of organisms suitable for culture in lagoons in relation to the salinity zones illustrated in Fig. 14.1. Fresh zone 0% carps tilapias 045% Macrobrachiurn sea-bass (Dicentrarchus) mullets miMish penaeid shrimp sea-perch (Lates) siganids groupers mud crab (Scylla) oysters mussels (Perna) cockles (Anadnra) Gracilaria Tridncna

Brackish zone

5%

25%

Sea water zone

Hypersaline zone

36%

60%

0-5s 0-25% 0-36% 638%

0-35% 0-3% 2042% 238% 2&3% 20-30% 12-40/50% 1130% 30-38s

clearly defined salinity zones as illustrated diagrammatically in Fig. 14.1. Based on salinity tolerance, a list of suitable species for aquaculture in each of the four lagoon zones indicated is provided in Table 14.1. Usually adding further to this spacial variation are temporal changes in salinity caused by climatic factors. These may be strong enough to completely alter salinity conditions throughout the lagoon system. The strength of the NE monsoon on the east coast of Peninsular Malaysia, for example, dominates the coastal topography and environmental conditions along much of this coastline. Here coarse sand is deposited as a bar or sand bank across the mouths of most rivers, creating numerous small, shallow lagoons (Chua, 1984).These lagoons and mangroves swamps are subjected to complete freshwater flooding during the NE monsoon, such that even the normal brackish water fisheries have to be suspended for 2 to 4 months. Obviously aquaculture would be virtually impossible under these physical and salinity conditions, except perhaps as a seasonal activity in the intermonsoon periods.

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Temperature Temperature is interactive with salinity in influencing the physiological tolerance of aquatic organisms to lagoonal conditions. While many of the cultivated brackish water species of fish, crustaceans, molluscs and seaweeds are tolerant of extremes of temperature or salinity, they may be less tolerant of these two parameters in combination. The post-larvae of Penaeus monodon for example are extremely tolerant of both salinity (5-40% in 72 hour survival tests) and temperature (20-36°C). However in combination, extreme temperatures narrow the salinity tolerance range of this important cultured shrimp (Valencia, 1976). In addition to survival, temperature is also an important parameter affecting the growth of cultured organisms. In terms of aquaculture economics, there may be a very narrow range of temperature at which temperature dependent variables such as growth rate and food conversion ratio give a favourable economic return. A common feature of tropical lagoons is that the annual range in water temperature is low (2 5°C) but, as noted above in the Malaysian example, salinity can vary considerably from predominantly marine conditions during the dry season to virtually freshwater conditions during the rainy season. Another case in point is Lagos Lagoon in Nigeria, where Ajana (1980) reported high mortalities of Crassostrea gusar from the oyster fishery during the wet season when salinity in the lagoon dropped to as low as 0.5%. In sub-tropical lagoons there may be significant fluctuations in both salinity and temperature with respect to the wet and dry seasons which will greatly restrict the range of species suitable for aquaculture, or limit their production to particular seasons. Figure 14.2 gives an example showing how salinity and water temperature vary from about 8 to 60% and from 22 to 36°C respectively in Caimanero Lagoon, Mexico (latitude 22'50' N).

Dissolved Oxygen Control of dissolved oxygen (DO) is critical to many aquaculture operations, particularly where fish or crustaceans are farmed at high density. In nature, DO may fluctuate diurnally and seasonally; it is also affected by several other water quality parameters. The oxygen saturation capacity of water decreases with increasing temperature and salinity. A DO level of above 5-6 mg 1-l is generally acceptable for aquaculture purposes. The lethal level for shrimp and for some species of cold water fish such as salmon is 2-3 mg 1-l (Boyd, 1982) and 1-2 mg 1-l for many warm water species. Low DO conditions are associated with calm areas of lagoons with little water movement, a n d o r areas where organic matter accumulates to create a high BOD (biological oxygen demand). DO levels may also be extremely sensitive to sediment disturbance through the suspension of oxygen-con-

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Fig. 14.2. Temporal changes in water quality in Caimanero Lagoon, Mexico (from Edwards, 1978):graphs show (a) monthly changes in average temperature and salinity in relation to rainfall, and (b) diurnal fluctuation in dissolved oxygen concentration.

suming bottom sediments during storms, or because of human activities, such as dredging, or the clearance of fringing mangroves. Figure 14.2shows the diurnal range of DO measured by Edwards (1978) in Caimanero Lagoon, Mexico. Values were at or above saturation (> 7 mg 1-9 for several hours during the day, but fell overnight to a low point of about 4.5 mg 1-l at 06.00hours. In eutrophic lagoons or aquaculture systems with high nutrient loading (such as fertilized ponds or fish cages), it is common for this early morning oxygen low to drop to critical levels due to nocturnal oxygen consumption by phytoplankton (Colt and Tchobanoglos, 1981). Even if lagoon oxygen levels remain above the lethal concentration for fish and shellfish, low oxygen conditions can induce sublethal effects detrimental to aquaculture. These include reduced appetite and growth (Chiba, 1966)and greater susceptibility to diseases or parasitic attack.

Turbidity Temporal and spatial variations in the turbidity of coastal waters are believed to be important to the distribution of fishes. Evidence from turbidity gradient studies indicates that many juvenile fish actually follow turbidity gradients as their means of locating estuarine nursery areas. This

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applies to the young of sciaenids, sparids and mugilids, whereas the adults of these and other coastal species prefer clearwater. Within lagoons, turbidity is affected by the water circulation pattern, and the type of sediments and vegetation. In the Boca de Rio Mandinga lagoodestuarine system on the Pacific coast of Mexico, Farias-Sanchez (1991) found that the water was turbid with suspended mud near the sheltered mangrove zone, but clear in other areas which had a hard substratum created from broken oyster shells. Because mangroves trap and bind fine sediment particles (Bird and Barson, 19771,the destruction of mangrove around the edges of lagoons is a common reason for increased sedimentation. In the Boca del Rio System, mangrove removal has led to some areas of the oyster beds being silted over (Farias-Sanchez, 1991).For site evaluation purposes, the acceptable levels of DO are indicated to be above 90% saturation for most fishes and shrimps (Huguenin and Colt, 1989).For the cockle, Anadara, which is subjected to particularly low oxygen tension in its natural mud flat habitat, the level is 80% (see Broom, 1985).

Choice of Species Not surprisingly, fish and shellfish species which occur naturally in lagoons and other coastal habitats show the greatest tolerances to the environmental conditions found in lagoon systems, particularly with respect to salinity and temperature fluctuations. Under tropical conditions the UILT, or upper incipient lethal temperature, is a useful measurement of temperature tolerance for aquaculture purposes as it indicates the maximum temperature which can be tolerated indefinitely, while CTM denotes the critical temperature at which death occurs. Menasveta (1981)tested 24 species of marine fishes in Thailand. He found that species associated with lagoons and mangroves had the highest thermal resistance, particularly the mullet Mugil dussurnkri (mean CTM and UILT 42.0,37.5).Other lagoon-associated fish which performed above average included the grouper, E. tauuinu (CTM 39.61, tarpon, Therupon theraps (42.3)and the scat, Scatophugus argus (41.2).In comparison, the range of CTM for other species was 36.7to 39.4(based on acclimation at 27 to 29°C). Similarly, the salinity tolerances of common lagoon fishes and shell fishes are generally much wider than those of related species restricted to open waters. Table 14.1 indicates the approximate salinity range of lagoon-associated species with proven aquaculture potential. As the great majority of these migrate between lagoons and coastal waters at one or more phases of their life cycles, their salinity and temperature requirements are also known to change with their stage of development. A feature common to the most important groups: milkfish, mullets, groupers, seabass, sea-perch, penaeid shrimp and mud crabs is that they utilize lagoons predominantly as nursery

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Aquaculture in Coastal Lagoons

areas and therefore from the juvenile stages until maturity they show a wide adaptability to lagoon conditions, particularly salinity fluctuations. This is a major reason why a number of species within these groups can be cultured successfully in lagoon environments. Aquaculture Practices in Lagoons: Some Examples

In lagoons with suitable environmental conditions, it is highly feasible to exploit the main water bodies for aquaculture by concentrating fish, shellfish or seaweed in floating or fixed structures. These include cages, pens and other enclosures for fish, shrimps and crabs; floating rafts, stakes and ropes or other artificial substrates for molluscs and nets for seaweeds (Fig. 14.3). In design, these structures are usually made from inexpensive locally available materials. Their common feature is that they are open so that they interrupt the natural lagoonal water circulation as little as possible. In this way the culture systems are dependent on natural water fluxes to maintain good water quality and to bring food in the form of plankton, detritus and nutrients, to the stock. To maintain this circulation, regular cleaning is necessary to remove encrusting organisms (e.g. barnacles), algae and sediments. Rates of encrustation are especially high in the tropics (see Chua and Teng, 1977). Examples of commercial open-water aquaculture systems operating in lagoons are described below, emphasizing their environmental requirements and the constraints on production.

Fish Culture in Cages Although commercial farming of fish in cages is a relatively recent development, there is a tremendous variation in cage design. A well illustrated review of the technical considerations in cage construction is provided by Beveridge (1987).Cage culture accounts for about 40% of the total production of farmed fish in marine and brackish waters. Caged fish culture has several advantages over other potential forms of aquaculture in lagoons: (a) The techniques for rearing fish in cages are well proven since there is already considerable experience of cage farming in major lagoon systems such as Lake Songkhla (Thailand) and Laguna de Bay (Philippines) as well as in freshwater lakes and reservoirs. Several major technical reviews of cage culture have already been published e.g. (Coche, 1979,1982;Beveridge, 1987). (b) The investment cost, relative to pond construction is modest, but stocking rates can be higher.

D.J. Ma