The Biology of Freshwater Wetlands (Biology of Habitats)

The Biology of Freshwater Wetlands THE BIOLOGY OF HABITATS SERIES This attractive series of concise, affordable texts...

Author: Arnold G. van der Valk

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The Biology of Freshwater Wetlands

THE BIOLOGY OF HABITATS SERIES This attractive series of concise, affordable texts provides an integrated overview of the design, physiology and ecology of the biota in a given habitat, set in the context of the physical environment. Each book describes practical aspects of working within the habitat, detailing the sorts of studies which are possible. Management and conservation issues are also included. The series is intended for naturalists, students studying biological or environmental science, those beginning independent research, and professional biologists embarking on research in a new habitat.

The Biology of Rocky Shores Colin Little and F. A. Kitching The Biology of Polar Habitats G. E. Fogg The Biology of Lakes and Ponds Christer Brönmark and Lars-Anders Hansson The Biology of Streams and Rivers Paul S. Giller and Bjorn Malmqvist The Biology of Mangroves Peter F. Hogarth The Biology of Soft Shores and Estuaries Colin Little The Biology of the Deep Ocean Peter Herring The Biology of Lakes and Ponds, Second ed. Christer Brönmark and Lars-Andres Hansson The Biology of Soil Richard D. Bardgett The Biology of Freshwater Wetlands Arnold G. van der Valk The Biology of Peatlands Håkan Rydin and John Jeglum

The Biology of Freshwater Wetlands Arnold G. van der Valk Ecology, Evolution and Organismal Biology Iowa State University

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Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With ofﬁces in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Oxford University Press 2006 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 2006 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, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available Typeset by Newgen Imaging Systems (P) Ltd., Chennai, India Printed in Great Britain on acid-free paper by Biddles Ltd., King’s Lynn ISBN 0–19–852539–7 ISBN 0–19–852540–0 (Pbk.) 10 9 8 7 6 5 4 3 2 1

978–0–19–852539–4 978–0–19–852540–0 (Pbk.)

Preface The recognition that freshwater wetlands are more than a transition zone between terrestrial and aquatic systems is relatively recent. If wetlands were considered at all, they were treated as either wet versions of terrestrial vegetation by ecologists or the shallow portions of lakes and ponds by limnologists. For both, wetlands were a peripheral topic. Even the term wetland is relatively new. Wetland is not found in the index of C. D. Sculthorpe’s (1967) The Biology of Aquatic Plants, G. E. Hutchinson’s (1975) A Treatise on Limnology: Vol. III. Limnological Botany, or the ﬁrst edition of Robert G. Wetzel’s (1975) Limnology. However, by the second edition of Limnology (Wetzel 1983), this had changed. In 1978, Freshwater Wetlands: Ecological Processes and Management edited by R. E. Good, D. F. Whigham and R. L. Simpson was published to which Robert Wetzel contributed a paper. In 1980, the Society of Wetland Scientists was founded. In the same year, the ﬁrst international meeting on wetlands was held in New Delhi, India, under the aegis of INTECOL. In 1983, the journal Wetlands was launched by the Society of Wetland Scientists, and John R. Etherington published his classic book, Wetland Ecology. Today, wetlands are widely studied around the world and the literature on wetlands is huge. Typing ‘wetlands’ into Iowa State University’s electronic library catalogue brings up nearly 1000 books, monographs, and government publications. I make no claim to have read all of them, or even most of them. Although it might seem that wetland ecology as a discipline is only about 25 years old, this is misleading. Wetlands had been studied for many years prior to the 1980s by a variety of people, including waterfowl or waterbird biologists, palaeoecologists, and especially peatland ecologists. In fact, Peatlands by Peter Moore and David Bellamy, which was published in 1973, is arguably the ﬁrst book published on freshwater wetland ecology. Peatland ecology and wetland ecology are very similar disciplines that continue to coexist but are beginning to unite. Peatland ecology originated and was initially centred in northern Europe. Wetland ecology originated and was initially centred in North America. From my perspective, peatlands are just a type of wetland and they are treated this way in this book. Nevertheless, peatlands with their organic soils, unique ﬂora, and unique hydrology cannot be covered

vi PREFACE

adequately in a book of this length. Consequently, there will be another book in this series on peatlands. In the meantime, Charman’s Peatlands and Environmental Change (2002) provides an excellent introduction to them. A complete account of the species in any given wetland would be much longer than this entire book. Consequently, only a small number of representative species of microorganisms, plants, and animals are described. Likewise, it was impossible to describe in any detail even one speciﬁc wetland. Fortunately, there are good books on a variety of wetlands around the world, including Australian wetlands (McComb and Lake 1990), the Pantanal in South America (Heckman 1998); the Great Dismal Swamp (Kirk 1979), pocosins (Richardson 1981), the Everglades (Lodge 2005), prairie potholes (van der Valk 1989), patterned peatlands (Wright et al. 1992) in North America; and the Kafue Flats in Zambia (Ellenbroek 1987). There are also some excellent and well-illustrated books on the wetlands of the world (Finlayson and Moser 1991, Dugan 1995). Today the importance of wetlands is widely recognized around the world, and the number of people who work in wetland-related jobs in universities, government agencies, non-governmental organizations, and private companies continues to increase. Universities have responded to this increasing need for people trained in wetland ecology by offering courses in the ﬁeld at both the graduate and undergraduate level. This book is designed to be a textbook for beginning courses in wetland ecology. It is, in fact, largely based on such a course that I teach. I have assumed that anyone taking a wetland ecology course has already had an introductory ecology course. Consequently, some ecological jargon and concepts are used that are not deﬁned or explained in detail in the text. For anyone whose background in ecology is limited, a glossary is included that deﬁnes these technical terms. This book focuses on the organisms found in wetlands. Wetland ecology is a ﬁeld that has developed unevenly, and some topics, for example, wetlands as nutrient sinks or as duck habitat, are much better developed than others. I have tied to provide a balanced overview of the discipline rather than to cover various topics in proportion to the amount of information available. Nevertheless, the information and ideas that have made it into this book are inevitably based largely on my own experiences and those of my colleagues from around the world whose work I know, respect, and admire. I am particularly indebted to colleagues with whom I have worked on research projects, with whom I have had the opportunity to talk about wetlands over the years at meetings and conferences, or whose writings have inspired me: Bruce Batt, Bill Crumpton, Craig Davis, Max Finlayson, Sue Galatowitsch, Dale Gawlik, Sylvia Haslam, Don Hey, Bob Kadlec, John Kadlec, Hal Kantrud, Wiley Kitchens, Dan Mason, Beth

PREFACE vii

Middleton, Henry Murkin, Bill Patrick, Eric Seabloom, Fred Sklar, Bill Streever, Jos Verhoeven, Milton Weller, Paul Wetzel, Robert Wetzel, Dennis Whigham, Tom Winter, and Joy Zedler. I would also like to thank my wife Suzanne whose editorial work improved the readability of the text signiﬁcantly. Any shortcomings and errors in fact or interpretation are my own. This book is dedicated to Suzanne who over the years has learned more about wetlands than any other English major. Ames, June 2005

Arnold van der Valk

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Contents 1

Introduction 1.1 1.2 1.3 1.4 1.5

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3

What is a wetland? Wetland classiﬁcation Wetland inventories Summary Practical experiments and observations 1.5.1 Wetland classiﬁcation

1 1 6 9 10 10 11

Wetland environments

13

2.1 2.2 2.3 2.4 2.5

14 20 22 30 30 30 31 32

Physical and chemical properties of water Macrophyte canopies Wetland hydrology Summary Practical experiments and observations 2.5.1 Water levels 2.5.2 Water chemistry 2.5.3 Temperature and oxygen gradients

Microorganisms and invertebrates

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3.1 Microorganisms 3.1.1 Bacteria 3.1.2 Bacterial metabolism 3.1.3 Dissimilatory metabolism 3.1.4 Assimilatory metabolism 3.1.5 Fungi 3.1.6 Algae 3.2 Invertebrates 3.2.1 Aquatic insects 3.3 Summary 3.4 Practical experiments and observations 3.4.1 Wetland algae 3.4.2 Wetland invertebrates

34 34 34 35 38 39 40 44 49 52 53 53 54

x CONTENTS

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Wetland plants and animals

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4.1 Wetland plants 4.1.1 Gas-space continuum 4.1.2 Heterophylly 4.1.3 Clonal growth 4.2 Birds 4.3 Fish 4.4 Amphibians, reptiles, and mammals 4.5 Summary 4.6 Practical experiments and observations 4.6.1 Macrophyte adaptations 4.6.2 Seed banks 4.6.3 Wetland birds 4.6.4 Wetland ﬁsh

58 62 67 69 71 77 81 84 85 85 85 86 86

Spatial and temporal patterns

87

5.1 Spatial patterns 5.1.1 Coenocline development 5.1.2 Fire 5.2 Temporal patterns 5.2.1 Macro-succession 5.2.2 Fluctuation versus Micro-succession 5.3 Development of wetland landscapes 5.4 Summary 5.5 Practical experiments and observations 5.5.1 Gradient analysis 5.5.2 Sediment cores

91 92 97 98 98 101 102 107 107 107 108

Wetland functions

109

6.1 6.2 6.3 6.4

109 115 118 123 129 131 132 133 133 134 134

Primary production Litter decomposition Food webs Nutrient cycling 6.4.1 Nitrogen and sulphur cycling 6.4.2 Phosphorus cycling 6.5 Summary 6.6 Practical experiments and observations 6.6.1 Primary production 6.6.2 Invertebrates and litter decomposition 6.6.3 Methane production

CONTENTS xi

7

The future of wetlands

136

7.1 7.2 7.3 7.4 7.5

137 139 141 143 149

Wetland losses Wetland degradation and invasive species Global climate change Wetland restoration Summary

GLOSSARY BIBLIOGRAPHY INDEX

150 155 169

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1 Introduction

1.1

What is a wetland? All around the world there are places with shallow water or saturated soils whose vegetation is dominated by species of large plants that are found nowhere else in the surrounding uplands. Plants sticking out of the water are usually the most conspicuous feature of such places, but closer inspection often reveals the presence of plants ﬂoating on the surface of the water and even plants growing under the water. There are also groups of animals, especially birds, that are more or less restricted to these areas. These places with their unique ﬂora and fauna are wetlands. The term wetland is a relatively new term that encompasses many kinds of wet areas that were locally called marsh, swamp, fen, slough, bog, glade, hammock, vlei, jheel, etc. Some of these areas are very large and cover hundreds or thousands of square kilometres and contain a variety of vegetation types, for example, the Florida Everglades, the Hudson Bay Lowlands, the Pantanal, the Okavango Delta, Mekong Delta, the Danube Delta, while others can be very small with areas less than 1 ha, for example, prairie potholes, California vernal pools, Australian billabongs. Wetlands come in an incredible variety of types (Fig. 1.1) that result from differences in their sources of water (hydrology) and location on the landscape (geomorphology). Their vegetation ranges from mossdominated bogs in the arctic to tree-dominated tropical ﬂoodplains in the Amazon Basin (Finlayson and Moser 1991). In wet climates, for example, north-western Europe, some kind of wetlands can be found on almost any landform from depressions to highland hills (Fig. 1.2), but in dry climates, for example southern Africa and most of Australia, wetlands are often restricted to river channels and depressions. In intermediate climates, for example, the north-central United States, most

4. Flat adjoining a channel

1. Basin within a plain (flat)

Flat, seasonally flooded = floodplain Channel, permanently flooded = river Basin, permanently inundated = lake Flat, seasonally waterlogged = palusplain 5. Flat adjoining a channel

2. Flat within a broad valley Flat, seasonally inundated = floodplain Flat, seasonally waterlogged = palusplain

Flat, seasonally waterlogged = palusplain Channel, seasonally flooded = creek

3. Basin within a plain, adjoining a slope Slope, seasonally waterlogged = paluslope Flat, seasonally flooded = floodplain Basin, permanently flooded = lake

Fig. 1.1

6. Tops of highland surfaces Seasonally waterlogged surfaces blanketing the uplands = palusmont

Landforms and the wetland types associated with them. (From Semeniuk and Semeniuk 1995.)

7. Slopes on highlands / hills Seasonally waterlogged slopes = paluslope

INTRODUCTION 3

Highlands

Very wet climates

Intermediate

Very dry climates

Highlands hills Slopes

Flats

Channels Basins Lowlands Fig. 1.2

Various landform types on which wetlands can develop under different climates. (From Semeniuk and Semeniuk 1995.)

wetlands are associated with either depressions or slopes (Fig. 1.3). Because of minor differences in their hydrology, in the same area, a variety of vegetation types can be found in similar landscape settings (Fig. 1.4). Wetlands have features in common with both aquatic (Chapter 3) and terrestrial systems (Chapter 4). Nevertheless, they have two features that together make them unique. The ﬁrst of these is anaerobic soils. Anaerobic soils develop because dissolved oxygen in water-saturated soils is quickly depleted by microorganisms (Chapter 2). It is their anaerobic soils that distinguish wetlands from terrestrial systems such as grasslands and deciduous forests. Not only are the soils anaerobic, but oxygen is often absent or found only in very low concentrations in the water above these soils. It is oxygen that is the limiting factor for many plant and animal species in wetlands. Consequently, organisms that live in wetlands must have anatomical, morphological, physiological, or behavioural adaptations to enable them to acquire, conserve, store, or ﬁnd oxygen (Chapters 3 and 4). The second feature that distinguishes wetlands from other aquatic systems is large plants, collectively called macrophytes (Chapters 5 and 6). Macrophytes, which mostly have terrestrial growth forms (tree, shrub, grass, fern, moss, etc.), are major primary producers in wetlands. Algae are the major primary producers in aquatic systems (streams, rivers, lakes). However, algae are also present in wetlands, where they can also be major primary producers (Chapter 6). In a wetland, however, macrophytes and their litter (Fig. 1.5) create much of the wetland’s physical structure, and they modify the local environmental conditions

Water table

Water table usually below wetland level Fig. 1.3

Evapotranspiration

Water table

Stream flow

Ground-water inflow

Evapotranspiration

Precipitation

Ground-water inflow

Overland flow

Precipitation

Overland flow

Overland flow

Evapotranspiration

Overland flow

Precipitation

Precipitation

Evapotranspiration

4 THE BIOLOGY OF FRESHWATER WETLANDS

Lake or river flood water

Water table usually below wetland level

Four common wetland types: depressional and slope wetlands with and without groundwater inputs. (Redrawn from Novitzki 1979.)

Fen, Marsh: Large GW inflow well-drained site

Cedar swamp: Moderate GW inflow moderate drainage

Wet meadow: Little GW inflow poor drainage

Fig. 1.4

Different vegetation types in ground water slope wetlands in Wisconsin, USA. (Redrawn from Novitzki 1979.)

INTRODUCTION 5

Solar energy

R

Above ground plant biomass

R

R

R

Roots

Fire

Herbivores

R

Herbivore-based food webs

R

Plant litter

Microbe-based food webs

Allochthonous organic matter

Microbes Peat

Fig. 1.5

Sources (primary production, allochthonous inputs) and loss (to herbivores, microbes, peat, ﬁre) of organic matter in wetlands. R represents loss due to respiration. (From Moore 1990.)

(e.g. water and soil temperatures, water velocities, water chemistry, wind velocity). Because it is dominated by macrophytes, the vegetation of a wetland has much in common with that of terrestrial systems. For many wetland plants and animals, especially those found in the water column, the chemical and physical characteristics of water per se also require adaptations to allow these organisms to move, acquire energy and nutrients, reproduce, and survive. In many wetlands, these organisms must survive or endure water-level ﬂuctuations, ice formation, and even the periodic absence of water. Many of the microorganisms, invertebrates, ﬁshes, and plants, especially algae, found in wetlands are also found in other aquatic systems, for example, rivers and lakes. In fact, boundaries between wetlands and adjacent aquatic systems are somewhat arbitrary and difﬁcult to delineate. Many organisms move freely across these boundaries occasionally, seasonally, or daily. Because the biology of aquatic organisms is already described in a companion volume in this series (Brönmark and Hansson 2005), aquatic organisms and their adaptations will not be discussed in detail, except for some species that are common in wetlands (Chapter 3). For most groups of organisms, relatively few species have the adaptations needed to live exclusively or mostly in wetlands. Only about 2–3% of angiosperm species are usually considered to be restricted to wetlands (Cronk and Fennessy 2001), although many more species may occasionally be found growing in wetlands. Only 3% of insects are estimated to be obligate wetland species with one or more life stages

6 THE BIOLOGY OF FRESHWATER WETLANDS

restricted to wetlands (Ward 1992). Only 1–2% of birds are classiﬁed as waterfowl (Owen and Black 1990) and another 3% as wading birds (Soothill and Soothill 1982), the major types of birds that are restricted to or are highly dependent on wetlands. Like plants, many other species of birds are occasionally found in wetlands. Wetland soils support a host of microorganisms, mostly bacteria, adapted to aerobic and/or anaerobic conditions, and they have a profound effect on soil chemistry and wetland nutrient cycles as described in Chapter 6. Many aerobic and facultative microorganisms are also found in the water column, including many types of algae, bacteria, and fungi.

1.2

Wetland classiﬁcation Several attempts have been made by national and international organizations to develop a formal deﬁnition of wetlands (Finlayson and van der Valk 1995). Although they differ in wording and emphasis, they are all surprisingly consistent. According to the Committee on Characterization of Wetlands (1995), ‘The minimum essential characteristics of a wetland are recurrent, sustained inundation or saturation at or near the surface and the presence of physical, chemical, and biological features reﬂective of recurrent, sustained inundation or saturation. Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation’. Although it is possible to deﬁne in principle what constitutes a wetland, in practice delimiting wetlands is often difﬁcult. In large part this is simply due to the immense number of wetland types, their dynamic nature, and the difﬁculty of determining the boundaries between wetlands and contiguous aquatic systems. Nevertheless, many wetland classiﬁcations have been developed and some have been used successfully to conduct national or regional wetland inventories (Finlayson and van der Valk 1995). Typical of these classiﬁcation systems is the one developed in the United States (Fig. 1.6). It is a hierarchical system which recognizes ﬁve major wetland types (systems) based on their hydrology: marine, estuarine, riverine, lacustrine, and palustrine (shallow depressions). (In this book we will be concerned primarily with the last three.) Because aerial photographs are used to map wetlands, different vegetation classes with distinct plant growth forms (mosses and lichens, emergents, submersed aquatics, shrubs and trees) are recognized that can be used to map wetlands in each hydrologic system. More recent classiﬁcation systems, for example, Brinson (1993), emphasize the importance of the geomorphological setting as well as the hydrology of wetlands. Wetlands exist in a landscape because some geomorphic features or landforms enable water to be stored or to ﬂow through some portion of the

INTRODUCTION 7

System

Subsystem Subtidal

Marine Intertidal

Aquatic bed Reef Rocky shore Unconsolidated shore

Subtidal

Rock bottom Unconsolidated bottom Aquatic bed Reef

Intertidal

Aquatic bed Reef Streambed Rocky shore Unconsolidated shore Emergent wetland Scrub-shrub wetland Forested wetland

Wetlands and deep water habitats

Estuarine

Tidal

Rock bottom Unconsolidated bottom Aquatic bed Rocky shore Unconsolidated shore Emergent wetland

Lower perennial

Rock bottom Unconsolidated bottom Aquatic bed Rocky shore Unconsolidated shore Emergent wetland

Upper perennial

Rock bottom Unconsolidated bottom Aquatic bed Rocky shore Unconsolidated shore

Riverine

Intermittent

Fig. 1.6

Streambed

Limnetic

Rock bottom Unconsolidated bottom Aquatic bed

Littoral

Rock bottom Unconsolidated bottom Aquatic bed Rocky shore Unconsolidated shore Emergent wetland

Lacustrine

Palustrine

Class Rock bottom Unconsolidated bottom Aquatic bed Reef

Rock bottom Unconsolidated bottom Aquatic bed Unconsolidated shore Moss–Lichen wetland Emergent wetland Scrub-shrub wetland Forested wetland

Hierarchical classiﬁcation system developed for American wetlands by Cowardin et al. (1979).

8 THE BIOLOGY OF FRESHWATER WETLANDS

landscape. Common landforms in which wetlands are found include slopes (Figs 1.1 and 1.2) where groundwater discharges, ﬂats or plains with saturated soils, depressions (palustrine wetlands) with or without inlets and outlets, shorelines of lakes (lacustrine), river channels and ﬂoodplains (riverine), deltas, and estuaries (estuarine). Sources of water for wetlands are normally precipitation, groundwater inﬂow or discharge, surface water inﬂow, surface water bi-directional ﬂow (tides), or some combination of these. The major sources of water in a wetland are determined by its geomorphic setting and local climatic conditions. In turn, the sources of the water in a wetland determine not only the amount present and when it is present, but also its chemistry (Fig. 1.7). The water chemistry of wetlands whose primary source of water is precipitation will be very different from that of a wetland whose primary source of water is groundwater discharge. This can have a major effect on the species composition of the vegetation and its primary production. A hydrogeomorphic classiﬁcation scheme that was developed in Australia (Semeniuk and Semeniuk 1995) combines landform and hydroperiod. Four hydroperiods are recognized: permanent inundation, seasonal inundation, intermittent inundation, and seasonal waterlogging; and ﬁve landforms: basin, channel, ﬂat, slope, and highland (Fig. 1.2). When combined there are 13 different wetland types that are possible. All of

Oligotrophic

Mesotrophic

Eutrophic

Total nutrient availability Production Decomposition Swamp Bog Poor fen Ba

Moderate rich fen

Freshwater marsh

se

Saline wetland

ity

lin

Mo

Extreme rich fen

a lk ,a H ,p w ns flo er at W

Fig. 1.7

Pe a ss tlan do mi ds na N ted W W ,P et at in la er s nd u lev rf s el ace flu w ct at ua er tio n

tio

ca

Treed Sphagnum Brown mosses

Tidal marsh

The relationship between Canadian wetland types and major water chemistry, biotic, and hydrological gradients. (From Zoltai and Vitt 1995.)

INTRODUCTION 9

these wetland types can be found in very wet climates, but in very dry climates only wetlands associated with basins or depressions and channels can occur. It should be noted that in areas with very wet climates, it is possible for wetlands to overcome their geomorphological constraints. In these situations, the wetlands which may originally have developed in a basin can, through the accumulation of peat, ﬁll up the basin and begin to expand into the surrounding uplands turning them into a wetland, a process known as paludiﬁcation. These types of wetlands themselves become local landforms (Charman 2002).

1.3

Wetland inventories Wetlands are found on every continent except Antarctica and in almost every climatic zone on these continents (Fig. 1.8). Because no detailed inventory of the world’s wetlands has ever been done, only rather crude estimates of the total area of wetlands are available (Mitsch and Gosselink 2000). The polar/boreal region is estimated to have about 2.5 106 km2, the sub-boreal/temperate region about 1.0 106 km2, and the subtropics/tropics 2.0 106 km2 for a total of around 5.5 106 km2. Overall, it is estimated that about 5% of the land surface of the earth is covered with wetlands. The distribution of these wetlands is

23

23

0

0

23

23

Fig. 1.8

Wetlands more than 10%

Wetlands less than 0.5%

Wetlands 0.5 – 10%

Islands with substantial wetlands

Global distribution of wetlands according to Gore (1983). (From Williams 1990.)

10 THE BIOLOGY OF FRESHWATER WETLANDS

uneven with major wetlands areas in the arctic and subarctic regions of North America, Europe, and Asia and in association with large tropical rivers and lakes in South America and Africa (Fig. 1.5). Of these wetlands probably more than half are peatlands (Charman 2002). Peatlands are distinguished from other kinds of wetlands by their soils which are layers of partly decomposed plant material (peat). The most detailed inventories of wetlands have been carried out in the United States, Canada, and some countries in western Europe (Finlayson and van der Valk 1995). What is clear from these inventories is that large areas of wetlands have been lost, typically due to drainage and conversion to farmland, but also to many other human activities, including stream channelization, dam construction, mining, ﬁlling for development, and sedimentation. Estimates of wetland loss range from about 50% (53% for the United States (Dahl 1990)) to more than 90% for New Zealand (Dugan 1993). Overall, wetland loss at the global scale is probably around 50% (Dugan 1993) with losses continuing in most parts of the world. In parts of the world where wetlands are protected by law from drainage and ﬁlling, as is mostly the case in the United States, these laws have been effective in reducing losses and in stimulating the restoration of wetlands to replace wetlands that are lost due to development (Chapter 7).

1.4

Summary Wetlands are characterized by three major features: (1) shallow water or saturated soils, (2) anaerobic soils, and (3) unique ﬂora and fauna adapted to environmental conditions in wetlands. It is their anaerobic soils that distinguish wetlands from terrestrial systems. It is the presence of vegetation dominated by trees, shrubs, grasses, mosses, and other large plants that distinguishes wetlands from aquatic systems. There are many different kinds of wetlands that are characterized by different hydrologies (source(s) of water and duration and timing of ﬂooding), geomorphological setting (ﬂats, basins, slopes, channels, etc.), vegetation (submersed aquatic beds, emergents, mosses, shrubs, trees), soils (mineral, peat), and water chemistries. Wetlands can be classiﬁed based on their hydrology, vegetation, and/or geomorphology.

1.5

Practical experiments and observations There is no substitute for ﬁrst-hand familiarity with wetlands. Consequently, visiting some local wetlands, observing their ﬂora and fauna, and doing some studies and/or experiments are strongly encouraged, and suggestions for speciﬁc observations and studies are included at the

INTRODUCTION 11

end of each chapter. Some wetlands, however, are easier to visit and study than others. It can be difﬁcult to work in some freshwater wetlands. Emergent vegetation is often very dense and very tall. Visibility can be very limited; it is easy to get lost. The bottoms of wetlands can be poorly consolidated; this can make walking very difﬁcult. Some wetland plant species have common names such as sawgrass and rice cut-grass; these names are well deserved and appropriate clothing may be needed. Mosquitoes can be a problem, especially along the edges of wetlands, but rarely in their interiors. In tropical and subtropical wetlands, it is possible to encounter dangerous animals such as alligators, crocodiles, or poisonous snakes. Human parasites with aquatic stages can also be a problem in some parts of the world. Finally as the name implies, wetlands are wet places. Sometimes this means only that the soil is saturated; boots may not be needed to visit such a wetland. On the other hand, some wetlands may have standing water in places 2 m or more in depth; boots or a boat will be essential. Although visiting and working in wetlands is no more difﬁcult or dangerous than visiting or working in grasslands or forests in the same region, it will be a much more pleasant and productive experience if you are properly dressed and equipped. It is not recommended that you visit most wetlands for the ﬁrst time, or anytime, on your own. If you are taking a university course in wetland ecology, class ﬁeld trips to local wetlands will undoubtedly be part of the course. If not, the ideal way to visit wetlands is by going to nearby parks or refuges with wetlands. Wetlands in parks and refuges often have boardwalks in them that make it easy to get into the wetland without any special gear such as boots or boats; they may have boat trails and boats to rent; or they may have nature trails along levees or dikes. Guided tours are often available in large wetlands such as the Everglades. Local birding and natural history clubs may occasionally offer trips to wetlands, and such trips may be the best way to ﬁrst visit large wetlands not in parks or preserves. Before you visit any wetland, it is highly recommended that you learn as much about it as is feasible. There is an incredible amount of information available about speciﬁc wetlands around the world; much of it is available through the World Wide Web. Borrowing or acquiring some local guides to wetland plants, birds, and invertebrates is also highly recommended in order to get the most out of your visit.

1.5.1

Wetland classiﬁcation There are many kinds of wetlands (Fig. 1.6). Before visiting a wetland, it very useful to become familiar with the local wetland classiﬁcation

12 THE BIOLOGY OF FRESHWATER WETLANDS

system and to determine what kind of wetland you will be visiting. Most national or regional wetland classiﬁcations systems are available on the Web. If there is no national or regional classiﬁcation system, the Ramsar Convention on Wetlands (see Chapter 7) has a classiﬁcation system that is designed for use around the world. It is available on their Website. Existing topographic maps can often provide most of the data needed initially to classify a wetland. Topo maps will show whether the wetland is associated with a lake or river, or whether it is isolated. They will also show whether it has surface inﬂows and outﬂows. Soils maps are useful for determining the kinds of soil in the wetland, at a minimum if they are organic (peat) or not.

2 Wetland environments

In any given climatic region, the environmental conditions that organisms in wetlands encounter are largely a function of four factors: the chemical and physical properties of water, the shape of the basin or channel in which the wetland is found, the volume of water in the basin or channel over time, and the vegetation in the wetland. Basin or channel morphometry determine the water depths or water regime experienced by wetland organisms. Water-level ﬂuctuations are common due to seasonal or year to year differences in water inputs. Water depth (Fig. 2.1) and changes in water depth ultimately determine which sessile organisms will be found and where they are found in wetlands. Plant canopies determine what happens to incoming energy from the sun. Because there are usually a variety of vegetation types in wetlands, the presence of different kinds of plant canopies creates a complex mosaic of microenvironments. Plant canopies alter light regimes, water temperatures, oxygen concentrations, water chemistry, etc. The direct and indirect effects of plant canopies ultimately determine the spatial and temporal distributions of most organisms in wetlands. The environmental complexity created by plant canopies enables wetlands to support a much greater diversity of species than would be found in a hydrologically comparable situation, for examples, the shallow area around a lake margin, without plants. In this chapter, the physical and chemical environments of wetlands will be examined. We will begin with the most ecologically salient properties of water: high density, viscosity, speciﬁc heat, solvency, transparency, etc. (Wetzel 1988; Brönmark and Hansson 2005). This will be followed with a discussion of wetland water budgets and water regimes. Finally, the effect of macrophyte canopies on environmental conditions will be considered.

14 THE BIOLOGY OF FRESHWATER WETLANDS

Height (m)

2

1

0 Submerged DRYWT. (g/m2) LAI CHL a+b (g m2) No. species Canopy height (cm) Fig. 2.1

2.1

200 71 2.8–4.0 0.24–0.92 3–10 20–30

Floating leaved 220 97 2.5–4.7 0.29–0.80 4–11 20–30

Emergent

Meadow

465 168 2.1–3.9 0.62–2.13 7–32 100–150

327 151 1.3–4.6 0.54–1.41 15–26 70–100

Distribution of macrophyte growth forms along the water depth gradient in Oxbow lakes in Alberta, Canada as well as the mean standing crop, leaf area, chlorophyll content, species diversity, and canopy height of vegetation zones dominated by submerged, ﬂoating leaved, emergent, and wet meadow species. (Redrawn from van der Valk and Bliss 1971.)

Physical and chemical properties of water Water is one of the few substances that is mostly in liquid form at temperatures and pressures found at the earth’s surface. Water is very dense, about 770 times denser than air. This means that water will displace air when it is pulled by gravity into soil pores and potentially into openings in plants and animals. Because most aquatic organisms are less dense or about the same density as water, they will ﬂoat in or on the water column. For example, submerged aquatic plants are so buoyant that they actually ﬂoat in the water column much like helium balloons in air. These plants do not need structural tissues and turgor pressure to support their canopies like their terrestrial counterparts. In fact, submersed plants need to be tethered to the bottom by their roots or they will ﬂoat to the surface. For some animals such as diving birds and reptiles, overcoming their buoyancy has required a variety of anatomical, morphological, and behavioural adaptations to increase their density. For examples, alligators and caimans ingest rocks and about 1% of their body weight can be rocks in their stomachs (Butler 1995). Although there is some debate whether reptiles do this primarily to aid their digestion or to adjust their buoyancy, lithophagy clearly does have an effect on their buoyancy. The speciﬁc heat capacity of water, the number of calories required to raise the temperature of 1 g of a substance by 1 C, is 1 cal deg1 gm1.

WETLAND ENVIRONMENTS 15

This is much higher than that for air (0.17 cal deg1 gm1) or dry soils (ca. 0.20 cal deg1 gm1). In theory, temperatures in the water column of wetlands should change more slowly than in terrestrial ecosystems, and the annual range of temperatures should be smaller. Thus plants and sessile animals that live in the water column are often said to experience a more uniform and benign temperature environment than their terrestrial counterparts or even emergent plant species in the same wetland. The absorption of light by submersed plant canopies in the water column can result in very high daytime temperatures in and above these canopies and reduced temperatures below them (Fig. 2.2). Because of the high thermal conductivity of water and the inability of submerged plants and most sessile animals in the water column to dissipate or reduce heat loads, the body temperatures of these organisms track water temperatures very closely. Water reaches its maximum density at about 4 C, not at its freezing point of 0 C. When water freezes, its density decreases signiﬁcantly; ice is about 8% less dense than water at 0 C. This unusual change in water density means that water freezes from the surface down and ice ﬂoats on water. For wetlands in climatic zones with signiﬁcant winters, this pattern has many implications for their plants and animals. Many plants (Boylen and Sheldon 1976) and animals (e.g. muskrats in North America) can survive and remain active under the ice during the winter as long as the ice does not freeze to the bottom. During exceptionally cold winters or dry periods when water levels are low, however,

Relative spectral photon flux

Daylight 30 cm water

Canopy

400

Fig. 2.2

600 Wavelength (nm)

800

Spectral attenuation of light by 30 cm of pure water and by an aerial plant canopy. (From Holmes and Klein 1987.)

16 THE BIOLOGY OF FRESHWATER WETLANDS

these wetlands can freeze to the bottom. During such winters, populations of some species can be temporarily eliminated. They may migrate to other wetlands (e.g. muskrats) or survive only in some resting or dormant stage (e.g. submersed plant species as seeds). Water is transparent allowing light to pass through (Fig. 2.3). Nevertheless, even in the absence of a plant canopy, light levels under water are lower, often much lower, than those above the water. In effect, organisms below the surface of the water are always in the shade. There are several reasons for this. First, some of the light will be reﬂected off the surface of the water. At low angles, little light will penetrate the water. The amount reﬂected depends inversely on the height of the sun. Second, light is scattered in water, some of it back to the atmosphere. The amount of light that is scattered depends on what is dissolved and suspended in the water. Because of the shallow water depths in wetlands, this back scattering of light is not a major factor in light attenuation. Third, wavelengths of light are differentially absorbed as they pass through the water column by the water molecules and by substances that are dissolved and suspended in the water. In pure water (Fig. 2.3), long wavelengths (e.g. oranges and reds) travel a shorter distance in water than short wavelengths (e.g. blue). However, dissolved substances in water tend to absorb short wavelengths preferentially. The total amount of light energy available for photosynthesis declines rapidly due to light extinction with depth and is reduced about 50% after passing through 1 m of clear water. Although algae and submersed plants growing near the surface of the water column are in a better light environment than those growing lower in the water column, at the water depths normally encountered in wetlands (1 m), the differential absorption of the various wavelengths of light with depth is often not of any great signiﬁcance. Light is much more reduced by the presence of a plant canopy below, on, or above the surface of the water column (Fig. 2.3). As anyone who has ever gone swimming knows, water is very viscous when compared to air. The frictional resistance of a particle moving in water is about 100 times greater than in air. Invertebrates and other small organisms in the water column use up a lot of energy just moving around and consequently rarely go very far solely as a result of their own locomotion. Water currents caused by gravity, wind, or thermal gradients are thus very important for the vertical and horizontal movement of microscopic organisms in wetlands, lakes, and rivers (Brönmark and Hanson 2005). Submerged and emergent plants in the water column can greatly reduce the velocity of water by increasing hydraulic resistance to ﬂow (Haslam 1978; Jarvela 2002). However, if water velocities are too high then macrophytes can be damaged or even torn out of the sediments. The hydraulic resistance of macrophytes depends on their structure and size. Their susceptibility to high water velocities is a function

WETLAND ENVIRONMENTS 17

Temperature (°C) 35

40

0

0

20

20

40

40

Depth (cm)

Dusk 30

35

40

0

5

10

15

20

0

5

10

15

20

20

25

30

35

40

20

25

30

35

40

80

0

0

20

20

40

40

60

60

80

80 5

10

15

20

20

25

30

35

40

0 5 15 20 Dissolved oxygen (mg l–1)

10

15

20

5

10

15

20

20

25

30

35

40

0

0

20

20

40

40

60

60

80

80 5

10

The unshaded area available to fish compresses and expands spatially and temporally

0

0

0

August

Oxygen Temperature

80

Depth (cm)

25

60

60

Fig. 2.3

20

July

25

September

Depth (cm)

20

Dawn 30

Temperature and dissolved oxygen proﬁles at dusk and dawn in July, August, and September in Eagle Lake, Mississippi, USA. The areas sampled were in a dense stand of the submersed macrophyte, Najas guadalupensis. (From Miranda et al. 2000.)

18 THE BIOLOGY OF FRESHWATER WETLANDS

of their anchoring strengths. Erosion of sediments by high water velocities or waves can prevent the development of wetlands. Water is an excellent solvent, and in no other naturally occurring liquid have so many different kinds of dissolved substances been found. Consequently, water is sometimes called the universal solvent. Water, oxygen, carbon dioxide, nutrients, etc. can move readily into and out of plant and animal cells and microorganisms from surrounding water either passively through diffusion or actively through some molecular pumping mechanism. In fact, algae and other aquatic microorganisms can obtain all of the elements and compounds that they require for growth directly from the water. The concentrations of substances in the cells of plants and animals, however, are rarely similar to their concentration in the surrounding water. Aquatic organisms have a variety of physiological mechanisms for coping with these concentration gradients. In freshwater wetlands, the concentration of various minerals salts in the water is much lower than it is within living cells. Consequently, wetland organisms have to expend energy to maintain their internal salt concentrations and to get rid of excess water. The chemical nature and concentration of various substances dissolved in the water determine its pH, hardness, salinity, nutrient content, and other measures used to categorize water chemistry, and can have a signiﬁcant impact on the ﬂora and fauna of the wetland. For example, there is little overlap in plant species between inland freshwater and saline marshes (Steward and Kantrud 1972). Even in freshwater wetlands, many groups of plants and animals are more or less restricted to wetlands with a speciﬁc kind of water chemistry. This has been extensively investigated in peatlands (Gignac 1994) in which pH and some measure of fertility (e.g. nutrient concentrations, Ca concentrations) can be used to delimit various peatland types such as oligotrophic fens and oligotrophic bogs (Fig. 2.4). Differences in water chemistry reﬂect different sources of water (precipitation, surface water, and groundwater). Groundwater typically has a very high concentration of dissolved substances while precipitation has a comparatively low concentration. Air contains about 21% oxygen and only about 0.036% carbon dioxide. The amount of oxygen and carbon dioxide present in water depends on temperature and atmospheric pressure. Saturated oxygen concentrations in water at temperatures normally encountered in wetlands (0–35 C) range from about 7 to 14 mg l1. Carbon dioxide, although much less abundant in the atmosphere, is about 200 times more soluble in water than oxygen. In fact, unlike oxygen, carbon dioxide has about the same concentration by volume in water as in air. Because water is so much denser than air, diffusion of dissolved gases in water is very slow; at 20 C diffusion of oxygen is about 10,000 times slower in water than air. This means that it would take oxygen

WETLAND ENVIRONMENTS 19

30

Fertility

Eutrophic fen

15

Mesotrophic fen

Mesotrophic bog

Oligotrophic fen

Oligotrophic bog

0

Fig. 2.4

3

5.5 pH

8

European peatland communities occurring along pH and fertility gradients. Points are actual peatland communities sampled in England. (From Charman 2002.)

dissolved in water about 7 h to move 1 cm by molecular diffusion. Because the concentrations of dissolved gases are low, concentrations of these gases, especially oxygen, can be quickly exhausted locally in the water column or soil due to the metabolic activity (aerobic respiration) of wetland microorganisms, plants, or animals (Figs 2.2 and 2.5). The transport of oxygen from the air into the water column is also very slow, especially in still air, and rates of oxygen consumption within the water column are often much higher than rates of diffusion into the water column. For many organisms, inadequate local supplies of oxygen are a limiting factor (resource) that restricts their distribution. For small wetland organisms (e.g. bacteria, algae, many invertebrates), their surface area to volume ratio is sufﬁcient to allow the diffusion of gases and other molecules into and out of their bodies at rates sufﬁcient to allow their normal metabolism. Larger wetland organisms (e.g. submerged plants and insects), especially those derived from terrestrial ancestors, have one or more adaptations in their anatomy, morphology, physiology, or behaviour to cope with little or no oxygen (Chapter 3). The absence of oxygen in the water column and soils of wetlands, however, allows a variety of microorganisms capable of anaerobic respiration to exist in wetlands (Fenchel and Finlay 1995). The close proximity of aerobic and anaerobic environments in wetlands is one of their deﬁning characteristics.

20 THE BIOLOGY OF FRESHWATER WETLANDS

0 Depth (cm)

20 0.2

40

5.0 2.0 0.2

2.0

60

0.2

80 100 0

100

200

300

400

0 Depth (cm)

20 40

2.0

60

0.2

0.2

80 100 0

100

200

300

400

500

0 5.0

Depth (cm)

20 2.0

0.2

40

2.0

60 0.2

80

0.2

100 0 Fig. 2.5

2.2

100

200

300 Distance (cm)

400

500

600

Oxygen concentrations along three transects from emergent vegetation to open water in August 1994 in Anderson Lake, Iowa, USA. (Redrawn from Rose and Crumpton 1996.)

Macrophyte canopies Within wetlands there are both aerial plant canopies and submersed plant canopies. Both can affect environmental conditions in a variety of ways. Plant canopies create three-dimensional structures with large

WETLAND ENVIRONMENTS 21

surface areas that can be utilized by microorganisms, invertebrates, and some vertebrates; they intercept light; they reduce the velocity of water and air within them; they produce plant litter; emergent plant canopies can transpire water; and the photosynthesis and respiration of submerged canopies can alter local concentrations of oxygen and carbon dioxide. Both emergent and submerged plant canopies can have leaf areas (Fig. 2.1), the total surface area of leaves and photosynthetic shoots per unit area of wetland, that are several times the surface area over which they are growing. Although many factors (leaf area, canopy height, canopy surface roughness, location of the water table in relation to the rooting zone, air temperature, wind velocity, relative humidity, etc.) can inﬂuence the dissipation of net radiation reaching an emergent canopy, most of it is dissipated (typically 50–75%) by evapotranspiration (Souch et al. 1998; Jacobs et al. 2002). Storage heat ﬂux, that is, the heating up of water and soil, and advection or sensible heat ﬂux dissipate the remainder of the net radiation. When standing water is present, it is the major sink for storage heat ﬂux. The amount of water lost by evapotranspiration can vary seasonally, depending on both the condition of the canopy and environmental conditions. Likewise the relative importance of evapotranspiration to the overall water budget of wetlands can vary widely. Within a wetland at any given time, differences in evapotranspiration from one part of a wetland to another can occur and this can create internal groundwater ﬂow patterns. For example in prairie potholes, higher evapotranspiration rates around the margins of potholes cause groundwater to ﬂow laterally from the centre to the periphery of these wetlands (Hayashi et al. 1998). This also results in a redistribution of solutes because they accumulate around the margins of these wetlands as their concentrations increase due to evapotranspiration of groundwater. The presence of an emergent plant canopy can greatly reduce the amount of light reaching the surface of the water (Fig. 2.3) and thus water temperatures. Because there are usually several different kinds of plant canopies (Fig. 2.1), the light and thermal environments of wetlands are highly variable spatially and temporally. The presence of plant canopies can also have a signiﬁcant effect on local oxygen (Figs 2.3 and 2.5) and carbon dioxide concentrations in the water column. During the day (Fig. 2.3), oxygen concentrations within canopies of submerged plants can reach saturation levels while oxygen levels below these canopies are very low and can approach hypoxic levels (1 mg l1). During the night (Fig. 2.3), oxygen levels in submersed plant canopies drop signiﬁcantly and can reach hypoxic levels just before dawn. Not only do oxygen concentrations vary diurnally, they also vary spatially (Fig. 2.5). Oxygen concentrations even during the day are often much lower within stands of emergents than in nearby areas without

22 THE BIOLOGY OF FRESHWATER WETLANDS

emergents. As with light (Fig. 2.2) and temperature (Fig. 2.3), wetlands can have strong concentration gradients of dissolved gases (Figs 2.3 and 2.4) created by the presence of plant canopies. In summary, macrophyte emergent and submerged canopies create very heterogeneous environments. The physical and chemical properties of water, especially the low solubility of oxygen, exacerbate the intensity of these environmental gradients. These complex and shifting temperature, light, and dissolved gas gradients in turn affect the distribution of sessile and mobile organisms in wetlands. Water’s ability to dissolve a wide variety of substances means that all of the elements needed for the growth and development of plants and animals are normally present and are continuously being transported into the wetland by surface and groundwater inﬂows and precipitation. The high density and viscosity of water creates both problems and advantages for the growth, movement, and dispersal of wetland plants and animals (Chapter 4). In wetlands, aerobic and anaerobic environments are often in close proximity, and this is one of the deﬁning characteristics of wetlands.

2.3

Wetland hydrology Most of the water on earth is in the oceans (about 97%) with large amounts also present in the polar ice caps and groundwater. Because they contain such small volumes of water, most freshwater aquatic systems (lakes, rivers, wetlands) are dynamic environments in which environmental conditions (water depth, water velocity, turbidity, temperature, oxygen concentrations, etc.) change daily, seasonally, and interannully. Because wetlands are shallow, even small changes in water levels, which would be inconsequential in large lakes and rivers, can result in signiﬁcant local environmental changes for sessile plants and animals. In wetlands, changes in water inputs and outputs, that is, their water budgets, and the water depths at any given place over time in a wetland, that is, the local water regime, are important descriptors of the entire wetland and of the physical environment experienced by sessile organisms found at different elevations within the wetland. Past and current depth of ﬂooding is the major determinant of the distribution of sessile plants and animals within a wetland. Hydrology is the study of the movement and storage of water. Because of the global cycling of water, a large quantity of water that evaporates from the oceans is carried to land by winds where it falls as rain or snow. Although some of this water is incorporated into glaciers and ice ﬁelds, most of it eventually ﬂows back to the oceans either on the surface of the ground (surface ﬂow) or underground (groundwater ﬂow). It is along these myriad surface and groundwater ﬂow paths that freshwater

WETLAND ENVIRONMENTS 23

wetlands are found. These paths consist of channels and depressions of various sizes, depths, and shapes as well as areas where ground water discharges on slopes (springs, seeps) or ﬂats (Chapter 1). Because the amount of precipitation in most parts of the world varies seasonally and because the total amount of precipitation can vary signiﬁcantly from year to year, the amount of water in a wetland along any ﬂow path often changes signiﬁcantly from season to season and from year to year. As a consequence, the ﬂora and fauna are constantly adjusting to changes in a wetland’s hydrology. As noted in Chapter 1, wetlands can be classiﬁed into broad classes (lacustrine, palustrine, riverine, etc.) based on their hydrology. Within these broad hydrological classes, however, a variety of wetlands can be found whose hydrology varies signiﬁcantly. Within a class, freshwater wetlands differ primarily in how long they are ﬂooded annually or interannually. When quantitative data on hydrology of wetlands are not available, a number of terms to describe the general hydrology of wetlands are commonly used (Fig. 2.6). These are primarily descriptors of the amount and length of time that water is stored in the wetland. Typical of these terms are those used in the American wetland classiﬁcation system (Cowardin et al. 1979): Permanently ﬂooded: standing (surface) water present throughout year. Intermittently exposed: standing water present throughout the year except in years of sever drought. Semipermanently ﬂooded: standing water present throughout the growing season in most years. Seasonally ﬂooded: standing water present for extended periods of time during the growing season. Saturated: the soil or substrate is saturated with water (waterlogged) to the surface during the growing season, but standing water rarely present. Temporarily ﬂooded: standing water is present only for brief periods during the growing season. Intermittently ﬂooded: standing water is present periodically but without any seasonal pattern. Similar terms are sued to describe the hydrology of wetlands in other parts of the world (Semeniuk and Semeniuk 1995). In reality, the vegetation present in a wetland is often used as an indicator of the water regime in a given elevation in the wetland. In any given wetland, vegetation at different elevations experiences different water regimes (Fig. 2.6). The vegetation in the deepest parts of the wetland can be permanently ﬂooded, while that at increasingly higher elevations is semipermanently, seasonally, temporarily, and

Upland

Aquatic bed

Emergent wetland persistent

Palustrine

Emergent wetland nonpersistent

Upland

Unconsolidated bottom

Forested wetland

Palustrine

Scrub/shrub wetland

Upland

Emergent wetland persistent

Palustrine

Scrub/shrub wetland

Upland

t Seepage zone a

a b

High water

a b

c d

e

a Temporarily flooded b Seasonally flooded c Semipermanently flooded d Intermittently exposed e Permanently flooded f Saturated

Fig. 2.6

Water regimes at different elevations in palustrine wetlands. (Redrawn from Cowardin et al. 1979.)

Average water Low water 2m

WETLAND ENVIRONMENTS 25

intermittently ﬂooded. Consequently, in wetlands there are different vegetation and soil types along elevation gradients (Fig. 2.6). Quantitative studies of a wetland’s hydrology attempt to measure or estimate all water inputs (precipitation, surface inﬂows, groundwater inﬂows (discharges)) and outputs (evapotranspiration, surface outﬂows, groundwater outﬂows (recharges)). Over any period of time, typically a year, the difference between the inputs and outputs is equal to the change in storage (volume) of water in the wetland. Detention or turnover time (how many days does it take for all the water in the wetland to be replaced) can also be estimated from hydrologic budgets. The data required to construct a detailed hydrologic budget are very difﬁcult and costly to obtain; consequently, hydrological budgets are available for only a small number of wetlands. The following equation is used to calculate the hydrologic budget for a wetland over some period of time, usually a year: SWI GWI P SWO GWO ET S where inputs SWI volume of surface water inputs GWI volume groundwater inputs (discharge) P precipitation (rain and snow) and outputs and storages are SWO volume of surface water outputs GWO volume of groundwater outputs (recharge) ET evapotranspiration output S change in volume of water in wetland (storage) total error or uncertainty. (It is an estimate of the reliability of the budget. If 0 then the budget is perfectly balanced and all inputs and outputs have been accurately measured or estimated.) An example of a hydrologic budget for the Monoma peatlands is given in Table 2.1. In this peatland, the major input of water in both years was precipitation ( 80%) and the major output was evapotranspiration ( 90%). The main sources of water for wetlands in a variety of landscape settings are given in Fig. 2.7. Surface water inﬂows and precipitation often dominate the inﬂows into wetlands and surface water outﬂows and evapotranspiration the outﬂows. While many wetlands have continuous layer of standing water through which surface water ﬂows, in peatlands this is often not the case. In peatlands, the upper peat is less compacted and has a higher hydraulic conductivity while the lower peat is denser

26 THE BIOLOGY OF FRESHWATER WETLANDS Table 2.1 Hydrologic budget for the Monona peatland, Wisconsin, USA Hydrologic component

1990 (June–November)

1991 (April–October)

cm

%

cm

%

Inputs Precipitation Surface runoff Surface inﬂow from river Groundwater input Total inputs

43.1 2.7 0.0 1.0 46.8

92.1 5.8 0.0 2.1

64.7 24.3 0.1 0.75 79.1

81.8 17.1 0.14 0.95

Outputs Evapotranspiration Surface outﬂow to river Groundwater output Total outputs

43.6 0.0 1.5 45.1

96.6 0.0 3.4

79.30 1.27 4.2 84.8

93.5 1.5 5.0

Storage Observed change in storage

4.9

Total inputs outputs

1.7

Error (change in storage (total inputs outputs))

3.2

1.9 5.7 7.1

3.8

4.5

Source: Adapted from Owen (1995).

and has a much lower hydraulic conductivity. This surface layer of peat is called the acrotelm while the layer below it is called the catotelm. Surface water often ﬂows preferentially through the acrotelm to and from the periphery of peatlands (Fig. 2.8; Moore 1990). A hydrograph (Fig. 2.9) is a plot of water depth or water discharge in riverine wetlands at a given point over time. A variety of instruments are available to measure water level from a simple staff gauge that is read periodically by eye to sophisticated electronic gauging stations that can record stage data continuously on data loggers and even radio the data to some central collection point every few minutes. The mean, maximum, and minimum water depths over some period of time are among the important statistics that can be calculated from hydrographs. The hydroperiod, that is, the number of days per year that there is standing water at the sampling point, can also be easily derived from a hydrograph (Fig. 2.9). Changes in the amount of water stored in a palustrine wetland or passing some point in a riverine wetland with directional ﬂow can also be estimated from hydrographs using data on basin morphometry or cross-sectional area of a channel. If a long-term hydrograph is available, then a depth–duration curve can be derived from it. The depth–duration curve is a plot of the amount of time (typically number of days per year or percentage of a year) that the sampling point was ﬂooded to a certain depth. Depth–duration curves are used

WETLAND ENVIRONMENTS 27

Riverine wetland Arid

Humid P

P

S

S

ET G

GR

ET

GU GR

GU

G

Lacustrine wetland Arid

Humid P

P S

S

ET G

GS

ET GU

G

GS

Arctic fen

GU

Ombrogenous bog P

Summer P Snow patch S G Active

S

SM

ET Layer

S

ET

GU

Permafrost Water gain

Fig. 2.7

Water loss

Wetland surface

Major inﬂows and outﬂows of water in riverine and lacustrine wetlands, arctic fens, and ombrogenous bogs. Precipitation (P), surface ﬂows (S), groundwater ﬂows (G), groundwater ﬂow to or from rivers (GR), groundwater ﬂows from uplands (GU), evapotranspiration (ET), surface ﬂow (S), groundwater ﬂows (G), and snowmelt (SM). (From Orme 1990.)

Precipitation P Evaporation E E

P

P Acrotelm (peat) Catotelm (peat) E

Ground-water G

E

E Run-off R G R

Fig. 2.8

Main features of the hydrology of peatlands. In peatlands, the surface ﬂow of water is often through the acrotelm, a layer of peat that is more porous than the denser layer of peat below, the catotelm. (Redrawn from Moore 1990.)

28 THE BIOLOGY OF FRESHWATER WETLANDS Hydroperiod (time of inundation) Cypress swamp (8 months) Marsh (6 months) Hydric pine flatwoods (2 months)

+2

Water levels (metres)

+1

Ground level

0

–1

–2

–3 O Fig. 2.9

N

D

J

F

M A Month

M

J

J

A

S

Hydrographs and hydroperiods (duration of ﬂooding) of three types of Florida wetlands. (Duever et al. 1986.)

to describe the water regime, that is, how long the area is ﬂooded and to what depths each year. In many parts of the world, there are cyclical changes in the amount of annual precipitation with years of below normal precipitation (droughts) alternating with years of above normal precipitation. The resulting changes in water depths in wetlands are often called wet–dry cycles. Wet–dry cycles often have a period of 10–20 years. Water depths at a given elevation in a wetland can vary signiﬁcantly from year to year at the same time of the year. Spring and fall water levels in two prairie potholes in southern Saskatchewan, Canada, are summarized in Fig. 2.10. One pothole is a seasonal wetland that goes dry almost every year. The other is a permanent wetland in which water levels over the 30 year period of record ﬂuctuated about 2 m. Such changes in mean annual water depths from year to year can result in major changes in the composition and functions (primary production, litter decomposition, nutrient cycling, etc.) of the wetland from year to year (Chapter 6). Consequently, the sessile plants and animals that are found in freshwater wetlands must cope in some way with a range of water depths from year to year. Richter et al. (1996) suggested a series of hydrological statistics that can be used to describe wet–dry cycles in wetlands (van der Valk 2005). These hydrological statistics are based on ﬁve fundamental characteristics of hydrologic regimes: (1) magnitude: the level or volume of the water at any given time; (2) timing: when speciﬁc water conditions (especially highs or lows) normally occur;

WETLAND ENVIRONMENTS 29

Seasonal wetland

Water depth (cm)

200 150 100 50 0 Permanent wetland

Water depth (cm)

200 150 100 50

1998

1996

1994

1992

1990

1988

1986

1984

1982

1980

1978

1976

1974

1972

1970

1968

0

Year Fig. 2.10

Water depths in seasonal and permanent prairie potholes in southern Saskatchewan, Canada. (Adapted from van der Camp and Hayashi 1998.)

(3) frequency: how often over some time interval do speciﬁc water conditions occur; (4) duration: how long does a speciﬁc water condition last; (5) rate of change: how quickly do water conditions go up or down. Of these characteristics, the rate of change has been most often ignored. Nevertheless, it can be very important for many species in wetlands. For example, in the Everglades, there is an annual cycle of water-level change due to the alternation of wet (May to October) and dry seasons (November to April). This annual pattern of water-level change results in what DeAngelis and White (1994) have called fronts. A front is a moving transition zone from low to high or from high to low water conditions. Fronts are an important feature of the Everglades and other wetlands because they change prey availability (ﬁsh and invertebrates) to wading birds and other carnivores. As water levels decline during the dry season, new zones of shallow water become available for wading birds and there is a continuous concentration of prey. The rate at which this drop in water levels occurs is critical. If water levels drop steadily, there is always an adequate supply of food in the front. In the case of wading birds, the decline in water level must last long enough for food to remain available till the current year’s hatchlings are ﬂedged.

30 THE BIOLOGY OF FRESHWATER WETLANDS

In summary, the hydrology of any wetland is a function of the various inputs of water (precipitation, surface inﬂows, groundwater inﬂows) and outputs of water (evapotranspiration, surface outﬂows, groundwater outﬂows). These determine not only the amount of water in a wetland at any time, but also the chemical characteristics of this water. As a ﬁrst approximation, simple hydrological descriptors can be used to describe a wetland’s hydrology. Hydrographs record the changes in water depth at a given point in a wetland over time. From hydrographs, many quantitative descriptors of the hydrology of a wetland can be derived. These include mean, minimum, and maximum water depths over some period. More detailed descriptions of the water regime can also be obtained such as depth–duration curves. Because wetlands are shallow, water-level ﬂuctuations, both seasonal and interannual, are common, and these ﬂuctuations can be so large that plants and other sessile organisms can experience a variety of water depths within any given year and especially from year to year.

2.4

Summary The properties of water have had signiﬁcant effect on the evolution of wetland species as well as on their distribution within and among wetlands. Environmental conditions experienced by organisms in wetlands are largely a function of the physical and chemical properties of water and of the sources and amounts of water entering, being stored in, and leaving a given wetland. Important physical properties of water include its high density, speciﬁc heat, transparency, and viscosity. Important chemical properties include its pH, nutrient content, and oxygen content. Plant canopies signiﬁcantly alter environmental conditions (water temperatures, oxygen levels) creating a series of microhabitats within each wetland. The overall hydrology of wetlands is described by its water budget. Within any given wetland, the vegetation found at different elevations experiences different water regimes. Water regimes, the duration of ﬂooding, at any point in a wetland can be determined from long-term hydrographs, plots of water depth over time.

2.5

Practical experiments and observations

2.5.1

Water levels One of the most important characteristics of a wetland is its hydrology. In the ﬁeld one easily measured aspect of a wetland’s hydrology is water level. Water depths will vary across the wetland, and the best place to

WETLAND ENVIRONMENTS 31

measure a wetland’s water level is in one of the deepest places in the wetland that is readily accessible or visible. Water level is measured using a staff gauge. A staff gauge is a vertical stick, narrow board or pipe on which a scale has been inscribed or to which a meter stick has been attached. They may be attached to piers or water control structures or driven into the ground. The lowest part of the scale should normally be at the wetland substrate and the upper end should be above the elevation of the highest anticipated water level. Staff gauges should be read as often as feasible. Binoculars may be very useful for reading staff gauges. The longer the record, the more informative it is. A plot of water level over time, a hydrograph, is used to summarize the data collected. Water-level data from the wetland can be compared with water-level data collected at gauging stations maintained by government agencies on nearby lakes and streams. It is often feasible to infer much about long-term water levels in the wetland by comparing the hydrograph from the wetland with long-term hydrographs from the nearest river or lake. What is the expected range in water level annually and over longer time periods? How might these annual and long-term water-level ﬂuctuations affect the vegetation and animals in the wetland?

2.5.2

Water chemistry Water chemistry has a major inﬂuence on the composition of wetland vegetation. Two commonly and easily measured chemical parameters of water are pH and conductivity. Both can be measured in the ﬁeld using suitable electronic meters or a water sample can be collected and brought back to the laboratory for measurement. pH, which is a measure of the concentration of H in water, is temperature dependent and is normally adjusted to a temperature of 25 C. Most modern electronic pH meters do this automatically. pH can vary during the day and also seasonally. A pH measurement is most useful in peatlands for distinguishing bogs from fens. Speciﬁc conductance or conductivity is a measure of the dissolved electrolytes in water. The amount of current that water will conduct is a direct function of the concentration of ions in solution. Speciﬁc conductance is measured in mhos, the reciprocal of ohms. The higher the value, the more the ions in solution; that is, the water is more conductive. Like pH, speciﬁc conductance is temperature dependent. Speciﬁc conductance can range from 500 mho in freshwater to over 60,000 mho in saline wetlands. In wetlands in dry climates that have seasonal or long-term water-level ﬂuctuations, speciﬁc conductance can vary

32 THE BIOLOGY OF FRESHWATER WETLANDS

dramatically with water level. It is low when water levels are high and high when water levels are low. How do pH and conductivity affect the classiﬁcation of the wetland? How do they affect the composition of the vegetation?

2.5.3

Temperature and oxygen gradients Oxygen and temperature gradients are common in wetlands. Both water temperature and oxygen can be easily measured in the ﬁeld with a portable electronic oxygen meter. Measure oxygen, temperature at the surface and near the bottom, and water depth along a transect from the shallow water emergent zone until you get well into a zone dominated by submerged aquatics. Make these measurements early in the morning, during the middle of the day, and late in the afternoon. How much do oxygen concentrations and temperatures vary along the transect? How much does water depth affect them? How much does the time of day affect them? What types of organisms are most likely to be affected by these oxygen and temperature gradients?

3 Microorganisms and invertebrates

Many organisms found in wetlands are also found in other nearby aquatic systems. These include probably all the microorganisms (bacteria, fungi, algae), most, if not all, invertebrates, and most ﬁsh. There are also organisms that are restricted to or are primarily found in wetlands. These include wetland plants or macrophytes, some groups of birds, and some vertebrates (many amphibians, some reptiles, and a small number of mammals). Organisms found in wetlands can also be grouped based on their size. Most wetland organisms that are either microscopic or just visible to the naked eye are also found in other aquatic systems. Unlike the more widespread microbiota, the macrobiota often have evolved speciﬁc adaptations for living in wetlands. Although not unique to wetlands, the microbiota are functionally important organisms in freshwater wetlands. They play major roles in wetland food chains and mineral cycling (Chapter 6). Although the distinction between microbiota and macrobiota is somewhat arbitrary, it does reﬂect the fact that wetlands contain microscopic species that also occur in other kinds of aquatic systems and larger organisms that are largely restricted to wetlands. The former species mostly evolved in water, the latter on land. Although aquatic insects have been classiﬁed as part of the microbiota, along with other invertebrates, they could just easily be considered as part of the macrobiota, especially because they are derived from terrestrial ancestors and they are easily visible to the naked eye. In this chapter, the microbiota (microorganisms and invertebrates) will be brieﬂy described. More detailed treatment of these organisms can be found in Brönmark and Hansson (2005) or any other limnology text. The larger organisms, the macrobiota, will be described in the next chapter. It is impossible in this chapter to deal in any detail with the biology of microorganisms and invertebrates. Instead, this chapter deals only with those aspects of the ecology of these organisms that is relevant to understanding their functional role in wetlands, which will be considered in Chapter 6.

34 THE BIOLOGY OF FRESHWATER WETLANDS

3.1

Microorganisms Wetlands contain many groups of microorganism (algae, bacteria, fungi, viruses (Rheinheimer 1992; Fenchel and Finlay 1995; Vymazal 1995)). Among these various groups of microorganisms, three are particularly important: bacteria, fungi, and algae. Although viruses are known to occur as pathogens of bacteria, algae, and various groups of aquatic plants and animals, their signiﬁcance in wetlands is completely unknown.

3.1.1

Bacteria Bacteria are very small (0.5–2 m typically). Consequently, they usually go unseen except for some colonial forms or occasionally when noncolonial forms are found in very high numbers. The bacterial ﬂora of a wetland is a function of its water chemistry (pH, salinity, nutrients, etc.), temperature, oxygen levels, and organic matter content. There are many kinds of bacteria. Ecologically, bacteria are important because of their varied metabolic pathways, and they are classiﬁed primarily based on their metabolic capabilities rather than on their limited morphological differences. Available evidence suggests that the bacteria found in wetlands are essentially the same as those found in nearby lakes and streams with similar water chemistries. Bacteria are found free-ﬂoating in the water column, growing on plant, litter, and soil surfaces, and in the soil. Bacteria have high metabolic rates and very short generation times. Their metabolic rates are temperature dependent. The generation time of a bacterium is in the order of 0.25–0.50 h while that of a protozoan, an invertebrate (copepod), and small ﬁsh would be 8 h, 10 days, and 1 year, respectively (Fenchel et al. 1998). Most wetland bacteria are non-photosynthetic, and most are probably saprophytes, organisms that break down living or dead plant and animal material. Some wetland bacteria are undoubtedly pathogenic, but not much is known about them. There are also bacteria that can photosynthesise; that is, they can reduce carbon dioxide or some other simple compound using an external source of energy. One group of photosynthesizing bacteria, the cyanobacteria (blue-green algae) can be common in wetlands.

3.1.2

Bacterial metabolism Metabolically, bacteria are much more versatile than any other group of organisms in wetlands. Like all organisms, bacteria require a source of energy and a variety of inorganic and organic substances from their

MICROORGANISMS AND INVERTEBRATES 35

surrounding environments to synthesize their cellular components (cell walls, proteins, lipids, DNA, etc.). Bacteria fall into two broad categories: heterotrophs and autotrophs. Heterotrophs use existing organic matter both as a source of energy and for the materials needed for the synthesis of all cellular components. They play an important role in the decomposition of plant and animal material. Autotrophs use C-1 molecules (carbon dioxide, methane, methanol) as a carbon source. There are two types of autotrophs that differ in the source of energy used for photosynthesis. Photoautotrophs use sunlight while chemoautotrophs obtain their energy by oxidizing inorganic substrates (e.g. methane, hydrogen gas) with an inorganic electron acceptor (e.g. oxygen, nitrate). They obtain their carbon from carbon dioxide or reduced C-1 molecules. Some bacteria can be both autotrophs and heterotrophs. They are often autotrophs in the light and heterotrophs in the dark.

3.1.3

Dissimilatory metabolism Within the bacteria, there are ﬁve general classes of energy-yielding or dissimilatory processes: photosynthesis, fermentation, aerobic respiration, anaerobic respiration, and methanogenesis (Fenchel et al. 1998). Two of these are unique to bacteria, anaerobic respiration and methanogenesis. Fermentation and aerobic respiration in bacteria differ only in detail from these same metabolic pathways in other microorganisms (algae, fungi), plants, and animals. Photosynthesis (photoautotrophy) can occur both in the presence of oxygen (oxygenic photosynthesis) and in its absence (anoxygenic photosynthesis). Oxygenic photosynthesis is one of the most important biochemical pathways on earth and is also found in algae, mosses, ferns, and vascular plants. Most of the organic matter ﬁxed in terrestrial, aquatic, and wetland ecosystems is through oxygenic photosynthesis in which carbon dioxide and water are converted to a sugar plus oxygen with the energy required for this reaction initially captured by some type of chlorophyll molecule. Among bacteria only the cyanobacteria (blue-green algae) have oxygenic photosynthesis. There are also bacteria that can carry out anoxygenic photosynthesis, for example, purple sulphur bacteria. In anoxygenic photosynthesis, a reduced compound (e.g. H2S) other than water (H2O) is used as the reductant or electron donor in photosynthesis. As a result of the accumulation of live and dead organic matter produced by photosynthesis, wetlands have a chemically reduced environment. The heterotrophic microorganisms in wetlands obtain the energy stored in these reduced carbon compounds by oxidizing them.

36 THE BIOLOGY OF FRESHWATER WETLANDS

Fermentation is an aerobic process in which organic substrates are sequentially transformed and in which there is no external electron acceptor. Fermentation yields only a small amount of energy when compared with respiration for which an external terminal electron acceptor is needed (Fig. 3.1). The ﬁnal products of fermentation are often lactate or ethanol. In aerobic respiration the terminal electron acceptor is oxygen (Box 3.1). Aerobic respiration yields the most energy of all energy-yielding metabolic pathways. Aerobic respiration in bacteria is essentially the same as that in other microorganism (algae, fungi) as well as in plants and animals. Most bacteria oxidize organic substrates. There are also bacteria, however, that can use substrates other than organic compounds in aerobic respiration (chemoautotrophs). In wetlands, there are four important groups of bacteria that fall into this category: methanotrophs, nitriﬁers, sulphide oxidizers, and iron and manganese oxidizers. Methanotrophs oxidize methane to carbon dioxide. Nitriﬁers oxidize ammonia to nitrate in a two-step process. Sulphide oxidizers oxidize sulphide to sulphate. Iron and manganese oxidizers oxidize ferrous iron (Fe2) and manganous manganese (Mn2) to their oxidized forms, ferric iron (Fe3), and manganic manganese (Mn3), respectively. The oxidation of Fe and Mn can occur without bacteria in the presence of oxygen.

CO2

2+

Mn

CO2

Acetate H2, CO2

Methanogenic bacteria Acetogenic bacteria

2–

S

Sulphate reducing bacteria

ia l

Fe

Iron reducing bacteria

te nt

Fe

2+

po

CO2 3+

x

Mn

SO42– Volatile fatty acids, alcohols

Eh, mV

CO2 4+

Manganese reducing bacteria

Fermentative bacteria

n

ge

y Ox

NO3– N2

do

Denitrifyingbacteria

–200 0 200 400 600 800

O2 H2O

Re

Hydrolysis

Oligo- and monomers

Monomers

Aerobic-bacteria and fungi

Depth

Hydrolysis Organic polymers

CO2 CO2 CH4 CO2

O2, µm 50 100 150 200 250

Syntrophic bacteria

Fig. 3.1

Major pathways of aerobic and anaerobic respiration which are responsible for the decomposition of organic matter in wetlands and the oxygen and redox levels (right panel) at which these pathways occur. (From Westermann 1993.)

MICROORGANISMS AND INVERTEBRATES 37

Box 3.1 Redox potential The order or sequence in which various terminal electron acceptors are used in respiration is determined by chemical thermodynamics. From thermodynamic theory, it is possible to predict if a given chemical reaction can occur and the magnitude of the energy released or used in the reaction (Fenchel et al. 1998; Vepraskas and Faulkner 2001). All aerobic and anaerobic respiration involves a series of coupled oxidation–reduction reactions, and each half reaction either consumes or produces electrons. During oxidation, a chemical element or molecule losses an electron, while during reduction it acquires an electron. In aerobic respiration, for example, glucose (C6H12O6) is oxidized to CO2 in the following reaction: C6H12O6 6O2 → 6CO2 6H2O

(3.1)

This chemical reaction is actually a coupled oxidation–reduction reaction that combines these half reactions: (1) An oxidation reaction: C6H12O6 6H2O → 6CO2 24e24H (2) A reduction reaction: 6CO224e24H → 12H2O

(3.2) (3.3)

The sum of the oxidation (3.2) and reduction (3.3) half reactions is the overall chemical reaction (3.1). By physically separating the two half reactions, if a suitable path connects them such as a salt bridge, it is possible to measure the electrical potential between the two half-cells. By measuring the electric potential for a half reaction relative to the potential of a hydrogen half reaction (H2 ↔ 2H 2e), the half reaction potentials can be calculated and measured with suitable equipment. The potential for the hydrogen half reaction at 1 atm at a temperature of 25C and a H concentration of 1 M is by deﬁnition 0 V. (In reality, half-cell potentials are measured relative to a calomel electrode and then corrected to a hydrogen reference electrode.) Because half reaction potentials are pH dependent, they are conventionally expressed at a pH 7. Redox potentials for half reactions (Eh) are measured in millivolts (mV). Redox, in other words, is a measure of an environment’s ability to supply or to take up electrons. These electrons are produced by the oxidation of reduced organic compounds eventually to CO2. The order in which various terminal electron acceptors are reduced for respiration is shown in Fig. 3.1. Table 3.1 gives the

38 THE BIOLOGY OF FRESHWATER WETLANDS

standard potentials (Eh) for the most important redox pairs in wetlands. By measuring reduction potential or redox (Eh) in the ﬁeld, it is possible to predict which terminal electron acceptor or other redox pair is currently favoured thermodynamically. Table 3.1 Standard potentials (Eh) at pH 7 and 25C for some common redox pairs in wetlands Redox pair O2/H2O Fe3/Fe2 NO3/N2 MnO2/MnCO3 NO3/NO2 NO3/NH4 SO42/HS CO2/CH4 S0/HS H2O/H2

Eh (mV) 820 770 520 430 380 220 240 270 270 410

From Fenchel et al. (1998).

Anaerobic respiration involves the use of an external terminal electron acceptor other than oxygen (Fig. 3.1). Only a limited number of other terminal electron acceptors can be used by different groups of bacteria (Fenchel and Finlay 1995). Three important types of anaerobic respiration occur in wetlands: nitrate reduction, sulphate reduction, and iron and manganese reduction, in which nitrate, sulphate, and oxidized forms of iron and manganese, respectively, are used as terminal electron acceptors (Fig. 3.1). Methanogensis occurs only in some groups of archaebacteria (methanogenic bacteria). There are different types of methanogenic bacteria that differ in the substrate they utilize (e.g. acetate or other methylated compounds or carbon dioxide).

3.1.4

Assimilatory metabolism Bacteria require a variety of elements (especially, C, O, N, H, P, and S) in order to synthesize the various compounds (sugars, amino acids, enzymes, proteins, lipids, etc.) needed for cellular structures and for their metabolism. When the uptake of these elements requires energy, it is called assimilatory metabolism. The most important assimilatory metabolic process, the assimilatory reduction of carbon dioxide, occurs in oxygenic photosynthesis in which carbon dioxide is incorporated into organic compounds in the Calvin cycle. Nitrogen ﬁxation, assimilatory reduction of N2, also occurs among some prokaryotes. Nitrogen

MICROORGANISMS AND INVERTEBRATES 39

ﬁxation can take place only under anaerobic conditions because one of the enzymes involved in nitrogen ﬁxation, dinitrogenase reductase, is destroyed by oxygen. Many groups of anaerobic bacteria can ﬁx nitrogen as can some cyanobacteria. Under aerobic conditions, nitrogen ﬁxation in cyanobacteria seems to be restricted to special cells (heterocysts) in which oxygen is not present. Some nitrogen-ﬁxing bacteria are known to form symbiotic relationships with a number of plants, especially plants in the legume family. These symbiotic bacteria are found in root nodules. At least two genera of wetland trees, Alnus (Alder) and Myrica (Wax Myrtle) are known to form such nodules. The nitrogen-ﬁxing cyano-bacterium, Anabaena azollae, forms symbiotic associations with the ﬂoating aquatic fern Azolla. Azolla species are widespread in many tropical and subtropical wetlands. Under anaerobic conditions, bacteria can assimilate ammonia, which is the form of nitrogen found in anaerobic environments, in order to synthesize amino acids and similarly reduced sulphur can be assimilated for metabolic use. Under aerobic conditions, bacteria assimilate nitrates and through a series of stepwise reduction processes have to convert nitrates to ammonia (assimilatory nitrate reduction) before the nitrates can be used in amino acid synthesis. Likewise under aerobic conditions, bacteria can assimilate sulphate and have to use energy to reduce it.

3.1.5

Fungi As with bacteria, there are many kinds of fungi in aquatic habitats. Some of these fungi are obligate aquatic species (e.g. most phycomycetes) that complete their entire life cycle in water. There are also semi-aquatic fungi whose vegetative forms are found in water, but they can only reproduce sexually out of water. Because of ﬂuctuating water levels, wetlands seem to be well suited for semi-aquatic fungi. Terrestrial species are also found in wetlands. These are often fungi that are ﬂooded when water levels rise in a wetland. They can endure ﬂooding for some period, but will die if water levels remain high too long. All fungi are heterotrophs that obtain energy and chemical compounds required for their growth from organic matter (Fig. 3.1). There are both saprophytic and parasitic fungi and some can be both depending on circumstances. Fungi are mostly aerobes, but a small number of species seem to be able to tolerate anaerobic conditions (Rheinheimer 1992). They are found both in acid and alkaline water and over a range of salinities. Most fungi are found growing on some kind of substrate, for example, litter, living plants, rocks, shells, etc. The exception is yeasts (saccharomycetes) and some other yeast-like fungi (deuteromycetes) which are found free-ﬂoating in the water column.

40 THE BIOLOGY OF FRESHWATER WETLANDS

Ecologically, fungi play a major role in decomposing plant and animal litter (Fig. 3.1). They can break down many large organic polymers and other high molecular weight compounds such as pectins, hemicelluloses, celluloses, lignins, and chitins as well as proteins, starches, and fats. Because most fungi require free oxygen for respiration, they are not common in wetland soils and in parts of the water column that are anaerobic. Bacteria do not have the ability to break down (hydrolize) many of the long-chain polymers found in plant litter. Consequently, the low populations or the absence of fungi under anaerobic conditions is in part responsible for the slow rate of decomposition of litter in wetlands. In other words, bacteria typically out compete fungi for oxygen in wetland soils and within dense stands of fallen litter, and this results in the build-up of organic matter in wetland soils. Although fungi may not grow under anaerobic conditions, their spores are often present. When water levels drop and plant litter is exposed to the atmosphere, it can become quickly colonized by fungi. There are a group of soil fungi called the mycorrhizae that form a symbiotic relationship with many species of terrestrial plants. These fungi are found growing on the roots of their host plants. The fungi provide the host plants with water and nutrients, especially phosphorus. The host plants in turn supply the fungi with organic compounds that the fungi break down to obtain energy. Mycorrhizae can signiﬁcantly increase the surface area available for water and nutrient uptake by host plants, and this enables the plants to exploit a much larger volume of soil. Mycorrhizae have been isolated from the roots of a number of wetland plants. Besides energy, these fungi presumably also obtain oxygen from the roots of wetland plants. It is unlikely, however, that these fungi always form a symbiotic relationship with wetland plants. In some wetlands, neither water nor phosphorus is in short supply. Many mycorrhizal fungi are also saprophytes, and the suppression of these fungi in some cases can actually increase the growth of their wetland host (Wetzel and van der Valk 2005). In short, the overall signiﬁcance of mycorrhizae in wetlands is still uncertain.

3.1.6

Algae The algae are a phylogenetically diverse group of photosynthetic organisms found in all aquatic systems. They range from microscopic one-celled organisms to large colonial forms that are visible to the naked eye (macroalgae) (Fig. 3.2). In fact, a precise ecological deﬁnition of what constitutes the algae in wetlands can be difﬁcult (Vymazal 1995). Most deﬁnitions of the algae stress several characteristics that are common to this group of organisms: (1) they are ‘simple’ eukaryotic plants (i.e. they lack roots, leaves, stems, ﬂowers, etc.), (2) they have chlorophyll a as a photosynthetic pigment, and (3) they have unprotected (uncovered) reproductive

MICROORGANISMS AND INVERTEBRATES 41

Navicula spp. Diatoms Anabaena spp. Blue-green alga

Asterionella spp.

Pediastrum spp.

Closterium spp.

Spirogyra spp. Green algae

Volvox spp.

Euglena spp. Fig. 3.2

Some common algae found in wetlands. (Redrawn from Caduto 1990.)

structures. This deﬁnition excludes the blue-green algae (cyanobacteria) from the true algae because blue-greens are prokaryotes and are thus considered to be photosynthetic bacteria. Some of the macro-algae, especially those belonging to the Charophyceae (Fig. 3.3), which superﬁcially resemble small, submerged aquatics, are treated for most purposes as macrophytes because they are sampled along with the macrophytes in most wetland studies. Metaphyton or mats of algae (see below) are also sometime treated as macrophytes because they are sampled with macrophytes.

42 THE BIOLOGY OF FRESHWATER WETLANDS

A

B

C

Fig. 3.3

Chara vulgaris. A. Plant, B. branchlet with male (lower) and female gametangia, and C. node. (Redrawn from Cook et al. 1974.)

Although blue-greens are easy to distinguish from true (eukaryotic) algae, separating them from other algae is often not done. Blue-green and other kinds of algae typically grow together and are sampled together. Blue-green algae and other algae are functionally similar, except that some blue-green algae can ﬁx nitrogen. Consequently, the treatment of blue-green algae in wetlands is somewhat inconsistent. When considering mineral cycling, they are treated as bacteria, but when considering primary production they are often lumped with the

MICROORGANISMS AND INVERTEBRATES 43

algae. Algae in wetlands can be grouped into a number of different assemblages (Fig. 3.4) depending on where they are found growing. Plankton are small (only the largest are visible to the naked eye) organisms suspended in the water column (Fig. 3.4). They are mostly sampled using nets. At any one time, the plankton contains many kinds of algae and invertebrates. Although many have some limited means of locomotion, they are carried passively within the wetland by water currents. Most members of the plankton are not actually ﬂoating (i.e. have neutral or positive buoyancy). They are denser than water and require currents to keep them in suspension. The plankton is not composed of a unique group of organisms that only occur in the water column. In reality, many of the plankton are also found growing on sediments, litter, or living plants. Those members of the plankton that are either true algae or blue-green algae are called phytoplankton. (The invertebrates in the plankton are called zooplankton.) When the density of algal cells in the water column is high and the phytoplankton become readily visible to the naked eye, this is referred to as an algal bloom. Various kinds of algae can bloom in wetlands, especially blue-greens. Metaphyton Epiphyton Epipelon

Plankton

Fig. 3.4

Algal assemblages found in wetlands in the water column (metaphyton, phytoplankton), on the surface of sediment (epipelon), and on the surface of plants (epiphyton). (Adapted from Kadlec and Knight 1995.)

44 THE BIOLOGY OF FRESHWATER WETLANDS

Periphyton are small organisms growing on (attached to) any kind of substrate under water (Fig. 3.4). This is the slippery, greenish-brown layer covering all submerged surfaces (plants, litter, soils, etc.) in wetlands. The periphyton is composed of many kinds of organisms including algae, fungi, bacteria, and various kinds of small invertebrates. Periphyton are often sampled using artiﬁcial substrates placed in the water column. Periphytic algae are the algal component of this community. Many of the same organisms found in the plankton can be found in the periphyton. The periphyton can be subdivided into different types depending on the nature of the substrate on which the organisms are attached. Among the types of periphyton commonly found in wetlands are the epiphyton (growing on macrophytes), the epipelon (growing on sediment or litter), and the epilithon (growing on rocks and stones). Two other types are occasionally encountered in wetlands: the epipsammon (growing on sand) and the epizoon (growing on animals, for example, turtle shells). Metaphyton are mats of ﬁlamentous and other algal that may be loosely associated with various substrates (Fig. 3.4). They often are found growing on the soil surface early in the growing season and with time move up in the water column as the mats become more buoyant. These mats sometimes develop in the canopies of submersed aquatics. The metaphyton differ from the periphyton in that the metaphyton are an easily visible mat or layer of algae in the water column that is not attached to a particular substrate and can be harvested by sampling the mats. Many of the species that are found in the metaphyton may also be part of the plankton and periphyton. In some wetlands, for example the Florida Everglades, the term periphyton is used more broadly for all non-planktonic assemblages of algae and associated organisms. The metaphyton is most abundant in wetlands that have gone dry during the ﬁrst year or so after they reﬂood. It is also well developed in many wetlands that go seasonally dry after they re-ﬂood. The species composition of the metaphyton, like that of the plankton and periphyton, depends on water chemistry and temperature. Because there are many kinds of algae in wetlands, their identiﬁcation to species or even genus is very time consuming (Vymazal 1995). Consequently, in many wetland studies no attempt is made to identify the species of algae found in the plankton, periphyton, or metaphyton. Instead, the size of these communities is estimated by estimating their chlorophyll content, total mass, or total number of cells or cell volume.

3.2

Invertebrates Like bacteria and algae, invertebrates found in wetlands are mostly a subset of those found in other aquatic systems. However, many terrestrial invertebrates, for example, grasshoppers, can also be found in

MICROORGANISMS AND INVERTEBRATES 45

wetlands, especially in the canopies of emergent macrophytes. A list of the major groups of invertebrates that are known to occur in wetlands (Thorp and Covitch 2001) is lengthy and includes protozoa, bryozoans, porifera, cnidaria, ﬂatworms, gastrotricha, rotifers, nematods, molluscs (gastropods and bivalves), annelids of various kinds, tardigrades, water mites and other arachnids, aquatic insects, and crustacea (ostracods, cladocerans, branchiopods, copepods, and decapods). Invertebrates (Figs 3.5–3.7) range in size from microscopic, for example most protozoans, to 10 cm or more, for example, crayﬁsh. Invertebrates (Fig. 3.8)

Mosquito larva Culex spp.

Phatom midge larva Chaoborus spp.

Tail appendage used for springing into the air Springtail Fish spider Dolomedes triton

Water flea Daphnia spp. egg sacs Cyclops Rotifer Fig. 3.5

Some neustonic invertebrate species commonly found at or on the water surface and nektonic species fond swimming in the water column. (Adapted from Caduto 1990.)

46 THE BIOLOGY OF FRESHWATER WETLANDS

Bloodworm (True midge larva) Rat-tailed maggot

Planaria

Tubifex worm (Sludge worm)

Nematode

Bloodsucking leech

Two horsehair worms (Gordians) Fig. 3.6

Aquatic earthworm

Some benthic species found on or in wetland substrates. (Redrawn from Caduto 1990.)

are common throughout the water column (plankton and nekton), on and in plants, litter, and rocks (periphyton), and in the soil (benthos). Most invertebrates have complex life cycles with multiple life stages that may be found in different parts of a wetland or even outside the wetland

MICROORGANISMS AND INVERTEBRATES 47

Scud (Sideswimmer)

Crayfish

Exterior view

Seed shrimp

Internal anatomy

Fig. 3.7

Some common crustaceans and molluscs found in wetlands. (Redrawn from Caduto 1990.)

48 THE BIOLOGY OF FRESHWATER WETLANDS

Neuston Plankton

Epiphyton Benthos

Fig. 3.8

Microhabitats in which assemblages of invertebrates are found in wetlands: the water surface (neuston), the water column (plankton and nekton), plant surfaces (epiphyton), and substrate (benthos). (Redrawn from Murkin and Ross 2000.)

entirely. The distribution of aquatic invertebrates within and among wetlands is affected by water chemistry, especially pH and salinity, temperature, water depth and ﬂuctuation, and oxygen levels. As with other animals in wetlands, one of the major problems facing wetland invertebrates is the low level or absence of oxygen in the water column and especially in the soil.

MICROORGANISMS AND INVERTEBRATES 49

3.2.1

Aquatic insects There are many types of aquatic insects found in wetlands (Williams and Feltmate 1992). Some major groups are the mayﬂies (Ephemeroptera), dragon ﬂies and damselﬂies (Odonota), stoneﬂies (Plecoptera), hemipterans, especially the Corixids, (Hemiptera), aquatic beetles (Coleoptera), true ﬂies (Diptera), and caddis ﬂies (Trichoptera). Aquatic insects associated with submerged, ﬂoating, and emergent vegetation include damselﬂies (Zygoptera), dragonﬂies (Anisoptera), caddis ﬂies, mayﬂies, many kinds of beetles, and some true ﬂies. As with many other aquatic groups of organisms in wetlands, they are mostly not unique to wetlands. Anaerobic conditions in wetlands appear to prevent some aquatic insects that are common in nearby streams and lakes from becoming established in most wetlands. In addition, many wetlands go dry regularly, and some aquatic insects cannot cope with the rapidly changing environmental conditions found in these wetlands. The aquatic insects in wetlands with ﬂuctuating water levels are typically species of Hemiptera, Coleoptera, Trichoptera, Odonata, Chironomidae, Culicidae (mosquitoes), and Ceratopogonidae (biting midges) that have eggs, larvae, or pupae that can survive periods without standing water. Most aquatic insects like most macrophytes (Chapter 4) are derived from terrestrial ancestors (Ward 1992). As with the macrophytes, there have been many independent invasions of aquatic ecosystems by insects. Compared with other groups of animals, for example, mammals, birds, and even many reptiles and amphibians, aquatic insects have become better adapted to life in water while still maintaining a ﬂying stage out of water. It is the immature forms that live in water that have become most adapted to this foreign environment. Many of the morphological adaptations of aquatic insect larvae are for locomotion. Williams and Feltmate (1992) list the following adaptations: (1) ﬂattening of limbs into oar-like structures, (2) fringes or setae on legs to increase surface area, (3) elongation of larvae to allow worm-like thrashing or wriggling movement, (4) strong claws to aid in crawling across rocks and plants, and (5) some truly exotic methods such as water jets from the rectum as in some dragon ﬂies. The most formidable problem faced by aquatic insects is a source of oxygen for respiration (Fig. 3.9). There are two kinds of aquatic insects that differ in how they obtain oxygen: aeropneusty (air breathers) and hydropneusty (water breathers). The aeropneusty can obtain oxygen in three distinct ways. There are surface breathers that remain permanently in contact with the air and use a miniature snorkel to obtain oxygen. Some diptera (Syrphidae or rat-tailed maggots) and Ephydridae (shore ﬂies) have a pair of posterior, telescopic breathing tubes that open in spiracles with hydrophobic hairs that prevent water from entering. These breathing

50 THE BIOLOGY OF FRESHWATER WETLANDS

Thoracic respiratory horns

Terminal breathing tube

A

C

E

Fig. 3.9

Trailing air bubble from underwing chamber

Plate-like gills

B

D

Respiratory attachment

Adaptations for respiration among aquatic insects. A. Mosquito pupa at air–water interface. B. Rat-tailed maggot with extended breathing tube. C. Diving beetle with trailing bubble of air. D. Shore ﬂy pupa obtaining oxygen from the submersed stem of a macrophyte. E. Mayﬂy with gills. (Redrawn from McCafferty 1981.)

tubes have a maximum extension of about 5 cm and thus this group is limited to very shallow water. A variation on this theme is found in a few insects, mostly dipterans, that obtain their oxygen from macrophytes. These have spiracles that are modiﬁed for piercing plants. There are insects that periodically come to the surface to breath like lungﬁsh. Many larvae of hemipterans obtain oxygen this way. When they dive, a ring of hydrophobic hairs that surround them closes their spiracles. This is how mosquito larvae obtain oxygen. Some aquatic insects such as adult gyrinid and dytiscid beetles carry a bubble of air with them when they dive. This air is stored under their wings in a sub-elytral cavity or in the hairs around their spiracles. Hydropneustic insects obtain oxygen through their cuticle directly from the water. Aquatic insects of this type are most common in

MICROORGANISMS AND INVERTEBRATES 51

streams and rivers and in well-aerated lakes. Nevertheless, hydropneusty is also found in some wetland groups such as the chironomids. Many insects in this group have various kinds of membranous growths that act effectively like gills and improve oxygen uptake, or like the chironomids containing haemoglobin to improve their oxygen uptake efﬁciency. Chironomids or midges (Order Diptera) are the aquatic insects best adapted to low oxygen levels in wetlands. There can be thousands of midges per square meter in wetlands (Fig. 3.10). Both the larval and pupal stages live in organic sediments (Fig. 3.11). When the adults are ready to emerge, the pupae rise to the surface. Adults live for only a few days during which they often do not eat. Their time is spent primarily mating (Oliver 1971). Females lay their eggs on the water surface or on vegetation where they hatch into larvae. Chironomid larvae have both behavioural and physiological adaptations that enable them to live in low oxygen levels. They construct burrows or tubes of sediment particles that are lined and held together by silky salivary secretions. Larvae of different species of chironomids have distinct substrate preferences. The larvae ventilate these burrows or tubes by undulating their bodies to create water currents. These currents carry not only oxygen but also food particles to the larvae.

Water levels Spring Summer

Phragmites Whitetop Cattail Bulrush Pondweed

Open water

Total annual emergence (×103 m–2)

15

10

5

0 Phragmites Whitetop Fig. 3.10

Cattail

Bulrush Pondweed Open water

Total annual chironomid emergence along an elevation/vegetation gradient in the Delta Marsh, Manitoba, Canada. (Redrawn from Williams and Feltmate 1992.)

52 THE BIOLOGY OF FRESHWATER WETLANDS

Adult male

Adult female

Egg mass Pupa Larva

Fig. 3.11

Typical Chironomid life cycle. (Redrawn from Koehler 1980.)

When the water becomes anoxic, chironomid larvae become inactive for extended periods of time (weeks or months). Many chironomids have haemoglobins that increase their respiratory efﬁciency. Chironomid haemoglobins have higher afﬁnity for oxygen than vertebrate haemoglobins, and oxygen stored on chironomid haemoglobins can serve as a temporary reservoir of oxygen. Chironomids often alternate periods of undulation during which the haemoglobins become saturated with oxygen with still periods when the oxygen stored on the haemoglobin is used. Chironomid larvae are an important part of the food chains of many types of wetlands. They are involved in the processing of litter and they are often major food items in the diet of larger aquatic invertebrates, ﬁsh, and birds.

3.3

Summary Although largely unseen and mostly a subset of species found in other kinds of aquatic systems, the bacterial, fungal, algal, and invertebrate communities of wetlands are functionally very important. Because of their metabolic diversity, bacteria play a central role in nutrient cycling in wetlands. They are involved in the decomposition of organic matter and also in important chemical transformations of nitrogen, sulphur, and other elements. Wetland fungi are also involved in the decomposition of litter, especially the early stages. Algae are major primary producers in wetlands. Invertebrates provide an essential link in wetland food webs between the primary producers and larger omnivorous and carnivorous organism (e.g. ducks and ﬁsh). With the partial exception of some protozoans, all invertebrates are heterotrophs that eat bacteria, fungi, algae, each other, or

MICROORGANISMS AND INVERTEBRATES 53

macrophytes. Different groups of algae and invertebrates are found in the water column, growing on plants, and on or in the substrate. Low levels of oxygen caused by the decomposition of litter can be a major problem for some invertebrates, especially aquatic insects. Aquatic insects have a variety of behavioural, morphological, and physiological adaptations that enable them to cope with low oxygen levels. The functional signiﬁcance of these groups in wetlands is examined in more detail in Chapter 6.

3.4

Practical experiments and observations

3.4.1

Wetland algae There are a number of algal assemblages in most wetlands: plankton, epiphyton, epipelon, and metaphyton. The relative importance of each assemblage varies from place to place within a wetland. Algae are important primary producers in many wetlands and are an important source of food for many invertebrates. Collect samples of the algae from all four assemblages, if possible, within the emergent and submerged aquatic zones of a wetland. Phytoplankton can be collected in a jar or other suitable container. The phytoplankton can be concentrated ﬁrst by passing the water through a plankton net. Collect plant parts from under the surface of the water (stems will normally be easier to handle and process than leaves), and place them in container ﬁlled with tap water or wetland water from which the plankton have been ﬁltered. Likewise, collect pieces of litter from the bottom of the wetland and, if present, a sample of metaphyton. In the laboratory, identify the algae in the phytoplankton, epiphyton, epipelon, and metaphyton. This may require scraping them off the living plants (epiphyton) and litter (epipelon). What are the most common kinds of algae in the phytoplankton, epiphyton, epipelon, and metaphyton? Which are the most similar assemblages? Do these algal assemblages have the same composition in the emergent zone and submerged zone?

Fig. 3.12

An activity trap for sampling nektonic invertebrates. (Redrawn from Murkin and Murkin 1989.)

54 THE BIOLOGY OF FRESHWATER WETLANDS

3.4.2

Wetland invertebrates Invertebrates are common in wetlands and are an important food for waterfowl and ﬁsh. The composition and distribution of invertebrate assemblages varies both spatially and temporally. There are many different assemblages of invertebrates and different sampling methods are needed to determine the species composition of these assemblages. Activity traps (Fig. 3.12) can be sued to sample free-swimming, nektonic, invertebrates. Emergence traps (Fig. 3.13) are used to sample aquatic insects. Sweep nets can be sued to collect neustonic and nektonic invertebrates. Soil corers or dredges can be used to sample benthic invertebrates. Examples of projects using activity and emergence traps are described. Similar projects can be done using sweep nets and dredges. For all ﬁeld projects, it is essential that the date, time, and location at which each sample was collected be recorded in a ﬁeld notebook at the time of collection. Descriptive information about each site sampled should also be recorded, including water depth, type of vegetation, weather conditions, etc. Activity traps. Place a series of activity traps midway in the water column along a transect from the emergent zone into the submerged zone. Activity traps should be ﬁlled with water before the funnel is put into place (Fig. 3.12). No air bubbles should be trapped in the jar. Traps

Fig. 3.13

An emergence trap for sampling aquatic insects in wetlands. (Redrawn from Murkin and Murkin 1989.)

MICROORGANISMS AND INVERTEBRATES 55

should be suspended horizontally in the water. They should be left for 24–30 h before being collected. When traps are collected they should be raised to surface and emptied into a ﬁne sieve. The contents of the sieve are then washed into small jar with a squeeze bottle. The samples can be stored in a refrigerator if necessary. Identify the macro-invertebrates and determine their methods of feeding. What are the common species in the nekton? Does the composition of the nekton vary among different plant communities? How do common nektonic species feed? Emergence traps. Emergence trap are simply a ﬂoating net that funnels insects into a jar at the top (Fig. 3.13). They can be built out of plastic tubing and window screening. Traps that cover an area of 0.5 m2 are commonly used. Place emergence traps in two different vegetation types. They should be left for several days to a week. Sometimes, it maybe necessary to trim the vegetation before traps can be placed. When traps are collected remove the jar and quickly screw a cap onto it. Identify the insects collected in the traps. What kinds of insects are found in this wetland? Do different insects emerge from areas dominated by different plant communities in this wetland? What environmental factors seem to control the distribution of aquatic insects?

4 Wetland plants and animals

As noted previously, it is the presence of large plants, macrophytes, that distinguishes wetlands from other aquatic ecosystems. The presence of macrophytes creates a habitat that is different both in biotic and abiotic conditions (Chapter 2). Wetlands are, in fact, composed of many microhabitats that differ quantitatively in water depth, water velocity, light levels, diurnal temperatures, diurnal oxygen concentrations, etc. (Table 4.1). Animals use habitats or microhabitats based on their requirements for food, oxygen, nesting sites, absence of predators, etc. Microhabitats can exist at spatial scales ranging from millimetres to kilometres, mostly depending on the size and mobility of the animal. Many larger animals use more than one habitat during their life cycles: some ducks use wetlands for feeding but nest in uplands. Many smaller animals use more than one microhabitat within a single wetland either daily or seasonally: some ﬁsh may use submerged aquatic plant beds during the day but move into vegetation-free or open areas at night. Because of their feeding, nest building, or other activities, animals not only have an impact on the distribution and abundance of macrophytes, but they can even modify a wetland’s topography. In this chapter, we examine the characteristics of the larger plants and animals found in wetlands, their macrophytes and macrofauna. These organisms are what distinguish wetlands from other ecosystems. This is not to imply that the microscopic organisms such as bacteria, fungi, most algae, small invertebrates, etc. are not important organisms in wetlands. They most certainly are, and their roles in primary and secondary production, nutrient cycles, and food webs will be discussed in the next chapter. Most microscopic organisms, however, typically comprise a subset of those found in other aquatic ecosystems in an area. Because they are well described in books dealing with other aquatic ecosystems (e.g. Brönmark and Hansson 2005 in this series), their biology will not be dealt with in any detail.

WETLAND PLANTS AND ANIMALS 57

Table 4.1 Environmental and morphological characteristics of wetland that can inﬂuence their use by various groups of animals Variable

Examples

Hydrology

Depth of water Seasonal water-level changes Interannual water-level changes Water velocity Connectivity to rivers and lakes Dissolved gas concentration (especially oxygen) patterns pH Ionic composition, salinity, conductivity Dissolved organic compounds, for example, humic acids Turbidity Seasonal patterns Horizontal patterns Vertical patterns Organic or inorganic (sand, silt, clay) Depth of fallen litter Amount of woody litter Herbaceous, shrubby, or forested wetland Amount of emergent, ﬂoating-leaved, submerged vegetation Canopy height of emergent vegetation Amount of persistent, standing litter Prey species Predators Competitors

Water chemistry

Water temperature

Substrate

Vegetation structure

Other species

The macrophytes and macrofauna of wetlands differ in how well adapted they are to life in wetlands. Based on their anatomical, morphological, and physiological adaptations and, for animals, their behaviour, wetland species can be placed into four broad categories: obligate, amphibious, facultative, and incidental wetland species. Obligate wetland species are found either in the water column or in ﬂooded soils. They are species that cannot survive, except in some resting stage, without water. Organisms in this group, for example, ﬁsh, are often not unique to wetlands and are also found in adjacent lakes and streams. Many macrophytes are in this category. Amphibious species spend at least part of their life cycle or spend some of their time in wetlands and the remainder in some terrestrial environment. This is a very large and diverse group that includes amphibians (e.g. leopard frogs), reptiles (e.g. American alligators), and birds (e.g. diving ducks). Facultative species are those that can be found both in wetlands and terrestrial environments. They differ from amphibious species in that they do not have any stage in their life cycle that must occur in water. If a wetland was drained, its facultative species would survive, but its amphibious species would not. The Asian water buffalo is an example of a facultative wetland species. Incidental wetland species include a host of species that are occasionally found in wetlands. Because obligate and amphibious species are

58 THE BIOLOGY OF FRESHWATER WETLANDS

the species that dominate the ﬂora and fauna of wetlands, in this chapter, their biology will be stressed.

4.1

Wetland plants The most obvious organisms in wetlands are macrophytes (Figs 4.1–4.4). Macrophytes are major primary producers; they provide much of the physical structure of wetlands; and they alter the wetland’s abiotic environment (Chapter 2), both physical (light levels, wind and water currents, oxygen levels, etc.) and chemical (pH, nutrient levels, dissolved organic concentration, etc.). There are many kinds of macrophytes (mosses, liverworts, ferns, conifers, monocots, dicots, etc.) that range in size from large tropical trees to minute duckweeds that are barely visible to the naked eye (Cook et al. 1974; Cook 1990; Cronk and Fennessy 2001). Various attempts to classify macrophytes have been proposed. They are all primarily based on the depth of water in which the species grow and whether they are rooted or free-ﬂoating (Figs 4.1–4.4). Since water levels in many wetlands change seasonally and from year to year, most wetland plants can be found growing in a variety of water depths, including no standing water at all. Consequently, these classiﬁcation systems are based on what are perceived to be ‘typical’ water depths at which a given species or group of similar species is found and reproduces. Common macrophyte growth forms include: Submerged plants (Fig. 4.1): all parts of these plants (leaves, stems, and roots) except the inﬂorescences (ﬂowers) normally grow underwater, for example, most pondweeds (Potamogeton spp.). (1) rooted submerged plants: plants rooted in the soil, water celery (Vallisneria spp.); (2) unrooted submerged plants: plants free-ﬂoating in the water column, for example, bladderworts (Utricularia spp.); (3) attached submerged plants: plants attached to a substrate but not by roots, for example, members of the Podostemaceae. Floating-leaved plants (Fig. 4.2): plants whose leaves ﬂoat on the surface of the water but that are rooted in the substrate, for example, water lilies (Nymphaea spp.). Floating plants (Fig. 4.3): plants that ﬂoat on the surface of the water and that are normally not attached to the substrate, for example, water hyacinth (Eichhornia crassipes). Emergents (Fig. 4.4): plants whose leaves and sometimes stems are found above the water, but whose roots are in ﬂooded or saturated soils, for example, cattails (Typha spp.), papyrus (Cyperus papyrus). (1) herbaceous emergent (includes wet-meadow species), for example, common reed (Phragmites australis), sedges (Carex spp.), sawgrass (Cladium spp.)

WETLAND PLANTS AND ANIMALS 59

A

B

C

D

Fig. 4.1

Submersed plants: A. Potamogeton richardsonii and B. Vallisneria americana, C. Ceratophyllum demersum, and D. Apinagia spp. (Podostemaceae). (Copyright, IFAS Publications, University of Florida.)

(2) emergent shrubs, for example, willows (Salix spp.) (3) emergent trees, for example, bald cypress (Taxodium distichum) Moist-soil species: terrestrial plants, mostly annuals, that temporarily become established in wetlands when they have no standing water. These are also called drawdown and mudﬂat species, for example, Polygonum spp.

60 THE BIOLOGY OF FRESHWATER WETLANDS

Fig. 4.2

Floating-leaved plants: Nuphar sp. (left) and Brasenia schreberi (right). (Copyright, IFAS Publications, University of Florida.)

1 inch 1 cm

1 cm

Fig. 4.3

Floating plants: Eichhornia crassipes (left) and Spirodela polyrizha (right). Drawings at very different scales. (Copyright, IFAS Publications, University of Florida.)

In general, macrophytes are similar in their general anatomy, morphology, and physiology to their terrestrial ancestors (Sculthorpe 1967; Cronk and Fennessy 2001). Herbaceous macrophytes, however, have several characteristics in common: (1) a gas-space continuum from leaves to roots; (2) heterophylly, and (3) clonal growth. These allow herbaceous macrophytes to overcome the two major environmental problems of rooted plants in wetlands, anoxic soils, and water-level ﬂuctuations.

WETLAND PLANTS AND ANIMALS 61

A

B

C

D

Fig. 4.4

Emergents: A. Typha domingensis, B. Ludwigia peruviana, C. Sphagnum cuspidatum, and D. Sarracenia purpurea. (Typha and Ludwigia. Copyright, IFAS Publications, University of Florida.)

62 THE BIOLOGY OF FRESHWATER WETLANDS

4.1.1

Gas-space continuum Roots of terrestrial plants obtain oxygen for respiration directly from the air in soil pore spaces. When soils are ﬂooded, the water in the soil pore spaces is quickly depleted of oxygen, and the roots of wetland plants are no longer able to obtain oxygen directly. It is anaerobic soils that prevent terrestrial plant species from growing in wetlands. Herbaceous macrophytes, especially submerged, ﬂoating-leaved, and emergent species, have a series of interconnected gas-ﬁlled spaces from leaf to root tip through which oxygen and other gasses can be redistributed internally (Figs 4.5 and 4.6). For example, the leaves of the emergent macrophyte Sparaganium americanum are mostly air spaces and, as with many emergent monocots, its leaf architecture resembles that of a skyscraper (Fig. 4.5). The porosity (percentage of a given tissue that is air space) of herbaceous macrophyte tissues can exceed 50%. Organ porosity is often a function of conditions under which it is formed, especially that of roots (Fig. 4.7). Air spaces can take two forms, lacunae and aerenchyma. Lacunae are open spaces full of gas. Aerenchyma is a porous tissue that is full of air spaces. The formation of aerenchyma is often triggered by submersion (hypoxia) and seems to be caused by changes in ethylene and other plant hormones (Evans 2003). Aerenchyma can form due to schizogeny during which cells separate to create gas space or lysigeny during which cells die to create gas spaces. The gas-space continuum not only

1

vpm nv

Fig. 4.5

vpc

Mid-section of the emergent leaves of Sparganium americanum showing its compartmentalized structure. Compartments (air spaces) are formed by various kinds of horizontal diaphragms: vpm, vascular, photosynthetic diaphragm with marginal bundle; nv, nonvascular diaphragm; and vpc, vascular photosynthetic diaphragm with cross bundle. (Adapted from Kaul 1972.)

WETLAND PLANTS AND ANIMALS 63

A Air space

B

Upper epidermis Air space beneath stoma

Palisade mesophyll

Lacuna of spongy mesophyll

Lower epidermis Fig. 4.6

Cross sections of (a) submerged and (b) ﬂoating leaves of Potamogeton nodosus. (Redrawn from Sculthorpe 1967.)

allows the movement of oxygen from the aboveground organs to the belowground organs, but also the movement of carbon dioxide and other gases (e.g. methane) from belowground organs to aboveground organs. Emergent plants are, in effect, pipelines connecting wetland sediments to the atmosphere (Fig. 4.8). Three mechanisms for the movement of oxygen to roots through the gas-space continuum have been proposed (Colmer 2003): diffusion, mass through ﬂow, and some combination of the two. There are strong chemical potential gradients for oxygen, carbon dioxide, and other gasses between the leaves and photosynthetic shoots and the rhizomes and

64 THE BIOLOGY OF FRESHWATER WETLANDS

Root porosity (%)

40 35 30 25 20 15 10 5

nd

0 C.

C.

C.

C.

C.

Ca

at a

a os

lim

str ro

a gr ni

lia na

r pe

m

a ne gi rru

se

l va da

fe

x re

re vi ns

Fig. 4.7

Root porosity (percentage of air space per volume of root tissue) of six hydroponically grown Carex species under high oxygen (aerated) conditions (light bars) and low oxygen (stagnant) conditions (dark bars) for three or more weeks. (From Visser et al. 2000.)

A

B Air enters via leaf sheath stomata

Air enters via short, broken culms (Influx)

Tall, dead broken culms (Efflux)

Soil Rhizome

Old dead broken culm (Efflux)

Young intact culm (influx) Soil Rhizome

O2 diffuses into roots and rhizome tips Fig. 4.8

O2 diffuses into roots and rhizome tips

Through-ﬂow pathways in Phragmites australis. A Differential air pressure caused by wind blowing across dead culms sucks air into the lower culms through the rhizomes and into the taller culms. B Pressurization of new culms due to a build up of vapour pressure or higher temperatures causes mass ﬂow of gasses down the culms into the rhizome and up into more porous older culms. The movement of oxygen from the rhizomes into the roots and out of the roots into the soil is due to diffusion. (Redrawn from Colmer 2003.)

WETLAND PLANTS AND ANIMALS 65

roots. Consequently, internal diffusion of these gasses through the gas-space continuum is always occurring, but rates of gas diffusion are low. Because diffusion over long distances is so slow, it may not be adequate to supply needed oxygen to roots and rhizomes. In living emergent and ﬂoating-leaved plants (Fig. 4.8), mass ﬂow results from pressure differences in new and old shoots caused by differences in their porosity. During the day gas pressure builds up in new shoots because of an increase in water vapour and possibly higher temperatures. Gas pressure builds up more in less porous, young shoots than in older shoots, and this causes gases to ﬂow down newer shoots, through the aerenchyma in the rhizomes, and up into more porous older shoots. As expected, through ﬂow results in much higher concentrations of oxygen (often at atmospheric levels) in the roots and rhizomes during the day than at night when photosynthesis is not occurring. In submersed aquatic plants, through ﬂow cannot occur, and diffusion alone is responsible for the internal redistribution of gasses through the gas-space continuum or through the direct exchange of gasses between water and plant tissues. Although convective through ﬂow signiﬁcantly improves the delivery of oxygen to belowground organs over molecular diffusion, there is a physical limit to how much oxygen can be delivered through the gas-space continuum, and this may ultimately determine the maximum depth at which different wetland species can grow (Visser et al. 2000). If internal oxygen redistribution mechanisms are not adequate to meet rhizome and root oxygen demands, fermentation in roots and rhizomes will begin. Fermentation produces less energy than aerobic respiration and also potentially results in the build-up of toxic metabolic end products, for example, ethanol that could adversely affect plant growth. Less efﬁcient internal redistribution of oxygen may largely explain the lower ﬂooding tolerances of some emergent species and their replacement by more ﬂooding tolerant species if hydroperiods increase, for example, the replacement of sawgrass (Cladium jamaicense) by the more ﬂood tolerant Typha domingensis in the Everglades (Chabbi et al. 2000). There are periods in the life cycle of wetland plants when internal gas redistribution cannot occur, for example, in early spring in the temperate zone before stems and leaves are fully developed. During such periods, wetland plants may temporarily switch to alcoholic fermentation until new shoots are formed and through ﬂow of gasses as a result of pressurized ventilation becomes possible. Through ﬂow can also occur in dormant plants with persistent, standing litter. This has been reported for Phragmites australis. Differences in wind speed at the top and near the bottom in the canopy create a differential internal pressure between tall and short dead shoots. The lower air pressure in taller shoots draws air into the shorter dead shoots, down into the rhizomes, and up the taller dead shoots (Fig. 4.8). In the

66 THE BIOLOGY OF FRESHWATER WETLANDS

temperate zone in the early spring, this may be an important mechanism for Phragmites to get oxygen to shoot and root meristems. In addition to oxygen, gasses such as methane and carbon dioxide can also diffuse from sediments into roots and from the roots through the gas-space continuum into the atmosphere or into the water column. Oxygen diffusing into the roots may not be completely used up in root respiration, and some of it may diffuse from the root into the surrounding soil. This is called radial oxygen loss. It creates a thin oxidized layer around the roots, and this oxidized layer reduces the accumulation of anaerobic phytotoxic products around the roots and makes mineral nutrients more available for plant uptake. Radial oxygen loss occurs primarily in young roots and its extent is both species dependent and soil redox-level dependent. Root channels are often easily seen in wetland soils because of the deposition of oxidized iron and other minerals around new roots (Colmer 2003). The large volume of air space within macrophytes also means that there is an internal reserve of oxygen and carbon dioxide that makes them less dependent on external sources for these gasses. Because of belowground respiration, internal concentrations of carbon dioxide needed for photosynthesis can be much higher inside wetland plants than in the atmosphere (Constable and Longstreth 1994). This internal storage seems to be most important for submerged plants in which there is a lot of internal recycling of carbon dioxide. Like oxygen, carbon dioxide can be lost to the surrounding atmosphere, water, or soil by diffusion. The gas-space continuum is best developed in herbaceous plants. In woody species, other morphological adaptations for getting oxygen to the roots are found: lenticels and pneumatophores. Lenticels are small pores or openings in the bark that permit gas exchange between the metabolically active cells below the bark and the atmosphere. They are found on both stems and woody roots. Pneumatophores are modiﬁcations of the roots that aid in the uptake and movement of oxygen. They take several forms such as vertical roots at intervals that stick up above the water; knees, which are sections of horizontal roots that loop up above the water; and plank roots whose upper parts remain out of the water (Cronk and Fennesy 2001). In wetland trees, oxygen is depleted in parts of the root system that are submersed and, because of its high solubility in water, some of the carbon dioxide produced by root respiration diffuses into the surrounding water. As a result, the partial pressure of gases in the ﬂooded root system is lower than in the atmosphere, and this draws gases from the atmosphere into the roots through the lenticels or pneumatophores that are above the water (Brix 1993).

WETLAND PLANTS AND ANIMALS 67

Flooding of some wetland woody and herbaceous emergents often triggers the production of adventitious roots at the water surface. This is also believed to be due to a change in plant hormone distribution with both auxins and ethylene implicated. These adventitious roots develop near the surface of the water where oxygen levels are highest. Adventitious roots enable wetland plants without well-developed gas-space continua or pneumatophores to cope with periods of high water.

4.1.2

Heterophylly Fluctuations in water levels are a common feature of wetlands (Chapter 2). Consequently, wetland plants can encounter a variety of water depths seasonally and interannually. Even submersed plants may have to endure periods without standing water and most have a terrestrial form. Not surprisingly, wetland plants show a great deal of phenotypic plasticity (Fig. 4.9), and this allows them to adjust their growth as water levels change. One type of phenotypic plasticity that is widespread among macrophytes is heterophylly (Wells and Pigliucci 2000; Minorsky 2003; Dorken and Barrett 2004). Heterophylly is the ability to produce different leaf types (Figs 4.6 and 4.9). Two kinds of leaves are commonly produced by herbaceous wetland species, submerged and aerial. Submerged leaves are thin, lack or have a greatly reduced cuticle, and lack functional stomata. Aerial leaves are thicker, have a cuticle, and have stomata (Fig. 4.6). Changes in leaf shape, size, and thickness and petiole or leaf/shoot length are common in facultatively heterophyllous species. The porosity of their roots can also change signiﬁcantly as soils become anoxic after ﬂooding in ﬂood-responders (Fig. 4.7). These morphological responses primarily serve as a way to improve oxygen uptake by leaves, the volume of internal gas storage, and the efﬁciency of internal gas redistribution by diffusion. As would be expected, macrophytes that ﬂoat on the surface of the water (e.g. E. crassipes) or are free-ﬂoating below the surface (e.g., Ceratophyllum demersum) are less affected by water-level ﬂuctuations and are less heterophyllous than rooted submerged aquatics and emergents. The latter may produce underwater, ﬂoating, and aerial leaves. Heterophylly increases the ability of many macrophytes to survive a wide range of water levels. Heterophylly is not restricted to aquatic macrophytes; many terrestrial plants are also heterophyllous. Most wetland plants, like many terrestrial plant species, produce different juvenile and adult leaves, but this is strictly a ontogenetic phenomenon and replacement of juvenile leaves by adult leaves is not induced by a change in water depth or other environmental conditions. Many other wetland and terrestrial plants are

68 THE BIOLOGY OF FRESHWATER WETLANDS

Fig. 4.9

Leaves from a single Sagittaria rigida plant. (Redrawn from Muenscher 1944.)

facultatively heterophyllous. They adjust their leaves in response to environmental conditions. One common type of facultative heterophylly found in terrestrial plants, sun and shade leaves, has much in common with the aerial and submerged leaf heterophylly found in macrophytes (Fig. 4.6 and Table 4.2). This is not surprising because submerged macrophyte leaves are found in a low light environment (Chapter 2). Besides changes in water depth, experimental studies have shown that aquatic macrophyte leaf characteristics are also affected by light intensity, temperature, photoperiod, osmotic pressure, water velocity, and nutrient levels (Wells and Pigliucci 2000; Dorken and Barrett 2004; Puijalon and Bornette 2004). Not all aquatic macrophytes, however, are facultatively heterophyllous. Leck and Brock (2000) have pointed out that some emergent wetland species are ﬂood-tolerators. Flood-tolerators are unable to change their anatomy or morphology in response to rising water levels. They endure rather than respond and may even go dormant if water levels remain

WETLAND PLANTS AND ANIMALS 69

Table 4.2 Characteristics of shade versus sun leaves of terrestrial plants and submerged versus aerial leaves of aquatic macrophytes Trait

Shade leaves of terrestrial plants

Submerge leaves of aquatic macrophytes

Leaf areaa Thickness Margins Stomate density Mesophyll Venation Cuticle Epidermal cells Chloroplasts

Larger Thinner Less lobed or toothed Lower Reduced palisade layer Reduced in density Reduced in thickness Larger More in epidermis

Larger Thinner Variablea Lower Reduced or no palisade layer Reduced in density Reduced or absent Larger Found in epidermis

a

Monocot leaves—longer and thinner; dicot leaves—more lobed and divided under water.

Source: Adapted from Wells and Pigliucci (2000).

high for long periods (van der Valk 1994). Species that are facultatively heterophyllous are ﬂood-responders and quickly adjust their anatomy and morphology to changes in water depth.

4.1.3

Clonal growth Most herbaceous macrophytes are perennials with clonal growth (Fig. 4.10), and clonal species occur more frequently in wetlands than in other kinds of habitat (van Groenendal et al. 1996). Changes in water levels can create conditions that decrease or eliminate chances for pollination, seed production, seed germination, and seedling establishment (Barrett et al. 1993; Santamaria 2002). Clonal growth is believed to increase the probability of these plants surviving locally from year to year in spite of rapidly changing water levels. It also facilitates the spread of clonal species within a wetland in the absence of seed production. Differential clonal growth rates also explain the changes in species abundance from year to year that occur in many wetlands. Wet years with high water levels favour the clonal growth of some emergent species, while dry years with low water levels favour the growth of other species. Changes in the clonal growth from year to year can result in changes in the dominant species, and these are called ﬂuctuations (van der Valk 1985). Clonal growth may not only facilitate the local spread and persistence of a species, but sometimes its dispersal to other sites as well. Two different modes of clonal growth are generally recognized in wetlands, splitter-spreader and packed-connected. New plants or ramets in splitterspreader species become established at the ends of rhizomes or stolons some distance from their parent plant while those of packed-connected species are produced close to the parent plant and are often grouped tightly. The former (Fig. 4.10) is common among submerged,

70 THE BIOLOGY OF FRESHWATER WETLANDS

free-ﬂoating, and herbaceous emergent plants. The dispersal of plant fragments by water currents can occur both within and among wetlands. The packed-connected growth form is restricted mostly to wet meadow and peatland species found in habitats that are rarely ﬂooded and often low in nutrients. It produces the tussocks that sometimes are common in these vegetation types. Clonal growth when combined with heterophylly seems to be responsible for the low plant species diversity in many wetlands as well as the low level of genetic diversity of these species within wetlands and high levels of genetic diversity among wetlands (Santamaria 2002). The meristems, points of origin of stems or leaves, of emergent clonal species are normally underground (Fig. 4.10); therefore they are protected from ﬁres. Fires are common in many kinds of wetlands, especially those that are subject to seasonal and interannual ﬂuctuations in water levels. For example, in the Florida Everglades (Gunderson and Snyder 1994), ﬁres occur each year during the dry season (November to April). Besides this annual ﬁre cycle, there is also a second, longer ﬁre cycles of 10–15 years that is caused by the wet–dry cycle in south Florida. During drought years, there are more ﬁres during the dry season than in wet years. Although their live and dead shoots are normally burned away by ﬁres, emergent canopies quickly regenerate after ﬁres from belowground meristems. Emergent species with clonal growth can be killed by prolonged periods of higher than normal water levels. For example, in prairie potholes

Fig. 4.10

Clonal growth in Scirpus paludosus, a splitter-spreader. (Adapted from Muenscher 1944.)

WETLAND PLANTS AND ANIMALS 71

in North America, during wet periods (Fig. 2.10), water levels may remain as much as 1 m above normal for several years. This can kill all or most of the emergent populations. With the exception of submerged species, the seeds of most wetland macrophytes do not germinate readily under water. Thus it takes a period of below normal precipitation, when the wetland goes completely or mostly dry, a drawdown, to re-establish emergent species. In wetlands subject to large water-level ﬂuctuations, most of the species in these wetlands have long-lived seed that can persist in their seed banks for decades. When these wetlands go dry, all of the emergent species become re-established simultaneously (van der Valk and Davis 1978). The long-term persistence of clonal emergent species in wetlands with ﬂuctuating water levels is due to the presence of their seeds in seed banks of these wetlands.

4.2

Birds Different vegetation types distributed along water depth gradients create a complex mosaic of microhabitats (Fig. 4.11) that can range from nearly terrestrial to open water. Each of these microhabitats or some combination of them can supply some group of birds (Table 4.1) with water, food, and shelter. During the evolution of the world’s avifauna, many groups of birds have become adapted anatomically, morphologically, and physiologically to different wetland microhabitats. Collectively, these are called wetland birds (Weller 1999). Common wetland birds include divers (loons), grebes, waterfowl or wildfowl (ducks, geese, swans), waders or herons (egrets, storks, ibises), shorebirds (sandpipers, stilts, plovers), cranes and rails, pelicans, ﬂamingos, passerines (sparrows, wrens, warblers), blackbirds, gulls and terns, kingﬁshers, and raptors (snail kites). Of these, the two groups most commonly associated with freshwater wetlands are ducks (Fig. 4.12) and wading birds (Fig. 4.13). It is not unusual to ﬁnd many non-wetland birds in wetlands, for example, ostriches grazing in African marshes. Table 4.3 summarizes the major evolutionary adaptations found in wetland birds. Many of these are adaptations for getting into and out of wetlands and for obtaining food in wetlands. These include the ability to land on, swim on, and take off from the water surface (ducks, geese, swans, coots, grebes, etc.) or the ability to land, walk in, and take off in shallow water (wading birds). Not surprisingly, the feet of wetland birds show a range of adaptation to life in wetlands: swimming (Fig. 4.14), diving, walking on mud or on ﬂoating plant leaves, wading in shallow water, and grabbing prey. There are also many adaptations for feeding. Among wetland birds, there are bills that strain, peck, spear, store, and grab. An examination of the beaks of ducks illustrate the range of adaptations found for feeding in this group alone (Fig. 4.15). Waterproof

Short-bill

Wrens Plovers

Long-bill

Upland Killdeer

Black

Terns

Forster’s

B.W.Teal Ducks

Mallard American

Bitterns

Ruddy redhead

Wooo duck Night heron

Least King R. Sora Virginia

Rails

Icterios

Coot gallinule

Bobolink Meadowlark Redwing Upland grasses

Fig. 4.11

Yellowhead

Oriole

Grackle Redwing Mink

Lowland grasses

Sedge

Cattail

Hardstem muskrat

Muskrat

Distribution of birds, minks, and muskrats along water depth and related vegetation gradients in a North American prairie pothole. (Redrawn from Weller 1999.)

WETLAND PLANTS AND ANIMALS 73

Fig. 4.12

On the left, a dabbling duck, Northern Pintail (Anas acuta) and on the right a diving duck, Canvasback (Aythya valisineria). (Redrawn from Bellrose 1976.)

Fig. 4.13

Wading birds: Wood Stork (Mycteria Americana), Great Blue Heron (Ardea heodius), and Cattle Egret (Bubulcus ibis). Table 4.3 Long-term (evolutionary) and short-term (behavioural) adaptations of birds to wetlands Long-term adaptations

Short-term adaptations

Rear leg placement for swimming or diving Bone and lung modiﬁcations for diving Eye modiﬁcation for nocturnal or underwater vision Flight adaptations for aerial divers Webbed and lobed feet Bill specialization for grabbing, straining, digging, etc. Water-resistant plumage Social/behavioural: breeding, spacing, aggression Long-range mobility: migration

Feeding strategies Food preferences Local ﬂight paths Nesting preferences Water depth preferences Roosting preferences

Source: Adapted from Weller (1999).

feathers are also a common adaptation of many wetland birds, although not all (e.g. anhingas). Wetland birds may be herbivores (e.g. geese), omnivores (e.g. dabbling ducks), and carnivores (e.g. mergansers, herons). Because of the

74 THE BIOLOGY OF FRESHWATER WETLANDS

heterogeneity and variability of food sources in wetlands (Table 4.4), many wetland birds are opportunistic feeders. Some exploit food resources found on or in the soil (invertebrates, seeds, tubers, etc.) and some exploit plants and animals found in the water column, on the surface of the water, on ﬂoating-leaves, or ﬂying above the wetland, especially insects. Common foraging strategies include surface swimmers that strain food with their beaks from mud or grab food on or in the water column (dabbling ducks); divers that grab or spear ﬁsh (mergansers); divers that strain, dig, or root food out of the soil (diving ducks); ﬂight feeders that catch insects (blackbirds), stationary spear ﬁshers (herons), plunge and perch divers for ﬁsh (kingﬁshers), and course hunters that seize prey with their feet (snail kites). These different foraging strategies result in the effective partitioning of available food sources among wetland birds and this facilitates their coexistence. However, as water levels rise or fall in any given wetland, the foraging strategies that will be successful will change and some groups of Lobed foot of coot

Lobed foot of grebe

Webbed foot Fig. 4.14

Adaptations of the feet of coots, grebes, and ducks for swimming.

Coots and grebes Fig. 4.15

Ducks and geese

Loons

Mergansers

Adaptations of waterfowl beaks for different feeding strategies. Coots and grebes eat by pecking; ducks and geese by grabbing; loons by spearing, and mergansers by catching ﬁsh. Table 4.4 Wetland microhabitats and the major types of foods associated with them that are eaten by birds Microhabitat

Major foods

Epibenthic Endobenthic Plankton Neuston Nekton Submerged plant beds Emergent plant beds Dry mudﬂats Wet mudﬂats

Seeds, crayﬁsh, invertebrates Roots, tubers, worms, clams Small invertebrates, ﬁlamentous algae Duckweeds, insects Fish, amphibian larvae Plant leaves and seeds, invertebrates Leaves, seeds, invertebrates Insects, seeds, seedlings, tubers, clams Worms, clams, tubers,

Source: Adapted from Weller (1999).

WETLAND PLANTS AND ANIMALS 75

birds will be favoured over others (Krapu and Reinecke 1992). Thus the assemblage of wetland birds (Fig. 4.16) found in a wetland changes seasonally and interannually (Murkin and Caldwell 2000). The diets of wetlands birds sometimes vary seasonally because of changing dietary needs. For example, while laying eggs female dabbling ducks A

Blackbirds 1

Increasing vegetation

RW 0 YH

–1

–1 B

0

1

2

3

Coots 1

Increasing vegetation

0

Fa –1

Su Sp

–1

Fig. 4.16

0

1 Increasing water depth

2

3

Habitat use during a wet–dry cycle in North American prairie wetlands by red-winged blackbirds (RW) and yellow-headed blackbirds (YH) during the spring and by American coots during the spring (Sp), summer (Su) and fall (Fa). (Redrawn from Murkin and Caldwell 2000.)

76 THE BIOLOGY OF FRESHWATER WETLANDS

such as pintails (Fig. 4.17) need more protein that cannot be supplied by their normally vegetarian diets. Therefore, female dabbling ducks increase their consumption of invertebrates with their much higher protein content and become carnivores while laying eggs. Likewise, their ducklings will be carnivores initially. Males of these species during the breeding season continue to be vegetarians. Other events in the life cycle of wetland birds that can result in changes in foraging behaviour include molting, which most ducks do in late summer, and migration. The degree to which a wetland bird is adapted to life in wetlands varies tremendously. Dabbling ducks (Anas spp.) such as pintails and diving ducks (Aythya spp.) such as canvasbacks (Fig. 4.12) are amphibious wetlands species. Diving ducks such as canvasbacks (Aythya valisineria) are a good example of a wetland bird species that is particularly well adapted to wetlands. They have large, webbed feet that are far back on their bodies; kick their feet in unison when diving; can squeeze the air out of their feathers to reduce their buoyancy when they dive; and have small wings to aid in swimming underwater. Because of their relative small wings, however, they must run along the surface of the water, ﬂapping their wings and kicking their feet to pick up enough speed to lift off (Fig. 4.12). Some groups of birds such as cranes seem to show obvious adaptation, long toes, long legs and necks, for exploiting shallow wetlands but often feed in grasslands. Some birds such as blackbirds, marsh wrens, and old-world warblers nest and feed in wetlands, but show few adaptation to wetland life. Nevertheless, even birds that have no anatomical, morphological, or physiological adaptation to life in wetlands can be completely dependent on them for food or nesting sites such as the Everglade snail kite. Everglade snail kites (Rostrhamus sociabilis) are found in subtropical and tropical wetlands from Florida to Argentina. They prefer to nest over

77

P > 0.05

Laying

P < 0.001

29 56

Pre-laying Fig. 4.17

P > 0.05

Post-laying

Percentage of invertebrates in the diet of female northern pintails (Anas acuta) prior to, while, and after laying eggs. (From Krapu 1974.)

WETLAND PLANTS AND ANIMALS 77

water usually in shrubs or small trees. In Florida, the apple snail (Pomacea paludosus) is the main food of the Everglade snail kite. Because it is a visual hunter, the snail kite prefers to hunt in less densely vegetated marshes with a mix of emergents and open water. Apple snails spend a lot of time at the air–water interface on emergent leaves or culms where they feed and breathe. Apple snails also lay their eggs on emergents just above the water line. Everglade snail kites ﬂy over wetlands at low elevations looking for apple snails or perch hunt from shrubs and trees for them. With their feet, they grab snails that are above, at, or just below the surface of the water. They will only plunge their feet into the water and never grab snails with their beaks. Because of its food and nesting requirements, snail kite populations are highly dependent on wetland water regimes (Kitchens et al. 2002). Prolonged low water can signiﬁcantly reduce apple snail populations because the snails require nearly continuously ﬂooded wetlands. Prolonged high water can eliminate emergents, shrubs, and trees and thus reduce snail kite nesting sites. Many upland birds will exploit wetlands seasonally or occasionally to obtain food and water. When wetlands go dry, they are abandoned by obligate wetland species that migrate to areas with wetlands with standing water. However, almost any local upland bird can be found in dry wetlands (Murkin and Caldwell 2000). Which types of non-wetland birds are found at any time is dependent on the type and extent of the plant cover.

4.3

Fish As with other aquatic organisms (Table 4.1), oxygen levels, water depth, water chemistry (e.g. pH, salinity), and water temperatures control the distribution of ﬁsh (Matthews 1998). The two major problems for ﬁsh in wetlands are periods with no or very low oxygen levels and with no or very shallow standing water. Fish are often absent from some kinds of wetlands, including some peatlands, small depressional (palustrine) wetlands, and slope wetlands. Other kinds of freshwater wetlands have signiﬁcant ﬁsh populations, and may even support commercial ﬁshing, for example, the Delta Marsh in Manitoba, Canada, or recreational ﬁshing, for example, Okavango Delta, Botswana. Wetlands associated with large lakes, such as the Delta Marsh, and large rivers, for example, the Amazon, are most likely to have signiﬁcant ﬁsh populations. In other words, the larger and more diverse a wetland, the more likely it is to have ﬁsh. For example, although it is a peatland, ﬁsh play an important part in the food chains of the Florida Everglades. Large wetlands can often support ﬁsh because their ﬁsh can ﬁnd refugia (deep holes with open water) that do not go dry during droughts where they can escape anoxia. Low oxygen and high water temperatures can prevent many ﬁsh species from entering wetlands and can limit the reproductive success of those

78 THE BIOLOGY OF FRESHWATER WETLANDS

that do. However, ﬁsh vary in their tolerance of low oxygen levels and this has a signiﬁcant impact on the species of ﬁsh found in wetlands. There are several ways in which ﬁsh have adapted to low oxygen levels (Graham 1997): First, they avoid anoxic conditions by moving to areas with oxygen. Such movements can be daily and seasonal. Second, they swim to the surface of the water to ventilate their gills in the thin layer of oxygenated water adjacent to the air–water interface. This is called aquatic surface respiration. Third, they can breathe air. Air-breathing ﬁsh have a variety of specialized organs (Fig. 4.18) that allow them to gulp air at the surface of the water (Fig. 4.19) or, in rare cases, to gulp air when they crawl onto land. Air-breathing ﬁsh are the most adapted to wetlands. There are over 350 species of air-breathing ﬁsh in about 125 genera and 50 families. Like the macrophytes, and wetland birds, air-breathing ﬁsh are polyphylletic in origin, and they can be found in both ancient and modern ﬁsh families (Graham 1997). Air-breathing ﬁsh are mostly found in subtropical and tropical wetlands in Africa, Asia, and South America. Most air-breathing ﬁsh are carnivores that take prey at or near the water surface. There are, however, also some planktivorous and herbivorous species. They vary in size from about 3 cm to 2 m. There are different kinds of air-breathing ﬁsh. Amphibious air breathers only breathe air when they are stranded on land due to dropping water levels. Aquatic air breathers surface periodically to gulp air. There are two types of aquatic air breathers, facultative and continuous. Facultative air breathers only do so when oxygen levels are unfavourable in the water column. Continuous air breathers gulp air more or less regularly, regardless of the oxygen levels in the water column. Some continuous air breathers can no longer obtain an adequate supply of oxygen from even well-oxygenated water. There are many different types of air-breathing organs. These include organs normally used for respiration such as gills and skin as well as modiﬁcations in the head and digestive tract (Fig. 4.18). In an African Papyrus swamp, the air-breathing catﬁsh, Clarias liocephalus, was able to cope with low oxygen levels and ﬂuctuating water levels while the only other ﬁsh, Barbus neumayeri, an aquatic surface respirator, had a much more limited distribution, especially during the dry season when water levels were low (Chapman 1995). In short, the respiratory capabilities of ﬁsh species have a deﬁnite effect on their ability to exploit wetlands and on how widely they are distributed within wetlands. Although clearly advantageous in low oxygen environments, air breathing does have an energy cost associated with it (travel to the surface) and also exposes ﬁsh breathing at the surface to aerial predation. Studies of the air-breathing ﬁsh, Ctenopoma muriei, in Africa indicate that the behaviour of this ﬁsh minimizes both problems. It stays close to the surface (within 15–30 cm) to minimize travel time and it reduces its susceptibility to aerial predation by being under a plant canopy (Randle and Chapman 2004).

WETLAND PLANTS AND ANIMALS 79

SBC

PD

L-GB

PS D

F

L

B Fig. 4.18

P

E

G

O

S

ST

I

A generic air-breathing ﬁsh equipped with all of the known air-breathing organs. These include modiﬁed epithelial surfaces in the buccal (B), pharyngeal (P), and opercular (O) chambers as well as gills (G), skin (S), stomach (ST), intestine (I). Other modiﬁed spaces and chamber include the suprabranchial chamber (SBC), pharyngeal sacs (PS), the pneumatic duct (PD), and lung or respiratory gas bladder (L-GB). Projections into these spaces to improve gas exchange include labyrinth (L), dendrites (D), and gill fans (F). (Redrawn from Graham 1997.) A

B

Fig. 4.19

Postures of two African ﬁshes using aquatic surface respiration. A. Synodontis afroﬁscheri and B. Synodontis nigriventris. (Redrawn from Chapman et al. 1994.)

Air-breathing ﬁsh, for example, lungﬁsh, often survive the dry season when wetlands can go completely dry in the mud in a hypometabolic, aestivating state. Aestivation resembles hibernation in that the ﬁsh enters a state of torpor by reducing its oxygen consumption, dropping its heart rate, and lowering its blood pressure. Most ﬁsh in temperate wetlands, however, show few or no adaptations for environmental condition in wetlands and are mostly anoxic avoiders

80 THE BIOLOGY OF FRESHWATER WETLANDS

(Fig. 2.3). In the summer during the day, yellow perch (Perca ﬂavescens), for example, will shelter and feed in lacustrine wetlands at the margins of lakes in Canada, but they avoid them at night when oxygen levels drop. Consequently, the ﬁsh found in wetlands are often just a small subset of those in contiguous waters. Except during periods when ﬁsh are spawning, the ﬁsh found in many temperate wetlands are small and are often juvenile forms of those found in adjacent bodies of water. In isolated palustrine wetlands like the prairie potholes in North America, there are often no ﬁsh and even the larger potholes only have two small species of minnows that can cope with low oxygen levels by aquatic surface respiration (Peterka 1989). Wetlands, especially those associated with lakes, can play an important role in the ecology of some ﬁsh species (Matthews 1998). Many ﬁsh lay their eggs in the wetlands fringing large lakes, sometimes on submerged plants. Juvenile often move into fringing wetlands during the day, presumably to avoid predators, and out of them at night when these zones become hypoxic or anoxic. Thus wetlands can be important refuges from predators. This is sometimes true for adult as well as juvenile ﬁsh. The introduction of a new predator, the Nile perch (Lates niloticus), into the lakes of the Lake Victoria basin in east Africa has resulted in the loss or major decline in population sizes of hundreds of indigenous ﬁsh species. Because the Nile perch cannot tolerate very low oxygen levels, they are normally not found in the Papyrus-dominated wetlands fringing these lakes and are also less abundant in transition zones (ecotones) between these wetlands and lakes. Consequently, some of the hypoxia-tolerant ﬁsh species that were eradicated from the lakes continue to be found in and at the edge of Papyrus swamps. For example, the lungﬁsh, Protopterus aethiopicus, was formerly abundant in lakes and adjacent wetlands. After the introduction of Nile perch, it was mostly found in fringing wetlands (Chapman et al. 1996). Although low oxygen levels are generally believed to make Papyrus swamps refugia for hypoxia-tolerant ﬁsh, it is also possible that these ﬁsh are able to survive in fringing wetlands because the plants make it more difﬁcult for the Nile perch to forage. When ﬁsh are found in wetlands they can compete with other wetland animals for food, especially with waterfowl, and their predation on larval forms (Lannoo 1996) may signiﬁcantly reduce the population size of some amphibians (e.g. frogs and salamanders). Some ﬁsh can also become major problems when they invade wetlands. One notable example of this is the European Carp (Cyprinus carpio) now found in many North American wetlands. Carp spawn in these wetlands in the spring and their spawning activities churn up wetland sediments. This greatly increases the turbidity of the water and uproots submerged macrophytes, often eliminating them from portions of the wetland.

WETLAND PLANTS AND ANIMALS 81

4.4

Amphibians, reptiles, and mammals Many amphibians and reptiles are found in wetlands but the number of species and their abundance varies considerably from wetland to wetland. Presumably because they are very low-nutrient ecosystems, peatlands appear to have the most depauperate fauna. Water chemistry, especially low pH, also seems to keep some amphibians out of some peatlands, that is, bogs. Large riverine wetlands generally have the greatest diversity of vertebrates, especially those in the tropics. The most common groups of amphibians and reptiles in freshwater wetlands are frogs, salamanders, alligators, crocodiles, turtles, and water snakes. Many are residents of wetlands for only part of their life cycle. For example, many frogs and salamanders use wetlands primarily for reproduction. They lay their eggs in wetlands, and their larval stages, which are aquatic, remain in them till they metamorphose into adults. The adults, however, spend most of their lives in terrestrial habitats. Because their eggs and larvae are food for many ﬁsh species, some amphibians do best in isolated wetlands that have few or no ﬁsh. Adult amphibians are often an important part of the diet of many carnivores animals in wetlands, for example, wading birds. Because amphibians and reptiles are poikilotherms that regulate their body temperature by getting in and out of the water, they do best in wetlands that provide places for them to sun themselves. Consequently, they are often most common along the edges of wetlands. Many reptiles, however, spend their entire lives in wetlands, for example, turtles and alligators. Crocodilians have the distinction of being the top carnivores in most tropical and subtropical wetlands around the world. Because they were nearly hunted to extinction, one of the best-studied crocodilians is the American alligator (Alligator mississippiensis) (Mazzotti and Brandt 1994). Alligators (Fig. 4.20) are amphibious and thus well adapted to wetlands. While in the water, alligators swim by sweeping their massive tails. On land they walk using their four short legs. They are dependent on water for feeding, courtship, and mating. Alligators and other crocodilians have numerous morphological and anatomical adaptations for living in wetlands (Grenard 1991), including elevated nostrils and eyes that protrude above the water when they are lying in wait or swimming just below the surface of the water. They can close their nostrils and auditory canals when they submerge. They also have a special eyelid called the nictating membrane that covers their eyes when they submerge. This membrane acts like built-in diving goggles and it enables the alligator to see under water. When under water, they close their glottic valve to prevent their lungs from becoming ﬂooded. This also enables them to swallow under water. Although they can remain submersed for longer periods, alligators rarely remain submersed for more than 10 or 15 min. Their circulatory system does have the

82 THE BIOLOGY OF FRESHWATER WETLANDS

Fig. 4.20

American alligator (Alligator mississippiensis). (Copyright Roger Hall.)

capability to shunt oxygenated blood to where it is most needed and de-oxygenated blood to organs where it is least needed. Changes in water levels from the wet to dry season have a major impact on alligator distribution. In the Everglades, during the wet season, male alligators normally are found in deeper, larger depressions while females and young alligators are found in smaller depressions. During the dry season, all alligators become concentrated in any remaining areas with standing water. Alligators are opportunistic feeders and will eat anything that moves and also carrion; the larger the alligator the larger its prey. Turtles, ﬁsh, snakes, birds, mammals, and even other alligators are all staples of their diets. On sunny days, alligators usually bask out of the water during the day and spend the night in the water. On cold or overcast days, they will extend the amount of time spent in the water. Of all reptiles, alligators have probably the greatest impact on the wetlands in which they live. They create depressions, alligator holes, which are often important refugia for ﬁsh, invertebrates, and other aquatic organisms, for example, turtles, during dry years when the wetlands have little standing water. They also create nests out of mounds of vegetation and soil that rise above the water in which they deposit their eggs; they guard these nests and recent hatchlings. Although there are many kinds of mammals (whales, dolphins, manatees, seals, sea lions, walruses, etc.) that are classiﬁed as aquatic (Howell 1930), surprisingly, with the possible exception of manatees in the Amazon Basin, none of these are found in freshwater wetlands. Freshwater wetlands are too shallow and too choked with emergent vegetation for these large mammals to utilize them. Only a couple of large amphibious mammals are associated with wetlands, the largest of which is the hippopotamus

WETLAND PLANTS AND ANIMALS 83

Fig. 4.21

Hippopotamus (Hippopotamus amphibious). (Copyright Roger Hall.)

(Hippopotamus amphibius) found in African wetlands (Fig. 4.21). Hippos spend most of the day in the water, primarily to stay cool. At night, they leave the water to graze on nearby grasslands. The wild water buffalo (Bubalus arnee) is the other. Today this is a very rare animal found in India, Thailand, and a few neighbouring countries. The domestic water buffalo (Bubalus bubalis), however, is common and found in Asia, Africa, and Europe and has been introduced into South America and Australia. Feral populations of the domestic water buffalo are sometimes found in tropical and subtropical wetlands in Asia and other parts of the world. Most of the amphibious and facultative mammals in wetlands are small. Typically they have long and narrow bodies and some webbing or other modiﬁcation of their feet to aid in swimming (Howell 1930). Most of them forage for food in freshwater wetlands, and they generally have their burrows or nests in them. Small mammals, mostly rodents and mustellids, that spend all or a good part of their lives in freshwater wetlands include muskrats (Ondatra zibethicus), capybara (Hydrochoerus hydrochaeris), nutria (Myocastor coypus), beaver (Castor canadensis), and Cape clawless otters (Aonyx capensis). In some wetlands, populations of muskrats can periodically rise to very high levels. When this happens, their feeding and lodge building activities can eliminate most of the emergent vegetation in a wetland (Fig. 4.22). The best documented examples of these ‘eat outs’ are for North American prairie potholes (Weller 1994). Many other large mammals use freshwater wetlands, but show little or no adaptations for living in wetlands (e.g. larger or longer hooves).

84 THE BIOLOGY OF FRESHWATER WETLANDS

75

300 Vegetation 200

50

100

25

1957 Fig. 4.22

100

Muskrat lodges

1958

1959

1960

1961

Percentage emergent vegetation

Total number of lodges

400

1962

Reduction in emergent vegetation in Goose Lake, a large prairie pothole in Iowa, USA, due primarily to an increase in muskrat population density. The number of muskrat lodges is highly correlated with muskrat population size. (From Weller and Spatcher 1965.)

Many of these are African ungulates (e.g. lechwe (Kobus spp.) and sitatunga (Tragelaphus spekei)) and they are normally not very abundant. However, large herds of herbivores in Africa and Asia are found often around the periphery of wetlands (Denny 1985). As water recedes with the onset of the dry season, the emergent vegetation of these wetlands is a major source of forage for large grazers in the wet–dry tropics. Finlayson and Moser (1991) provide a good overview of the various types of large mammals found in African wetlands and also in other parts of the world. Unfortunately, more often than not, these animals are declining in abundance as wetlands on which they depend shrink or disappear due to human encroachment.

4.5

Summary Plants and vertebrate animals have to overcome two major environmental constraints in wetlands, low oxygen levels in the water column and soils and ﬂuctuating water levels. Macrophytes have overcome these constraints either by the internal redistribution of gasses or by the direct exchange of gasses between roots and the atmosphere. Heterophylly and clonal growth allow macrophytes to cope with ﬂuctuating water levels. Macrophytes create a mosaic of microhabitats in wetlands that can be exploited by animals. Wetland birds such as waterfowl and wading birds have a variety of anatomical, morphological, and physiological adaptations that enable them to feed in wetlands. Although many ﬁsh avoid wetlands because of low oxygen levels in the water column and ﬂuctuating water levels, some ﬁsh are able to use wetlands during the day as refugia from predators. In subtropical and tropical wetlands in Africa, Asia, and South America, air-breathing ﬁsh are able to live in wetlands despite low oxygen levels in the water column. Amphibians and reptiles

WETLAND PLANTS AND ANIMALS 85

are common in wetlands, with some reptiles, alligators, and crocodiles, often being top predators. There are few large mammals that are restricted to wetlands with the hippopotamus being a notable exception. Many small rodents and mustellids, however, can be found in wetlands and some like the American muskrat can have a signiﬁcant impact on wetland vegetation when their populations are high.

4.6

Practical experiments and observations

4.6.1

Macrophyte adaptations The presence of macrophytes is one of the deﬁning features of wetlands. Macrophytes have evolved a variety of adaptations to ﬂooded conditions, especially anoxic soils. Submerged Aquatics. Collect as many species of submerged aquatics from a local wetland as possible. Identify these species and separate them into broad taxonomic groups, for example, monocots and dicots, based on their gross morphology. Try to collect roots and rhizomes, shoots and leaves, and ﬂowers and fruits. Make cross sections of rhizomes, shoots and, if possible, leaves of a representative species from each group and examine them under a binocular scope. Draw these cross sections and estimate the percentage of each organ that is air space. How do aquatic monocots differ from aquatic dicots in leaf morphology (shape)? What percentage of the organs of monocots and dicots are air spaces? What kinds of pollination syndromes (wind, insect, water) are found among submersed aquatics? Emergents. Collect leaves, rhizomes, and roots from some emergent species growing in shallow and deep water. Species of Typha, Sparganium, Scirpus (Schoenoplectus), and Sagittaria would be suitable. Make cross sections of them and examine these cross sections to see how much of each tissue is air space. Cut a piece of each organ about 10–15 cm long. Immerse it in a graduated cylinder full of water to determine its volume. Now ﬂatten it with a heavy weight or rolling pin and re-measure its volume. Estimate the percentage of the organ that is air space. How much of the leaf, rhizome, and root of emergents is air space? Do plants growing in deep water have a higher percentage of air space than plants in shallow water?

4.6.2

Seed banks Water-level ﬂuctuations are common in many wetlands and they can rapidly alter the composition of the vegetation. One way that species can survive adverse conditions caused by water-level ﬂuctuations is as

86 THE BIOLOGY OF FRESHWATER WETLANDS

seeds in the seed bank. The seed bank is all the viable seed found in the surface soil at any given time. Collect enough of the surface soil (upper 5 cm or so) from several vegetation zones in a wetland to ﬁll two small trays (ca. 25 cm 25 cm). Record the plant species growing around each sampling site. Mix the soils from each location thoroughly and ﬁll two trays with it. Place these trays in a sunny location. Keep the soil in one tray moist and keep the soil in the other tray ﬂooded. The ﬂooded trays can be placed in aquaria or in a wading pool. After several months identify and count the plants that have become established in each tray. How many species were found in trays from each vegetation zone? How does ﬂooding affect seed germination? Do all of the dominant species in each vegetation zone have seeds in the seed bank? Are there species in the seed bank that were not present in the vegetation?

4.6.3

Wetland birds Many bird species use wetlands. Some of these species are restricted to wetlands while many others are casual visitors. Identify and record the birds seen in different vegetation types in a wetland. This will require visiting the wetland a number of times and at different times of the day. Make a list of all the species seen in each wetland zone. If feasible, several different wetlands could be surveyed. What were the most common species in each vegetation type? What were they doing (nesting, feeding, loaﬁng, mating)? How much overlap in bird use is there among vegetation types?

4.6.4

Wetland ﬁsh Fish may or may not be common in wetlands. When they are common, they are often important food items for wading birds. Place ﬁsh traps in two or more ﬂooded areas with different kinds of vegetation during the day and at night. Many kinds of ﬁsh traps have been developed and the most appropriate design will depend on the size of the ﬁsh in the wetland and the kind of vegetation in which the traps will be deployed. Collect these traps after several hours or more and identify the ﬁsh in them. In wetlands with high turtle populations, precautions will be needed to prevent turtles from entering the traps. What species of ﬁsh are present? Are they mostly juvenile ﬁsh or adults? In which vegetation types were ﬁsh most abundant during the day or at night? What organisms might be eating these ﬁsh?

5 Spatial and temporal patterns

Wetlands normally have a variety of vegetation types or plant communities arranged in readily observable zones or patches that are dominated by different growth forms and that occur at a variety of scales (Fig. 5.1). Although often occurring in fairly simple zones along transects within a wetland, when mapped in detail, the vegetation of E144 21⬘ 45⬙ E144 21⬘ 40⬙ N43 06⬘ 18⬙

N43 06⬘ 15⬙

N43 06⬘ 12⬙

N43 06⬘ 08⬙ N E

W 0m Fig. 5.1

50

100

S

Detailed vegetation map of the Akamuma marsh, which is part of the Kushiro wetland, in northeastern Japan. Ten different plant communities are recognized and most of these have a number of compositional variants. (From Miyamoto et al. 2004.)

88 THE BIOLOGY OF FRESHWATER WETLANDS

wetlands usually has complex distributions because of slight topographic irregularities, small-scale disturbances, and the patterns of plant dispersal (Fig. 5.1). At the macroscale (Fig. 5.2), the most widespread plant communities form the matrix within which microscale plant communities are often embedded. At the microscale, micro-topographic relief, for example, hummocks in Sphagnum-dominated peatlands (Fig. 5.3), or openings created by disturbances, for example, animal feeding activities, allow species of both plants and animals to persist that otherwise would not be present in the macroscale plant community (Wheeler 1999). A series of plant communities found along an environmental gradient are referred to as a coenocline. Coenoclines can be found at both the macro- and microscale in wetlands (Figs 5.2 & 5.3). Coenoclines are not stable, and they are constantly adjusting to changes in abiotic or biotic conditions. Sometimes these changes are due to the growth of plants from seedlings to adults. This kind of change is especially important in forested wetlands. Herbaceous species from one year to another can adjust the number or size of individual plants or culms depending on whether environmental conditions have improved or deteriorated. As a result of a major change in environmental conditions, Phragmites karka Cyperus papyrus

Typha domingensis

Vossia cuspidata Eichhornia crassipes

Najas pectinata

Submerged and free floating vegetation Fig. 5.2

Anchored with floating rafts. Emergent vegetation

Rooted emergent plants Pools with submerged and floating-leaved vegetation

Cross-section showing the major vegetation zones (not to horizontal scale) in a wetland along the Upper Nile in the southern Sudan. Redrawn from Armentano (1990) based on a ﬁgure in Denny (1984).

SPATIAL AND TEMPORAL PATTERNS 89

species may be killed or new species may become established along a coenocline. For example, in prairie potholes, during prolonged periods of high water, emergent species may be extirpated along some portions of the coenocline. During drawdowns in the same wetlands, however, emergent species can become established or re-established (van der Valk and Davis 1978; van der Valk 2000). At the plant community level, these population responses produce three kinds of changes: maturations, ﬂuctuations, and successions (Fig. 5.4). These have been deﬁned operationally for perennial plant communities (van der Valk 1985, 1987) as: maturation: an increase in the biomass in area from one year to another due to the growth of plants from seedlings to the adults; ﬂuctuation: a change in the relative abundance of herbaceous species in an area from one year to another in response to a change in environmental conditions; micro-succession: a change in the species composition in an area from one year to another due to the establishment of new species or extirpation of established species or both. It is common for two or all three to occur simultaneously. For example, maturation may be accompanied by succession as some species are

Height (cm)

75

Hummock pH 2.7–3.1

Lawn intermediate pH

Hollow pH 3.4–4.0

50

25 maximum Water table 0

minimum

Mosses:

Sphagnum fuscum Sphagnum capillifolium Polytrichum strictum Aulacomnium palustre Pleurozium schreberi Dicranum undulanum Pohlia nutans

Sphagnum magellanicum Sphagnum capillifolium Sphagnum angustifolium Sphagnum rubellum Sphagnum papillosum

Sphagnum cuspidatum Sphagnum majus Sphagnum fallax Sphagnum angustifolium Sphagnum pulchrum Warnstorfia fluitans var fluitans Cladopodiella fluitans

Vascular plants:

Rubus chamaemorus Kalmia angustifolia Ledum groenlandicum

Carex oligosperma Eriophorum angustifolium Eriophorum virginicum Scirpus cespitosus Andromeda glaucophylla

Carex limosa Carex oligosperma Eriophorum virginicum Rhynchospora alba Utricularia cornuta Andromeda glaucophylla

Fig. 5.3

Micro-relief in an eastern Canadian peatland. (From Faubert (2004) and based on a ﬁgure in Campbell and Rocheford (2001.)

90 THE BIOLOGY OF FRESHWATER WETLANDS

extirpated and other species become established. Animal populations will respond to changes in coenoclines, but they may also move both within and among wetlands for other reasons such as changes in seasons or habitat requirements over their life cycles. In this chapter, we will examine (1) the characteristics and development of spatial patterns (coenoclines); (2) temporal changes in wetland vegetation (ﬂuctuation and succession); and (3) differentiation of wetland landscapes.

A

B

C

Year one Fig. 5.4

Year two

Types of community-level vegetation changes that can occur along wetland coenoclines from year to year: A, micro-succession; B, maturation; and C, ﬂuctuation. (Adapted from van der Valk 2005.)

SPATIAL AND TEMPORAL PATTERNS 91

5.1

Spatial patterns Field and experimental studies suggest that in recently created or restored wetlands all the zones expected to develop along a wetland coenocline begin to develop simultaneously (van der Valk and Bliss 1971; Seabloom and van der Valk 2003). In other words, emergent species become established at the edge of the wetlands while at the same time submerged species become established in the deepest areas. All the zones in a coenocline may not develop at the same rate because of the differences in propagule dispersal rates. After zones are established they may undergo many changes in composition due to both internally (autogenic) and externally (allogenic) induced changes in environmental conditions. Although vegetation zones along wetland coenoclines often appear visually to be very distinct entities, in reality their boundaries are often very diffuse. Zones in wetlands appear distinct because they are typically dominated by one species or several species with the same growth form while adjacent zones are dominated by one or more species with different growth forms. A variety of studies have shown that, as is the case with terrestrial coenoclines, each species along a wetland coenocline has a unique distribution (Fig. 5.5). The zones that are seen are simply sections of the coenocline where the density of species with the same growth form is highest. Because transitions from species with one growth form to another are often fairly abrupt, this creates the impression that these zones are distinct entities. In his classic work, ‘The zonation of plants in freshwater lakes’, the late D. H. N. Spence (1982) identiﬁed water depth as the main factor controlling the distribution of macrophytes along coenoclines. Water depth or elevation has often been used as a surrogate for water regime in wetlands that do not undergo signiﬁcant seasonal or interannual waterlevel ﬂuctuations. In riverine wetlands (Haslam 1978; Dawson 1988) and

Importance value

300

1

2 2

3

200

5

6

100 7

4 0 100

Fig. 5.5

200

300 400 Metres

500

600

700

Distribution of species along an elevation gradient in a swamp forest. Species are 1. Larix larcina, 2. Thuja occidentalis, 3. Ulmus americanus, 4. Fraxinus nigra, 5. Acer saccharinum, 6. Acer saccharum, and 7. Fraxinus americanus. (Adapted from van der Valk 1982.)

92 THE BIOLOGY OF FRESHWATER WETLANDS

many peatlands (Wheeler 1999; Charman 2002), water velocity gradients can also result in the development of coenoclines (Fig. 5.6) as can water chemistry gradients (Fig. 5.3) in all kinds of wetlands. Many other factors, however, can affect plant distributions along coenoclines. These include light extinction for submerged plants, sediment characteristics (physical and chemical), competition among the plants, and competition between submerged plants and algae for light. Competition among emergent plants has been investigated experimentally, and these studies suggest that the depth distribution of species along coenoclines seems to be more restricted than would be suggested by the studies of their waterdepth tolerances when grown alone (Wheeler 1999). Gopal and Goel (1993) have also suggested that chemicals released by plants (allelopathy) can inhibit the seed germination or growth of other plants. Other biotic factors that could limit the distribution and abundance of plant species are herbivores and plant pathogens. These can both decimate local populations of wetland species. Finally, dispersal problems may prevent some species from occurring along a speciﬁc coenocline even though conditions for it are suitable. Although most wetland coenoclines develop along water regime, velocity, and/or chemistry gradients, the location of species along coenoclines, even within one wetland, can vary signiﬁcantly (Wheeler 1999). Nevertheless, several models have been developed that can successfully predict the distribution of species in wetlands (Seabloom et al. 2001). Coenoclines in peatlands have many parallels to those in other types of wetlands. Although there can be water depth or water regime gradients in peatlands, water chemistry gradients, as noted in Chapter 2, are the main environmental gradient producing coenoclines in peatlands (Charman 2002). These water chemistry gradients may have different proximate causes (groundwater inputs, nearness to the edge of the peatland, composition of the vegetation, etc.), but they result in a pH, fertility, and/or base richness (calcium, magnesium) gradient that is strong enough to affect plant establishment, survival, and growth (Fig. 5.3). Although water chemistry gradients in peatlands are often described as pH gradients, this may be more because pH is relatively easy to measure than nutrient concentrations. pH is often correlated with other features of peatland water chemistry. In fact, nutrient gradients may be more important in structuring some peatland coenoclines than pH per se. Water ﬂow can also affect zonation patterns in peatlands (Wheeler 1999; Charman 2002), and this will be discussed in the Section 5.3.

5.1.1

Coenocline development Although many studies of the distribution of plant species along coenoclines in wetlands have been done, there have been very few studies

Increasing velocity Fig. 5.6

Distribution of plants in shallow and deep ﬂowing water. Water velocity increases from slow to fast away from the bank. (Redrawn from Dawson 1988.)

94 THE BIOLOGY OF FRESHWATER WETLANDS

of the development of coenoclines. The best opportunities to study wetland coenocline development in detail have been provided recently by restored wetlands (Middleton 1999) and by studies of wetlands that undergo cyclical changes in their vegetation due to wet–dry cycles (Fig. 5.7). In both cases, a coenocline develops along an environmental gradient that previously supported a coenocline. Unlike during primary coenocline development, secondary coenocline development occurs in areas with wetland soils and the seeds of many plant species may already be present. The presence of viable seeds in the seed bank along the developing coenocline eliminates many of the vagaries of seed dispersal that can occur during the development of primary coenoclines. One of the most detailed studies of secondary coenocline development was done in the Delta Marsh in Manitoba, Canada. In a series of large impoundments in the Delta Marsh, a long-term experimental study was conducted of the impacts of water-level ﬂuctuations on prairie wetlands (Murkin et al 2000). The existing coenoclines (Fig. 5.8) in these impoundments were destroyed by raising water levels 1 m for 2 years. These coenoclines were composed primarily of four emergent zones: wet meadow (Scolochloa festucacea or Phragmites australis), shallow emergent (Typha spp.), deep emergent (Scirpus [Schoenoplectus] Dry marsh

Regenerating marsh Normal Rainfall

Mudflat

Drought or drawdown

Mudflat emergent

Submersed and free-floating

Marsh seed bank

Submersed and free-floating Submersed and Submersed and free-floating free-floating

Emergent submersed and free-floating Senescence disease insects high water Emergent submersed and free-floating

Muskrat Damage Lake marsh Fig. 5.7

Seed inputs Seed germination

Degenerating marsh

Changes in coenoclines during the four stages of an idealized wet–dry cycle in a semipermanent prairie pothole. (Redrawn from van der Valk and Davis 1978.)

SPATIAL AND TEMPORAL PATTERNS 95

lacustris), and a submerged aquatic zone. During the subsequent drawdown, a new coenocline developed due to the re-establishment of emergent species. After the drawdown, the impoundments were reﬂooded for 5 years during which time the development of the new coenocline continued to be monitored (van der Valk 2000). Many factors (seed distribution, seed germination, ﬂooding tolerances, etc.) could affect the position of emergent species along the new coenocline (Fig. 5.9), and the relative importance of the following factors needed to be assessed: 1. Seed dispersal patterns: Since this is secondary coenocline development, the distribution of seeds in the seed bank could reﬂect the distribution of species on the coenocline prior to its destruction by high water. In other words, the coenocline that re-develops is a result of seed distribution patterns and nothing more (Fig. 5.7). 2. Seed germination patterns: The seeds of all species are evenly distributed along the entire coenocline, but differential seed germination causes seeds of different species to germinate along different portions of the gradient. 3. Seedling survival patterns: If seedling are evenly dispersed along the coenocline, zones could form because of differential seedling survival during the drawdown. 4. Adult ﬂooding tolerances: When the standing water returns, species not able to tolerate being ﬂooded for long periods or species growing at water depths too deep or shallow for them to tolerate will be extirpated from parts of the coenocline, creating zones.

Upland

Water levels Spring Summer

Phragmites Whitetop Cattail

Bulrush

Pondweed Open water

Fig. 5.8

Typical coenocline in the Delta Marsh, Manitoba, Canada. Phragmites Phragmites australis, Whitetop Scolochloa festucacea, Cattail Typha glauca, Bulrush Scirpus [Schoenoplectus] lacustris, and pondweed Potamogeton spp. (Redrawn from Batt 2000.)

Similar seed bank 1 2 Differential seed germination 3

Different seed banks 1 Seed germination

ffe

Di ial

nt

re

Similiar seed generation

al viv ur

gs

lin

ed

se

4

Exploitative competition

5

l

a viv ur ts ul ing ad od ial lo nt ef re er r ffe aft

Di

Similiar seedling survival

Adult survival unaffected by reflooding Fig. 5.9

Possible pathways for the development of a secondary coenocline and the data needed to determine which pathway(s) play an actual role. (Redrawn from van der Valk and Welling 1988.)

SPATIAL AND TEMPORAL PATTERNS 97

5. Exploitative competition: Flooding per se may not be enough to eliminate species from the coenocline, but competition among species may eliminate species from portions of the coenocline where they are not the best adapted to environmental conditions. In the Delta Marsh, each species eventually became re-established along the new coenocline for different reasons (van der Valk 2000). Differential seed dispersal patterns had some inﬂuence, especially for Scolochloa whose seeds were most common at the upper end of the coenocline. Differential seed germination and seedling survival had some inﬂuence with Scirpus and Typha seedlings at their highest density at the lower end of the coenocline. The re-ﬂooding of the marsh resulted after several years in the elimination of adults of various species from parts of the coenocline at water depths that they could not tolerate. This allowed populations of species that were able to tolerate these water depths to expand into open sections of the coenocline. In the Delta Marsh, there was no indication that exploitative competition played any major role in the development of the coenocline. The ﬁnal position of species along the coenocline was typically the result of a combination of factors (seed distribution patterns, seedling survival patterns, and adult ﬂooding tolerances). Of these, however, adult ﬂooding tolerance was the most important and largely obliterated the initial distribution patterns created by differences in seed distribution, seed germination, and seedling survival. In short, this study supported Spence (1982) conclusion that water depth is the single most important factor controlling the distribution of plants in wetlands with standing water.

5.1.2

Fire Fire is common in wetlands during seasonal or interannual dry periods, and ﬁres often alter coenoclines. Although ﬁres occur in many kinds of wetlands, including prairie potholes, ﬁre frequency and extent have rarely been documented in any wetland. One notable exception is the Florida Everglades where ﬁre records have been kept by a variety of state and federal agencies with management jurisdiction of various parts of the Everglades. The majority of ﬁres in the Everglades occur, as expected, during the dry season. Lightning and humans are the major sources of ignition. April and May at the end of the dry season are the months with the largest burns. Besides an annual season cycle in ﬁre frequency, there is also a longer 10–14 year cycle that corresponds with periodic major droughts in South Florida. Fire in the Everglades re-sculpts the wetland surface and, in effect, alters local coenoclines by creating deeper water areas. The landscape-level signiﬁcance of ﬁres in wetlands is still poorly understood, but ﬁre clearly has an impact on both the short- and long-term distribution and abundance of plants and animals along coenoclines.

98 THE BIOLOGY OF FRESHWATER WETLANDS

5.2

Temporal patterns In wetlands, temporal changes can occur over different timescales. Long-term changes over hundreds or thousands of years are normally accompanied by permanent changes in a wetland’s coenoclines due to the deposition of sediment or peat. These long-term changes will be referred to as macro-successions. Superimposed on these macrosuccessions are often short-term changes over a few years or decades that are caused by reversible changes in environmental conditions such as ﬂuctuations in water levels during wet–dry cycles. These microsuccessions often slow down macro-successions because organic matter accumulation may be reduced or even negative during microsuccessions that involve drawdowns. The development of a new secondary coenocline in the Delta Marsh described in the previous section is an example of a micro-succession.

5.2.1

Macro-succession Most studies of macro-succession have examined the development of the vegetation in one place, usually in the deepest part of a wetland basin. Although these studies are informative, they provide little insight into the regularity or predictability of macro-successions. When comparable multiple sites are examined, the kinds of plant communities that were present over time in similar wetlands can vary considerably. For example, Walker’s (1970) study of the development of East Anglian peatlands examined the development of the vegetation at multiple sites and compared actual changes in plant communities over time to the predicted developmental sequence. The predicted pattern in East Anglia was algae, submerged aquatic communities, ﬂoating-leaved communities, emergent communities (reedswamp), fen/bog, shrub (carr), and ﬁnally terrestrial or dry woodlands. In reality, there were numerous variations on this theme at different sites. Some plant communities did not occur at all at some sites, communities sometime showed up in the wrong order, and no dry woodland ever developed at any site. Similar patterns have been observed in other studies of macro-succession such as those in Estonian peatlands (Fig. 5.10). These studies suggest that timing of arrival of propagules may play a signiﬁcant role in the kinds of communities that are found at any given site during macro-succession. Macro-successions vary from site to site within and among wetlands just like coenoclines. Macro-succession is often described as terrestrialization; that is, the wetland becomes shallower with time as it ﬁlls with sediment or peat and eventually the wetland turns into dry land. This is not what normally happens, especially for peatlands. Terrestrialization has not

SPATIAL AND TEMPORAL PATTERNS 99

0.45

Bryales

Carex/Bryales 0.4

0.10

Phragmites 0.26

0.1 4

0.10 0.16

Betula/ Carex

0.22 0.15

20

6

19

0.

0.1

0.42

0.36

Eriophorum/ Sphagnum

Calluna/ Eriophorum 0.2 6

0.23

2

0.5

3

0.3

Pinus 0. 25

13

0.

Mixotrophic peat

Sphagnum

4

Betula/ Pinus

0.28

0.1

0.

Carex/ Sphagnum

Carex

0.27

0.17 0. 15

13 0.

Minerotrophic peat

0.19

0.1 8

9

Carex

0.26

8

0.2

Betula or Phragmites

0.13

0.3

Betula/Salix or Betula/Picea 0.1 3

0.39

Phragmites/ Carex 0.14

Sphagnum

0.23

7

Betula or Salix/grass 0 .1 3 0.25

0.19

Carex/Sphagnum

Sphagnum magellanicum

Calluna/ Sphagnum 0.10

0.40

0.6

8

Sphagnum fuscum Ombrotrophic peat

0.34

0.33

Complex (Mix of hummock and hollow species) 0.36

0.66

Hollow Fig. 5.10

Macro-successional changes in Estonian peatlands. The number beside each arrow is the probability of that transition from that plant community to another. (From Charman 2002 based on data from Ilomets et al. (1995))

happened in the East Anglian fens, in prairie potholes, in the Everglades, nor in most other wetlands. Over geological timescales, losses of wetlands were most often caused by changes in climate, down cutting by rivers, or sea level changes. In the last few thousand years, losses of wetlands have been due to human activities, primarily drainage and peat cutting (Dugan 1993).

100 THE BIOLOGY OF FRESHWATER WETLANDS

Wetlands are located at low spots in the landscape and they continue to collect or channel water even as they ﬁll in with allochthonous or autochthonous sediment. In most landscapes, wetlands are of the same age or older than the terrestrial vegetation around them. In fact, it is possible for uplands to turn into wetlands (Fig. 5.11), a process known as paludiﬁcation (Wheeler 1999; Charman 2002). Paludiﬁcation is the result of a change in local hydrology that results in the development of waterlogged soils. This can occur for many reasons, including a decrease in evapotranspiration due to the destruction of forests by deforestation and possibly by ﬁre. One common mechanism is podsolization, that is, the leaching of iron and aluminium oxides from the upper layers of the soil downward where they are deposited in a horizon that becomes increasingly less permeable to water. In many parts of the boreal zone in North America and Eurasia, podsolization has resulted in an increase in soil moisture levels and a decrease in soil pH. This allows Sphagnum species to become established. The establishment of a Sphagnum layer further inhibits drainage and results in more waterlogging because of a reduction in litter decomposition rates due to lower redox levels and (a)

(b)

(c)

(d)

(e)

Plant communities

Aquatic community Water level

Clay Fig. 5.11

Gyttja Open Gyttja water macrofossils

Swamp forest

Bog mat

Wood peat

Shrub peat

Lateral expansion as of two peatlands into one as a result of paludiﬁcation of the upland originally separating them. (Redrawn from Anderson et al. 2003.)

SPATIAL AND TEMPORAL PATTERNS 101

lower pHs. This positive feedback loop, without any change in climate, results in the development of a peatland in areas that were previously dry land (Anderson et al. 2003). Because of continued paludiﬁcation along the edges of the newly formed peatlands, peatlands can expand laterally to form raised or blanket bogs that historically covered large areas in northern Britain, Scandinavia, and Canada (Crawford et al. 2003). As these peatlands spread laterally, their vegetation becomes increasingly more locally differentiated into zones due to the development of topographic irregularities, water chemistry gradients, and ﬂow ways on the surface of the peat (see Section 5.3).

5.2.2

Fluctuation versus micro-succession As noted, the most common cause of changes along coenoclines is interannual water-level ﬂuctuation. However, in some wetlands such as monsoonal wetlands in India even annual changes in water levels can be so large that coenoclines change seasonally (Middleton 1999). In most wetlands, seasonal water-level ﬂuctuations seem to have no effect on their coenoclines. The magnitude of interannual water-level ﬂuctuations determines the kind of vegetation change observed (Wheeler 1999; van der Valk 2005): small ﬂuctuations—no detectable change; medium ﬂuctuations—ﬂuctuations; and large ﬂuctuations—micro-successions. In many wetlands, small interannual water-level ﬂuctuations do not appear to cause any detectable change in the distribution or composition of plant communities along a coenocline. Most wetland studies are short (3 years). The assumption is often made that the coenoclines in the wetlands studied for such a short period are stable. This is questionable. It can take several years for plant species to respond to a waterlevel change, and many species can be found growing in wetlands under suboptimal conditions along coenoclines (Wheeler 1999). Because responses to small water-level ﬂuctuations are not expected, more subtle responses to them, for example, number of ﬂowers produced or seed produced per ﬂower, have not been investigated. Consequently, little is known about the minimal magnitude of water-level ﬂuctuations that alter plant growth along coenoclines. One of the best-studied examples of the effects of water-level ﬂuctuations on coenoclines, however, are for prairie potholes in North America (van der Valk 1989; Murkin et al. 2000; Euliss et al. 2004). The magnitude of water-level ﬂuctuations depends on the amount of annual precipitation and the type and degree of connectivity to local groundwater ﬂows (Euliss et al. 2004). In prairie potholes, plant communities along coenoclines can undergo either ﬂuctuations or successions depending on the magnitude of interannual water-level ﬂuctuations. If the range of

102 THE BIOLOGY OF FRESHWATER WETLANDS

interannual water-level ﬂuctuations is small (ca. 50 cm or less), then fluctuations, changes in the relative abundance of species, are the primary kind of change observed along coenoclines. If the range is large (ca. 100 cm or more), then successions, changes in species composition, occur. Along a coenocline in a prairie pothole with large interannual water-level ﬂuctuations, however, plant communities at the upper end of the elevation gradient, which experience a smaller range of water levels than those at lower elevations, can undergo ﬂuctuations while those at the lower end simultaneously undergo micro-successions (van der Valk 2005). The most common kind of micro-successional change is a reduction in width or loss of emergent zones (Fig. 5.7). Because their seeds will not germinate underwater, many emergent species cannot become reestablished until the next drought when the wetland goes dry. Wetlands without emergent zones are described as being in the lake stage. It takes several years for emergent zones to fully redevelop after the dry marsh reﬂoods. This is called the regenerating marsh stage. Changes in the magnitude of water-level ﬂuctuations from one wet–dry cycle to another can result in the development of unusual zonation patterns. Inverted zones, duplicated zones, and missing zones are all possible. These changes in vegetation are always accompanied by changes in invertebrates, amphibians, muskrats, and birds (Murkin et al. 2000; Euliss et al. 2004).

5.3

Development of wetland landscapes The considerable impact of some animal species on wetlands is well known and several examples were noted earlier in this chapter. Overgrazing is the most common short-term impact. High population densities of muskrats (Ondatra zibethicus) and nutria (Mycastor coypus), two aquatic rodents, can ‘eat out’ or eliminate the emergent vegetation in wetlands. However, once muskrat or nutria population sizes have declined, emergent species usually become re-established. Sometimes overgrazing can have more long-term consequences. In North America, the increase in snow geese population during the last three decades of the twentieth century had a signiﬁcant impact on coastal wetlands in their breeding grounds around Hudson Bay in Canada. Snow geese feed on grasses and sedges. After grass and sedge shoots have been consumed, their roots and rhizomes become the geese’s primary food. Excessive grubbing has destroyed the vegetation in large areas of coastal marsh. Because of increased evaporation from bare sediments, the destruction of the vegetation results in increased soil salinities. Increased soil salinities in turn prevent the re-establishment of grasses and sedges (Srivastava and Jefferies 1996; Handa et al. 2002). Geese are not the only animals responsible for macro-successions in wetlands.

SPATIAL AND TEMPORAL PATTERNS 103

Dams built on streams and rivers by the American beaver (Castor canadensis) can convert sections of them into a series of shallow reservoirs. As they ﬁll up with sediment, many of these shallow reservoirs are colonized by wetland plants, including emergent plants and even on occasion peat forming species. Beavers will abandon these sites as they become wetlands, and this often results in the eventual washing out of the unmaintained dams and the wetlands that formed behind them. However, in some lowgradient areas where ﬂow is not sufﬁcient to scour away abandoned dams, permanent wetland or peatland complexes can develop. In the Everglades alligators (Alligator mississippiensis) excavate depressions, appropriately called alligator holes (Fig. 5.12), that retain water during the dry season and thus become important refugia for ﬁsh, reptiles, amphibians, and aquatic invertebrates that are in turn preyed on by wading birds. Even the vegetation in and around alligator holes is different from that of the surrounding area. The hole itself is kept free of vegetation by the alligator, and a ring of emergents and shrubs develops on the soils pushed up around the hole (Palmer and Mazzotti 2004). Hippos (Hippopotamus amphibius) seem to play a similar role in some African wetlands by keeping areas free of vegetation. The movement of hippos to feeding grounds results in the development of open channels within these wetlands, and these channels can alter water ﬂow patterns and vegetation distribution (McCarthy et al. 1998). Because any increase in topographic diversity can increase wetland biodiversity (Vivian-Smith 1997), even small changes in local topography created directly or indirectly by animals or plants are important in wetlands. In a number of large wetlands around the world, the focused redistribution of nutrients and other materials by plants and animals can result in the establishment and growth of tree islands (Wetzel 2002). These tree islands develop around some point of formation, that is, a 4x vertical scale 2 0

4

6

8

North to south transect (m) 10 12 14 16

18

20

22

24

26

Depth (cm)

20 40 60 80 100 120 0 50 100 150 True scale Fig. 5.12

Musk

Water

Transects of two alligator holes in the Everglades. (From Campbell and Mazzotti 2004.)

104 THE BIOLOGY OF FRESHWATER WETLANDS

local topographic high spot. This differentiation of wetland landscapes, which can take hundreds or thousands of years, has been best studied in the Okavango Delta, one of Africa’s largest wetlands. In the Okavango, the point of formation of many tree islands is a termite mound. Colonies of the mound-building termite (Macrotermes michaelseni) build subterranean nests during prolonged dry periods in seasonally ﬂooded areas (Fig. 5.13). These nests are eventually crowned by a turret connected to the nest by a series of air passages. These turrets or mounds can be 4 m tall and have a basal area of 50 m2 (Dangerﬁeld et al. 1998). Termite colonies redistribute resources by moving ﬁne soil particles to their nests to build up the mound, thus changing the local topography and soil textural properties of the wetland. Termites also carry organic matter to their nests, thus redistributing nutrients locally. Because of their elevation above the ﬂoodplain, termite mounds can be colonized by tree species. During the wet season, the termite colonies may be killed. However, any trees on the mounds survive because they are growing above the mean water level. The presence of trees attracts birds and other animals that carry seeds and nutrients to the newly formed island (Fig. 5.13). Consequently, tree islands become biodiversity hotspots that are colonized or used by a variety of plant and animal species. Trees on the islands have roots that reach the groundwater. Because of their evapotranspiration, these trees act as pumps that remove large quantities of groundwater from underneath the islands, but leave the dissolved mineral salts in the groundwater (Fig. 5.14). Thus groundwater salinity beneath the islands increases from the edge of the island to the centre. The less soluble compounds (e.g. calcium carbonate) precipitate (as calcite) around island perimetres, causing the islands to expand, Flat floodplain subject of periodic flooding

Ungulate herbivory

Several years without flood allows termite colony to establish

Plant succession

Next flood kills colony

Carbonate deposition

Mound erodes and relief created

Island ‘grows’

Favoured for subsequent colonization Fig. 5.13

Landscape development

Model of the development of tree islands in the Okavango Delta, Botswana. See text for details. (From Dangerﬁeld et al. 1998.)

SPATIAL AND TEMPORAL PATTERNS 105

while the more soluble salts (e.g., NaHCO3) collect in the centre of the islands. Most of an island’s growth is lateral rather than vertical. On some islands in more permanently ﬂooded areas, the accumulation of salts in the groundwater beneath the centres of the islands can kill the trees (McCarthy and Ellery 1994). These processes result in a characteristic vegetation zonation pattern on these islands (Fig. 5.14) with tall dense woody vegetation along the water’s edge that decreases in stature and diversity towards the centre of the island until it gives way to herbaceous species or even bare ground (Ellery et al. 1993). In addition to island growth due to transpiration and precipitation of solutes, the trapping of dry fallout by trees on these islands also contributes to their growth directly (Krah et al. 2004) as well as indirectly through the accumulation of nutrients. Dry deposition becomes a more important mechanism for island growth as islands get larger. As they grow laterally, tree islands can coalesce into larger island platforms. It has been hypothesized that after the islands coalesce in an area, new channels will form on these large platforms, initiating a new cycle of wetland formation (Gumbricht et al. 2004). At the scale of the entire wetland, these islands collectively also have a signiﬁcant impact Transpiration

Transpiration

Evaporation

Salt crusts

Swamp

Swamp Capillary fringe Water table Groundwater flow

Swamp vegetation

Fig. 5.14

Phonix reclinata

Groundwater flow

Salin ground water

Broad leaved evergreen species

Deciduous trees

Hyphaene petersiana

Cynodon Sparobolus doctylon spicotus

Mature tree island in the Okavango Delta, Botswana. Evapotranspiration from trees pumps groundwater into the islands from the surrounding swamp. This results in the precipitation of calcite from the groundwater under the islands that results in the growth of the islands both vertically and horizontally. Saline water accumulates under the centre of the islands, and this results in the development of a salt panne in the middle of the islands. (Redrawn from Ellery et al. 1998.)

106 THE BIOLOGY OF FRESHWATER WETLANDS Elongate tree-island

D

C

B

Sawgrass strand A

Peat

Limestone Fig. 5.15

Model of the development of tree islands on sawgrass strands or ridges in the Everglades. Island is shown in cross section. Water ﬂow is perpendicular to island. A sawgrass strand or ridge that develops because peat accumulation rates are higher than in slightly drier areas. B. As peat accumulates ridge ferns become established. C. Highest portions of ridge, for example, fern tussocks, become colonized by trees. D. Forest develops that is still seasonally ﬂooded during rainy season. (Redrawn from Stone et al. 2002.)

on the water chemistry of the Okavango. Dissolved mineral salts are sequestered in tree islands, so the accumulation of mineral salts in the surface water of the wetlands is minimized, and this has prevented the Okavango from becoming a saline wetland. Because annual rainfall is less than 500 mm and potential evapotranspiration is 2000 mm, this wetland without tree islands could have extensive areas of saline surface water.

SPATIAL AND TEMPORAL PATTERNS 107

The development of tree islands in wetlands because of the focused redistribution of materials directly or indirectly by animals and plants is not limited to the Okavango. Tree islands are found in a variety of large wetlands around the world, including the Everglades (Wetzel 2002). In the Everglades as well as in many other peatlands, differential accumulation of peat produces topographic irregularities that alter water ﬂow patterns (Fig. 5.15). With time as some areas become higher and drier and others lower and wetter, these differences increase in magnitude creating ridges and sloughs. Ferns and eventually shrubs and trees can invade these ridges creating tree islands that can grow in size because they differentially accumulate nutrients.

5.4

Summary Wetlands are often a complex mix of vegetation types. The ever-changing vegetation along a hydrologic or other gradient is called a coenocline. Wetland vegetation is very dynamic and temporal changes in vegetation are common, especially in wetlands dominated by herbaceous species. Maturation is the increase in biomass from year to year due to the plant growth. Fluctuation is a change in the abundance of species from year to year caused by changes in environmental conditions. Micro-succession is a change in species composition from year to year. The distribution of species along coenoclines is dependent on many factors, including seed distribution, seed germination patterns, seedling survival, and adult ﬂooding tolerances. Permanent changes in wetland vegetation, macro-succession, can also occur due to changes in environmental conditions. The fauna and ﬂora of wetlands can alter them in several ways. Alligators excavate holes in wetlands that contain water during the dry season. These holes become refugia during the dry season for amphibians, other reptiles, ﬁshes, and many invertebrates. Beavers can turn riverine wetlands into palustrine wetlands. Animals and plants are also involved in the formation of tree islands in many large wetlands.

5.5

Practical experiments and observations

5.5.1

Gradient analysis Plant species in wetlands are found growing along water-depth gradients. One way to determine the water-depth tolerances of different emergent species is to determine the depths at which they are growing. In theory, competition is supposed to prevent similar species from overlapping in their distributions.

108 THE BIOLOGY OF FRESHWATER WETLANDS

In a wetland or a number of similar wetlands, establish multiple transects across the emergent zone from shallow to deep water. Along the transect measure the water depth at which a given species is ﬁrst found and last found. As many transects should be done as possible. For each species plot its minimum and maximum distribution along each of the transects. For each species, calculate the mean minimum depth and the shallowest and deepest minimum depths at which it was found along all the transects. Also calculate their mean maximum water depth and their shallowest and deepest maximum water depths. Is a species always found at the same range of water depths along each transect? Rank the species by their mean minimum water depths. Rank the species by their mean maximum water depths. Are these rankings the same? How much overlap is there in the distribution of these emergent species? How could you quantify the degree of overlap between two species? What makes it possible for emergent species to overlap in their distributions?

5.5.2

Sediment cores Changes in the vegetation of wetlands over time are recorded in the wetland sediments. Major changes in vegetation can often be reconstructed by examining sediment cores. Collect one or more sediment cores from a wetland with a suitable sampler or corer. As the core is collected try to keep it intact. In the ﬁeld describe the core from top to bottom, that is, sections that differ in colour, texture, or presence of identiﬁable plant or animal parts. Cores can be stored in a refrigerator. In the laboratory, sample the core from top to bottom making sure that all parts of the core that differ in colour and texture are sampled. This can be done by taking slices out of the core or by taking small amounts from it. Examine each core sample under a microscope to see if it contains identiﬁable organisms, especially plant parts. Identiﬁcation of partly decomposed plant parts is difﬁcult and may not be possible without help. A reference collection of sediment from the wetland in areas dominated by submersed species, ﬂoating-leaved species, and emergent species might be useful. In general, this type of study will work better in wetlands with highly organic soils. How much sediment has accumulated in this wetland? What colour are the layers in the core? Is this sediment mostly organic or inorganic? What evidence exists in the core for major changes in vegetation?

6 Wetland functions

Functions are measurable characteristics of a wetland such as its primary production, food chains, nutrient cycling, water budget, duck production, etc. Wetland functions produce wetland services. These are beneﬁts derived either by individuals or by society from wetlands such as timber, fur, ﬁsh, ducks, peat, carbon sequestration, ﬂood reduction, water puriﬁcation, etc. Because of these beneﬁts wetlands have economic value to individuals or to society or both (Woodward and Wui 2001). Unfortunately, there have been relatively few detailed studies of wetland functions and most of those have only examined a limited number of functions in a given wetland. These studies have often been driven by practical considerations. For example, the importance of some types of wetlands as waterfowl habitat has generated many studies of their food chains. Concerns about the impacts of increased nutrients or altered hydrology on some major wetlands, for example, the Everglades, have generated numerous studies dealing with nutrient cycling. Macrophytes dominate wetlands. They capture most of the solar energy entering wetlands, create most of their physical structure, and are the largest living reservoir or compartment for nutrients such as phosphorus and nitrogen. In short, a wetland’s functions are largely the product of its macrophytes. In this chapter, three functions that result from the presence of macrophytes in wetlands will be examined: primary production, food webs (secondary production), and mineral cycling.

6.1

Primary production Ultimately, the living or dead plant material that is present in wetlands is primarily the result of oxygenic photosynthesis. Oxygenic photosynthesis is the reduction by macrophytes, algae, and cyanobacteria of carbon dioxide, the most oxidized form of carbon, to organic matter

110 THE BIOLOGY OF FRESHWATER WETLANDS

using light energy captured by chlorophylls and water as the reductant. An important by-product of oxygenic photosynthesis is the release of oxygen. Oxygenic photosynthesis:

CO2 H2O

→

[CH2O] O2

The reduced organic matter produced by photosynthesis can be oxidized by heterotrophic organisms during aerobic or anaerobic respiration to obtain the chemical energy stored in organic compounds produced by photosynthesis. Aerobic respiration:

→ [CH2O] O2

→

CO2 H2O

The total amount of carbon that is fixed by photosynthesis over some period is referred to as gross primary production (GPP). Plants, however, are also respiring during the light and dark in order to provide energy needed for their maintenance and growth. Consequently, the amount of organic matter that actually accumulates is less than that fixed by photosynthesis by the amount lost to respiration (R) during the same period. The amount of organic matter that actually accumulates over a given period is the net primary production (NPP GPP R). Although it is possible to measure NPP, in reality most estimates of NPP are based on harvesting the plants growing in a wetland. The amount of plant material present per unit area at any given time is called the standing crop or biomass. Although the maximum standing crop is often used to estimate seasonal or annual NPP, such estimates are always too low because of unaccounted for losses of organic matter due to herbivores, diseases, and shedding of plant parts (leaves, flowers, roots, etc.). Although a number of methods for estimating annual NPP in wetlands are theoretically possible, nearly all of the available data are based on harvest methods (Westlake et al. 1998). Because different harvesting techniques are used to collect above- and belowground standing crop data, it is customary to present biomass data for both above- and belowground plant parts. To estimate annual primary production, below- and aboveground biomass are collected over the growing season (Fig. 6.1). The seasonal translocation of organic matter from above- to belowground or vice versa complicates making estimates of annual total NPP. Annual total net production is not normally just the sum of maximum above- and belowground standing crops (Schierup 1978). Total net annual production can be estimated, however, from above- and belowground biomass data by using seasonal changes in belowground standing crop to correct for translocations (Fig. 6.2). For this Phragmites australis wetland in Denmark, its maximum aboveground standing crop was 1160 g m 2. From seasonal changes in above- and belowground

WETLAND FUNCTIONS 111

Standing crop biomass (gm m–2)

1800

1200

600

0

Fig. 6.1

50 100 150 200 Days from start of growing season

Seasonal changes in aboveground (solid line) and belowground (dashed line) standing crop of cattail (Typha) in a New Jersey wetland. (From Jervis 1969.)

0

Aboveground

1180

1025

425 2000

1575

2690 Underground

2000 MAR. Fig. 6.2

600 AUG.

OCT.

Seasonal changes in above- and belowground standing crop (g m 2) and estimated translocation from below- to aboveground in spring (425 g m 2) and from above- to belowground in fall (1025 g m 2) in a Phragmites australis wetland in Denmark. Total annual NPP is estimated to be 1760 g m 2 ( 1160 1025 – 425 g m 2). (Adapted from Schierup 1978.)

standing crop, translocation from below- to aboveground in spring (425 g m2) and from above- to belowground in fall (1025g m2) can be estimated. When translocation is taken into account, annual total net primary production is estimated to be 1760 g m 2 ( 1160 1025 – 425 g m 2). Aboveground standing crops signiﬁcantly underestimate annual primary production. Unfortunately, most of the data on macrophyte NPP in wetlands is based on aboveground biomass estimates. For some macrophytes, especially emergents, belowground standing crops are often much larger than aboveground standing crops (Fig. 6.2).

112 THE BIOLOGY OF FRESHWATER WETLANDS

The estimated annual total primary production of wetlands is highly variable and ranges from 100 to 9000 g m2 (Table 6.1). As in terrestrial ecosystems, primary production is a function of plant growth form, nutrient levels, light levels, temperatures, length of the growing season, etc. Even within the same wetland different vegetation types have different maximum standing crops (Fig. 2.1). In wetlands that undergo signiﬁcant changes in water levels from year to year like prairie potholes (Fig. 5.7), both aboveground (Fig. 6.3) and belowground (Fig. 6.4) biomass can vary signiﬁcantly from year to year. Table 6.1 Range of macrophyte total annual primary production (g m2) of different types of wetlands Wetland type

Net primary production

Sphagnum peatlands Sedge/shrub peatlands, fens Forested peatlands Emergent tropical wetlands (e.g. Cyperus papyrus) Emergent wetlands (e.g. Phragmites, Typha, Salix) Floating plants (e.g. Eichhornia crassipes) Submerged plants

100–400 200–1000 500–1500 6000–9000 500–7000 4000–6000 500–1600

Source: Adapted from Vymazal (1995).

500

Mean aboveground biomass (g m–2)

400

300

200

100

0 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Year Fig. 6.3

Mean maximum aboveground standing crop during a simulated wet–dry cycle in experimental cells in the Delta Marsh, Manitoba, Canada. Water levels were raised 1 m over normal (1980) in 1981 and 1982. Water levels were lowered to 50 cm below normal in 1983 and 1984, and there was no standing water. From 1985 till 1989, water levels were restored to normal. (Adapted from van der Valk 2000.)

WETLAND FUNCTIONS 113

1000

Mean belowground biomass (g m–2)

800

600 Fall 400

Spring

200

100 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Year Fig. 6.4

Mean spring (solid line) and fall (dashed line) belowground standing crop during a simulated wet–dry cycle in experimental cells in the Delta Marsh, Manitoba, Canada. Water levels were raised 1 m over normal (1980) in 1981 and 1982. Water levels were lowered to 50 cm below normal in 1983 and 1984, and there was no standing water. From 1985 till 1989, water levels were restored to normal. (Adapted from van der Valk 2000.)

Macrophytes are, of course, not the only primary producers in wetlands. Algae including cyanobacteria are also signiﬁcant contributors to the overall primary production of many wetlands. The signiﬁcance of algal production is often overlooked because algal standing crops are usually only a fraction of those of macrophytes. In the ﬁrst 5 years after reﬂooding, mean algal standing crop in a series of experimental wetlands made up only about 20% of their total standing crop (Robinson et al. 2000). Among the four assemblages of algae present, metaphyton made up 90% of the algal standing crop and the epiphyton made up most of the remainder while phytoplankton and epipelon had negligible standing crops. Algal standing crops do not provide a meaningful estimate of their total annual production. The average turnover time of algal standing crop in these same wetlands was estimated to be 4.3, 12.4, 25.7, and 32.6 days for phytoplankton, epiphyton, epipelon, and metaphyton, respectively. When adjusted for turnover, algal annual NPP is comparable to macrophyte production or even higher. As expected, metaphyton and epiphyton had the highest annual net production. Like macrophyte production to which it is closely linked, algal production can vary from year to year. During wet–dry cycles in prairie wetlands

114 THE BIOLOGY OF FRESHWATER WETLANDS

(Fig. 5.7), algal production is highest during the early regenerating stage when emergent macrophyte populations are still expanding. When macrophytes begin to decline during the degenerating stage, annual net algal primary production can begin to exceed macrophyte production. Algal production is low during both the dry marsh and lake stages. The relative contribution of different algal assemblages (Fig. 6.5) during a wet–dry cycle has been estimated by Robinson et al. (2000). Epipelon dominate the dry marsh stage, metaphyton the regenerating stage, epiphyton the degenerating stage, and phytoplankton the lake stage. Unfortunately, there have been very few comparative studies of algal and macrophyte production outside the prairie pothole region of North America. The few studies done in other kinds of wetlands also indicate that algal biomass and production make a signiﬁcant contribution to their overall primary production. In the Everglades (McCormick et al. 1998), algal standing crops are very high (100–1600 g m 2) in sloughs and areas dominated by sparse stands of Eleocharis cellulosa, but much lower (0–50 g m 2) in areas dominated by dense stands of emergents such as Cladium jamaicense and Typha domingensis. Both during the wet and dry season the epipelon were the highest contributors to the total algal biomass followed by epiphyton and metaphyton. Overall, algae are estimated to account for 1⁄3 to 1⁄2 of the total annual NPP of the Everglades.

Proportionate contribution to total algal production

In summary, algal production in wetlands can be a signiﬁcant percentage of the total primary production in wetlands. Algal primary production varies signiﬁcantly among vegetation zones in wetlands and from year to year in the same wetland. Although it is certain that the relative importance of algae production also varies signiﬁcantly among different

Phytoplankton Metaphyton

Epiphyton Epipelon + Plocon Dry marsh

flooding

Regenerating marsh

Degenerating marsh

Lake marsh

drought or drawdown

Fig. 6.5

Relative contribution to overall algal net annual primary production by epipelon and plocon, epiphyton, metaphyton, and plankton over a wet–dry cycle in a prairie pothole. Plocon are crusts of cyanobacteria and diatoms on the soil surface. The characteristics of the other algal assemblages are described in the text. (From Robinson et al. 2000.)

WETLAND FUNCTIONS 115

types of wetlands, there is too little data available to estimate the range in algal production. Most peatlands have little or no standing water and algal production will be considerably less important in these wetlands than in riverine, palustrine, and lacustrine wetlands that have large expanses of standing water.

6.2

Litter decomposition Eventually macrophytes and algae are either eaten by herbivores or they die. When they die, the resulting litter (dead organic matter) becomes available to heterotrophic microorganisms as an energy source. The break down or decomposition of this litter releases the remaining energy ﬁxed and the elements taken up or assimilated by the macrophytes and algae. The quantity and quality of organic matter produced in wetlands greatly inﬂuences other wetland functions, especially food chains and nutrient cycling. Litter is a chemically very complex mix of compounds that can range from simple sugars and amino acids that are very easy for microorganisms to metabolize to long-chain polymers (e.g. lignins). When litter decomposes, there is generally a rapid initial loss of small molecules that are leached from the dead tissue. Litter is quickly colonized by microorganisms, and, as microbial biomass increases, the rate of litter decomposition also increases. As the more easily metabolized molecules are used up, however, litter decomposition rates begin to slow down (Fig. 6.6). Litter decomposition (Fig. 6.7) is a

Percent (%) remaining

100

A 10 B

F E D

C

1 0

Fig. 6.6

2

4

6

8 10 Years

12

14

16

Idealized breakdown curves for the major constituents of plant litter: A, phenols; B, waxes; C, lignin; D, cellulose; E, hemicellulose; F, sugars. (Adapted from Fenchel et al. 1998.)

116 THE BIOLOGY OF FRESHWATER WETLANDS

contact phenomenon because microorganisms, especially fungi, need to be in contact with the surface of litter for them to metabolize it or to take up molecules released by decomposing litter. Because of the colonization of litter by microorganisms, an inseparable mix of litter and microorganisms develops that is called detritus. Because of these microorganisms, which can have densities as high as hundreds of million per square centimetre, detritus often has more nitrogen and phosphorus per carbon than leached litter. While litter per se is not very nutritious—it is nutritionally equivalent to a piece of cardboard—detritus is. Detritus is the staple diet of many wetland invertebrates collectively called detritivores. Wetland plants differ signiﬁcantly in their chemical composition, even within the same wetland litter decomposition rates can differ signiﬁcantly among woody, emergent, ﬂoating-leaved, and submerged species (Fig. 6.8). Submerged and ﬂoating-leaved aquatic litter with little structural tissue can completely decompose in weeks or months while woody litter may take many decades or centuries. For microorganisms to break down litter, they need not only a source of energy

Fungi

Plant primary production

Senescence

Bacteria

Microbial colonization

Litter

Conditioned litter

Leaching (K+, Mg++) Macroinvertebrate Nutrient cycling

Accumulated organic matter

Predators

Collectors Filterers Gatherers

Community Grazers

Weathering and biological fragmentation

Shredders

Oi Oe Oa

Soil

CPOM FPOM

Fig. 6.7

Nitrogen and phosphorus accumulation

Litter decomposition results initially from leaching and then from the metabolism of microorganisms (fungi, bacteria) that colonize the surface of the litter. The fragmentation of litter by invertebrates or water currents creates new surfaces that can be colonized by microorganisms. Over time litter that is not decomposed by microorganisms accumulates in the wetland. CPOM, coarse particulate organic matter; FPOM, ﬁne particulate organic matter. (From Magee 1993.)

WETLAND FUNCTIONS 117

(organic mater), but also a source of other essential elements (e.g. N, P, S). Some macrophyte litter may initially be too low in some essential elements to support the growth of microorganisms. The ratio of carbon to nitrogen (C:N) or carbon to phosphorus (C:P) can be much higher in this litter than it is in microorganisms. Before the decomposition of the litter can occur, the microorganisms have to acquire sufﬁcient N or P from their surrounding environment. Typically, C:N ratios have to be 30 before mineralization will occur. During the initial stages of litter decomposition, the assimilation of nutrients by microorganisms may exceed the release of these nutrients due to the mineralization of the litter. Litter during this phase of decomposition is actually a sink for nutrients. Not only do intrinsic characteristics of the litter such as lignin, nitrogen, phosphorus, and tannin content effect decomposition rates but so do many environmental factors (oxygen, temperature, pH) because they affect the types and mass of microorganisms found on the litter. Godschalk and Wetzel (1978) did a classic study of the effects of environmental conditions on litter decomposition (Fig. 6.9). In their study, they examined the effects of oxygen (aerobic versus anaerobic) and temperature (10 C versus 25 C) on decomposition of a number of species. Decomposition was slowest under the 10 Canaerobic treatment and fastest under the 25 C-aerobic treatment. At 10 C under both aerobic and anaerobic conditions, much of the litter remained in particulate form. Only in the 25 C-aerobic conditions would most litter decompose completely to CO2. Thus, the fate of litter in wetlands is a function of both environmental conditions and litter characteristics.

100

Percent original mass

80 Pin oak k = 0.50 60

40 American lotus k = 1.81

Black willow k = 0.55

20 Watershield k = 6.17 0 Oct Nov Dec Jan Feb Mar Apr May Jun Fig. 6.8

Jul Aug Sep

Decomposition curves for four plant species. The annual decay coefﬁcient (k) is derived by ﬁtting a negative exponential decay curve to the data points. (From Magee 1993.)

118 THE BIOLOGY OF FRESHWATER WETLANDS Table 6.2 Intrinsic characteristics of litter and environmental factors controlling rates of litter decomposition Intrinsic characteristics

Environmental factors

Lignin content C:N ratio C:P ratio Nitrogen content Tannic acid content Polyphenol content

Presence of absence of water Oxygen level Temperature pH Nutrient (N, P) concentrations Invertebrate biomass

Warm CO2

RDOM A N A E R O B I C

CO2

LDOM

RDOM

Sediment

LDOM

Sediment >18°C – O2 + O2

CO2

RDOM

<18°C

LDOM

RDOM

Sediment

CO2

A N E R O B I C

LDOM

Sediment Cold

Fig. 6.9

6.3

Effect of temperature and oxygen concentrations on the decomposition of submerged macrophyte litter. LDOM, labile dissolved organic matter; RDOM, resistant dissolved organic matter. (From Godshalk and Wetzel 1978.)

Food webs Wetland primary production supports a large number of heterotrophic organisms that range in size from bacteria to hippos. For most wetlands, relatively little living macrophyte biomass (5–10%) seems to be utilized by grazers, primarily small mammals and invertebrates. There

WETLAND FUNCTIONS 119

are, however, many exceptions such as southern African wetlands that are grazed by large herds of antelopes, zebras, wildebeests, buffalos, etc. as water levels decline during the dry season (Ellenbroek 1987). Some wetlands are also grazed by domestic animals to eliminate shrubs, to make them more suitable habitat for shore birds or simply to provide forage for cattle and horses (Vulink 2001). Domestic cattle and water buffaloes can signiﬁcantly alter the composition and structure of wetland vegetation and thus the other kinds of animals that can utilize the wetland. Unfortunately, the fate of algal production in freshwater wetlands has been little studied. However, studies from lakes and the few studies done in wetlands indicate that invertebrate grazers and ﬁlter-feeders consume a signiﬁcant amount of algal biomass (Liess and Hillebrand 2004). Nevertheless, much of the primary production in wetlands, both macrophyte and algal, is not eaten by herbivores and eventually become litter. There are thus two different paths along which energy is transferred from one trophic level to another in wetlands, the autotrophic (herbivore) and the heterotrophic (detritivore) (Merritt et al. 1999). The autotrophic pathway involves the consumption of living macrophytes or algae by herbivores. The heterotrophic involves the decomposition, that is, consumption, of macrophyte, algal, and animal litter by microorganisms and then the consumption of microorganisms by invertebrates. The relative importance of these two pathways in wetlands has been much debated. Historically, the heterotrophic pathway was believed to be the dominant trophic pathway in wetlands. As the magnitude of algal production in wetlands became better documented, however, the autotrophic pathway is increasingly being considered equally if not more important. Food chains or webs in their simplest form are an accounting of who eats whom. Figure 6.10 is an example of a food web that was constructed for the Everglades (Rader 1999). Although this food web appears complex, it is, in fact, highly simpliﬁed and many complex and important assemblages of organism are lumped into a single entity, for example, algae, macrophytes, detritus, etc. The emphasis in this food web is on invertebrate interactions and, as in most food webs, on especially valued birds, mammals, and ﬁsh. The emphasis on invertebrate assemblages in food webs is warranted because invertebrate assemblages that differ in how they obtain food (shredders, ﬁlter-feeders, scrapers, predators) are the key link between primary producers and higher trophic levels (e.g. carnivores such as ducks and ﬁsh). Studies of food webs are complex because there are many different microhabitats with different assemblages of algae, invertebrates, and vertebrates within any wetland and there is considerable temporal

Adult alligators

Large fish (> 8 cm)

Otters

Frogs

Salamanders

Turtles

(5 species)

(2 species)

(6 species)

1° Piscivorous/Carnivores Largemouth Bass Gar Pickeral

Juvenile alligators

Wading birds

1° Invertivores* Adult Sunfish (7 species) lctalurus (4 species)

1° Piscivore Wood Stock Great Blue Heron Great Egret

Small Fish/Invertivores Small Herons Egret (9 species)

1° Invertivores White Ibis Roseate Spoonbill

Small Fish (> 8 cm) Invertebrates

Crayfish & Shrimp Procambarus Palaemonetes* >10mm

Predator Agabetes* Bidessonotus* Celina* Desmopachria* HYdroporus* Hydrovatus* Dybius* Laccophilus*

Gyrinus* Berosus (larvae) Chaetarhria (larvae) Crenitulus (larvae) Tropisternus* Helophorus (larvae) Hydrocanthus* Hydrocanthus* Suphis* Suphisellus* <10 mm

Bezzia/Palpomyla Ablabesmyia Labrundinia Larsia Parachironomus Paramerina Procladius Tanypus

Tabanus Polymera Pelocoris* Mooreobdella* Helobdella* Enallagma* Ischnura* >10 mm

Cybister* Belosioma* Lethocenus* Coryphaeschna* Telebasis* Erythernis* >10mm

Pachydiplax* Libellula* Lesies* Perithernis* Celithemis*

Ferrissia Gyraulus Littoridonops Planorbella Fossarin Planorbula Mieromenetus Pseudosuccinae Palnacorixa* Drepanocrema Trichocorixa* < 10mm

Herbivores Poecilia latipinna Cyprinodon variegatus

Fishing spider

Oravelia* Mesavelia*

Scraper Endochironormus Lymnaea Physella Pomacea Biomphalaria Helisoma Viviparus > 10mm

Omnivores Gambusia affinis Heterandria formosa Jordinella floridae Lcania goodei Notropis (2 species)

Algae

Collector-gatherer Shredder

Acentria Parapoynx >10mm

Limonia Tipula Simyra Neelopsyche Oecetis Polycentropus >10mm

Haliplus* Peltodytes* Berosus (adults)* Oxyethira <10mm

Donacia* Helophorus (adult)* Hydropyris Leptoceris <10mm

Piercer/Shredder

Odontomyia >10mm

Live macrophytes

Filterer Hyalalla* Crangonynx*

Dasybeles Forcipornyia Asheum Pelenomus* Chironormus Enochrus* Cladopelma Chaetarthria (adult)* Dicrotendipts Crenitulus (adult)* Goeldichironomus Derallus* Kiefferulus Helebata* Parakiefferiella Paracymus* Tropishermus* <10mm Prionocyphon*

Cladocerans*

Polypedilum Pseudochironomus Pericoma Cullibaetis* Caenis* Caeccidoiea* Oligochaeta Ostracoda* Psychomia

Copepods*

Plumatella Spongilla Corbicula Musculium >10mm Cladotanytarsus Tanytarsus Nimbocera <10mm

Rotifers*

Senescent macrophytes Detritus

Producers/food base

Acarina

Invertivores Elussorna evergladel Ennescanthus gloriosus Young Lepomis (5 species) Fundulus (4 species) Young lctalunus (4 species) Ethcostoma fusiforne

Bacteria DOC

Protozoans

Consumers Fig. 6.10

A food web developed for the Florida Everglades. Vertebrate species are grouped by size and/or food preferences. Invertebrate species are grouped by how they obtain their food (collector-gatherer, shredder, scraper, piercer-shredder, ﬁlterer, predator). Invertebrates play a central role in this and all other wetland food web. (From Rader 1999.)

WETLAND FUNCTIONS 121

variability (seasonal and interannual) within each microhabitat. There are also many different kinds of wetlands, and the relative importance of different microhabitats varies considerably among wetlands. Understanding wetland food chains is often made even more complicated because many of the organisms may utilize different foods at different stages of their life cycles or when they are different sizes. Female dabbling ducks eat primarily invertebrates while they are laying eggs, but mostly plant seeds or tubers for the rest of the year. Filter-feeding invertebrates may select food based primarily on its the size, not whether it is alive or dead. Developing food webs is an essential ﬁrst step in understanding trophic dynamics in wetlands. To quantify the movement of energy and materials from one trophic level to another, however, requires a different approach. There are basically three ways to determine what organisms are eating in wetlands: (1) observing individuals in the ﬁeld or laboratory to see what they are eating; (2) examining the guts of organisms to see what they contain; and (3) utilizing stable isotope ratios or other biochemical markers that remain essentially unchanged through the food web. Observational and gut content studies have been used most effectively with larger organisms such as waterfowl and ﬁsh. They are useful for determining the kind and amount of food that a speciﬁc species, such as a species of dabbling ducks, is eating, but they are impractical for quantifying entire wetland food webs. An examination of stable isotope ratios at different trophic levels in a food web can provide useful insights into the transfer of energy and essential elements from producers to consumers (Box 6.1). Food webs of prairie potholes (Fig. 6.11) have been studied using stable isotope ratios to determine whether various groups of invertebrates obtain their energy and nutrients primarily from macrophytes/macrophyte litter or from algae. In this study, ␦13C signatures of algal assemblages other then the epipelon were more deleted (negative) in 14C than those of the macrophytes. The ␦13C signatures of most of the aquatic invertebrates were more similar to those of the algae than to those of the various assemblages of macrophytes. Using stable isotopes, however, it was not possible to determine the relative importance of the different algal assemblages for various invertebrates. Nevertheless, this study indicated that food web pathways for invertebrates in these prairie potholes were primarily autotrophic. Even in prairie potholes, however, it is likely that the heterotrophic food web pathway may occur in parts of the wetland with the densest emergent vegetation that have little algal growth. This was the case in wetlands associated with the Kissimmee River in Florida (Merritt et al. 1999). In prairie potholes, heterotrophic pathways may also predominate during periods of their wet–dry cycles when emergent vegetation dominates these wetlands.

122 THE BIOLOGY OF FRESHWATER WETLANDS

Box 6.1. Many elements (including C, H, O, N, and S) have two or more stable isotopes. The lightest isotope is much more abundant than heavier ones. For example, the heavy isotopes of carbon and nitrogen are 13C and 15N, respectively, and the light isotope are 12C and 14N. For a given element, the stable isotopic signature of an organism or its litter is determined by comparing the ratio of heavy to light isotopes in a sample of the organism (Rsample) to this ratio of a standard (Rstandard). This value is referred to as the delta (␦) value and is expressed in parts per thousand (‰): ␦ (‰) (Rsample/Rsample 1) 1000 Delta (␦) values indicate whether the organism has more or less of the heavy stable isotope than the reference. A positive value means that the sample contains more of the heavy isotope than the standard; a negative value means that the sample contains less of the heavy isotope. In other words, a ␦15N value of 25‰ means that there are 25 parts per thousand or 2.5% more of 15N in the sample relative to the standard. Stable isotope values can be used to examine the transfer of organic matter in food webs because different primary producers often have different stable isotope values and these stable isotope values remain essentially unchanged when primary producers are eaten by consumers. The stable isotope signatures of herbivores, omnivores, or carnivores reﬂect the stable isotope signatures of the foods that they have eaten.

Many of the organisms at the higher trophic levels of wetlands are carnivores that eat macro-invertebrates and small ﬁsh. These include some duck species, many wading birds, and sometimes ﬁsh. Invertebrates may not always exist is densities that are adequate to support these species. Wading birds in the Everglades rely on falling water levels to concentrate their prey while they are breeding so that they can obtain adequate amounts of food to feed their young. One of the reasons that prairie potholes in North America are referred to as ‘duck factories’ is that these wetlands contain few if any ﬁsh and hence the ducks do not have to compete with ﬁsh for invertebrates. There are some experimental studies that indicate that duck reproduction is adversely affected if larger ﬁsh occur in a wetland and that some ducks avoid nesting on wetlands with ﬁsh (Eadie and Keast 1982, Eriksson 1979). More detailed studies of wetland food webs are needed to identify and document such trophic constraints.

WETLAND FUNCTIONS 123

16 Floating Emergents 12

Metaphyton Submersed

d15N (‰)

Periphyton

2 5 9 4 7 11 6 10 1 12 3 8

8

14 15

4 13

Phytoplankton

Epipelon

0

–4 –40

–35

–30

–25

–20

d13C (‰) Fig. 6.11

6.4

Stable carbon and nitrogen isotope ratios for algae, macrophytes, and aquatic invertebrates in 10 prairie potholes in North Dakota, USA. The shaded boxes represent the range of values obtained for different groups of algae (phytoplankton, periphyton, epipelon, and metaphyton) and macrophytes (ﬂoating, emergent, and submerged). The mean values and range are also given for different groups of aquatic invertebrates: (1) Cladocera, (2) Copepods, (3) Glyptotendipes (Chironomid), Tanypodinae, (5) Hydracarina, (6) Ephemeroptera, (7) Chaoboridae, (8) Chironomus (Chironomid), (9) Zygoptera, (10) Corixidae, (11) Notonectidae, (12) Dysticidae, (13) Hydrophilidae, (14) Anisoptera, and (15) Gastropoda (snails). (From Euliss et al. 1999.)

Nutrient cycling In studies of nutrient budgets of wetlands (Table 6.3), the mass of nitrogen or phosphorus entering a wetland in precipitation and surface and groundwater inﬂows is compared with that in surface and groundwater outlows. Such studies indicate that many wetlands, especially palustrine wetlands, are nutrient sinks or traps. Sometimes wetlands are good nutrient traps simply because they have very little or no outﬂow. This was the case in Eagle Lake in 1978 when rainfall was low and there was no outﬂow at all from this wetland (Table 6.3). Even in 1979, however, when there was signiﬁcant outﬂow, Eagle Lake retained most of the NO3–N (86%) and NH4–N (78%), and some PO4–P (18%). The mass of dissolved organic carbon (DOC) in its outﬂow, however, was larger than in its inﬂows. To understand the nutrient budgets of Eagle Lake, or any other wetland, it is necessary to understand its hydrology and nutrient cycles. Nutrient cycles are an account of where nutrients are found in wetlands, of how nutrients are transported from one storage compartment or pools to another within the wetland, and of the chemical transformations that occur. The most important nutrient cycles in wetlands are carbon (Fig. 6.12), nitrogen (Figs. 6.13 and 6.14), sulphur (Figs. 6.13 and 6.15), and

124 THE BIOLOGY OF FRESHWATER WETLANDS Table 6.3 Annual inputs and outputs of water (mm) and nutrients (kg ha 1) from Eagle Lake, Iowa, USA Year

Inputs

Water 1978 1979 NH4–N 1978 1979 NO3–N 1978 1979 PO4–P 1978 1979 DOCa 1978 1979

Outputs

Precipitation

Inﬂows

Total

709 944

163 1620

872 2564

0.0 1758

5.0 6.6

0.8 2.5

5.8 9.1

0.0 2.0

5.0 6.7

25.4 202.8

30.4 209.5

0.0 29.8

0.24 0.32

0.75 3.10

0.99 3.42

0.0 2.81

ND ND

19 302

19 302

0.0 516

a

DOC, dissolved organic carbon; ND, no data.

Source: From Davis et al. (1981).

to atmosphere

CO2 H2O, H2S, S

respiration

respiration Aerobic

O2, S°, SO4 Autotrophs and chemolithotrophs death, excretion

Aerobic microbes

POC + DOC

DOC (low M.W. fatty acids) H2 H2O

SO4

NO–3 N2

Denitrifiers

HS–

Sulphate reducers

H2O

HS–SO

4

CH4

Methanogens

Fig. 6.12

diffusion ebullition

Intensity of reducing conditions

Fermenters

H2

Carbon cycling in aerobic (water column) and anaerobic (sediment) environments. Boxed are either carbon pools or carbon processors (various assemblages of microorganisms). Arrows are chemical transformation or physical transports. Note the links between the nitrogen and sulphur cycles. (From Valiela 1984.)

WETLAND FUNCTIONS 125

A Nitrogen cycle

Reduction

org-N

NH4+

NH2OH

ammonium

hydroxylamine

N2 nitrogen gas

N2O nitrous oxide

NO

NO2–

NO3–

nitric oxide

nitrite

nitrate

Oxidation –3

–1

0

+1

+2

+3

+5

Reduction

B Sulphur cycle org-S

H2S

S°

S2O32–

SO32–

SO42–

hydrogen sulphide

elemental sulphur

thiosulphate

sulphite

sulphate

FeS

Oxidation

iron sulphide etc.

–2 Fig. 6.13

0

+2

+4

+6

Oxidation states of various species of nitrogen A. and sulphur B. and chemical transformations associated with various changes in oxidation states. Arrows indicate known chemical transformation among species. (From Fenchel et al. 1998.)

phosphorus (Fig. 6.16). The dominant cycle is the carbon cycle (Fig. 6.12). Carbon is by far the most common element in all living organisms and typically accounts for 45–55% of their dry weight while nitrogen accounts for only 2–10% and sulphur and phosphorus even less. Most of the nutrients found in wetlands are in the soil compartment followed by the macrophyte compartment. Most macrophytes are rooted and they assimilate nitrogen, sulphur, and other essential elements from the soil. In effect macrophytes are nutrient pumps moving nitrogen, sulphur and phosphorus from the soil. The production of organic matter by photosynthesis and the distribution, movement, and decomposition of this organic matter directly or indirectly inﬂuences nearly every aspect of a wetland’s structure and function. The carbon cycle is central to understanding why wetland soils are anaerobic, why

126 THE BIOLOGY OF FRESHWATER WETLANDS (a)

N2

4

NO3–

1

Organic N

5

NH4+

3

Transformation processes 1. N-Fixation 2. Ammonification 3. Nitrification 4. Denitrification 5. Immobilization

2

(b) Air

N2

1

2 5

Anaerobic soil

Organic N

2

NH4+

3

NO3– Diffusion

Organic N

NO3– Leaching Diffusion

Aerobic soil

Organic Leaching N Decomposition

Diffusion

Algae Water

NH4+

NO3–

4

N2

5

Leaching Fig. 6.14

The nitrogen cycle in wetlands. Major chemical transformation are given in A and the locations of these transformation in B. Not all intermediate steps are shown for some transformations. (From Vepraskas and Faulkner 2001.)

wetlands store or sequester carbon, and why they release methane and other greenhouse gases. The carbon cycle is coupled to other microbial cycles, and the energy required for chemical transformations in nitrogen and sulphur nutrients comes from compounds produced by the carbon cycle. In effect, most of this chapter and much of Chapter 3 (Fig. 3.1) have already dealt with carbon cycling. Most of the carbon found in wetlands is the product of oxygenic photosynthesis by macrophytes and algae. Although some of this organic matter is eaten by herbivores, from a carbon cycling perspective (Fig. 6.12), this is just a brief detour on the way to either the DOC or particulate organic carbon compartments (POC). DOC are soluble compounds that are leached or excreted directly into the water columns where they can be taken up by aerobic microorganisms

WETLAND FUNCTIONS 127

A

H2S (DMS) 4

3

5 1

SO42–

Plants Organic S

2

Carbon-Bound S 2

4

B

Air

Reduced inorganic S (sulphides; S°)

Transformation processes 1. Immobilization (Assimilatory S Reduction) 2. Aerobic mineralization 3. Sulphate reduction SO42– 4. Sulphide oxidation 5. Anaerobic mineralization

SO42–

Water

4

Leaching Diffusion

Aerobic soil

Organic S

2

H2S

3

SO42–

1

Organic S

Anaerobic soil

RIS

Diffusion

SO42– 1

Fig. 6.15

5

5

The sulphur cycle in wetlands. Major chemical transformation are given in A and the locations of these transformation in B. Not all intermediate steps are shown for some transformations. (From Vepraskas and Faulkner 2001.)

and eventually mineralized. POC can take many forms that range from standing emergent litter several metres tall to microscopic particles. POC normally is broken down into smaller and smaller particles over time and eventually much of it can be converted to DOC and CO2. Most standing litter ends up as fallen litter on the surface of the wetland and eventually becomes buried by subsequent litter inputs. Once buried, the litter is in an anaerobic or reducing environment. Under anaerobic conditions, fermenters hydrolyse larger organic molecules (lignins, celluloses, hemicelluloses) to smaller molecules (e.g. fatty acids, formate, acetate, alcohols). These compounds can be utilized as an energy source and oxidized to CO2 by a variety of microorganisms capable of anaerobic respiration as long as a suitable terminal electron

128 THE BIOLOGY OF FRESHWATER WETLANDS A

1

Plants Organic P

Microbes

3

Soil organic matter

4

Fixed mineral P

Soluble organic P Transformation processes 1. Mineralization 2. Immobilization 3. Fixation 4. Reduction-Release

Ca - P

Fe - P Al - P

Air

Aerobic soil

Ortho P n

Organic P sedimentation

sio

Decomposition

ffu

Water

Di

B

Orthophosphate

2

Decomposition

Organic P

1

Ortho P

Organic P

1

Ortho P

4

2

Anaerobic soil

Dec

omp

.

3

Fixed mineral P

2

Fe2+–P

Fig. 6.16

Fe3+–P

The phosphorus cycle in wetlands. Major chemical transformation are given in A and the locations of these transformation in B. Not all intermediate steps are shown for some transformations. (From Vepraskas and Faulkner 2001.)

acceptor (e.g. NO3 , SO2 4 ) is available. Under very reducing conditions, when other terminal electron acceptors have been exhausted, methanogenic bacteria can utilize the products of fermentation to produce methane (CH4) in some cases using CO2 as a terminal electron acceptor. Many of the diagnostic features of wetland soils are the result of anaerobic respiration (Box 6.2). Much of the organic matter in wetlands is eventually utilized as an energy source in bacterial anaerobic respiration, and it fuels many of the chemical transformation that drive the cycling of nitrogen and sulphur. One common characteristic of wetlands is the high organic content of their soils compared with that of adjacent terrestrial ecosystems; that is, in wetlands less of the organic matter produced annually is completely

WETLAND FUNCTIONS 129

mineralized. Because much of the decomposition of organic matter in wetlands occurs under anaerobic conditions, litter decomposition is less efﬁcient in wetlands than uplands. This is due to the limited availability of terminal electron acceptors that is caused by slow diffusion rates or other chemical transport mechanisms, lags in the growth of bacteria when suitable substrates for their metabolism become available, and poor environmental conditions for microbial growth such as low pHs or soil temperatures. Because of the annual accumulation of organic matter, the soil surface of many wetlands increases slightly, typically a few millimetres or less, in elevation every year.

6.4.1

Nitrogen and sulphur cycling Both nitrogen and sulphur can undergo a number of oxidation–reduction reactions (Fig. 6.13) that involve oxidizing reduced forms such as NH 4 or H2S or reducing highly oxidized form such as NO3 and SO42. As with all oxidation–reduction reactions, some occur primarily under aerobic and other under anaerobic conditions. The ﬁve most important of these reactions (Fig. 6.14) are Nitrogen ﬁxation: Nitrogen gas (N2) can be converted to NH4. In wetlands, many anaerobic bacteria, some facultative bacteria, and cyanobacteria are the primary nitrogen ﬁxers. This is an energy requiring reaction. Nitriﬁcation: Nitriﬁcation is the conversion of NH 4 to NO3 . It only occurs under aerobic conditions. Normally the only place that NH4 is found in wetlands is in anaerobic (low redox) soils. It can sometimes be found in the water column due to diffusion from soils, surface inﬂows, or leaching or mineralization of litter, but in an aerobic zone it is quickly oxidized to NO3 by bacteria (Nitrosomonas, Nitrosococcus, Nitrobacter) in a multistep process (Fig. 6.13). Denitrﬁication: When nitrates (NO3) diffuse into anaerobic zones, NO3 can be used as a terminal electron acceptor in anaerobic respiration by many groups of bacteria. Denitriﬁcation is the conversion of nitrate to nitrogen gas (N2) or nitrous oxide (N2O). Denitriﬁcation removes nitrogen permanently from wetlands. Immobilization: The conversion of inorganic forms to organic nitrogen by plants. Ammoniﬁcaton: This is the conversion of organic-N to NH4 by bacteria. Ammoniﬁcation can occur under either aerobic or anaerobic conditions. Wetlands are such good nitrogen sinks because ammoniﬁcation when linked with nitriﬁcation in aerobic zones and denitriﬁcation in anaerobic zones can remove any kind of inorganic or organic nitrogen compounds from a wetland by converting it to nitrogen gas (Fig. 6.14).

130 THE BIOLOGY OF FRESHWATER WETLANDS

Box 6.2 Redoxymorphic features As noted in Chapter 3, oxidized forms of both iron (Fe3) and manganese (Mn4) can also be used as terminal electron acceptors in soils after nitrates are depleted. Oxidized forms of both compounds are insoluble in water while their reduced counterparts (Fe2 and Mn2) are highly soluble. As ﬂooded wetland soils become more reduced due to microbial respiration, this causes more and more of the iron and manganese to go into solution. Because of water movements, both iron and manganese can become depleted in some portions of the soil column and become concentrated in others. A

B Dead root Fill Channel with water

Soil matrix brown color (oxidized Fe3+)

Root channel

Fe2+ reduced in soil along channel

Drain Soil

Fe Depletion (Gray color) D

Fe Mass (Red oxidized Fe3+ compounds)

Reduced Fe2+ moves into matrix C

Fig. 6.17 Formation of a soil redox depletion around a root channel. A. The soil matrix is uniformly red due to the presence of oxidized Fe3. The root channel contains a dead root that is being decomposed by bacteria. B. When the soil is ﬂooded, the water quickly become anaerobic and bacteria begin to use oxidized iron as a terminal electron acceptor in anaerobic respiration. This reduces the oxidized Fe3 around the root channel to Fe2. C. The soluble reduced Fe moves away from the channel leaving the soil around the channel free of Fe. D. The reduced Fe2 in the soil matrix is subsequently be oxidized back to Fe3 when the soil drains. This produces an Fe mass in the matrix. (Redrawn from Vepraskas 2001.)

WETLAND FUNCTIONS 131

When reduced, these compounds are largely responsible for the greyish-bluish or gleyed colour of many wetland soils. In fact, many of the characteristic features of wetland soils, for example their high organic matter content, are due to various oxidation–reduction reactions, and they are collectively called redoxymorphic features. Common redoxymorphic features include redox depletions (Fig. 6.17) and redox concentrations. These often result from the movement of soluble, reduced forms of Fe or Mn by water and their precipitation when these elements are re-oxidized around roots or root channels when water levels drop temporarily allowing oxygen to enter the soil. Information about the development and characteristics of wetland soils can be found in Richardson and Vepraskas (2001).

The sulphur cycle (Fig. 6.15) has many parallels with the nitrogen cycle (Fig. 6.14). Various chemical species of sulphur can occur in wetlands, and it is either the oxidation of reduced forms (H2S) or reduction of oxidized forms (SO42) that drives the sulphur cycle (Fig. 6.13). As in the nitrogen cycle, sulphur can be lost from wetlands because hydrogen sulphide (H2S) is a gas. Under anaerobic conditions, SO42, which can act as a terminal electron acceptor in anaerobic respiration, can be reduced to H2S. H2S often interacts with iron to form insoluble iron sulphides.

6.4.2

Phosphorus cycling The phosphorus cycle (Fig. 6.16) is not comparable to that of carbon, nitrogen, or sulphur. Unlike microbial-driven cycles, the phosphorus cycle does not involve a series of oxidation–reduction reactions. The phosphorus cycle is a sedimentary cycle. In wetlands, P is found in two forms, inorganic, usually called orthophosphate, and organic. Inorganic phosphorus is taken up by organisms and converted to organic phosphorus (immobilization). Organic phosphorus is converted to inorganic phosphorus by mineralization. Phosphorus is rapidly lost from new litter by leaching and taken up by microorganisms. Both immobilization and mineralization can occur under aerobic and anaerobic conditions. The main mechanisms for removing inorganic phosphorus from the water column are soil adsorption and co-precipitation with carbonates. The latter only occurs in hard water with high carbonate levels and pHs 7. The adsorption capacity of wetland soils seems to be a function of the amount of amorphous aluminium and iron oxides and hydroxides present. Phosphates bound on soil particles can become

132 THE BIOLOGY OF FRESHWATER WETLANDS

solubilized if redox levels drop low enough for Fe3 to be reduced to Fe2 . In any case, wetland soils have only a ﬁnite capacity to adsorb inorganic phosphorus. As occurred at Eagle Lake in the spring of 1979 (Table 6.3), there is often a seasonal export of phosphorus from wetlands if the production of litter corresponds to a period of high water outﬂow. Ultimately, it is the burial of litter with its adsorbed phosphorus that makes wetlands phosphorus sinks.

6.5

Summary Many wetland functions are a consequence either directly or indirectly of macrophyte production. Although various algal assemblages can be major contributors to the overall primary production of wetlands, especially when emergent vegetation is sparse, most of the living (biomass) and dead (litter) standing crop in wetlands at any given time was produced by macrophytes. Much of the physical structure of wetlands is produced by macrophytes, for example, the plant canopies in which birds nest and the living and dead stems colonized by algae, fungi, bacteria, and invertebrates. Although there are both vertebrate and invertebrate grazers in wetlands that consume macrophytes, most of the organisms that are found in wetlands do not obtain their energy from them. The base of food chains in many wetlands seems to be various assemblages of algae, especially phytoplankton, epiphyton, and epipelon. The decomposition of litter is the most important process in wetlands. Litter decomposition has two stages, leaching and microbial mineralization. The former removes readily soluble small molecules such as sugars and amino acids and makes them available to microorganisms. The later requires the colonization of the litter surface by microorganisms capable of breaking down various kinds of organic molecules. Fungi are able to hydrolyse complex, long-chain polymers. Bacteria are able to metabolize smaller molecules. If the litter is in an aerobic environment, it can be completely mineralized. The large amount of litter produced every year quickly exhausts the limited amount of oxygen at the soil surface of ﬂooded wetlands. Consequently, most litter decomposes in anaerobic environments. Under anaerobic conditions, fermenters metabolize long-chain polymers to fatty acid, ethanol, or other shortchain organic molecules. These molecules are further broken down by various bacteria capable of anaerobic respiration. Many of these bacteria use oxidized forms of nitrogen (NO3), sulphur (SO42) or carbon (CO2) as terminal electron acceptors in anaerobic respiration. Under very reduced conditions, one of the products of litter decomposition is the greenhouse gas, methane (CH4).

WETLAND FUNCTIONS 133

Nutrient cycling in wetlands is either due to microbial cycles (carbon, nitrogen, and sulphur) or sedimentary cycles (phosphorus). Microbial cycles involve a series of oxidation–reduction reactions carried out by bacteria or cyanobacteria. In microbial cycles, one or more of the products of these oxidation–reduction reactions is a gas, CO2 and CH4 in the carbon cycle, N2 and NO2 in the nitrogen cycle, and H2S in the sulphur cycle. Thus carbon, nitrogen, and sulphur compounds added to wetlands can theoretically be permanently removed from them. In reality, because of spatial and temporal heterogeneity, transport problems, and lags in the growth of microbial populations, these elements, especially carbon, often can accumulate over time. Phosphorus, however, does not undergo oxidation–reduction reactions. When added to wetlands, phosphorus is either adsorbed on soils or co-precipitated with carbonates under some conditions. Phosphorus accumulates until the adsorption/precipitation capacity of the wetland is exceeded. After that, it stays in solution and can potentially leave the wetland in surface or groundwater outﬂows. Stirring up wetland sediments can release this phosphorus and put it back into solution, especially under anaerobic conditions.

6.6

Practical experiments and observations

6.6.1

Primary production Macrophytes vary signiﬁcantly in size and growth form from small freeﬂoating species to trees. How they partition organic matter produced by photosynthesis above- and belowground can vary considerably and this can have signiﬁcant ramiﬁcations for other organisms. Harvest the above and belowground standing crop in a small area (ca. 0.25 m2) of different kinds of wetland plants such as emergents, submerged plans, and ﬂoating-leaved species. Harvesting wetland plants is both locally very destructive and messy. (Pick an out of the way place and dress appropriately.) Clean the belowground parts to remove soils and litter in the ﬁeld as much as possible. The belowground parts will need to be thoroughly cleaned in the laboratory to remove all soil and litter. Samples can be stored refrigerated in plastic bags for several weeks. Dry the samples at 60–100 C in an oven until dry and weigh them. Which kind of plant has the highest aboveground standing crop? Which has the highest belowground standing crop? Is there any relationship between standing crop and water depth?

134 THE BIOLOGY OF FRESHWATER WETLANDS

6.6.2

Invertebrates and litter decomposition Invertebrates can affect litter decomposition by breaking up litter and creating new surfaces that can be colonized by microorganisms and by excreting nutrients needed by these microorganisms. Collect some standing litter from above water from a wetland. Litter of cattails (Typha) or some other emergent should be used. Litter decomposition is measured by determining the loss of litter in the container between the start and end of an experiment. Experiments should run from 3 to 10 weeks. The required length will depend on the type of litter. Place an equal amount of litter by weight it in some shallow containers. Cover the litter with water that has been ﬁltered to remove macroinvertebrates from the same wetland. Allow the litter to stand for at least a week so that it begins to decompose. Collect macro-invertebrates from the wetland and separate them into groups, for example, aquatic insects, mollusca, and crustucea. Until ready for use, they will need to be in aquaria with aged litter as a food source. Bubbling air into the aquaria will probably be necessary to keep them alive. Some possible studies that could be done to examine the effects of invertebrates on litter decomposition: 1. Compare decomposition rates of litter with and without invertebrates. 2. Compare the decomposition of litter with different quantities of invertebrates. 3. Compare the rates of litter decomposition with a single type of invertebrate and with several types. 4. Compare rates of decomposition in containers without invetebrates with that in containers in which the water is replaced periodically with water from containers in which invertebrates are being raised. What effects do different kinds of invertebrates have on litter decomposition? Does adding more invertebrates increase the rate of litter decomposition? What effect do invertebrate excretions per se have on the rate of litter decomposition?

6.6.3

Methane production Because of their anaerobic soils, wetlands are major producers of methane (CH4). Methane is a combustible gas. Methane can be collected in wetlands with standing water using the equivalent of an invertebrate activity trap (Fig. 3.12). Place a funnel over the mouth of a bottle ﬁlled with water, the larger the funnel, the better. Lower the bottle and funnel under water and

WETLAND FUNCTIONS 135

carefully turn them over without trapping any air. While moving around in the wetland, catch bubbles of gas that rise from the bottom in the bottle. When the bottle is full of gas, remove the funnel and cap it under water. Bring the bottle out of the water, point it away from people, light a match, and remove the cap. The methane will ignite and produce a brief, pale blue ﬂame. Was methane present? What does this indicate about the redox potential of the soils of this wetland? Did you smell any other gas while collecting the methane sample? If so, what was it? If gas chromatography equipment is available, more sophisticated studies of methane release are possible. For example, patterns of methane gas production in wetlands can be examined by placing samplers in various parts of the wetland with standing water. Sampling chambers are also available to collect gases in areas that are not ﬂooded. Do different parts of the wetland produce more methane than others? Is there a relationship between vegetation type and methane release? Between water depth and methane release?

7 The future of wetlands

About 50% of the world’s wetlands have been lost, and in some regions wetland losses approach 99%. In response to these losses, wetland ecologists have made a considerable effort to demonstrate the local, regional, national, and international value of wetlands; initially most of this effort was aimed at conserving wetlands as waterfowl habitat. Ed Maltby’s (1986) book, Waterlogged Wealth, which was the ﬁrst book to document multiple values of wetlands, signiﬁcantly increased interest in wetlands and wetland conservation around the world. Table 7.1 lists some common functions of wetlands and their economic beneﬁts. Wetlands are valuable because they are habitats for many animal species (Chapter 4) and are often centres of biodiversity, are locally and regionally important features in hydrological cycles (ﬂood reduction, recharging groundwater), are nutrient and sediment traps (Chapter 6), are global carbon sinks, are producers of commodities (timber, furs), and are important recreational areas. Many studies of the economic value of Table 7.1 Wetland functions and their associated economically valuable goods or services Function

Economically valuable good and/or service

Groundwater recharge Groundwater discharge Flood control Nutrient sink or trap Primary production Habitat for wetland species

Aquifer protection, protect local water supply Maintain ﬂow in rivers and streams, protect ﬁsheries Reduce ﬂood damage and insurance Reduce need for water puriﬁcation Timber, furs, food Commercial and recreational ﬁsheries Recreational hunting Non-use appreciation (e.g. bird watching) Recreational hunting Non-use appreciation (e.g. bird watching) Erosion reduction Amenity values due to proximity to wetlands

Habitat for non-wetland species Shoreline stabilization Landscape heterogeneity

Source: Adapted from Woodward and Wui (1991).

THE FUTURE OF WETLANDS 137

wetlands have been done and even meta-analyses of these studies (Woodward and Wui 2001). Because many of the values of wetlands are non-market values, estimates of wetland economic values are highly dependent on the location of the wetland. Although wetlands are now widely recognized as important and valuable ecosystems, they are still largely unprotected and are still disappearing around the world. Not only are wetlands still being lost, but those remaining are often being degraded. In this chapter, threats due to continued wetland losses, invasive species, and global climate change will ﬁrst be examined. To try to reverse the historic losses of wetlands, efforts to restore wetlands have begun. The last section of this chapter examines wetland restoration.

7.1

Wetland losses Dugan (1993) provides a continent by continent account of the threats to each continent’s wetlands. Overall, not much has changed since its publication. In countries that have laws to protect wetlands from drainage or ﬁlling such as the United States, wetland losses have slowed down dramatically. In order to allow development of areas with wetlands, mitigation of wetland losses through the restoration of wetlands is often required (Committee on Mitigating Wetland Losses 2001). Although far from perfect in execution, mitigation has enabled a balance to be struck between wetland conservation and economic development interests. This balance, however, is constantly changing due to court challenges to existing laws protecting wetlands (van der Valk and Pederson 2003) and changes in public policy. In most of the world, wetlands are not provided much legal protection from destruction. One of the most effective programmes for protecting major wetlands around the world is the Ramsar Convention. The Ramsar Convention requires countries that are signatories (1) to designate wetlands of international importance for inclusion on a list of Ramsar sites; (2) to promote the wise use of all the wetlands in their territory; (3) to designate wetlands as nature preserves and to promote training in wetland research, management, and protection; and (4) to consult with each other about the implementation of the Convention and about transfrontier wetlands and other aquatic systems. As of June 2005, there were 145 signatories to the convention and there are over 1400 Ramsar sites. The criteria for being a wetland of internal importance are given in Table 7.2. When it was adopted in 1971, the Ramsar Convention was concerned with the protection of critical habitats for migrating birds. This waterbird bias is still reﬂected in the current list of criteria. Other

138 THE BIOLOGY OF FRESHWATER WETLANDS Table 7.2. Criteria for the identiﬁcation of wetlands of international signiﬁcance under the Ramsar Convention adopted at the seventh Conference of Contracting Parties in 1999 Group A of the Criteria. Sites containing representative, rare or unique wetland types Criterion 1: A wetland should be considered internationally important if it contains a representative, rare, or unique example of a natural or near-natural wetland type found within the appropriate biogeographic region. Group B of the Criteria. Sites of international importance for conserving biological diversity Criterion 2: A wetland should be considered internationally important if it supports vulnerable, endangered, or critically endangered species or threatened ecological communities. Criterion 3: A wetland should be considered internationally important if it supports populations of plant and/or animal species important for maintaining the biological diversity of a particular biogeographic region. Criterion 4: A wetland should be considered internationally important if it supports plant and/or animal species at a critical stage in their life cycles, or provides refuge during adverse conditions. Criterion 5: A wetland should be considered internationally important if it regularly supports 20,000 or more waterbirds. Criterion 6: A wetland should be considered internationally important if it regularly supports 1% of the individuals in a population of one species or subspecies of waterbird. Criterion 7: A wetland should be considered internationally important if it supports a signiﬁcant proportion of indigenous ﬁsh subspecies, species or families, life-history stages, species interactions, and/or populations that are representative of wetland beneﬁts and/or values and thereby contributes to global biological diversity. Criterion 8: A wetland should be considered internationally important if it is an important source of food for ﬁshes, spawning ground, nursery, and/or migration path on which ﬁsh stocks, either within the wetland or elsewhere, depend.

than various migratory bird treaties, the Ramsar Convention is the only international mechanism available for the protection of wetlands and it has resulted in the protection of many large wetlands around the world. Although conversion of wetlands to agricultural land continues to be the major problem in parts of Africa, Asia, and South America, water projects are another major threat. The need to ﬁnd additional water supplies in many countries with expanding populations and economies is threatening some of the world’s major wetlands. For example, water entering the Okavango Delta in Botswana comes from Angola and passes through Namibia before it reaches the Delta. All three countries are short of water and various plans to divert water from the Delta have been proposed. Many wetlands are already adversely affected by irrigation projects that extract so much water that the hydrology of downstream wetlands has been altered. Water projects can alter not only hydrology but also the delivery of sediments to deltaic wetlands. The reduction of sediment to wetlands in coastal Louisiana, USA, has

THE FUTURE OF WETLANDS 139

resulted in major losses of freshwater wetlands because they are no longer able to keep up with sea-level rise.

7.2

Wetland degradation and invasive species Increased inputs of nutrients, especially agricultural run-off entering wetlands, altered hydrologies, and invasive species are responsible for the degradation of many wetlands. Invasive species are species that were not historically found in an area and whose introduction displaces species that were found in the area. Invasive species grow rapidly and spread easily. Invasive species may be either native species that can take advantage of changes in environmental conditions or introduced species from another part of the world. In other words, an invasive species may be introduced or may spread outside its normal range through its normal dispersal mechanisms. The impacts of nutrients in agricultural run-off are widespread, but often have not been studied in detail. One wetland in which they have been well studied is the Everglades (Fig. 7.1). In the last 100 years, the historic Everglades have been altered in numerous ways. A large area south of Lake Okeechobee was drained for agriculture and is now known as the Everglades Agricultural Area. Canals carry water from this area into the remaining northern parts of the Everglades that have been converted into a series of impoundments called Water Conservation Areas. The water draining the Everglades Agricultural Area is much higher in nutrients, especially phosphorus, than the water that historically entered the Everglades from Lake Okeechobee. The increased nutrients entering the Water Conservation Areas have resulted in the replacement of sawgrass (Cladium jamaicense) with dense cattail stands (Typha) in areas adjacent to canals. Cattail previously had been only a minor component of the vegetation of the Everglades. Studies of factors responsible for cattail becoming dominant in the Everglades indicate that increased nutrients are a major factor but changes in hydrology and ﬁre frequency also play a role (Newman et al. 1998). To protect the Everglades from degradation by nutrient inputs from the Everglades Agricultural Area, a series of treatment wetlands have been build to remove phosphorus from this run-off before it enters the Water Conservation Areas. Cattails are not the only invasive species problem in the Everglades. Melaleuca (Melaleuca quinquenervia) has invaded large areas. It was originally planted to promote drainage. Water hyacinth (Eichhornia crassipes) and Water lettuce (Pistia stratiodes) are two other common invasive species that are common in canals. Invasive plants such as cattail are a common problem in many wetlands. Other widespread

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THE FUTURE OF WETLANDS 141

problem species in North American wetlands include Purple Loosestrife (Lythrum salicaria), Phalaris (Phalaris arundinacea), and Phragmites (Phragmites australis). Water hyacinth, water lettuce, and salvinia (Salvinia molesta) are widespread invasive species in tropical and subtropical wetlands around the world. The spread of invasive species due to the alteration of wetland environments has resulted in signiﬁcant declines in biodiversity in wetlands. The management of invasive species is becoming increasingly necessary to preserve existing wetlands.

7.3

Global climate change Global climate change may affect wetlands in two ways: ﬁrst changes in temperature and precipitation will alter wetland hydrology and thus the wetlands vegetation and fauna. Second, they could cause the release of large amounts of carbon currently stored in wetlands.

Altitude, in metres above sea level

To evaluate the potential effects of global climate on a wetland’s vegetation requires coupling three kinds of models: (1) a global climate change model that predicts local changes in temperature and precipitation, (2) a hydrology model that predicts wetland hydrology, especially water levels, from climate data, and (3) a wetland model that predicts changes in vegetation caused by changes in hydrology. All three types of models have shortcomings and are constantly being improved. Poaini and Johnson (2003) have used this approach to investigate potential changes in prairie potholes in North Dakota, USA. For these potholes, signiﬁcant changes in wetland hydrology are predicted to occur (Fig. 7.2). Water levels are predicted to be much lower and these wetlands will be without any standing water for considerable periods of time. These predicted changes in hydrology would result in major changes in

Fig. 7.2

558.9 558.7

Enhanced greenhouse gas climate

558.5

Current climate

558.3 558.1 557.9 557.7

1

2

3

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8 5 6 7 Year of simulation

9

10

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Water levels in a North Dakota wetland under current climatic conditions and those predicted under an enhanced greenhouse gas climate. (From Poaini and Johnson 2003.)

142 THE BIOLOGY OF FRESHWATER WETLANDS

the vegetation of these wetlands (Fig. 7.3). In one permanent pothole, the predicted change is a decline in the amount of open water from 51% of the basin under current climatic conditions to 0% after 4 years under enhanced greenhouse conditions. In effect, this wetland would be converted from a permanent to a semi-permanent wetland. The loss of open water would signiﬁcantly reduce the use of this wetland by waterfowl. When the potential impacts of hydrological changes are examined over a much larger part of the prairie pothole region, it is predicted that there will be fewer wetlands with standing water most years and that this will result is much smaller duck populations in the region. Wetlands annually store carbon at rates that can be 10–20 times faster than in terrestrial systems. These high rates are due to slower decomposition rates under anaerobic conditions and cooler temperatures in

Current

1

4

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Enhanced

Open water

Wet meadow/shallow marsh

Mixed emergents

Upland

Deep marsh Fig. 7.3

Predicted emergent vegetation and open water areas under current climatic conditions and under enhanced greenhouse climate in years 1 and 4 of the simulated water levels in Fig. 7.1. (Adapted from Poaini and Johnson 2003.)

THE FUTURE OF WETLANDS 143

boreal and arctic regions where most wetlands are found (Fig. 1.8). As a consequence, today wetlands may contain as much as 40% of global soil carbon, most of it in peatlands and forested wetlands. Although they cover only 3% of the world’s land area, peatlands alone may contain as much as 25% of the soil carbon pool. So much carbon is stored in northern peatlands that changes in the amount of carbon stored in peatlands have been hypothesized to be linked to global iceage cycles (Frazen 1994). Although wetlands are an important sink for carbon, they are not expected to sequester much additional carbon as global CO2 levels increase. In other words, wetlands are not expected to ameliorate global climate change. In fact, the opposite is predicted. Global climate change could have several serious consequences for carbon sequestered in wetlands. Increases in global temperatures will increase the annual thaw depth of permafrost in northern wetlands. This will expose organic soils that are currently frozen year round to oxidation (Gruber et al. 2004). This could signiﬁcantly increase global CO2 and CH4 emissions. Wetlands have been estimated to be the source of as much as 40% of the annual emission of CH4. Because water-level ﬂuctuations have signiﬁcant effects on the emission of greenhouse gases, a drop in water levels (Fig. 7.1) could result in increased CO2 emissions as more organic matter is oxidized under aerobic conditions and a decrease in CH4 emissions (Canadell et al. 2004). Increasing temperatures will result in an increase in dissolved organic carbon (DOC) in the run-off from peatlands. Much of this DOC is likely to be oxidized. Increased temperatures could also result in more ﬁres in wetlands, especially in tropical wetlands. In Indonesia in 1997–1998, the burning of forested peatlands resulted in the emission of the equivalent of 13–40% of the emission from fossil fuels during the same period (Canadell et al. 2004). The continued conversion of wetlands, especially peatlands, to farmland will also result in increased emissions of greenhouse gases, primarily CO2. In summary, global climate change will alter the hydrology of wetlands, which will cause signiﬁcant changes in their vegetation. Because one of the potential consequences of these hydrological changes will be to increase the oxidation of organic mater in wetlands, especially in peatlands, the increase in emissions of greenhouse gases from wetlands may exacerbate the rate of global climate change.

7.4

Wetland restoration One of the most heartening developments in wetland ecology has been the increasing interest in the restoration of wetlands, especially in the

144 THE BIOLOGY OF FRESHWATER WETLANDS

United States (Kusler and Kentula 1990) and Europe (Wheeler et al. 1995) but also in many other parts of the world (Streever 1999; Hey and Philippi 1999). Although most wetland restoration projects are still relatively small, a few hectares or less, recent and proposed projects such as the ongoing Kissimmee River restoration (Dahm 1995) and the Comprehensive Everglades Restoration Plan (Lodge 2005) are restoring or will restore thousands or tens of thousands of hectares of wetlands. The restoration literature contains a bewildering number of terms to describe various kinds of wetland repair and building projects. Terms such as enhancement, rehabilitation, replacement, reclamation, restoration, creation, mitigation, management, etc. are all in use and are often used interchangeably. Most projects can be grouped into three broad classes: enhancements, restorations, and creations. Enhancements are projects in which an existing wetland is improved. This can include removing unwanted species, establishing wanted species, and altering the wetland’s hydrology. Restorations are projects in which a wetland is re-established on a site where a wetland previously existed. The re-establishment of a prairie pothole by breaking or plugging drainage tiles that had been used to drain the wetland 50 years ago is a restoration project. Wetland creations are projects in which a wetland is established on a site where previously a wetland had not existed. For example, a shallow excavated basin planted with wetland species in an area that was formerly upland is a creation project. Enhancement, restoration, and creaton projects are always done for a purpose. One common goal is to restore or replicate as closely as possible a wetland type that previously existed on the site or still exists at a nearby location. These are historical projects. The main criterion for success of historical projects is the demonstration that the plant and animal species that characterize the desired wetland type are present. Projects are also done to establish or re-establish a wetland that has certain function, for example to be a sink for pollutants in agricultural run-off. These are functional restorations or creations. The main criterion for success of functional projects is the demonstration that the wetland functions as expected, for example, 90% of the phosphorus entering the wetland is retained. All projects involve determining the goal of the project and then identifying and removing the constraints that prevent the restoration or creation of a wetland on a site (Fig. 7.4). These constraints can include physical conditions, for example, unsuitable hydrology, or biological constraints, for example, lack of seeds or other propagules. The removal of these constraints will need to be prioritized if there are multiple constraints. The actual project involves systematically removing these constraints, for example, ﬁlling in drainage ditches to restore the required

1. Determine restoration goal Restore what? (species composition, system type, service, structure) 4. Address constraints

2. Identify constrainsts Disturbance regime and/or physical conditions? Biogeochemical feedbacks owing to exotic/novel species? Herbivory or other trophic-level interactions? Seed source/species pool/landscape connectivity? Global/regional/environmental changes?

3. Prioritize constraints

Re-establish disturbance regime. Restore physical conditions

Single constraint? Address it

Remove specific species. Change resources to address feedbacks

Multiple constraints? Set priorities What constraints can be addressed simultaneously?

Exclude/re-introduce herbivores, symbionts, pathogens, etc. Add seeds/species. Establish landscape corridors Large-scale mitigation requiring social and political, as well as ecological, action 5. Characterize changed system

What actions require the least cost (time or money) to cross recovery thresholds?

What actions address dependencies and break feedbacks?

Have the goals been met? Do constraints remain? Have new constraints emerged? Is the goal not possible/feasible? Re-evaluate goals 6. Maintain the system

Fig. 7.4

Restoration planning: (1) determine restoration goal; (2) identify constraints that must be removed to achieve goal; (3) prioritize multi-constraints; (4) address or remove constraints; (5) characterize changed system and determine if any restraints remain or if new restraints have emerged; and (6) maintain system. (From Suding et al. 2004.)

146 THE BIOLOGY OF FRESHWATER WETLANDS

hydrology. The restored wetland then needs to be characterized to determine if the goal of the project has been met. If not, some constraints may remain or new constraints may have developed during the course of the project. Practical guidelines for designing and techniques for carrying out restorations and creations projects are available for many different types of wetlands (Northern Prairie Wildlife Research Center 2004). Some types of restorations and creations are relatively easy to do while others are both conceptually and practically very difﬁcult, for example, peatland restorations. How closely restored and created wetlands can resemble natural wetlands is still a debated question. The constraints faced by functional and historical projects can be signiﬁcantly different. In functional projects, getting the hydrology right is the main concern so that the wetland will function properly, for example, for phosphorus removal. In functional projects often little attention is paid to the vegetation. As a consequence, the vegetation of many of them is dominated by a small number of easily dispersed species. Historical projects have to get not only the hydrology right but also the vegetation. There are often major constraints to establishing plant species in these projects, and the establishment of desired plant species is often their most time-consuming and expensive part. Historical projects fall into two classes with regard to how plant species are re-established. For some types of wetlands, it is assumed that wetland species will become re-established from seed banks or from dispersal to the project from nearby wetlands. Natural re-vegetation sometimes works but often does not or only partially. In restored prairie potholes in North America, in which no species are planted, some species assemblages became established very quickly, most notably submerged aquatics, while others do not, for example sedge meadow species (Fig. 7.5). The former are widely dispersed by waterfowl but the latter are not. In projects in which plants are established from seed or other propagules, the major constraints are often ﬁnding suitable plant materials, planting them, and preventing them from being killed by herbivore and poor initial conditions for growth (too wet or too dry). For historical projects, one of the major uncertainties is how comparable the created or restored wetland will be to the reference natural wetland. Because of irremovable constraints, many created or restored wetlands may never be comparable to natural wetlands. Irremovable constraints may be both biological and environmental. Some wetland species or ecotypes may have been extirpated locally or, much more rarely, have gone extinct. New wetland species may have become established in the area. Regional changes in water chemistry, especially nutrient levels, may have occurred. Regional changes in hydrology, due to lowering or raising of regional water tables or changing precipitation patterns, may have occurred. Such irreversible changes make it

THE FUTURE OF WETLANDS 147

Submersed aquatic Deep emergent Shallow emergent Sedge meadow Wet prairie

No. of species

100 80 60 40 20 0 Fig. 7.5

Pristine

Natural

Restored

Species richness of ﬁve types of wetland species in restored prairie potholes, natural prairie potholes receiving agricultural runoff, and pristine prairie potholes in North America. From Galatowitsch and van der Valk (1995).

impossible to establish wetlands that are identical to those that existed previously. Consequently, the goal of historic restoration projects should not be to establish wetlands that are identical in every respect to a reference wetland, but to preserve as much of the biodiversity and genetic diversity of those wetlands as feasible. Even well-designed and executed projects may not closely resemble their natural counterparts. Zedler and Callaway (1999) examined longterm data from Sweetwater Marsh, a created coastal wetland in Southern California, to see if this wetland was similar to natural wetlands in the area (Fig. 7.6). Sweetwater Marsh was constructed to create habitat for several endangered species, including the light-footed clapper rail. One of the major problems with this restored wetland was that the grass that was established did not grow as tall as expected due to the low nutrient status of the coarse, sandy substrate. Because of the low stature of the vegetation, the light-footed clapper rail was not attracted to the site (Malakoff 1998). Adding nutrients to improve the growth of the grass, however, resulted in the growth of an unwanted species that out-competed the grass. Zedler and Callaway (1999) concluded that functional development of created or restored wetlands does not necessarily follow the simple trajectory models that are widespread in the restoration ecology literature. In other words, it was not possible to remove all the constraints preventing the development of the created wetland into a wetland with a plant canopy comparable to a natural wetland. Many created and restored wetlands are failures not because they failed to identify and remove all constraints, but simply because they were

148 THE BIOLOGY OF FRESHWATER WETLANDS

2

1

15

10

5

C 300

D 200 # Stems >90 cm

0

Total steam length

0

200

100

0

Fig. 7.6

B Soil organic matter (%)

3

Soil TKN (mg g–1)

A

150 100 50 0

0 2 4 6 8 10 Time (years)

0 2 4 6 8 10 Time (years)

Soil (A, B) and plant canopy characteristics (C, D) of natural (solid line) and created (dashed line) tidal wetlands in San Diego Bay, CA, USA. A. Total Kjeldahl nitrogen in surface soils. B. Soil organic matter content. C. Spartina foliosa total stem length at end of growing season. D. Number of S. foliosa shoots taller than 90 cm. (From Zedler and Callaway 1999.)

poorly designed and executed (Malakoff 1998). For example, in a study of wetlands constructed for mitigation purposes in Portland, OR, these wetlands sometimes had no natural analogues in the area. Their basins often had very steep sides. They often had plant communities dominated by invasive plant species and a low diversity of native wetland plant species. In some cases, the wetland was never constructed or was smaller than required. These results and other similar studies suggest that wetland creation and restoration practices often leave much to be desired (Salvesen 1994). Although a created or restored wetland may never be identical to a given natural wetland, it is, nevertheless, possible to create or restore wetlands that will closely resemble natural wetlands in function and composition. To do this, however, requires an understanding of the ecology of wetlands, careful planning and execution of projects, and a management strategy to keep the wetland on its desired developmental trajectory. Common management interventions include manipulations of water levels, ﬁre, grazing, fertilization, and various weed control methodologies.

THE FUTURE OF WETLANDS 149

7.5

Summary The drainage of wetlands to create land suitable for crops is still a problem in some parts of the world as are water projects that divert water from wetlands. Laws to protect wetlands from drainage and ﬁlling have successfully protected wetlands in some countries. The Ramsar Convention, an international effort to protect wetlands, has provided protection for many large wetlands around the world. Existing wetlands are being degraded by alterations in their hydrology and by increased nutrient inputs. These environmental changes have often been accompanied by major changes in vegetation, often due to the rapid spread of invasive species that were previously not present. Modelling efforts suggest that hydrological changes caused by projected global climate change will also alter wetland vegetation. The carbon stored in peatlands may be oxidized more rapidly due to these hydrological changes and this may accelerate global climate change. The restoration of wetlands promises to undo some of the wetland losses that have occurred. Unfortunately, many wetland restoration and creation projects have not been successful.

Glossary Acrotelm: the upper layer of peat in which the water table ﬂuctuates, which can be periodically aerobic, and in which water can move relatively rapidly. The layer below it is called the catotelm. Aerenchyma: air spaces in the roots, stems, and leaves of some hydrophytes that allow the diffusion of gases inside these plants. Aerenchyma production is often stimulated by the ﬂooding of plant organs. Allochthonous: pertaining to materials, usually litter, that were transported into an ecosystem from some other ecosystem. Anaerobic: occurring in the absence of molecular oxygen, for example, anaerobic respiration. Anaerobic respiration: See fermentation. Anoxic: without molecular oxygen. Autochthonous: pertaining to materials, usually litter, produced or originating within an ecosystem. Biomass: the mass of living microbes, plants, or animals present per unit area at any given time. It sometime includes dead parts of trees such as bark. Similar to as standing crop. Catotelm: the layer of peat below the acrotelm that is permanently saturated with water, that is anaerobic, and in which water moves very slowly. Climax: the ﬁnal, self-perpetuating stage in a Clementsian successional sequence in which the vegetation is in equilibrium with the climate. Coenocline: a series of plant communities along an environmental gradient. Creation: establishing a wetland in an area where a wetland did not formerly exist. Decomposition: the breakdown of litter or dead plant or animal material into smaller molecules or particles. Delineation: establishment of the boundary of a wetland for jurisdictional purposes. Typically the upper limit of hydric soils and/or hydrophyte abundance are used to establish wetland boundaries. Denitriﬁcation: conversion of nitrate to nitrogen gas. Detritivory: the consumption of dead organic matter (litter) and associated microorganisms by animals to obtain energy and nutrients. Detritus: partly decomposed plant or animal litter with its associated microorganisms. Discharge: the volume of water ﬂow per unit time; can also refer to groundwater entering a wetland as in discharge wetland.

GLOSSARY 151

Emergent hydrophytes (Emergents): erect, rooted, herbaceous hydrophytes whose bases may be permanently or seasonally ﬂooded, but which cannot survive longterm inundation of the entire plant, for example, cattail (Typha spp.). Enhancement: increasing or improving one or more functions of an existing wetland. Epipelon: assemblage of algae growing on the surface of soils, litter, and rocks. Epiphyton: assemblage of algae growing on living plants. Estuarine wetlands: wetlands found along the shores and in shallow water where rivers enter the ocean. Estuaries are transitional zones between freshwater and marine systems that are characterized by salinity gradients and daily tides. Eutrophic: nutrient rich. Evapotranspiration: the combination of evaporation and transpiration. Fermentation (glycolysis): the biochemical pathway by which starches and sugars are partially broken down to pyruvic acid. Under anoxic conditions, pyruvic acid is usually converted to acetaldehyde and CO2. Acetaldehyde is then typically reduced to ethyl alcohol. Floating-leaved plants: rooted, herbaceous hydrophytes with at least some leaves ﬂoating on the surface of the water, for example, water lilies (Nuphar spp. or Nymphaea spp.). Flood duration: the length of time that a wetland is ﬂooded. Flood frequency: the number of times a wetland is ﬂooded during a given period of time. Fluctuation: any reversible change in the abundance of species in a plant community from one year to another. Flux: the ﬂow or movement of material (e.g. litter, nutrients) from one compartment to another in an ecosystem. Free-ﬂoating plants: hydrophytes not anchored to a substrate that ﬂoat freely in the water column or on the surface of the water, for example, duckweeds (Lemna spp.) or water hyacinth (Eichhornia crassipes). Gleyed: neutral grey, bluish, or greenish colour of the soil matrix due to reduction of iron and other elements in soils that developed under anoxic conditions. Gross primary production: the total amount of organic matter produced through photosynthesis per unit area over some period of time. Histisol: Same as organic soil. Hydrochory: dispersal of seeds by water currents. Hydrogeomorphic classiﬁcation system (HGM): a system for classifying wetlands based on their geologic setting and resulting hydrology. It is designed to facilitate the evaluation of wetland functions and values. Hydrology: study of the distribution and movement of water. Hydric soil: soil that is saturated or ﬂooded long enough during the growing season to develop anaerobic conditions in the upper part. Hydrograph: a graphical record of the rise and fall of water levels during a given period of time.

152 GLOSSARY

Hydrophyte (hydric plant): a plant growing in soils that are ﬂooded or saturated long enough during the growing season that the soils go anoxic. Hydroperiod or water regime: the depth, duration, seasonality, and frequency of ﬂooding. The hydroperiod is usually expressed as either the mean number of days ﬂooded per year or the percentage of the year the wetland is ﬂooded. Hypoxic: with dissolved oxygen levels less than 2 mg l1. Lacustrine wetlands: wetlands fringing large lakes. Landscape: gross features of the land surface, including slope, aspect, topography, land use, etc. Litter: dead plant or animal material. Macrophytes: aquatic plants or hydrophytes large enough to see without a microscope, includes algae, mosses, liverworts, ferns, and angiosperms. Maturation: accumulation of biomass from one year to another due to the growth of individuals in an area. It is usually accompanied by changes in plant density and/or height. Metaphyton: algae that form mats visible to the naked eye. See also periphyton. Mitigation: avoiding, minimizing, rectifying, or compensating for wetland losses. Necromass: the mass of litter or dead plant and animal material per unit area at a given time. Muck: an organic soil in which virtually all of the organic material is decomposed, not allowing for identiﬁcation of plant forms. Net primary production: the amount of organic matter that accumulated per unit area over a period of time. Net primary productivity: the rate of organic matter accumulation per unit area. Nitriﬁcation: conversion of organic nitrogen and ammonium to nitrate. Nitrogen ﬁxation: conversion of nitrogen gas to ammonium. Nutrient budget: an accounting of the mass of a nutrient entering and leaving a system during a given interval. Nutrient cycling: the movement of nutrients among various components (soils, plants, litter, etc.) of an ecosystem. Oligotrophic: nutrient poor. Organic soils (Histisols): soils composed predominantly of decomposing plant litter (organic matter). Oxidation: a chemical reaction in which the oxidation state of a chemical is increased because of the loss of electrons. Oxidation number or state: Oxidation numbers are used to balance redox reaction equations. An oxidation number is the charge assigned to the more electronegative atom in a covalent bond. In effect, an oxidation number is an apparent charge; it is the charge the atom would possess if the bond were actually ionic. Some basic rules for assigning oxidation numbers to elements: (1) uncombined elements have an oxidation state of 0, for example, O2; (2) H in organic compounds is usually 1, for example, CH4, H2O; (3) O in compounds is usually 2, for example, H2O, CO2; and (4) the sum of all the oxidation states of the component atoms must equal the total charge on the molecule or ion.

GLOSSARY 153

Paludiﬁcation: the process in which an upland area is converted to a wetland, mostly a peatland, due to a rise in water table. Palustrine wetlands: wetlands found in shallow depressions, typically 20 acres in size. Peat: An organic soil composed primarily of partially decomposed plant organs that can still be identiﬁed. Periphyton: an assemblage of algae forming visible mats that often ﬂoat at or below the surface of the water for part of the growing season. Photosynthesis: conversion of carbon dioxide and water into sugars and oxygen by plants using energy from the sun. Plankton: algae, microbes, and invertebrates that are free-ﬂoating in the water column. Recharge: addition of water to an aquifer from a wetland. Redox (redox potential): the potential difference, usually expressed in millivolts, between a platinum electrode and a reference electrode in a solution. It is a measurement of reducing conditions or electron pressure (availability) in a solution. Redoximorphic features: soil features that are the results of reduction and translocation of iron and manganese in hydric soils. Reduction: a chemical reaction in which the oxidation state of a chemical is lowered by the addition of electrons. Restoration: the return of a wetland that had been destroyed (drained or ﬁlled) to its pre-disturbance state. Riverine wetlands: wetlands found in river channels. Saturated soils: soils in which all pore spaces are full of water and are thus anaerobic, but which have no standing water on the surface. Secondary production: production of herbivores, carnivores, and detritivores. Seed bank: the viable seeds or other propagules present in the soil at any given time. Soil matrix: the portion of the soil that has the dominant colour; usually more than 50% of the soil has to be of the same colour for it to be considered the matrix. Standing crop: the total amount of living or dead plant or animal material per unit area present at any given time. Similar to biomass. Submerged (submersed) plants: hydrophytes that normally grow entirely underwater, except for their reproductive parts for some species, for example, eel grass (Vallisneria americana). Succession: any non-seasonal change in the species composition of the vegetation in an area. Treatment wetlands: wetlands constructed to remove nutrients from the efﬂuent of wastewater plants or from polluted agricultural or urban run-off. Water budget: quantiﬁcation of inputs, storages, and outputs of water in a wetland or other water body over a given period of time, normally one year. Water quality: the physical, chemical, and biological properties of water. Water quality is usually judged from a human-use perspective. Water regime: See hydroperiod. Water table: the upper surface of a zone of saturation.

154 GLOSSARY

Wetland: an ecosystem that is constantly or recurrently shallowly ﬂooded or whose soil is constantly or recurrently saturated at or near the surface. Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation. Zonation: the distribution of species in more-or-less distinct bands along an environmental gradient. The various zones are often distinguished by differences in the life form, height, or colour of dominant species.

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Index

Page numbers in italics refer to boxes, ﬁgures and tables. Acrotelm 26, 150 Adventitious roots 67 Aerenchyma 62, 150 African wetlands and large grazers 84 and hippopotamus trails 103 Air-breathing ﬁsh 78, 79 amphibious air breathers 78 and predation 78 catﬁsh 78 evolution 78 lungﬁsh 79 survival during dry season 79 Algae 3, 18, 40, 41, 42, 43, 52, 166, 169 annual production 114 biomass 113 identiﬁcation 44 primary production 113 sampling 53 Allelopathy 92 Alligator 81, 82, 103 holes 82, 103 Alligator mississippiensis 81, 82, 103 Allochthonous 150 Ammoniﬁcation 129 Amphibians 81 Amphibious species 57 Anaerobic 150 Anas 76 Angiosperms, aquatic 5 Anhingas 73 Animal adaptations 57 buoyancy 14 distribution 57 Anisoptera 49 Anoxic 150 Antelopes and African wetlands 84 Aonyx capensis 83 Apple snail 77 Aquatic insects 33, 49, 50 activity traps 54 adaptations 49, 50 aeropneusty 49 emergence traps 54, 55

evolution 49 hydropneusty 49 Aquatic microorganisms 18 Aquatic plants buoyancy 14 Autochthonous 150 Aythya valisineria 76 Bacteria 34, 35, 38, 40, 132 assimilatory metabolism 38 autotrophs 35 dissimilatory metabolism 35 heterotrophs 35 Bald cypress 59 Barbus neumayeri 78 Beaver 83, 103 dams 103 Benthos 48 Birds. See Water birds Biodiversity 136 and topography 103 Biomass 110, 150 Blackbirds 71 Blue-green algae. See Cyanobacteria Bubalus arnee 83 Bubalus bubalis 83 C:N ratio 117 C:P ratio 117 Canvasbacks 76 Cape clawless otters 83 Capybara 83 Carbon cycling 124, 125, 126 Carbon dioxide 143 diffusion 66 in air 18 in water 18 production 127 Carbon storage 142 Carex 58 Carnivores 73 Carp 80 Castor canadensis 83 Catotelm 26, 150

170 INDEX Ceratophyllum demersum 67 Chironomidae 49 Chironomids 45, 46, 51, 52 larvae 51 life cycle 52 Cladium 58, 139 jamaicense 114 Clarias liocephalus 78 Clonal growth 69, 70, 71 and dispersal 69 and macrophyte genetic diversity 70 Coenocline 88, 89, 91, 95, 96, 150 development 92 gradient analysis 107 peatland 89, 92 Coleoptera 49 Competition 97 Conductivity. See Water Chemistry Cranes 71 Creation 144, 150 Ctenopoma muriei 78 Culicidae 45, 49 Cyanobacteria 41 Cyperus papyrus 58 Dabbling ducks 76 diets 76 Decomposition. See Litter Delta Marsh 77, 94, 95, 97 Denitriﬁcation 129, 150 Depth of water. See Hydrology Detritivory 150 Detritus 116, 150 Diptera 49, 51 Dissolved organic carbon 126 global climate change 143 Ducks 71, 73, 74, 76 adapatations 73, 76 and ﬁsh 122 dabbling 73 diving 73, 74, 76 foods 121 foraging strategies 74, 76 Eagle Lake 123, 124, 132 Eat outs, muskrat 83, 84 Economic value 136 Egrets 71 Eichhornia crassipes 58, 67, 139 Eleocharis cellulosa 114 Emergent vegetation 11 Emergents 58, 59, 61, 151 morphology 85 water depth tolerances 70 Energy budgets, heat ﬂux 21 Enhancement 144, 151 Ephemeroptera 49 Ephydridae 49 Epilython 44 Epipelon 43, 44, 152 Epiphyton 43, 44, 114, 151 invertebrate 48 Epipsammon 44

Epizoon 44 Evapotranspiration 21, 151 prairie potholes 21 Everglades 1, 77, 82, 103, 107, 109, 114, 120, 122, 140 and agricultural runoff 139 and ﬁre 97 restoration 144 tree islands 106 Facultative wetland species 57 Fermentation 19, 36, 38, 65, 110, 151 Fire 70, 97, 143, 161 Fish 33, 77, 78, 158, 159, 163, 166. See also Air-breathing ﬁsh and low oxygen 78 and wetlands 80 aquatic surface respiration 78 as competitors for waterfowl 80 sampling 86 Flamingos 71 Floating plants 58, 60 Floating-leaved plants 58, 60, 151 Flood reduction 136 Fluctuation 89, 101, 151 Food chains. See Food webs Food webs 74, 109, 118, 119 autotrophic pathways 119 Everglades 120 heterotrophic pathways 119 prairie potholes 121, 123 role of invertebrates 119 stable isotope ratios 122, 123 Free-ﬂoating plants 151 Functions. See Wetland functions Fungi 39, 132 Gas diffusion in water 18 Gas-space continuum 60, 62, 64, 65, 66 Geese 71 Global climate change 141, 142, 143 and carbon 143 and carbon storage 143 Gradient Analysis 107 Grazing 102 Groundwater 25, 27 chemistry 18 Gulls 71 Gut content analysis 121 Harvest method 133 Hemiptera 49 Herbivores 5, 73 Herons 71 Heterophylly 60, 63, 67, 68, 69, 84 facultative 68 ontogenetic 67 Hippopotamus 83, 103 Hippopotamus amphibius 83, 103 Hydric soil 6, 130 Hydrochoerus hydrochaeris 83 Hydrogen sulﬁde 131 Hydrogeomorphic classiﬁcation system 151

INDEX 171

Hydrograph 26, 28, 152 Hydrological budget. See Water budget Hydrology 4, 6, 8, 22, 28, 29 and prey availability 29 and the distribution of organisms 22 hydrograph 28, 31 hydroperiod 8, 28 statisitical description of 28 water level 30 water regime 22, 28, 29 Hydroperiod. See Water regime Hydrophyte. See Macrophytes Ibises 71 Immobilization 129 Incidental wetland species 57 Invasive species 139 Invertebrates 16, 33, 44, 45, 52, 122 activity traps 53, 54 and litter decomposition study 134 sampling 53, 54 terrestrial 44 Iron 131 oxidizers 36 reduction 38 Kingﬁshers 71 Kissimmee River 121 restoration 144 Kobus 84 Lacunae 62 Lacustrine 153 Landforms 2, 3, 8 Lates niloticus 80 Lechwe 84 Lenticels 66 Light 15, 16 extinction 15, 16 reﬂected by water 16 scattering by water 16 under plant canopies 16 Light-footed clapper rail 147 Lignin in litter 117 Lithophagy 14 Litter 115, 116, 117, 118, 153 decomposition 40, 115, 117, 118, 129 fallen 127 leaching 131 macrophyte 117 standing 127 study 134 Lungﬁsh. See Air-breathing ﬁsh Lythrum salicaria 141 Macrobiota 33 Macrofauna. See Animal Macrophyte canopies 13, 20 and environmental gradients 22 evapotranspiration 21 leaf area 21 light reduction 21

Macrophytes 3, 56, 58, 63, 109, 153 adaptations 57, 85 and grazing 118 as nutrient pumps 125 classiﬁcation 58 dicots 85 ﬂooding tolerances 95 ﬂood-tolerators 68 growth forms 14, 58, 59 hydraulic resistance 16 monocots 85 nutrient concentrations 116 phenotypic plasticity 63, 67 pollination 85 porosity 62, 64 primary production 113 primary production study 133 seeds 71 Macro-succession. See Succession, macroMacrotermes michaelseni 104 Mammals 82, 83 Manganese 130 oxidizers 36 reduction 38 Maturation 89, 90, 153 Melaleuca 139 Metaphyton 43, 44, 114 Methane 126, 143 production study 134, 135 Methanogenesis 38 Methanotrophs 36 Microbiota 33 Microhabitats 56, 71 Microorganisms 6, 33, 34 Moist-soil species 59 Muskrats 83, 84, 102 Mycorrhizae 40 Myocastor coypus 83, 102 Nekton 46 Neuston 48 Nitogen ﬁxation 38 Nitrate reduction 38 Nitriﬁcation 129, 153 Nitriﬁers 36 Nitrogen cycling 125, 126, 129 Nitrogen ﬁxation 129, 153 symbiotic 39 Nutria 83, 102 Nutrient budget 123, 153 Nutrient cycling 109, 123, 133, 153 carbon 125, 126 nitrogen 128 phosphorus 130, 133 sulfur 131 Nutrient removal 123 Nymphaea 58 Obligate wetland species 57 Odonota 49 Okavango 1, 77, 104, 105, 107, 138 tree island development 104 Omnivores 73

172 INDEX Ondatra zibethicus. See Mukrats Oxygen and ﬁsh 17, 77 and litter decomposition 17, 117 as a limiting factor 19 diffusion in macrophytes 63 dissolved in water 18, 20 gradients 20, 32 in air 18 mass ﬂow in macrophytes 63 radial oxygen loss 66 Paludiﬁcation 9, 100, 154 Palustrine 154 Palustrine wetlands 26 and ﬁsh 77 Papyrus 58 Papyrus swamp Africa 78, 80 Particulate organic carbon 126 Peatland 10, 18, 26, 81, 101 and pH 19 coenoclines 92 East Anglian fens 98 Pelicans 71 Perca ﬂavescens 80 Perch, Nile 80 Perch, yellow 80 Periphyton 44, 155 pH 18, 31, 34, 48, 58, 77, 81, 92, 100, 117 Phalaris arundinacea 141 Phosphorus cycling 128, 131, 133 removal mechanisms 131 Photosynthesis 35, 38, 110, 154 Phragmites australis 58, 64, 65, 94, 110, 141 Pistia stratiodes 139 Plankton 43, 46, 154 invertebrate 48 Plant canopies. See Macrophyte canopies Plecoptera 49 Plovers 71 Pneumatophores 66 Podostemaceae 58 Pomacea paludosus 77 Potamogeton 63 Practical Experiments and Observations 10, 30, 53, 85, 107, 133 Prairie potholes 1, 97, 121, 122, 141, 142 and global climate change 141 Primary producers 3, 5. See also Algae and Macrophytes Primary production 109, 110, 111, 112, 113, 153 algal 114 and wet–dry cycles 113 annual 112 belowground 113 harvest method 110, 111 net 110 Purple Loosestrife 141

Rails 71 Ramsar Convention 12, 137, 138 Rat-tailed maggots 49 Redox 37, 38, 125, 154 measuring 37 potential 37 oxidation states 125 reactions 35, 129 Redoxymorphic features 130 Reed canary grass 141 Refugia and ﬁsh 77 Reptiles 81 Respiration aerobic 36, 37 anaerobic 36, 38 Restoration 143, 144, 145, 148, 149, 154 constraints 144 establishing plant species 146 functional 144 historical 144 prairie potholes 148 problems and failures 147 Riverine 155 Riverine wetlands 26 and amphibians and reptiles 81 Rostrhamus sociabilis 76 Sagittaria 85 Salix 59 Salvinia molesta 141 Sandpipers 71 Sawgrass. See Cladium Schoenoplectus 85 lacustris 94 Scirpus 85 lacustris 94, 97 Scolochloa festucacea 94, 97 Sedge. See Carex Sediment and vegetation change 108 trapping 136 Seed dispersal 95 germination 95 Seed banks 71, 154 sampling 85 Seedling survival 95 Shorebirds 71 Sitatunga 84 Snow geese 102 Soils anaerobic 3 organic matter content 128 Sparganium 85 Spatial patterns. See Coenocline Sphagnum 88, 100 Stable isotope ratios 121, 122 Standing crop 154 Stilts 71 Storks 71

INDEX 173

Submerged plants 58, 59 and algae 92 and oxygen levels 21 internal air spaces 85 internal gas storage 66 light extinction 92 Succession 90, 98, 154 Succession, macro- 98. See also Paludiﬁcation East Anglian fens 98, 99, 100, 108 Estonian peatlands 98, 99 Succession, micro- 89, 90, 94, 102 Sulfur cycling 125, 127, 131 Sulphate reduction 38 Sulphide oxidizers 36 Swans 71 Sweetwater Marsh 147 Syrphidae 49 Taxodium distichum 59 Temperature and litter decomposition 117 and plant canopies 20 water 15 Temporal changes. See Succession Termite mounds 104 Terns 71 Terrestrialization. See Succession, macroTragelaphus spekei 84 Tree islands 103 and water chemistry 106 development in Okavango 104, 105 Trichoptera 49 Turtles 81 Typha 58, 85, 94, 97, 139 Typha domingensis 114 Utricularia 58 Vallisneria 58 Vegetation dynamics. See Succession, micro- and Wet–dry cycles Wading birds 6, 71, 122 and amphibians 81 hydrology 29 Water currents 16 density 13, 14, 15 speciﬁc heat 14 temperature 17, 31 thermal conductivity 15 transparancy 16 velocity 93 viscosity 16 Water budget 22, 25, 26, 27, 109, 154 Water buffalo wild 83 domestic 83

Water chemistry 8, 18, 31, 81 gradients 92 plant distribution 18 speciﬁc conductance 31 water source 18 Water hyacinth 139, 141 Water lettuce 139, 141 Water regime 24, 26, 28, 29, 153. See Hydrology and elevation 23, 24 intermittently ﬂooded 23, 24 permanently ﬂooded 23, 24 saturated soil 23, 24 seasonally ﬂooded 23, 24 semipermanently ﬂooded 23, 24 temporarily ﬂooded 23, 24 Waterfowl. See Wetland birds Water-level ﬂuctuations 13, 101 impact on vegetation 101 Waterlogged Wealth 136 Wet–dry cycles 28, 70, 94, 98 and global climate change 141, 142 Wetland birds 6, 71, 73, 74 adapatations 71 diets 74, 75, 76 distribution 72, 75 observation 86 Wetland classiﬁcation 6, 11 American 6, 7 and hydrology 23 hydrogeomorphic 6, 8 Ramsar 138 Wetland degradation agricultural runoff 139 Wetland functions 109, 136. See also Food Webs, Primary production, and Nutrient cycling Wetland inventories 9, 10 Wetland losses 10, 136, 137 Wetland organisms factors controlling distribution 13 Wetland resoration. See Restoration Wetland soil. See Hydric soil Wetland species obligate 5 Wetland values 136 Wetlands as refugia for ﬁsh 80 characterstics 3, 4 conversion 138 deﬁnition 6 global carbon storage 143 types 1, 2 Willow 59 Winter 15 Zonation. See Coenocline Zooplankton 43 Zygoptera 49