PREFACE
Muddy coasts are land-sea transitional environments commonly found along lowenergy shorelines which either receive large annual supplies of muddy sediments, or where unconsolidated muddy deposits are being eroded by wave action. Muddy coasts are found in all kinds of climates and under any tidal conditions. Accordingly, their geographic distribution ranges from low tropical to high sub-arctic latitudes and from microtidal to macrotidal coastal settings. The most conspicuous examples are the vast mangrove swamps of the tropics and the extensive salt marshes fringing the shores of estuaries and back-barrier lagoons of mid-latitudinal coasts. Muddy coastal environments harbour highly variable and fragile ecosystems which, for the most part, are still poorly understood. Today these ecosystems are not only threatened by the growing economic interests of man (e.g., tourism, fisheries, aquaculture, land reclamation) but also by the prospect of an accelerating sea-level rise in the wake of global warming. While the detrimental effects of the former are increasingly becoming evident, those of the latter are still largely unknown. In order to provide an up-to-date review of the state of the art in muddy coast research, and to identify gaps in our knowledge, both in a scientific and geographic sense, and to define priorities for future research, an international conference entitled "Muddy Coasts 97" was convened in Wilhelmshaven, Germany, in September 1997. The conference was co-sponsored by the Senckenberg Natural History Society (Frankfurt), the Terramare Research Centre (Wilhelmshaven), the Federal Ministry of Science and Technology (Berlin), the Deutsche Forschungsgemeinschaft (Bonn), and last but least the Scientific Committee on Oceanic Research (SCOR) under the able participation of Working Group 106. The book "Muddy Coast Dynamics and Resource Management" forms part of the proceedings and has been edited by the conference organisers. It presents 21 regional case-studies from different parts of the world, including the southern Baltic Sea of Germany (6), the German Wadden Sea (6), the Wash in the U.K. (1), Portugal (1), the U. S. A. (1), Cameroon (1), Tanzania (1), Korea (1), and China (3). The studies deal with hydrodynamics and suspended particulate matter in bays and back-barrier tidal basins, erosion, deposition, and sediment budgets on tidal flats, primary production, nutrient fluxes and mineralisation in shallow lagoons (Bodden), sediment geochemistry of salt marshes and Holocene marine deposits, impacts of sea-level rise and land reclamation, and resource management of muddy coasts. The book is designated as a companion volume to the proceedings of the SCOR Working Group 106 published under the the title "Muddy Coasts of the World: Processes, Deposits and Function" edited by Terry Healy (New Zealand) and Ying Wang (China). The editors wish to express their sincerest gratitude to the numerous unnamed referees who have contributed substantially to the high standard of the contributions.
Burg Flemming, Monique Delafontaine, and Gerd Liebezeit Wilhelmshaven, August 2000
CONTRIBUTORS (current addresses) M.O. Andreae Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany C.A. Angwe
Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon I. Austen Mittelstr. 26 25709 Kronprinzenkoog Germany H.-D. Babenzien Institut fi~r Gew~isser6kologie und Binnenfischerei Alte Fischerh~tte 2 16775 Neuglobsow Germany H.W. Bange Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany A. Bartholomii Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany S. Berghoff Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
H.J. Black Institut f6r Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.I. Ca~ador
Instituto de Oceanografia Departamento de Biologia Vegetal Universidade de Lisboa 1700 Lisboa Portugal K.-S. Choi
Department of Oceanography Seoul National University Seoul 151-742 Korea M. Collins Department of Oceanography Southampton Oceanography Centre University of Southampton SO14 3ZH Southampton U.K. S. Dahlke Institut fiir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.T. Delafontaine
Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany B.W. Hemming
Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany
xii C.E. Gabche
Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon S. Gerbersdorf Institut fi.ir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany M.-K. Han
Department of Geography Peking University Beijing 100871 P.R. China X. Ke
Department of Urban and Resources Science Nanjing University 22 Hankou Road Nanjing 210093 P.R. China B.-K. Khim Polar Research Center Ocean Research and Development Institute P.O. Box 29 Ansan 425600 Korea M. Kb'ster Institut ffir Okologie Universit~it Greifswald Schwedenhagen 6 18565 Kloster Germany
Y.-F. Liu Department of Geography Peking University Beijing 100871 P.R. China M.I. Madureira IPIMAR Av. Brasflia 1400 Lisboa Portugal S. Mai Eifelstr. 46 60529 Frankfurt-Schwarnheim Germany A.J. Mehta Coastal and Oceanographic Engineering Department University of Florida P.O. Box 116590 Gainesville, FL 32611 U.S.A. ]. Meyercordt Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany L.-A. Meyer-Reil Institut fiir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany N. Mimura
Department of Urban System Engineering Ibaraki University Hitachi 316 Japan
xiii
O.U. Mwaipopo Institute of Marine Sciences University of Dar Es Salaam P.O. Box 668 Zanzibar Tanzania N. Nyandwi Institute of Marine Sciences University of Dar Es Salaam P.O. Box 668 Zanzibar Tanzania T.M. Parchure Coastal and Hydraulics Laboratory U.S. Army Engineer Waterways Experiment Station Vicksburg, MS 39180 U.S.A.
P. Santamarina Cuneo Senckenberg Institute Schleusenstr. 39a 26382 Wilhelmshaven Germany G. Schlungbaum Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
U. Selig Department of Biology Rostock University Freiligrathstr. 7/8 18051 Rostock Germany
Y.-A. Park Department of Oceanography Seoul National University Seoul 151-742 Korea
I. Stodian Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany
R. Ramesh Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany
C. Vale IPIMAR Av. Brasflia 1400 Lisboa Portugal
S. Rapsomanikis Max Planck Institute for Chemistry P.O. Box 3060 55020 Mainz Germany
A. Voigt Institut f~ir Gew~isser6kologie und Binnenfischerei Alte Fischerh~itte 2 16775 Neuglobsow Germany
T. Rieling Institut fihr Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany
I. Wang National Marine Data and Information Service State Oceanic Administration 93 Liuwei Road Tianjin 300171 P.R. China
xiv Y. Wang State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
c. Wolff Institut ftir Okologie Universit/it Greifswald Schwedenhagen 6 18565 Kloster Germany L. Wu
Department of Geography Peking University Beijing 100871 P.R. China T.J. Youmbi Research Centre for Fisheries and Oceanography PMB 77 Limbe South-West Province Cameroon J. Zhang National Marine Data and Information Service State Oceanic Administration 93 Liuwei Road Tianjin 300171 P.R. China D. Zhu State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
X. Zou State Pilot Laboratory of Coast and Island Exploitation Nanjing University 22 Hankou Road Nanjing 210093 P.R. China
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000Elsevier ScienceB.V. All rights reserved.
Hydrodynamics of Chwaka Bay, a shallow mangrove-fringed tropical embayment, Tanzania N. Nyandwi* and O. U. Mwaipopo
University of Dar Es Salaam, Institute of Marine Sciences, P. O. Box 668, Zanzibar, Tanzania
ABSTRACT
Time-series data of currents, sea levels and temperatures from Chwaka Bay, Zanzibar were analysed with the view of understanding the water circulation of the bay. The analyses show that there is a tidal asymmetry in the bay, with peak ebb tidal currents in the deep channels (45 cm s being stronger than flood tidal currents (35 cm s-l), and ebb periods (7 hours) being longer than flood periods (5 hours). The velocity and time asymmetry as well as the asymmetry in the current direction are controlled by the morphological variations of the tidal basin. It was found that, as the water flows from the inner bay during the ebbing tide, it first drains towards the main tidal creek which leads to concentrated but delayed flows. The temperature variations in the inner part of the bay are predominantly diurnal, whereas at the mouth of the bay they are semi-diurnal. There is a general temperature gradient between the inner bay and the mouth, the highest temperatures being recorded in the inner bay (30.14~ This indicates high residence times of the bay waters, presumably resulting from entrapment.
1. INTRODUCTION The hydrology of many tropical, mangrove-lined bays are characterized by salinity gradients even in areas without visible river supply, and by the entrapment of water in the mangrove forests (e.g., Wolanski et al. 1980; Wolanski 1989). Similarly, spatial and temporal variations in tidal current velocities are commonly observed. Thus, in Coral Creek, Australia, peak current velocities are generally higher than 1 m s in the tidal creek, whereas they hardly exceed 0.07 m s-1 in the mangroves (Wolanski et al. 1980). Indeed, a tidal velocity asymmetry was actually reproduced in a numerical model using the Coral Creek data. Furthermore, it was observed that human activities such as land reclamation and the felling of mangrove trees tend to reduce the magnitude and asymmetry of the tidal currents (e.g., Wolanski 1992).
* Corresponding author: N. Nyandwi e-mail:
[email protected]
4
Nyandwi and Mwaipopo
Salinity variations are usually observed between the inner and outer parts of bays and creeks. Several factors which may produce salinity gradients have been identified, including groundwater infiltration, evapotranspiration, and surface freshwater influx (e.g., Wolanski et al. 1980; Mazda et al. 1990; Ridd et al. 1990). Dilution by freshwater influx into mangrove areas usually produces a pronounced salinity gradient between the bay and the mangroves. Groundwater infiltration, which commonly occurs along the landward reaches of tidal creeks, can have a similar effect. It is also thought to be an important flushing mechanism of salts left behind by evapotranspiration (Wolanski & Gardiner 1981). The only exception to the above rules are associated with the conditions in hot and dry environments where evapotranspiration may cause an increase in salinity landwards of mangrove creeks (Wolanski et al. 1980; Ridd et al. 1990; Wattayakorn et al. 1990). A landward increase in salinity under such circumstances can be attributed to the extraction of freshwater from seawater by mangroves (Wolanski & Gardiner 1981). The saline water resulting from evapotranspiration may thus induce an inverse estuarine circulation (Wolanski 1992). Another factor which may affect the circulation pattern is the trapping of water in mangrove ecosystems (Okubo 1973), the amount of trapped water appearing to determine general flushing rates (Wolanski 1992). In the case of Chwaka Bay, a mangrove-lined embayment along the east coast of Zanzibar Island, Tanzania (Fig. 1A), the existence of a velocity asymmetry was observed but not verified because data on current variations in the tidal creeks and mangrove areas were lacking at the time (Wolanski 1989). Similarly, water entrapment and groundwater infiltration have been suggested as possible factors contributing to the offshore decrease in water temperature and the increase in salinity in the bay (Wolanski 1989). The exchange of water between Chwaka Bay and the open sea is not well understood, and there is no information on the heat budget of the area. The collection of data on temperature distribution and temporal variation would therefore be an important first step towards establishing a local heat budget. At the same time, a better knowledge of current patterns in the bay would not only contribute towards a better understanding of nutrient dynamics, waste dispersal, and water quality in general, but could also help explain why muds accumulating in the mangrove forests are never flushed out to impair coral reef growth at the mouth of the bay.
2. S T U D Y AREA
Chwaka Bay is located on the east coast of Unguja Island (Zanzibar) which is situated off the East African coast centred around 6~ and 39 ~ 30'E (Fig. 1B). It is a shallow embayment with an area of approximately 50 km 2 at high water springs (HWS). Its mouth is barred by a living offshore coral reef. A dead reef lines the southern landward end wl~ch is fringed by a 1 to 3-km-wide mangrove forest. The bathymetry of the bay was first studied by Wolanski (1989), using a portable echo sounder from a small boat operating along east-west transects in the bay and the mangrove creek. Water depths relative to mean sea level (MSL) are mostly less than
Hydrodynamics of Chwaka Bay, Tanzania
5 m along the eastern side of the bay. There are several tidal creeks in the open water of the bay, some of which connect to the mangrove creeks in the south.
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Figure 1. A: Location of the study area off the African east coast. B: Position of measurement stations in Chwaka Bay. Current meters and tide gauges were deployed at stations I and 2, whereas a tide gauge only was deployed at station 3.
The water movement in Chwaka Bay is controlled mainly by tidal motions. According to tidal records from the harbours of Dar Es Salaam and Zanzibar, the tide in this part of the Indian Ocean is semi-diurnal, being dominated by the M 2 component (e.g., Lwiza & Bigendako 1988). Older measurements indicate that peak ebb currents are stronger than peak flood currents, suggesting a tidal asymmetry in the bay (Wolanski 1989). The mean spring tidal range in the bay is 3.2 m. The main ecosystems in the bay include mangrove swamps, coral reefs and seagrass meadows. There are large intertidal areas which have recently attracted seaweed farming. Although there is no obvious freshwater supply to the bay, salinity measurements in Mapopwe creek showed values of 29.5-35%o (Wolanski 1989), suggesting some freshwater input to the mangrove swamps. Since no surface runoff exists, freshwater can only be supplied by groundwater seepage. This type of freshwater input was, in fact, suggested by Mazda et al. (1990). Being part of the East African region, Chwaka Bay is subject to two alternating seasons, the south-eastern (SE) and the north-eastern (NE) monsoons. The former
6
Nyandwi and Mwaipopo
begins in April and ends in October, whereas the latter begins in November and ends in March. During the SE monsoon the winds blow predominantly from the south-east, being accompanied by heavy rains and thunderstorms. Heavy rains are particularly common between March and June. During the NE monsoon the winds blow mainly from the north-east. This season includes the 'short rains' between October and November. Normally the area is cool with occasional light rains between June and September. From December to March the weather is relatively hot and dry with only very occasional rain. Meteorological data from Tanzania indicate that the mean values for rain and evaporation along the coast are 120 mm month -~ and 4 mm day -~, respectively. The East African Coastal Current (EACC) flows northwards throughout the year but differs markedly between the two monsoon seasons (Newell 1959). Thus, during the SE monsoon surface current velocities reach 4 knots (2 m s-~),being amplified by the trade winds of the Indian Ocean. During the NE monsoon, by contrast, the EACC still flows to the north but its speed is reduced to about 0.5 knots (0.25 m s-~) due to the domination of north-easterly winds (Newell 1959).
3. MATERIALS A N D M E T H O D S
Water fluxes and circulation patterns in Chwaka Bay were measured with selfrecording current meters and tide gauges equipped with temperature sensors. The temperature data were used to study the heat flux in both Mapopwe creek and the bay. Sea-level data obtained from the tide gauges were used to show the temperature and flow variations in the course of a tidal cycle. Three tide gauges of the type Micro-Tide, and two Sensordata SD6000 recording current meters were deployed in Chwaka Bay for one month in August-September 1992. The Sensordata SD6000 is a compact vector averaging current meter with memory capacity for up to 6000 combined data sets of current speed, direction and water temperature. The tide gauges have a memory capacity of 200 MB, and can measure and record combined data sets of pressure (water level) and temperature. A tide gauge and a current meter were deployed at the entrance of Mapopwe creek, a mangrove creek in the south-western part of the bay (station 1). A similar set was deployed in the middle of the bay (station 2), whereas a third tide gauge was located at the mouth of the bay (station 3). Three reference points (Security House at Chwaka village, Ras Juja and Ras Michamwe; cf. Fig. 1B) were used to determine the exact positions of the instruments by means of triangulation. The tide gauges and recording current meters were programmed to measure and record at 10-minute intervals. The tide gauges essentially recorded without interruption over the whole sampling period of about one month. The current metres at stations 1 and 2 experienced short interruptions when their propellers were fouled by seaweed. A set of manually operated gelatine pendulum current meters (Haamer 1974; Cederl6f et al. 1995) were deployed from a boat at a number of different stations within the bay during peak tidal flow in order to compile a map of spatial current
Hydrodynamics of Chwaka Bay, Tanzania
speed and direction patterns. Measurements with the pendulum current meters were also undertaken on several occasions between 1992 and 1994, particularly at times of maximum ebb or flood currents (i.e. approximately 3 hours after high and low tide, respectively).
4. RESULTS 4.1. F l o w patterns
The temporal patterns of the tidal currents at stations 1 and 2 are illustrated in the time series of Fig. 2, whereas the spatial patterns within the bay are shown in Fig. 3. From Fig. 2 it is observed that the maximum ebb currents (positive values) at station 1 are stronger than the maximum flood currents, and the ebb phase is longer (about 7 hours) than the flood phase (about 5 hours), indicating both velocity and time asymmetry. At station 2, however, there is no time asymmetry, and the velocity asymmetry was found to be less, with the peak flood velocities being slightly higher than the ebb velocities. The flow directions in Fig. 3 suggest an asymmetry in the tidal current direction, especially on the west bank where the flood current flows southwards whereas the ebb current flows about north-north-eastwards.
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Figure 2A. Time-series plot showing the temporal pattern of the current velocity and the tidal elevation at station 1 in the period 21-25 August 1992. Negative values indicate the flood period, positive values the ebb period.
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Hydrodynamics of Chwaka Bay, Tanzania
4.2. Variations in water temperature A comparison of the temperature records from the three stations (Table 1) reveals a progressive decrease in the temperature maxima from the inner to the outer parts of the bay. Thus, at station 1 the maximum temperature is 30.14~ at station 2 it is 29.24~ and at station 3 it is 28.05~ The average temperatures at stations 1, 2 and 3 are 27.25, 26.96, and 26.27~ respectively. Minimum temperatures, by contrast, are almost identical at all three stations. In the inner bay (station 1), the lowest and highest daily temperatures occur at about 05 hours in the morning and 15 hours in the afternoon, respectively.
Table 1. Summary of temperature (~ 1992.
records for the period 17 August-21 September
Station I
Station 2
Station 3
Maximum
30.14
29.24
28.05
Minimum
25.38
24.93
25.00
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27.25
26.96
26.27
Maximum range
4.76
4.31
3.05
Temporal temperature patterns relative to tidal elevations recorded in the period 18-22 August 1992 are shown in Fig. 4. The tidal elevation curves are almost identical at all three stations. The temperature variations, by contrast, follow a different pattern at each station. Thus, in the shallow inner part of the bay (station 1), a distinct daily heating (day) and cooling (night) trend is observed (Fig. 4A). This daily signature is greatly modified at the other two stations. At station 2, the pattern clearly begins to depart from the daynight rhythm (Fig. 4B), and at station 3 the pattern is fully synchronized with the tidal motion of the water body (Fig. 4C). In the inner part of the bay the temperature trend thus shows a distinct diurnal pattern controlled by the daily heating and cooling process. By comparison, the central part of the bay shows a mixed diurnal/semi-diurnal pattern which is partly still dominated by the daily temperature curve, whereas in the outer bay the water temperature follows a diurnal pattern which is completely dominated by the tide.
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Hydrodynamics of Chwaka Bay, Tanzania
11
5. DISCUSSION AND CONCLUSIONS Chwaka Bay is sheltered from the open ocean, and hence from the larger-scale oceanic circulation by the presence of a coral-reef barrier in front of the bay entrance. Several coves along the southern shore of the bay are lined by mangrove forests in which mud is trapped and confined. By contrast, the outer part of the bay is dominated by living coral reefs. Their existence provides direct evidence that mud is evidently not exported from the mangroves towards the open sea. There are several mechanisms by which the mud may be confined to the mangroves. The results of the present study reveal an asymmetry in the current directions in the shallow western part of the bay, flood currents flowing southwards and ebb currents north-north-eastwards. Given the fact that ebb currents are stronger, this results in a net eastward mass transport, i.e. away from the western shore of the bay. As a result of this circulation pattern, any muds resuspended in the western mangrove coves are transported towards the east where they are probably trapped in the neighbouring coves. By this mechanism the retention of the mud within the bay can be explained. The finding that the heating of the water mass in the outer bay follows a semidiurnal cycle, while that of the inner bay is diurnal, demonstrates that the two water masses are not regularly exchanged. The diurnal heating of the water mass in the inner bay suggests a high residence time, a feature which strongly enhances the trapping efficiency of fine particles in the bay. Resuspended muds are thus simply recirculated in the bay until they are once more deposited under favourable conditions in sheltered locations. Water trapping also has positive implications for the retention of nutrients in the bay, supporting a high productivity and thereby favouring local fisheries. In addition, it enhances the health of the coral reef by preventing mud from reaching the outer bay.
ACKNOWLEDGEMENTS
The financial assistance provided by the Institute of Marine Sciences (University of Dar Es Salaam, Zanzibar) through the donation from SAREC is acknowledged.
REFERENCES
Cederl6f, U., Rohde, J., Rydberg, L. & Sehlstedt, P. (1996) Performance study of the Haamer gelatin pendulum current meter. J. Sea Res. 35: 55-61. Haamer, J. (1974) Current measurements with gelatine pendulums. Vatten 1 (74). Lwiza, K.M.M. & Bigendako, P.K. (1988) Kunduchi tides. Tanz. J. Sci. 14: 65-76. Mazda, Y., Sato, Y, Swamoto, S., Yakochi, H. & Wolanski, E. (1990) Links between physical, chemical and biological processes in Bashita-Minato, a mangrove swamp in Japan. Estuar. Coast. Shelf Sci. 31: 817-833.
12
Nyandwi and Mwaipopo
Newell, B.S. (1959) The Hydrography of the British East African Coastal Waters Part II. Fish. Publ. Colon. Off. No. 12, 18 p. Okubo, A. (1973) Effect of shoreline irregularities on streamwise dispersion in estuaries and other embayments. Neth. J. Sea Res. 6: 213-224. Ridd, P.V., Wolanski, E. & Mazda, Y. (1990) Longitudinal diffusion in mangrovefringed tidal creeks. Estuar. Coast. Shelf Sci. 31: 541-544. Wattayakorn, G., Wolanski, E. & Kjerfve, B. (1990) Mixing, trapping and outwelling in the Klong Ngao mangrove swamp, Thailand. Estuar. Coast. Shelf Sci. 31: 667-688. Wolanski, E. (1989) Measurements and modelling of the water circulation in mangrove swamps. UNESCO-COMARAF S6rie Document. No. 3, 43 p. Wolanski, E. (1992) Hydrodynamics of mangrove swamps and their coastal waters. Hydrobiologia 247: 141-161. Wolanski, E. & Gardiner, R. (1981) Flushing of salt from mangrove swamps. Aust. J. Mar. Freshwat. Res. 32: 681-683. Wolanski, E., Jones, M. & Bunt, J.S. (1980) Hydrodynamics of tidal creek-mangrove swamp systems. Aust. J. Mar. Freshwat. Res. 31: 431-450.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
13
T i d a l c h a r a c t e r i s t i c s of a n a c c r e t i o n a l t i d a l flat ( T h e W a s h , U.K.) X. Ke a'b* and M. Collins b
aDepartment of Urban and Resources Science, Nanjing University, 22 Hankou Road, Nanjing 210093, P.R. China bDepartment of Oceanography, Southampton Oceanography Centre, University of Southampton, S014 3ZH Southampton, U.K. ABSTRACT
Tidal current velocities and water depths have been surveyed repeatedly across an intertidal flat/saltmarsh in a rectangular coastal embayment (The Wash) over three consecutive years to study the tidal hydrodynamics. The tide in the bay has a standing wave character. In response to the changing elevations, the standing tidal wave over the tidal flat is shown to normally be associated with tidal current velocity maxima (i) at mid tide at/below mean low water, and (ii) at or shortly after the end of the ebb and beginning of the flood at/above mid-tide level. Tidal asymmetry is prominent, the ratio between the duration of the flood and ebb phases being 0.65-1.0. This indicates that the flood tidal flows are generally of shorter duration and of higher current velocities. The tidal currents have different velocities in the offshore channels (ca. 1.2 m s-l), over the exposed intertidal flats (ca. 0.5--0.7 m s-l), and in the saltmarshes (<0.2 m s-~). There are abrupt changes in velocities at the various sedimentary/geomorphological boundaries where steep gradients exist. The hydrodynamic situation is characterised by flood dominance, being important in controlling water and sediment advection as well as sedimentation in this tide-dominated environment. Based on the tidal current velocities, seabed sediment grain size, and the Sundborg (1967) sediment transport model, the sediments on the intertidal flats were found to be transported in suspension for 63% of the duration of a tidal cycle, there being an absence of any prolonged transport for the remaining 37%. Over the saltmarshes, sediments were transported in suspension only, the observed net deposition of the suspended load corroborating the long-term accretional nature of the study area. Bedload transport was not observed at any tidal stage.
* Corresponding author: X. Ke e-mail:
[email protected]
14
Ke and Collins
1. INTRODUCTION Early investigators studying sedimentation mechanisms in the Dutch Wadden Sea developed the 'settling and scour lag' hypothesis to explain observed sedimentation patterns of mud on intertidal flats (Postma 1954; van Straaten & Kuenen 1957, 1958). This concept suggests that (i) a sedimentary particle carried landwards by a particular flood current will not be resuspended by an ebb current which has the same magnitude as the original flood current, and (ii) a resuspended particle is not transported back to the same location from which it originated. According to this concept, net landward transport results in progressive sediment sorting into decreasing size fractions from the low water (LW) mark to the high water (HW) mark. The 'settling and scour lag' model was subsequently used to, amongst others, explain the sediment dynamics of saltmarshes (Pestrong 1972; Frey & Basan 1985; Elliott 1986), in spite of the fact that saltmarshes differ from open intertidal flats in terms of their geomorphological and sedimentological characteristics. Furthermore, sedimentation processes on saltmarsh surfaces also appear to differ between regions. For example, Stumpf (1983) showed that in a microtidal environment the short slack-water period and the associated turbulence in the waters overlying the marsh prevented deposition in response to 'settling lag'. Carling (1982), in turn, demonstrated that in a macrotidal inlet the consideration of mean current velocities alone could not explain the depositional rates of marshes, but that storm-induced events had a significant influence on the overall sediment budget. French & Spencer (1993), by contrast, argued that sedimentation under 'normal' tidal conditions on a macrotidal back-barrier saltmarsh was able to satisfactorily account for the maintenance of the marsh elevation, while storm events affected a significant fraction of the long-term sedimentation process over the highest marsh surface. More recent studies have revealed much more complicated patterns of water and sediment movement over intertidal flats than originally suspected. Thus, tidal currents over intertidal flats (i) were found to incorporate an alongshore component, rather than simply advancing onshore and retreating offshore perpendicularly to the coastline (Anderson 1973; Evans & Collins 1975; Collins et al. 1981; Zhang 1992), and (ii) were observed not to decrease gradually from LW to HW, but to show different flow strengths or levels in the offshore channels as well as on the intertidal flats and the saltmarsh (Ke 1995). Likewise, intertidal sediment distribution patterns in The Wash were shown to be more complicated than would be predicted by the 'settling and scour lag' mechanism (van Smirren & Collins 1982; Bearman 1989; Ke 1995). Clearly, other mechanisms must also be invoked if the higher depositional rates of fine sediments in the shallow parts of certain intertidal flats are to be explained, in particular decreased erodibility due to compaction during LW (Creuzberg & Postma 1979; Dyer 1994), wave resuspension (Amos & Collins 1978; Ehlers & Kunz 1993), biological binding/trapping (Krumbein et al. 1994), land reclamation (Kamps 1963; Flemming & Nyandwi 1994), and tidal asymmetry (Groen 1967).
Tidal characteristics of The Wash
15
Tidal asymmetry and the resulting sediment transport asymmetry between flood and ebb tides have been widely noted in various environments, e.g., on continental shelves (Stride 1982), in back-barrier environments and lagoons (van Straaten 1964; Groen 1967; Sha 1990; Oost & Boer 1994), and in estuaries (Postma 1967; Dronkers 1986; Dyer 1986, 1994). Although some observations on tidal asymmetry over tidal flats are available (Collins et al. 1981; Zhang 1992), it is still unclear how this process, together with other tidal processes, controls the dynamic regime of tidal flats where sediment erosion, transportation and deposition are very active. Similarly, wave process also plays an important role in the sediment dynamics of The Wash (Amos & Collins 1978). The combination of waves and tidal currents will significantly enhance erosion and transportation of intertidal sediments (cf. Ke 1995), making hydrodynamics and sediment dynamics more complicated than would be the case with tidal or wave processes alone. However, the combined effect of waves and tidal currents can only be meaningfully studied if the individual processes are understood (e.g., Shi & Chen 1996). In any given situation it is therefore advisable to begin with an investigation of the tidal currents before tackling the hydrodynamics as a whole. With this in mind, the purpose of the present study is to unravel the tidal characteristics along an intertidal flat and saltmarsh profile characterised by long-term accretion.
2. S T U D Y AREA
The profile investigated in this study is located at Freiston Shore along the NW flank of The Wash, U.K. (Fig. 1). Here, tidal ranges are 6.5 m at spring tide and 3.5 m at neap tide. The Wash has been a depositional environment for several thousand years, receiving sediments mainly from offshore sources in the form of suspended material (Evans 1965; Evans & Collins 1975, 1978; Chang & Evans 1992; Ke et al. 1996). This depositional process has been exploited by man in the form of saltmarsh reclamation since Roman times (Kestner 1962). A pronounced zonation is observed between HW and LW, commencing with saltmarshes and proceeding through upper mudflats and Arenicola marina sandflats to lower sandflats. In the areas between the Arenicola sandflats and the lower sandflats (the so-called lower mudflats), muddy deposits may occur in some places as the result of creek/channel sedimentation or rapid sedimentation. The area in the vicinity of the study profile has intermittently been investigated over the past 30 years (Evans 1965; Amos 1974; Evans & Collins 1975; Amos & Collins 1978; Collins et al. 1981; van Smirren 1982; Ke 1995). The profile has accreted vertically at a mean rate of 2-3 cm yr 1 (between 1973 and 1993), while the LW and HW marks along the profile have prograded seawards at a mean rate of 2-18 m yr -1 (from 1950 to 1981). A brief period of short-term erosion took place in the 1970s (Ke 1995). In the long term, the Freiston Shore tidal-flat profile can therefore be regarded as being accretional.
16
Ke and Collins
7
Figure 1. Location of the Freiston Shore profile (dashed line) in The Wash embayment along the east coast of England. Crosses indicate the locations of stations 21 and 23 surveyed by HRS (1974).
3. M E T H O D S
The Freiston Shore profile was selected for this study because of easy access and the fact that numerous other studies have concentrated on this area in the past. The survey stations were located on the intertidal flat and saltmarsh, creeks and channels being avoided (see Fig. 2). The morphological zones (i.e. the saltmarsh, mudflat, and sandflat) are parallel to, and persistent along, the coastline (Evans 1965; Collins et al. 1981; Ke 1995), the Freiston Shore profile being typical of the tidal-flat systems in the wider embayment, particularly those located along its northern and southwestern flanks. Hydrographic surveys were undertaken around spring tide (except at station 9) between 1991 and 1993 (Table 1). Both water depths and tidal currents were measured along the profile. In March 1991, a direct reading current meter (DRCM) was used from an inflatable boat. Readings were obtained within the near-bed layer (at 0.3-0.5 m above the seabed), at mid depth, and at the surface (0.3-0.5 m below the water surface).
Tidal characteristics of The Wash
17
Tidal current velocities and directions were measured and averaged over 3 minutes at half-hourly intervals. Two Braystoke velocity gradient rigs, in which the impellers were fixed at logarithmically spaced distances above the bed, were used on the other occasions to establish the more detailed velocity structure throughout the water column. In all, 11 surveys of the latter type were undertaken, 4 in June 1992 and 7 in June 1993 (Fig. 2 and Table 1). Current directions measured in 1991 showed that these are predominantly either perpendicular to or ca. 10 ~ oblique to the shoreline throughout the water column. These patterns are very similar to those documented in the 1970s which showed strong longshore components ca. 25 ~ oblique to the shoreline (Evans & Collins 1975; Collins et al. 1981; Ke 1995). Due to the limitations of the instrumentation used, current directions were not recorded during the surveys in 1992 and 1993, but it is reasonable to assume that they were similar to those observed in the 1970s and in 1991. Details of the 1992 deployment and measurement techniques have been described elsewhere (Ke et al. 1994). The tidal current velocities measured at different levels above the seabed have been averaged to obtain mean current velocities of the flood and ebb phases as well as of the whole tidal cycle. Most of the tidal current velocities at mid depth were derived from the velocity profiles using linear integration (Collins et al. 1998). The maximum flood and ebb tidal current velocities presented in this study are those actually observed during the field survey, and not the maximum velocities of the averaged values.
Station Number
4
~3 Z
I
2 34
,~~___
0 2
5
6
7
8
9
21 & 23
u-i ~r
E1
I
,
i !
|
C o 0
I 1
: :
>
i
:
! channel
!
iUperl
Saltmarsh ~udflal( -3
~
0
Arenicola
sandflat
Lower
sandflat
i
\
i
500
1000
1500
2000
Distance to dyke (m) Figure 2. Location of survey stations (arrows) along the Freiston Shore profile (profile extracted from Amos 1974). Stations 21 and 23: see Fig. 1. ODN: Ordnance Datum (Newlyn).
18
Ke and Collins
Table 1. Hydrographical surveys undertaken during the present study (for locations, see Fig. 2). Station
Date 24/6/93
Time (GMT) 07:24-10:53
Tidal height (m, ODN) 3.7
1 2
21/6/93
05:15-07:00
3.5
3a
18/6/92
06:20-09:20
3.4
3b
20/6/93
17:23-19:43
3.2
4a
20/6/93
04:30-07:45
3.3
4b
23/6/93
19:03-22:43
3.4
5a
17/6/92
05:35-09:20
3.5
5b
19/6/93
16:15-19:15
3.0
6a 6b
01/3/91 19/6/93
16:30-21:20 03:45-07:15
4.2 3.0
Instrument Rig BFM001 Rig BFM001 Rig BFM004 Rig BFM001 Rig BFM001 Rig BFM001 Rig BFM001 Rig BFM001/ 004 DRCM Rig BFM001 Rig BFM001 DRCM
Weather, sea conditions Fine, calm Fine, calm Cloudy, rain Calm Fine, light W/NW breeze Calm, light breeze, swell Cloudy, light breeze Fine, light breeze
Fine Fine, strong breeze 6c 21/6/93 17:26-21:00 3.3 SE breeze, calm 7 05/3/91 07:10-11:20 3.5 Cloudy, light breeze 8a 15/6/92 03:43-08:55 3.3 Rig Cloudy, BFM001 breeze 8b 22/6/93 17:48-22:53 3.4 Rig SE breeze, BFM001 0.6 m swell 9 13/6/92 14:20-19:45 2.9 Rig Fine, BFM001 calm ODN: Ordnance Datum (Newlyn); BFM 001 and BFM 004: Velocity gradient rigs Series No. BFM001 and BFM004, respectively; DRCM: direct reading current meter.
4. RESULTS 4.1. Stations 1 and 2 Over the saltmarsh, the tidal curves were nearly symmetrical, with flood to ebb phase ratios (Tf/Te) lying between 0.93 and 1.0, and the tidal inundation amounting to only 2.7-3.0 hr (Fig. 3). Tidal current velocities were generally weak, with mean values of 0.10--0.12 m s 1 during the flood, and 0.07-0.08 m s-1 during ebb phase. A m a x i m u m velocity of 0.19 m s -~was measured during the flood. Near-bed current velocities were extremely low and mostly <0.08 m s -~ (U7 denotes the current velocity at 7 cm, and U12
19
Tidal characteristics of The Wash
at 12 cm above the bed). Generally, the tidal current maxima occurred when the surface of the saltmarsh was either just being inundated or just about to be exposed, being associated with water depths of less than 0.2-0.4 m (Fig. 3). In addition, a marked difference in the time at which the mid-depth and near-bed current maxima occurred was evident at station 1.
0.5-
.1.2
0.5~ a Station 1, 1993
_--'-
s
0.4 6-'
"~
so"*
l"
0.3-
"" ~
,,
b
Station 2, 1993
-I.2 /e---e,,
0.4p
%
0.8 ~- - -
',
%
0.8
~.._, 0,3"o ~
o" //
%~ %
.,.,,
iI ,=
0.2-
0.2-
;-,
Z
0 -, / -IJ -1.0
'*-
o-o
.... -0.5
,x J
~ 0.10 1.5
,-.7,._._-~---,,~ 0 0,5 1.0
0 ,-'~ -1.5
Water depth
0---0
U (told-depth)
;,
I /
~" .o--"~ "~x--~-Z~
, -1.0
, -0.5
o..
, 0
/
'..... , 0.5
o.4
L\ ,
, 1.0
0
' 1.5
Time (hrs, relative to HW)
Time (hrs, relative to HW) e--e
%%
x;
0.4
.~. ..=...
~
U7
9 --I
Water depth
0---.0
U (mid-depth)
~
U12
Figure 3. Water depths and tidal current velocities over the saltmarsh at Freiston Shore in 1993: a) station 1; b) station 2 (for locations, see Fig. 2). U7 and U12: tidal current velocities at 7 and 12 cm above the seabed, respectively.
4.2. Station 3 Station 3 was located at the boundary between the saltmarsh and the upper mudflat (Fig. 2). As illustrated in Fig. 4, the tides were symmetrical to slightly asymmetrical at this station, Tf/Te ratios ranging from 0.81 to 1.0 (cf. also Table 2). Tidal current velocities were rather weak, mean velocities characteristically being <0.14 m s -1. Strong fluctuations were recorded, implying higher turbulence and more eddy action (Zimmerman 1981) at station 3 than at stations 1 and 2 (Fig. 3). The maximum tidal current velocities occurred immediately after the beginning of the flood and just before the end of the ebb (Fig. 4). Flood current velocities were generally lower than those of the ebb phase during the 1992 survey, but the trend reversed during the 1993 survey (Fig. 4). This change may have been due to the difference in weather conditions during the two surveys (Table 1), whereby the offshore wind (N or NW) during the 1992 survey possibly restrained the development of the flood. Nevertheless, in both cases the maximum tidal current velocities were higher during the flood (0.14 and 0.29) than during the ebb (0.13 and 0.26 m s 1, respectively; Fig. 4 and Table 2).
20
Ke and Collins
T a b l e 2. T i d a l d u r a t i o n (hrs) a n d t i d a l c u r r e n t v e l o c i t i e s ( m s
-1)a c r o s s t h e p r o f i l e
at F r e i s t o n S h o r e (for s t a t i o n l o c a t i o n s , see Fig. 2). Station 1
PhaSe Duration Tf/Te Ub Um Umax Hood 1.3 0.05 0.12 0.19 Ebb 1.4 0.04 0.08 0.14 Cycle 2.7 0.93 0.05 0.10 2 Hood 1.5 0.11 0.10 0.19 Ebb 1.5 0.07 0.07 0.15 Cycle 3.0 1.00 0.09 0.09 3a Hood 1.3 0.09 0.12 0.29 Ebb 1.6 0.12 0.14 0.26 Cycle 2.9 0.81 0.10 0.13 3b Flood 1.3 0.08 0.12 0.14 Ebb 1.3 0.04 0.07 0.13 Cycle 2.6 1.00 0.07 0.09 4a Hood 1.5 0.10 0.14 0.20 Ebb 2.2 0.04 0.09 0.13 Cycle 3.7 0.68 0.06 0.11 4b Hood 1.5 0.14 0.19 0.51 Ebb 2.2 0.08 0.10 0.27 Cycle 3.7 0.68 0.10 0.14 5a Flood 1.7 0.09 0.11 0.23 Ebb 2.3 0.08 0.11 0.16 Cycle 4.0 0.74 0.09 0.11 5b Hood 1.7 0.10 0.16 0.20 Ebb 2.2 N/A 0.09 0.14 Cycle 3.9 0.77 N/A 0.12 6a Flood 2.0 0.24 0.25 0.32 Ebb 3.0 0.27 0.24 0.35 Cycle 5.0 0.66 0.26 0.24 6b Hood 1.9 0.14 0.17 0.33 Ebb 2.2 0.11 0.11 0.25 Cycle 4.1 0.86 0.12 0.14 6c Hood 1.8 0.18 0.16 0.45 Ebb 2.1 0.11 0.12 0.20 Cycle 3.9 0.86 0.14 0.14 7 Flood 1.8 0.20 0.23 0.56 Ebb 2.3 0.17 0.17 0.19 Cycle 4.1 0.78 0.19 0.19 8a Flood 2.7 0.11 0.24 0.48 Ebb 3.5 0.09 0.20 0.33 Cycle 6.2 0.77 0.10 0.22 8b Hood 2.0 0.27 0.30 0.53 Ebb 3.1 0.14 0.15 0.29 Cycle 5.1 0.65 0.21 0.23 9 Flood 2.4 0.23 0.27 0.43 Ebb 3.5 0.18 0.21 0.29 Cycle 5.9 0.69 0.20 0.23 Tf and Te: flood and ebb tidal duration, respectively; Lib and Um: m e a n tidal current velocities at near-bed (7-35 cm above bed) and mid-depth, respectively; Umax: m a x i m u m tidal current velocity observed; N / A : not available.
21
Tidal characteristics of The Wash
0'5
a
- 0,4"~
-
Station 3, 1992 I
1,5
0,2
!
-I,0
~',,,
I
0.1-
4
,,...,, ,,,,.. ,,,,,.
o--e
Water
depth
, ,,.e,,,e o- . - %
--- 0.2
,~.K,~_oo,.~
~C~" ~ , / \ ^
Np,,- 0,1
IKi
-1.0 -0.5 0 0.5 1.0 Time (hrs, relative to HW)
.,r a. da
-0.5 ~"
0 -2.0 -1.5
1,0 ~" ._.,.
0,3
-~ =.,. .,...,,
~",
....o .-...I
9
1.5
b Station 3, 1993
0.4
%
0,3
i==
0,5
1.5
2.0
~
U7
0
o
....
.,,.,
,,
0.5
"%,,
I'"
i
!
9
0
Time (hrs, relative to HW)
0---~ U (mid-depth)
e--o
Water
depth
0---,0
U (mid-depth)
~
UIO
Figure 4. Water depths and tidal current velocities at station 3 at the boundary between the saltmarsh and the upper mudflat, Freiston Shore: a) 1992; b) 1993 (for location, see Fig. 2). U7 and U10: tidal current velocities at 7 and 10 cm above the seabed, respectively.
4.3. Station 4 Both data sets from the upper mudflat show very similar patterns. The tidal curves were asymmetrical, with a Tf/Te ratio of 0.68 and a tidal inundation of 3.7 hr on both occasions (Fig. 5 and Table 2). There was a consistent asymmetry in the tidal currents, with flood velocities being higher than those of the ebb. The maxima occurred either immediately after the beginning of the flood and before the end of the ebb (Fig. 5a) or at the very beginning of the flood and end of the ebb (Fig. 5b). 0.5
9 1.8
a Station 4, 1993, 1st survey
0.5
b
'
?-0,..
Q
' 0.4
0.4 o" e ~~ o-Q,,.
"1.2 ~
~ 0,3
-1.8
Station 4, 1993, 2nd survey
--...
,
i9
~ 0,3
9
~..
-,.,
,
-1.2
9
r
=,.
~_,~.
0.2 ..
r
/,t"
J/ ,~'
p-
~,
Xb-o.-a_ x-- x
i
0
-2.S
o'"
"~ / N x"x-%O"X:) N x.-x-X"~'~x
-o'.s b o:s
.i.s
l.S "-q
"0.6 ~
~\
"~ x
I---
0.1
/
2,5
~'.s
0
' .....
-2,5
,f.,.._ - ~ . , -o I "K~O.,o.-O.-O .-~ ) ~ ......... .1.s 0'.s i o15
Water
depth
0--<)
U (mid-depth)
I
~ 6
_~ o.6 ~=
',_ "
o
2.s
Time (hrs, relative to HW)
Time (hrs, relative to HW) e--e
",, ;~ ~. ~o~ o-~'7, ,,. / ~ I
~
U9
O--o
Water
depth
0--.0
U (mid-depth)
x~x
U9
Figure 5. Water depths and tidal current velocities at station 4 on the upper mudflat, Freiston Shore in 1993: a) first survey; b) second survey (for location, see Fig. 2). U9: tidal current velocity at 9 cm above the seabed.
Ke and Collins
22
The maximum tidal current velocity during the second survey was 0.51 m s -~, which is much higher than the maximum value recorded during the first survey (Fig. 5). The difference between the two survey results may partly have been due to the amplitude of the tides, the second survey having been undertaken on the highest spring tide of the month, whereas the first survey took place 3 days before this event (Table 1). The tidal current maximum during the second survey at this station was also much higher than those at stations 1-3 further landwards, presumably due to tidal energy dissipation over the intertidal flat (Figs 3-5 and Table 2). 4.4. Station 5 In 1992 and 1993 the tidal curves at station 5 were asymmetrical, the Tf/Te ratios fluctuating between 0.74 and 0.77 (Fig. 6 and Table 2). Tidal current velocities were highest at the beginning of the flood and at the end of the ebb but low during HW in the 1992 survey. This is reflected in the smooth, concave (upward) tidal current curve (Fig. 6a), being similar in this respect to the patterns at station 4 on the upper mudflat (Fig. 5b), and stations 1 and 2 in the saltmarsh (Figs 3 and 4). There was a marked change during the 1993 survey when the flood tidal current maxima seem to have occurred just before HW, there being no marked ebb current maxima (Fig. 6b). A possible explanation for this is that no current velocity measurements were made during the first phase of the flood and the last phase of the ebb when water depths were less than 0.3-0.4 m, as a result of which higher current velocities may have been missed (Fig. 6b). Nonetheless, flood tidal current velocities were generally higher than (or equal to) those of the ebb during both surveys, with mean values of <0.12 m s -1 and a maximum value of 0.23 m s -1 (Fig. 6 and Table 2).
a
0,5
Station 5, 1992
,.....
T.. ,=
.,,
0.4
~---e,,
y
;
~ 0.3
xe.. e
/
-1,2
-o.8
\/
"1.6
0.5-
,=
0,4"
\
~ o.20.4
N0.1-
.o:s ; ols
-2.5
Time (hrl, relative to HW) e--e
Water depth
0--.0
U (mid-depth)
6
0 2.5
x----x U7
,e-,"
9/ I
~ O.3.
"K
-,_,
-1,2 \
..,.-..
-o.s .~ ;,...~D..O-.O"O-. O
e~e
"0.4 ~
N 0.I
;'/
p-
0
,-
>,.. .i.,.
\
i Is
N o.2-
b Station 5, 1993
-1,6 -.,
0 / .2.5
a--e
-1,5
~176176
"b~ ' , ,
0 2.5
-0.5 0 0.5 1.5 Time (hrs, relative to HW)
Water depth
O--.O U (mid-depth)
x~x
U9
Figure 6. Water depths and tidal current velocities at station 5 at the boundary between the upper mudflat and the Arenicola sandflat, Freiston Shore: a) 1992; b) 1993 (for location, see Fig. 2; also see Figs 4 and 5).
Tidal characteristics of The Wash
23
4.5. Stations 6 and The tidal curves at stations 6 and 7 showed a distinct asymmetry over the Arenicola sandflat on four occasions. The tidal inundation ranged from 3.9 to 5 hr and showed Tf/Te ratios of 0.66-0.86, the tides being longer in duration and more asymmetrical than those at the more landward stations (Fig. 7 and Table 2). One of the characteristics over the Arenicola sandflat is that tidal currents gradually decreased from the water surface to the bed during the decelerating stage of the tide, i.e. at HW-1 hr and 0.5-1 hr after HW at station 6 (1991; Fig. 7a), and during the flood phase at station 7 (Fig. 7d). However, during the accelerating stage of the tide, the pattern changed into Unear-bed and/or Usurface >Umid-depth, or simply Unear-bed >Umid-depth >Usurface, i.e. at between HW-0.5 hr to HW and HW+I.5 hr towards the end of the ebb at station 6 (Fig. 7a), and during the ebb phase at station 7 (Fig. 7d). The variable character of the velocity profiles during tidal inundation could have been due to secondary flows or local topographically induced eddies. The mean tidal current velocities varied mostly in the range 0.11-0.25 m s -~, the flood tidal currents being significantly higher than those of the ebb (Table 2). .--. 1.0- a Station 6, 1991
2.5
9
,..s
;"
0.8P= 0.6--
I
:=,-
2.0
",
"'
"--"-
it
0.4-
~i,.
.~ 0.2-
.t
/'
...-,,.
~
2.5
b Station 6, 1993, 1st survey
2.0
0.8
1.5 ~
_~ 0.6
1,0 .P,
~ 0,4
/e-e"
-1,5
9
r
"e
,,%.
/
'-"
._. 1.0
"~..~
. . . .
~
,?
xl
.,
-"~--~.'-:.-=- 9
0,5
'V,.
~,,
-1.0
,,
\
-0,5
.p= 0,2 p._
Ii/
0 -3
..........
-i
-2
i
i
i
30
0
t"
, -2
-3
Time (hrs, relative to HW) 9 ---~
Water depth
0---0
U
z~----Z~
U
(surface)
~
U
(near-bed)
(mid-depth)
;
i
-,
i
3
0
Time (hrs, relative to HW) e---e
1.0" c Station 6, 1993, 2nd survey
2,5
1.0
0.8"
2,0
0.8
d
Water depth
0.--0
U (mid-depth)
=
-~ U 9
Station 7, 1991
i
,= ~ "~ o
._
e-e,.
0,6-
F9
0.4-
o.9 "9 X
/
x~ d '
~-~" 0 . 2 :l
i
-3
-2
e--4
.gE 1.5 ~
'
9 u
i
~
-I 0 1 Time (hrs, relative to HW)
Water depth
-1.0 ~=
o-.-0
U
(mid-depth)
u
2
~
3 U9
2.0 sO~
.,..,
~ 0.6
/i ;
~ 0.4
9 I.
9 //
0,5
N o.2
0
0
.1,5
"'"o
ot
,~
i~
,~
-1,0 ~r
~
;? ~ . ~ - ~ -i
-3
-0,5
-I 0 1 Time (hrs, relative to HW)
e---e
Water depth
O---.o
U
(mid-depth)
~..--a ~
~
t=z.
2
U
(surface)
U
(near-bed)
3
0
Figure 7. Water depths and tidal current velocities at stations 6 and 7 on the Arenicola sandflat, Freiston Shore: a) 1991 survey at station 6; b) first survey in 1993 at station 6; c) second survey in 1993 at station 6; d) 1991 survey at station 7 (for locations, see Fig. 2; also see Fig. 5).
Ke and Collins
24
During a tidal cycle, tidal currents often showed a decrease from the beginning of the flood (Fig. 7), with a sharper decrease in the first phase of the flood which was maintained essentially throughout the whole flood tide. The currents became even weaker during the ebb, with a slight increase at the end of this phase (Fig. 7a-c). Under extreme circumstances, tidal currents decreased from the very beginning of the flood until the end of the ebb (Fig. 7d). Over the Arenicola sandflat, the maximum tidal currents varied in the range 0.19-0.56 m s -1, mainly in response to the varying tidal ranges. The maxima occurred mostly during the first phase of the flood, and were much stronger than those observed over the saltmarsh and the upper mudflat (Table 2). It is possible that higher current velocities occurred at the beginning of the flood phase of the tides, and these are likely to have been missed because most of the surveys were undertaken only when the water depths were in excess of 0.2-0.3 m (cf. Fig. 7). In three of the four surveys undertaken over the Arenicola sandflat, minor peaks of tidal currents (0.15-0.3 m s -1) occurred at around HW (Fig. 7a-c). This may be a consequence of the tidal pulse caused by the abrupt increase in the tidal volume when the tidal water begins to flow into the saltmarsh near HW. 4.6. Station 8 The tidal curves for this part of the intertidal flat were asymmetrical in character, the duration of the ebb tide being almost an hour longer than that of the flood. The mean tidal current velocities at mid depth fluctuated between 0.15 and 0.30 m s -1, and at near bed between 0.09 and 0.27 m s -1. The results of the second survey show that tidal current velocities at mid depth and near bed (7-11 cm above the bed) were similar (Fig. 8b). They were also similar to those of the Arenicola sandflat, but noticeably higher than those on the saltmarsh and upper mudflat (Table 2).
0.6
,,"
I~ ~ 0.4
i ~.I
e" '~
b Station 8, 1993 .v
""
'
Jr r
"",t
d
0.4
2,0
'~..~
iI
9
0.6
3,0
a Station 8, 1992
"K
.f
e..>
2,0
",
.r
it.
N\o~o~.~o ^;r
~ O.2
1.0 N
&
~O.2-
: %
m
o.o.
;";,+47.
1.0
~;,~
L,g
i---
j--
0
'N,,,,,/'
Z -3.5
4.5
-1.5
-O.S 0 0.5
1.5
2.5
'i.. 3.5
-3.5
Water depth
0-..-0
U
(mid-depth)
-2;5-6-o{sio]s
~;s
Ii
!..
f.s '3
Time (hri, relative to HI)
Time (hri, relative to HW) I--.a
"~176
tl
,"
0
~
~- U 7
o-4
Water depth
0-.-0
U
(mid-depth)
~
~
U(7-11)
Figure 8. Water depths and tidal current velocities at station 8 in the lower section of the intertidal flat, Freiston Shore: a) 1992; b) 1993 (for location, see Fig. 2). U7: tidal current velocity at 7 cm above the seabed. U7-11" tidal current velocity at 7 to 11 cm above the seabed.
25
Tidal characteristics of The Wash
At station 8, the higher tidal current velocities occurred during the early stage of the flood phase and the late stage of the ebb phase, with a distinct minor peak at about 0.5 hr after HW. As in the case of stations 6 and 7, the lowest velocities were 0.5-1.5 hr before HW and 0.5-1.5 hr after HW, respectively. Generally, the flood tidal current velocities were noticeably higher than those of the ebb (Fig. 8 and Table 2). The highest velocities (with a m a x i m u m value of 0.53 m s-~) usually occurred at the beginning of the flood at very shallow water depths (U7; Fig. 8b). 4.7. Station 9 Tidal curves at this location were asymmetrical, with a Tf/Te ratio of 0.69 (Table 2). Throughout the tidal cycle the currents displayed a smooth progression with few abrupt changes, although a minor tidal current peak immediately after HW was again detectable. Current velocities were similar to those on other parts of the intertidal flat, and were higher than those over the saltmarsh (an order of magnitude in terms of the near-bed velocities Ub, and twice the value of the maximum tidal current velocity Umax; Table 2). The maximum tidal current velocities at this station (0.43 and 0.29 m s-1 for flood and ebb, respectively) occurred at the very beginning of the flood and at the end of the ebb. Again, the flood currents were stronger than the ebb currents (Fig. 9). 0.5
3.0
Station 9, 1992
A
'-.,. 0.4
,
o
.
ss
,'=~'"K
'~
2.0 -~ "03 o
r
,'
"~ 0.2 /'
,-
__.-~ x
"o. . . . .
-o.--o"
\x--"~'-..~....~J
x
/--
=..
',,
-01 0
3.s
t'
', ...... I
-2.s
-o.s o o'.s
'5
f.s
3.5
Time (hrs, relative to HW) e---e
Water depth
0--.-0
U
(mid-depth)
n . . . . -~
U21
Figure 9. Water depths and tidal current velocities at station 9 on the lower sandflat, Freiston Shore, surveyed in 1992 (for location, see Fig. 2). U21- tidal current velocity at 21 cm above the seabed.
5. DISCUSSION 5.1. Tidal wave types and velocity maxima The tides within The Wash embayment are characterised by a standing wave, with offshore m a x i m u m tidal current velocities around mid tide and slack water near HW and LW (Pond & Pickard 1978; Pugh 1987; Fig. 10). The present study has shown that
Ke and Collins
26
the propagation of such a wave onto the intertidal flats and saltmarshes causes maximum tidal current velocities to occur (i) immediately after the beginning of the flood and near to the end of the ebb at/between MLW and MTL; and (ii) at the very beginning of the flood and end of the ebb at/above MTL where inundation occurs only after mid tide (i.e. at the same time as they reach their maxima in the offshore waters). Hence, the tidal current curves change gradually from 'sinusoidal' in the offshore to 'co-sinusoidal' or 'trochoidal' (with a shorter tidal inundation, i.e. out of phase) over most of the area above MTL (Fig. 10). Such changes take place within very short distances, since the corresponding topographic profile between MTL and MLW is very short due to the steeper slope of the offshore channel here. The velocity pulse recorded at about 10 cm above the seabed during early/late stages over intertidal flats, and during the over-marsh flow stage across the saltmarshes (cf. Figs 7 and 8) have been modelled by a continuity-based exploratory model for the hydraulic regime of tidal saltmarshes. This predicts that the tidal current maxima ('velocity pulses') occur during the earlier and later flow stages when water depths are small, and at over-marsh flow stages (Allen 1994). Stations
1.2
S t a t i o n s
6--9
S t a t i o n =
21
,23
,""*", I
"
0'5"I I
/
.w "X
"
i..== t = I ~=
>
0.5
~ / IX
'\
'
UtOOcr
i/i ,i~l!i
Io -1 0 +I *2 Tame (hrs rel tire to HW}
:"
-2 -I 0 +1 +2 +3 Time (hrs relative to HW)
0
-,= ~
'
""~ 0,5 -~
I
U1OOcr
'
0 , -6
I
I
I I
i I
I ~3
i,
10 =
,i J v 9 III Ill , ,' , '. , !', 8 -4 -2 0 +2 +4 +6 Time (hrs relative to HW}
, i l!l
I
I I
=-= 1 :] Saltmarsh ; "~'t (Mz=0.008(Mz=0.mm) 008mm) 1 ~'-3 , 0 500
. . . . . .
"''''"''''""'"
"
"
I
O f f s h o r e channel Intertidal flat ..... : ~ [Mz=0.18 ram} (Mz=0,08 mm) i .....~::::"" ZMLW , 1000 1500 Distance to dyke (m)
2o'o0
Figure 10. Conceptual hydrodynamic regime across the accretional Freiston Shore profile, The Wash. Top: tidal current velocity (continuous line), Ucr (lower horizontal line), U100cr (upper horizontal line), and water depth (dashed line) over spring tide for the saltmarsh (stations 1-2), the intertidal flats (stations 6-9), and the offshore channel (stations 21 and 23). Bottom: the Freiston Shore tidal-flat profile. Hydrographical data for offshore channel extracted from survey undertaken at stations 21 and 23 in the Boston Deep (HRS 1974; Ke et al. 1996; for locations, see Fig. 1). Mean grain diameters (Mz) for saltmarsh and intertidal flats are average values for the three years of study (Ke 1995). Mz data for the offshore channel were extracted from BGS (1988). Ucr and U100cr are based upon Sundborg (1967), Miller et al. (1977), Kapdasli & Dyer (1986), and Ke et al. (1994). At stations 1 and 2, it was not practicable to plot U100cr because the water depths are generally less than 100 cm in this case. ODN: Ordnance Datum (Newlyn).
Tidal characteristics of The Wash
27
5.2. Tidal current structure
The maximum and mean tidal current velocities do not change gradually along the tidal-flat profile, but rather in a 'step-like' manner, i.e. current velocities in the offshore channels and over the intertidal flats and saltmarsh occur at different levels, with maximum tidal current velocities decreasing landwards. Rapid changes also occur over the narrow transitional zones between these sub-environments (Fig. 11). These locations coincide with sedimentary and/or geomorphological boundaries and distinct topographic or bathymetric changes, i.e. the presence of steeper slopes in the profile. Interestingly, clay contents of the surficial seabed sediments were found to be >10% landwards, and <8% seawards of the (hydrodynamic) boundary located between the sandflats and the saltmarshes, i.e. over the upper mudflat (cf. Fig. 11; Ke 1995). 1.5 a U m a x (1970)
o Umax (1980-93)
- Umean (1980-93)
+ HR d a t a (1975)
E
___~___
>, o mO
r
/
. . . . . . . . . .
0.5 ~
,?" ~ ,./O (~ ----o . . . . . d o e "
o
..
~
o
- ~ . . . . . . . . . a-o
No data
.,_
0
I
I
i
!
500
1000
1500
2000
2500
4 3 Q
o tO
2
0
i
Upper "'
Saltmarsh ~udfldl,, t
-2
Intertidal sandflat
O f f s h o r e channel
-3
0
500
1000
1500
2000
2500
Figure 11. Tidal current velocities (Umax and Umean) as a function of distance from the dyke (top) and topographic elevation along the study profile (bottom).
The sharp decrease in current velocities from the open intertidal flats across the mudflats to the saltmarsh (see above) may be attributed to a variety of factors. For an initially smooth intertidal-flat profile, the uppermost section will only be inundated during HW slack. Sediment deposition is thus more likely to occur on the upper intertidal flats rather than on those located more seawards. The different vertical accretional rates between the two areas will enhance differences in surface elevations. However, once the surface of the upper part of the flats has reached a height which permits pioneering halophytes to grow, a saltmarsh will develop. In the presence of vegetation, additional sediment will be trapped and the surface of the saltmarsh will be
28
Ke and Collins
raised further. As a result, saltmarshes will always occupy higher ground than the open intertidal flats relative to the regional tidal frame. At the same time, a steeply sloping area has been created between the saltmarsh and the seaward intertidal sandflat, i.e. the so-called upper mudflat at Freiston Shore. The latter feature explains why the tidal current velocities within the saltmarsh are so low. Meanwhile, due to the steeper slope, the surface of the upper mudflat is strongly scoured, and gullies and oval-shaped depressions occur. These increase the seabed roughness considerably (see Ke et al. 1994) which, in turn, causes a decrease in the tidal current velocity. Abrupt changes in the current patterns have been predicted on the basis of analytical models for flow and sedimentation over tidal saltmarshes (Woolnough et al. 1995; Allen 1996). As a result, hydrodynamic energy (in terms of tidal current velocities a n d / o r (?) wave heights) in the offshore channel, and over the open intertidal flats and the saltmarsh reaches different levels. In a sense, these have a 'quantum' character, and the observations or predictions in the case of the macrotidal environment of The Wash are therefore inconsistent with the 'settling and scour lag' hypothesis (see Postma 1954; Figs 10 and 11 in van Straaten & Kuenen 1957; the "lag effect' of van Straaten & Kuenen 1958). Instead, the present study has shown that mechanisms for the deposition of sediments over tidally dominated intertidal environments include (i) flood tidal current dominance, (ii) a 'step-like' shoreward hydrodynamic energy gradient, and (iii) the occurrence of strong tidal currents especially at the beginning of the flood, and at the end of the ebb. 5.3. Tidal asymmetry The results of the present study show that the tidal curve is asymmetrical in the lower part of the intertidal zone, and that it becomes gradually more symmetrical across the intertidal flats towards the saltmarsh (Fig. 10 and Table 2). A similar trend was recorded during the course of an earlier investigation (Evans & Collins 1975). However, some other studies show an almost opposite trend (Collins et al. 1981; van Smirren 1982). Such variation may, at least in part, be due to the fact that the early surveys were undertaken during spring tides, as was the case in the present study. In contrast, all the stations surveyed on the lower intertidal flats in most subsequent studies were undertaken at neap tides. Moreover, the earlier measurements were carried out high on the matured saltmarsh prior to reclamation, and these may have been affected by the presence of creeks and local flow patterns. The implication of such observations is that tidal symmetries change in response to tides, seasons and local control mechanisms which require further study if they are to be clarified. Furthermore, tidal currents in the region are also asymmetrical in character. Flood currents are dominant, with both the mean and maximum velocities being generally higher than those of the ebb (Figs 10, 12, and Table 2). Perhaps as a result of prevailing onshore winds, the velocity pulse of the final phase of the ebb was not observed on some occasions and hence, tidal current velocities decrease from the beginning to the end of a tidal cycle.
Tidal characteristics of The Wash
29
1.2 0
1E >" ===,
0.8 -
0 0 (D
0
> 0.6
-
0
o9
r-
(D t._
L040
+ .,
N "0 0 0
0.2
-I.-
o
o Maximum
-
+ Mean O
1
I
I
I
I
I
0
0.2
0.4
0.6
0.8
1
1.2
Ebb tidal current velocity (m/s)
Figure 12. Maximum and mean tidal current velocity asymmetries for stations from the saltmarsh to the offshore channel along the Freiston Shore profile, The Wash. Data for the saltmarsh and intertidal flats are based on the surveys undertaken in 1980 (van Smirren 1982) and in 1991-1993 (present study). Data for the offshore channel at stations 21 and 23: see HRS (1974) and Ke et al. (1996).
Similar tidal curves and tidal current patterns have been observed previously in the region (Collins et al. 1981), and can also be identified in data sets of other accretional intertidal flat/saltmarsh environments (Table 3). At least during spring tides under calm weather conditions, such tidal patterns may characterise hydrodynamic regimes. Above MTL during the early stage of the flood tide, current flow acts as a strong resuspension mechanism, whereby maximum velocities exceed the threshold velocity Ucr for sediments with a mean grain diameter Mz of 0.08 mm in the present case (Fig. 10). Similarly, the generally stronger flood flows would transport suspended particulate matter (SPM) originating offshore in a landward direction. The majority of the coarser-grained particles, together with some of the finer-grained material within the water column, may settle during the following slack-water period (cf. 'settling and scour lag' hypotheses). These may be resuspended at the end of the ebb when current velocities are generally above Ucr (Fig. 10).
30
Ke and Collins
Table 3. Various accretional intertidal flat/saltmarsh environments associated with hydrodynamic characteristics similar to those observed in the present study. Locations Intertidal flat (Ameland, Wadden Sea, The Netherlands) Saltmarsh (Norfolk, England)
Data sources van Straaten & Kuenen 1957
Bayliss-Smith et al. 1979; French & Clifford 1992 Saltmarsh (Burry Inlet, Wales) Carling 1982 Saltmarsh (west-central Florida, USA) Leonard et al. 1995 Intertidal flat (southern New Hampshire, USA) Anderson 1973 Mudflat (Avon River, Canada) Amos 1995 Intertidal flat(Jiangsu coast, China) .......... .......Zhu & Xu 1982; Zhang 1992
Since the ebb currents are generally weaker than the flood currents, there will be an enhanced difference between the flood and ebb values of (U-Ucr) 3 which is proportional to the bedload transport rate (Gadd et al. 1978). Hence, net flood (i.e. landward) bedload sediment transport and SPM flux will occur over the intertidal flats throughout each tidal cycle. Together with the 'step-like' shoreward hydrodynamic energy gradient, the present hypothesis emphasises the tidal asymmetry in explaining features such as the mudflat zone and sharp sedimentary boundaries. This is different from the 'settling and scour lag' concept which assumes a gradual decrease in tidal currents from LW to HW, and which emphasises the difference between critical scour velocity and critical settling velocity. Furthermore, the lag effect should exist not only on intertidal flats but also in many other sedimentary environments. Within the context of the present study, tidal asymmetry may be more important than the 'settling and scour lag' effect in explaining sediment movement on the intertidal flats of The Wash. 5.4. Tidal transport modes Using the near-bed current velocity and sediment grain size, the Sundborg (1967) model clearly illustrated the mode of sediment movement and transport. It has provided a useful tool to identify seabed sediment dynamics (modes), and has been applied extensively in sediment dynamics (Sternberg 1972; Reineck & Singh 1980; Sternberg et al. 1983). On the basis of established patterns of tidal currents over the saltmarshes, intertidal flats and offshore channels (Fig. 10), and in combination with the Sundborg model, the 'modes' of sediment transport during different stages of the tidal cycle may be identified (Fig. 10 and Table 4). The results show that (i) over the saltmarsh, Mode II (transport in suspension and net deposition of the suspended load) is the only mechanism to be anticipated; (ii) over the intertidal flat, Mode I (entrainment of grains from the bottom and transport in suspension with resulting net erosion) occurs during the first phase of the flood and at the end of the ebb, Mode IV (absence of prolonged transport with resulting deposition of bedload or suspended
31
Tidal characteristics of The Wash
load) around HW, and Mode II in between; (iii) in the offshore channel, symmetrical and periodic changes from Mode IV (see above) occur at the beginning and end of the tidal cycle, through a short period of Mode III (entrainment of grains from bottom and transport of bedload with resulting net erosion or net accumulation) to Mode I at mid flood and mid ebb (Fig. 10). In terms of duration, only Mode II occurs throughout the tidal cycle over the saltmarsh. Over the intertidal flats, Mode I is dominant during the flood (45% of the time), whereas Mode IV is dominant during the ebb (43%). Finally, in the offshore channel, Mode I is dominant (48% of the time) throughout the tidal cycle, followed by Mode IV (36%) and Mode III (16%; cf. Table 4).
Table 4. The time of occurrence (%) of different transport modes during a typical spring tidal cycle along the Freiston Shore profile, The Wash (see also Fig. 10). .......Environment Saltmarsh
Intertidal flats
Offshore channel .........
Tidal phase Flood Ebb Cycle Flood Ebb Cycle Flood Ebb Cycle
.
.
.
.
.
.
.
......... Transportmodepercentage I II III 0 100 0 0 100 0 0 100 0 45 27 0 27 30 0 34 29 0 48 0 15 47 0 17 48 0 16 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
IV 0 0 0 28 43 37 37 36 36
On the basis of this analysis, resuspension of seabed sediments into the overlying waters and net erosion of the seabed can occur only in offshore channels during mid flood and mid ebb, and over the intertidal flats during the early flood and late ebb phases. On the saltmarsh, by contrast, suspended sediment transport and net deposition take place throughout the tidal cycle. During the whole cycle, suspended sediment transport and deposition are the dominant processes along the profile, bedload transport being limited to the offshore channel, and being of short duration. This can also be verified by both field observations and laboratory experiments. Although sand ripples are commonly found on intertidal flats, field observations indicate that they can easily be erased by the tidal front at the very beginning of the flood tide. X-ray photographs of cores collected from the lower sandflat also show that horizontal bedding is the dominant sedimentary structure, and that cross bedding occurs only occasionally as a diagnostic sedimentary structure (Ke 1995). Although the asymmetry of both the tidal curve and current over the saltmarsh is not as marked as over the open intertidal flat at Freiston Shore, flood domination still persists, supporting a net landward SPM flux (Ke 1995). Elsewhere, tidal velocity
32
Ke and Collins
surges and asymmetry have been observed in the Warham tidal channel at Stiffkey Marshes, North Norfolk (Pethick 1980). At spring tide and under calm sea conditions, muddy sediments of the saltmarshes of The Wash are not resuspended because, throughout the various tidal cycles, current velocities were less than the threshold velocity Ucr for sediments with a Mz of 0.008 mm (Fig. 10). The dominant sedimentation process on the saltmarsh is therefore settling of SPM from the water column, in the present case. This leads to the accretion of the saltmarsh surface throughout each spring tidal cycle, particularly during slack water and on the ebb (see also Table 4). The above mechanism can be verified by the presence of well-developed muddy laminae which dominate the saltmarsh deposits in The Wash (Ke 1995). However, the low suspended sediment concentration (SSC is generally <100 mg 1-1)of the overlying waters during calm sea conditions in summer (for example, in 1992 and 1993), in association with the interlaying mud/sandy mud structures of the upper mudflat sediments and the occasional sandy deposits on the saltmarsh at Freiston Shore (Flavell 1995; Ke 1995), strongly imply either (i) a seasonal variation in sediment supply or (ii) a significant contribution by storm events to the accretion of the saltmarsh (see also Carling 1982; Stumpf 1983). Studies in the 1970s have shown that, for comparable tidal ranges, there was an order of magnitude increase in SSC from quiescent to storm conditions (Evans & Collins 1975).
5.5. Short and long-term sediment dynamics Like many other coastal embayments in the world, The Wash is experiencing a rise in sea level as shown by the extreme tidal levels recorded at King's Lynn in the south corner of The Wash (Fig. 1) since 1860 (Fig. 13a). Owing to the trapping effect of the embayment, and suspended sediment supply from the seabed of the North Sea (Ke et al. 1996), the intertidal fiats and saltmarshes have accreted and prograded for centuries, as witnessed by the land-reclamation history. The rise of the reclaimed saltmarsh surface seawards (Kestner 1962; Evans & Collins 1978) shows that accretion of the saltmarshes is keeping pace with sea-level rise. In the long term, the saltmarshes and intertidal fiats in The Wash are accretional. However, against this background of longterm accretion, short-term erosion of the intertidal fiats occurs from time to time, such as in the period 1972-1980 (Fig. 13d). Comparison of the local MHW-level change (Fig. 13b) and wave regime (Fig. 13c) with the tidal-fiat accretion rate (Fig. 13d) for this period reveals association with higher tidal levels and stormy sea conditions. The effects of the stormy, erosional period of 1972-1980 were also documented in other parts of the North Sea. For example, major dune erosion events occurred along the Dutch coast in that period (Oost, pers. comm.).
Tidal characteristics of The Wash
6
33
(a)
0
B
D
> 5
"0
E
,- 4 U.I
.
1860
1880
3.2 + /x~>~,
>
1900
.
.
1920
.
1940
+
(b) -=
.
1960
1980
- - + - - Measured
Xxx x/\
xxX~
---x-.+ Predicted+
3 -I-XXx
+*+.Xx >~x +.~+
"lp,
+
+ ~^x~x/x~X
+
1-
~ 2.8
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2.5
12
E u) "1-
2
-
_~1.5 eC
o
1
(c)
--x--Jan.
-
He50 +
-
0.5-
~
0
8g -6
X
;
E "
10
x
---+---July
;
+,.+-.
4
~ "
u 2 0
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 4
(d)
AI.
xx
x xxx xxx xx:~ ~:~ ~ ~:~
5
2
= =.
0
=
++
,
,
,
~
~
,
,
p. .0m
L
-2
-
x Saltmarsh + + + + + + + + +
0 0
+ Intertidal flat
<: "4
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 Year
Figure 13. Sea-level changes, wave regimes and accretion rates in The Wash and adjacent North Sea: a) extreme tidal level at King's Lynn, The Wash (extracted from HALCROW 1991); b) mean HW level at Lawyer's Sluice, The Wash (extracted from Richards et al. 1993); c) mean monthly significant wave height (Hs), and 50-year values of Hs (Hs50) at Dowsing LV in the North Sea (based upon Bacon 1989; DOE 1990); d) mean accretion rates for the intertidal flats and saltmarshes along the Freiston Shore profile (based upon Evans 1965; van Smirren & Collins 1982; Ke 1995).
34
Ke and Collins
Field surveys along the Freiston Shore profile show that the tidal current velocities measured at 10 cm above the tidal flats in 1981 were 2-3 times higher than those recorded in 1972 (van Smirren & Collins 1982). It is possible that the normal tidal character, i.e. tidal asymmetry favourable to onshore sediment transport (like that observed in the present study) may have been interrupted or even reversed to ebb domination during the stormy period. In other words, the main cause of short-term tidal-flat erosion is not long-term sea-level rise but short-term (i.e. decadal) fluctuations in regional wave climate and HW levels. In the present case, this particular erosional phase may also have been associated with the construction of a new embankment in 1979-1980.
ACKNOWLEDGEMENTS
The authors are grateful for the fieldwork assistance provided by M. Crowfoot, A. Flavell, S. Gao, C. Hobson, K. Padley, M. Poulos, and R. Stringer of the Department of Oceanography, University of Southampton, and T. Savill of MAREX (Isle of Wight). Thanks are extended also to the late C. Harness and friends at Freiston Shore, together with the officers of North Sea Camp (Boston), for their generous support during the field campaigns. We also would like to thank G. Evans and A.P. Oost for their sharp constructive comments on the manuscript. During the study, one of the authors (X. Ke) was in receipt of an ORS Award (Overseas Research Students Awards Scheme) from the Committee of Vice-Chancellors and Principles of the Universities of the United Kingdom (CVPC, U.K) (Reference No. ORS/913710), and supported by a NSFC (Natural Science Foundation of China) grant (Contract No. 49976026) and a MOE (Ministry of Education, China) grant (Project No. 99055).
REFERENCES
Allen, J.R.L. (1994) A continuity-based sedimentological model for temperate-zone tidal saltmarshes. J. Geol. Soc. (London) 151: 41-49. Allen, J.R.L. (1996) Shoreline movement and vertical textural patterns in saltmarsh deposits: implications of a simple model for flow and sedimentation over tidal marshes. Proc. Geol. Ass. 107: 15-23. Amos, C.L. (1974) Intertidal flat sedimentation of the Wash, E. England. Ph. D. thesis, University of London, 441 p. Amos, C.L. (1995) Siliciclastic tidal flats. In: Perillo, G.M.E. (ed.) Geomorphology and Sedimentology of Estuaries. Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 273-306. Amos, C.L. & Collins, M.B. (1978) The combined effects of wave motion and tidal currents on the morphology of intertidal ripple marks: the Wash, U.K.J. Sediment. Petrol. 48: 849-856.
Tidal characteristics of The Wash
35
Anderson, F.E. (1973) Observation of some sedimentary processes acting on a tidal flat. Mar. Geol. 14: 101-116. Bacon, S. (1989) Waves recorded at Dowsing Light Vessel 1970-1985. Institute of Oceanographic Sciences, Deacon Laboratory, Rep. No. 262, 60 p. Bayliss-Smith, T.P., Healey, R., Lailey, R., Spencer, T. & Stoddart, D.R. (1979) Tidal flow in saltmarshes. Estuar. Coast. Shelf Sci. 9: 235-255. Bearman, G. (1989) Waves, Tides and Shallow-Water Processes. Pergamon Press, Oxford, 187 p. BGS (British Geological Survey) (1988) East Anglia Sea Bed Sediments (Sheet 52N00). HMSO, London. Carling, P.A. (1982) Temporal and spatial variation in intertidal sedimentation rates. Sedimentology 29: 17-23. Chang, S.C. & Evans, G. (1992) Sources of sediment and sediment transport on the east coast of England: significant or coincidental phenomena? Mar. Geol. 107: 283-288. Collins, M.B., Amos, C.L. & Evans, G. (1981) Observation of some sediment transport processes over intertidal flats, the Wash, U.K. Spec. Publs Int. Ass. Sediment. 5: 81-98. Collins, M.B., Ke, X.K. & Gao, S. (1998) Tidally-induced flow structure over intertidal flats of the U.K. Estuar. Coast. Shelf Sci. 46: 233-250. Creuzberg, F. & Postma, H. (1979) An experimental approach to the distribution of mud in the southern North Sea. Neth. J. Sea Res. 13: 99-116. DOE (Department of Energy, U.K.) (1990) Metocean Parameters - Wave Parameters (supporting document to 'Offshore Installation: Guidance in Design, Construction and Certification- Environmental Consideration'). HMSO, London, 113 p. Dronkers, J. (1986) Tidal asymmetry and estuarine morphology. Neth. J. Sea Res. 20: 117-131. Dyer, K.R. (1986) Coastal and Estuarine Sediment Dynamics. John Wiley & Sons, London, 342 p. Dyer, K.R. (1994) Estuarine sediment transport and deposition. In: Pye, K. (ed.) Sediment Transport and Depositional Processes. Blackwell, Oxford, pp. 193-218. Ehlers, J. & Kunz, H. (1993) Morphology of the Wadden Sea: natural processes and human interference. Coastlines of the southern North Sea. Proc. 8th Symp. Coastal Ocean Management, pp. 65-84. Elliott, T. (1986) Siliciclastic shorelines. In: Reading, H.G. (ed.) Sedimentary Environments and Facies. Blackwell, Oxford, pp. 155-188. Evans, G. (1965) Intertidal flat sediments and their environments of deposition in the Wash. J. Geol. Soc. (London) 121: 209-245. Evans, G. & Collins, M.B. (1975) The transport and deposition of suspended sediment over the intertidal flats of the Wash. In: Hails, J. & Carr, A. (eds) Nearshore Sediment Dynamics and Sedimentation. John Wiley & Sons, London, pp. 237-306. Evans, G. & Collins, M.B. (1978) Sediment supply and deposition in the Wash. In: Doody, P. & Barnett, B. (eds) The Wash and its Environments. Nature Conservancy Council, U.K., pp. 48-63. Flavell, A.N. (1995) Sediment fluxes over intertidal flats: influence of first phase flood/last phase ebb. M. Sc. thesis, University of Southampton, 67 p.
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Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth. J. Aquat. Ecol. 28: 299-307. French, J.R. & Clifford, N.J. (1992) Characteristics and 'event-structure' of near-bed turbulence in a macrotidal saltmarsh channel. Estuar. Coast. Shelf Sci. 34: 49-69. French, J.R. & Spencer, T. (1993) Dynamics of sedimentation in a tidal dominated backbarrier salt marsh, Norfolk, UK. Mar. Geol. 110: 315-331. Frey, R.W. & Basan, P.B. (1985) Coastal salt marshes. In: Davis, R.A. Jr. (ed.) Coastal Sedimentary Environments. Springer, New York, pp. 225-301. Gadd, P.E., Lavelle, J.W. & Swift, D.J.P. (1978) Estimates of sand transport on the New York shelf using near-bottom current meter observations. J. Sediment. Petrol. 48: 239-252. Groen, P. (1967) On the residual transport of suspended matter by an alternating tidal current. Neth. J. Sea Res. 3(4): 564-574. HALCROW (Sir William Halcrow & Partners Ltd., U.K.) (1991) The Anglian Sea Defence Management Study Report. HRS (Hydraulic Research Station) (1974) The Wash Water Storage Scheme, Field Studies, Part Three (data collected in 1972). Vols I & II, No. DE15 (March 1974), 115 p. Kamps, L.F. (1963) Mud distribution and land reclamation in the eastern Wadden Sea. H. Veenman & Zonen N.V., Wageningen, 91 p. Kapdasli, M.S. & Dyer, K.R. (1986) Threshold conditions for sand movement on a rippled bed. Geo-Mar. Lett. 6: 161-164. Ke, X. (1995) Sediment dynamics of saltmarshes and intertidal flats, southern and eastern England. Ph.D. thesis, University of Southampton, 342 p. Ke, X., Collins, M.B. & Poulos, M. (1994) Velocity structure and sea bed roughness associated with intertidal (sand and mud) flats and saltmarshes of the Wash, U.K.J. Coast. Res. 10: 702-715. Ke, X., Evans, G. & Collins, M.B. (1996) Hydrodynamics and sediment dynamics of The Wash embayment, eastern England. Sedimentology 43: 157-174. Kestner, F.J.T. (1962) The old coastline of the Wash. Geogr. J. 128: 457-471. Krumbein, W.E., Paterson, D.M. & Stal, L.J. (eds) (1994) Biostabilization of sediments. BIS-Verlag, Oldenburg, 526 p. Leonard, L.A., Hine, A.C. & Luther, M.E. (1995) Surficial sediment transport and deposition processes in Juncus roemerianus Marsh, west-central Florida. J. Coast. Res. 11: 322-336. Miller, M.C., McCave, I.N. & Komar, P.D. (1977) Threshold of sediment motion under unidirectional currents. Sedimentology 24: 507-527. Oost, A.P. & de Boer, P.L. (1994) Sedimentology and development of barrier islands, ebb-tidal deltas, inlets and backbarrier areas of the Dutch Wadden Sea. Senckenbergiana marit. 24: 65-115. Pestrong, R. (1972) Tidal flat sedimentation at Cooley Landing, southwest San Francisco Bay. Sediment. Geol. 8: 251-288.
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Pethick, J.S. (1980) Velocity surges and asymmetry in tidal channels. Estuar. Coast. Shelf Sci. 11:331-345. Pond, S. & Pickard, G.L. (1978) Introductory Dynamic Oceanography. Pergamon, Oxford, 241 p. Postma, H. (1954) Hydrography of the Dutch Wadden Sea. Ph. D. thesis, Groningen University. Arch. N6erlandais Zoologie, 46 Livraison 10: 405-511. Postma, H. (1967) Sediment transport and sedimentation in the estuarine environment. In: Lauff, G.A. (ed.) Estuaries. Am. Ass. Adv. Sci., Washington D.C., pp. 158-179. Pugh, D.T. (1987) Tides, Surges and Mean Sea Level. John Wiley & Sons, Chichester, 472 p. Reineck, H.-E. & Singh, I.B. (1980) Depositional Sedimentary Environments. Springer, Berlin, 551 p. Richards, L.D., Windle, S.A., Blackman, D.L., Flather, R.A & Woodworth, P.L. (1993) Analysis of high waters and tides at Lawyer's Sluice in the Wash, eastern England. Proudman Oceanographic Laboratory, Rep. No. 31, 26 p. Sha, L.P. (1990) Sedimentological studies of the ebb-tidal deltas along the West Frisian Islands, The Netherlands. Ph. D. thesis, Geologica Ultraiectina, Med. Inst. Aadwetensch. Rijksuniversiteit Utrecht 64, 159 p. Shi, Z. & Chen, J.Y. (1996) Morphodynamics and sediment dynamics on intertidal mudflats in China (1961-1994). Cont. Shelf Res. 16(15): 1909-1926. Sternberg, R.W. (1972) Predicting initial motion and bedload transport of sediment particles in the shallow marine environment. In: Swift, D.J.P., Buane, D.B. & Pilkey, O.H. (eds) Shelf Sediment Transport. Hutchinson & Ross, Stroudburg PA, pp. 61-82. Sternberg, R.W., Larsen, L.H. & Miao, Y.T. (1983) Near-bottom flow conditions and associated sediment transport on the East China Sea continental shelf. In: Acta Oceanol. Sinica. Proc. Int. Symp. Sedimentation on the Continental Shell with special reference to the East China Sea. China Ocean Press, Beijing, pp. 486-498. Stride, A.H. (ed.) (1982) Offshore Tidal Sands: Processes and Deposits. Chapman & Hall, London, 222 p. Stumpf, R.P. (1983) The processes of sedimentation on the surface of a saltmarsh. Estuar. Coast. Shelf Sci. 17: 459-508. Sundborg, A. (1967) Some aspects of fluvial sediments and fluvial morphology. I. General views and graphic methods. Geogr. Ann. 49A: 333-343. van Smirren, J.R. (1982) Hydrodynamic and sedimentary characteristics of a predominantly sandy intertidal zone: The Wash, Eastern England. M. Sc. thesis, University of Wales, 144 p. van Smirren, J.R. & Collins, M.B. (1982) Short-term changes in sedimentological and hydrographical characteristics over a sandy intertidal zone, The Wash, U.K. GeoMar. Lett. 2: 55-60. van Straaten, L.M.J.U. (1964) De bodem der Waddenzee. In: Anderson, W.F., Abrahamse, J., Buwalda, J.D. & van Straaten, L.M.J.U. (eds) Het Waddenboek. Thieme, Zutphen, pp. 75-151. van Straaten, L.M.J.U. & Kuenen, P.H. (1957) Accumulation of fine grained sediments in the Dutch Wadden Sea. Geol. Mijnb. 19: 329-354.
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van Straaten, L.M.J.U. & Kuenen, P.H. (1958) Tidal action as a cause for clay accumulation. J. Sediment. Petrol. 28: 406-413. Woolnough, S.J., Allen, J.R.L. & Wood, W.L. (1995) An exploratory numerical model of sediment deposition over tidal salt marshes. Estuar. Coast. Shelf Sci. 41: 515-544. Zhang, R.S. (1992) Suspended sediment transport processes on tidal mud flats in Jiangsu Province. Estuar. Coast. Shelf Sci. 35: 225-233. Zhu, D.K. & Xu, T.G. (1982) Development and exploitation of the middle Jiangsu coast. J. Nanjing Univ. (Nat. Sci.) 334:1-24 (in Chinese with English abstract). Zimmerman, J.T.F. (1981) Dynamics, diffusion and geomorphological significance of tidal residual eddies. Nature 290: 549-555.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
39
Quantifying concentration and flux of suspended particulate matter through a tidal inlet of the East Frisian Wadden Sea by acoustic doppler current profiling P. Santamarina Cuneo* and B. W. Flemming
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
The seasonal transport of suspended particulate matter through a tidal inlet of the East Frisian Wadden Sea was recorded over complete tidal cycles using a 1.2 MHz acoustic doppler current profiler (ADCP). Suspended sediment concentrations and fluxes were estimated on the basis of the acoustic backscatter and the time-integrated current velocity along a cross-sectional transect of the main channel near the inlet. An optical transmissometer and a centrifuge p u m p were deployed to collect data and material for compositional analysis and calibration of the ADCP. Analyses of the p u m p samples revealed that substantial quantities of both fine sand and m u d were transported in suspension during peak flow. Under fair weather conditions (wind speeds <4 Bft), m a x i m u m concentrations of suspended particulate matter (SPMC) were relatively low (=60 mg 1-1), with no significant net import or export being evident. Under more windy conditions (=6 Bft), the total SPMC more than doubled (>130 mg 1-~), the concentration of suspended sand increasing by two orders of magnitude. In this case a mean net import of up to 2950 tonnes was recorded over individual tidal cycles, of which up to 1640 tonnes were contributed by the sand fraction, and up to 1310 tonnes by the m u d fraction. These results show that calibrated ADCPs can be used to quantify SPM concentrations and net fluxes of particulate matter in tidal channels. It was demonstrated that net transport was strongly dependent on local weather conditions, being evenly balanced during fair weather but showing a net sand-dominated import during more windy conditions. With increasing wind the proportion of suspended sand increased more rapidly than that of mud. It is tentatively suggested that the postulated longterm net export of fine-grained material from the Wadden Sea is linked to episodic storm action. Calibration of ADCPs for different size fractions is recommended. Furthermore, the resolution of the analog-to-digital conversion of the backscattering signal intensity should be upgraded to achieve more precise assessments of SPMC.
* Corresponding author: P. Santamarina Cuneo e-mail:
[email protected]
40
Santamarina Cuneo and Flemming
1. INTRODUCTION The East Frisian barrier island system is a mesotidal depositional environment that has been accreting while migrating landwards in response to the local sea-level rise for several thousand years (e.g., Streif 1989; Flemming & Davis 1994). In general, island migration is controlled by the sediment deficit created in a back-barrier tidal basin by sea-level rise in conjunction with the local sediment budget. To maintain the dynamic equilibrium between tidal prism, catchment area and basin elevation in the course of sea-level rise, sediment has to be imported into the basin. The sources for such sediment may be remote (e.g., updrift river discharge, coastal and shallow marine erosion), local (e.g., beach and upper shoreface erosion in front of a barrier island), or a combination of both. As a consequence, the rate of barrier island migration is directly related to the relative contributions of local and remote sediment sources, i.e. the higher the contribution from remote sources, the lower the local sediment turnover and hence, the slower the migration rate of the island. In the case of the East Frisian barrier island system there is little or no bedload input from remote sources (Flemming & Davis 1994; Flemming & Bartholom/i 1997). The sand deficit created by sea-level rise must therefore be compensated by beach and shoreface erosion with subsequent transfer into the back-barrier basin. Finegrained sediments (grain sizes <0.063 mm), on the other hand, are supplied from both local (shoreface) and remote (open North Sea) sources. Suitable mechanisms for the landward transfer of suspended matter in the Wadden Sea are the settling lag/scour lag mechanism (van Straaten & Kuenen 1957; Postma 1961) and tidal asymmetry with shoreward-directed residual flow on tidal flats (Groen 1967). However, Flemming & Nyandwi (1994) and Flemming & Bartholom/i (1997) have recently demonstrated that land reclamation over past centuries has strongly reduced the accommodation space for fine-grained sediments along the mainland shore, arguing that the Wadden Sea can no longer be regarded as a net sink for such material. Instead, they postulate that, with some local exceptions, muds imported in the short term must be eliminated in the long term as long as sea level continues to rise (at present the mean sea-level rises at 18 cm/century, the mean high-tide level at 25 cm/century in the wider study area). This hypothesis is supported by the widespread absence of mud flats and natural salt marshes along the diked shoreline of the Wadden Sea (Dijkema 1989; Flemming & Nyandwi 1994; Flemming & Bartholom/i 1997). Furthermore, recent investigations have revealed a marked seasonal variation in mud contents of local back-barrier sediments, gains in the course of the calmer summer months being counterbalanced by substantial losses over the more energetic winter months (Kr6gel & Flemming 1998; Kr6gel et al. 2000; Bartholom~i et al. 2000). Such apparent fluxes, however, have not been quantified on a basin-wide scale, and the fate of the resuspended material thus remains obscure. This brief introduction highlights the need for the establishment of quantitative import/export budgets on the scale of tidal catchments and subcatchments in order to accurately predict and realistically model the response of the Wadden Sea barrier-
Suspended particulate matter flux
41
island system to a rising sea level. This means that suspended sediment transport into and out of such systems has to be quantified along representative channel or inlet cross sections, and over sufficiently long time-intervals to achieve an integration of the different fluxes associated with periodic tidal rhythms (e.g., individual tidal cycles, spring-neap cycles), frequent aperiodic events (e.g., onshore and offshore winds ), and rare episodic events (e.g., severe storms). Although the existence of small dunes in the channels indicates that bedload transport also contributes to the overall sediment budget in the study area (e.g., Davis & Flemming 1991; Flemming et al. 1992), this aspect was not investigated in the present study because tidal flat accretion in the Wadden Sea is mainly associated with deposition from suspension transport (e.g., Postma 1961; Groen 1967). In the past, studies of sediment fluxes were hampered by severe instrumental and other technical limitations. Thus, quantitative point measurements had to be first interpolated vertically before being extrapolated horizontally, this awkward and semi-quantitative method producing gross estimates at best (e.g., Jay et al. 1997; Bartholdy & Anthony 1998; Suk et al. 1998). This has changed fundamentally with the recent advent of high-frequency acoustic doppler profilers (ADCPs) coupled with routine high-resolution navigation systems (differential GPS). ADCPs not only provide continuous vertical current profiles along a survey track but, in addition, the intensities of the backscattered signals are directly proportional to the concentration of scatterers (i.e. suspended matter) in the water column, the main problem to date having been a sufficiently accurate calibration of the instrument to allow conversion of the intensity of the backscattered signal into suspended material mass. The use of the backscattering signal of ADCPs for measuring SPMC is relatively new (e.g., Hales 1995; Austen et al. 1998; Holdaway et al. 1999). In this paper the results of such a calibration and conversion experiment are presented and discussed.
2. STUDY AREA
The study site was located in the main channel to the south of the western head of Spiekeroog island (Fig. 1). This channel drains the larger of the two major subcatchments (i.e. the eastern one) of the Otzum tidal basin situated between the islands of Langeoog and Spiekeroog. Although water exchange with neighbouring basins can be substantial at times of strong westerly or easterly winds, the catchment divides are of similar length and it can thus be assumed that the inflow across one watershed is counterbalanced by an equal outflow across the other. As a consequence, the Otzum basin can be regarded as a semi-closed system with only one inlet. The survey transect was 700 m long and oriented perpendicular to the channel axis between the intertidal Janssand and the western head of Spiekeroog island (Fig. 1). The channel cross section is asymmetrical at this location, reaching a maximum depth of about 15 m on the island side at high tide (Fig. 2). The tidal range reaches -1 3 m at spring tide, maximum current velocities attaining 1.5 m s in the centre of the
42
Santamarina Cuneo and Flemming
main channel. An anchor station was located in the deepest part of the main channel, just east of the transect.
Figure 1. Location of the study area and the measuring site in the Otzum inlet.
3. MATERIALS A N D M E T H O D S
Common approaches for measuring SPMC involve the use of optical backscatter devices, optical transmissometers, acoustic backscatter sensors and water samplers (e.g., Hanes et al. 1988; Osborne et al. 1994; Austen et al. 1998; Bunt et al. 1999). All optical and acoustic methods require careful calibration by measuring suspended matter concentrations in water samples from the respective survey sites. Water sampling, however, has many drawbacks in this respect, one of the biggest problems being the marked spatial and temporal variability of the SPMC in turbulent flows. Time-series measurements (e.g., Kr6gel 1997) and direct observations confirm that suspended matter is often transported in clouds, strong fluctuations in concentration often occurring within seconds and over distances as small as a few centimetres. The confounding effect of this variability has to be minimized if an acceptable level of accuracy is to be achieved for calibration. The small-volume instantaneous watersampling procedure commonly used for this purpose can clearly not meet this
Suspended particulate matter flux
43
requirement and, as a result, calibration curves for transmissometers and ADCPs often show considerable scatter of data (e.g., Holdaway et al. 1999). In the present case the aim of the study was to develop a quantitative procedure of accurately and efficiently determining SPM concentrations and fluxes through a tidal channel cross section. To achieve this a 1.2 MHz acoustic doppler current profiler (ADCP) was used. The instrument integrates the return signals of four instantaneous sound pulses emitted at I second intervals and oriented at 20 ~ from the vertical. Both current speed (cf. Fig. 2) and signal intensity (cf. Fig. 3) were recorded online at a sampling interval of 4 seconds while moving along the survey transect. In each case the return signals of 4 pings were averaged for each of the insonified cells spaced vertically at 25 cm, and horizontally at about 20 m intervals. At the same time the position of the survey boat was recorded by differential GPS.
Figure 2. Composite picture of the cross-sectional shape of the channel at the measuring site as recorded by the 1.2 MHz ADCP. The current pattern corresponds to the flow near low tide shortly after the turn of the tide, showing the flow separation between the flood current in the shallow channel section to the south (left), and the ebb current in the deep part of the channel to the north (right).
Since the intensity of the backscattered signal is directly proportional to the concentration of sound scatterers in each of the insonified cells, the signal can be converted into cell-specific SPMC once suitably calibrated. This was achieved by deploying a multi-sensor probe at the anchor station for 20 minutes every 2 hours over complete tidal cycles at two-monthly intervals over two years, measuring conductivity, temperature, pressure (depth) and turbidity (670 n m wavelength, and over a 5 cm path) at a water depth of 5 m in the deepest part of the main channel
44
Santamarina Cuneo and Flemming
(average depth 13 m). Simultaneously, a pump centrifuge for bulk sampling of suspended matter was used, the intake nozzle being placed at the same depth but a short distance downstream of the turbidity sensor. Suspended sediments were collected by online centrifuging a pumped water volume of 300 litres, corresponding to a pumping time of 20 minutes every 2 hours. At the same time continuous ADCP profiles were recorded at the anchor station. In addition to the spot measurements at 5 m water depth, vertical profiles were measured at hourly intervals with the multisensor probe. The seston samples were deep frozen and stored at-25~ being later separated in the laboratory into their respective sand (>63 1am) and mud (<63 pm) fractions.
Figure 3. Composite sound picture illustrating the intensity of the backscattered signals shortly before peak flow along the survey transect, and showing more intense backscattering (higher SPM concentrations) in the shallow southern section (left), and close to the bottom in the deeper northern section (right).
As pointed out above, the backscattered signals have to be averaged over several minutes at the very least in order to filter out the effect of high-frequency variability in SPMC. Since large-volume water samples (300 litres) were taken over long time intervals (20 minutes) during which the ADCP and the turbidity meter were both recording, the requirement of sampling and measuring at sufficiently large temporal and spatial scales was more than met. The turbidity meter had previously been calibrated in the laboratory using unfractionated seston samples from the study area. This involved the successive dilution of highly concentrated SPM samples with clear sea water, resulting in a highly correlated (r >0.99) calibration curve (Kr6gel 1997).
Suspended particulate matter flux
45
Since the calibration of ADCPs in the laboratory is rather elaborate, amongst others requiring a deep water tank, the instrument was calibrated in the field in the present study, the intensity of the backscattering signal being compared with the suspended matter concentrations obtained from the fractionated seston samples and the labcalibrated turbidity meter. Calibration factors were calculated for each of two grainsize groups (i.e. sand and mud). As pointed out by Thorne et al. (1991), a more complex calibration is required for high SPM concentrations or bigger particles. This was not considered necessary in the present study because the estimated sound absorption by the particles was much lower than the sound spreading and absorption in the sea water. On the basis of these data an algorithm converting the intensity of backscattered sound into SPMC (and components thereof) was developed (Santamarina Cuneo 2000), thus achieving an hitherto unparalleled accuracy in the calibration of an ADCP for this purpose. In addition to spot measurements at the anchor station, two cross-sectional transects were run consecutively with the ADCP at hourly intervals over complete tidal cycles (12.5 h), the two data sets being subsequently averaged in each case. Suspended matter concentrations and fluxes were then calculated for every 25 cm cell in each vertical profile by means of the calibration algorithm. These data were integrated over the whole cross section and the time intervals between successive hourly surveys. In this way mass fluxes of total suspended particulate matter as well as individual size fractions through the channel were calculated for each tidal cycle. Measurements were conducted at intervals of 1-3 months from April 1996 to December 1997. In the present study a selected data-set is presented which contrasts the fluxes recorded in March and May 1997 under fair weather and mild storm conditions, respectively.
4. RESULTS
The circulation patterns in the back-barrier basins of the wider study area are controlled by the interaction of the semi-diurnal tides with wind-induced water level changes which modulate the tidal prism. At the study site the ADCP surveys revealed a distinct temporal pattern in the lateral circulation structure, the current flowing along the deeper, northern side of the channel during the late ebb phase and along the shallower, southern side of the channel during the initial flood phase, probably reflecting at least in part the influence of the Coriolis force at low current velocities (cf. Fig. 2). In addition, a marked flow separation is evident at the turn of the tide. At full flow, however, the current occupies the whole channel during both tides. Suspended sediment analyses show that, in general, the sand content varied more strongly than the mud content, both varying with current speed (Santamarina Cuneo 2000). This pattern is not unexpected because the sand has higher settling velocities than the mud, the latter never settling out completely at the measuring site, even at slack tide. Furthermore, the suspended sand fraction showed a better correlation
Santamarina Cuneo and Flemming
46
with the backscattering signal (r >0.9) than did the suspended mud fraction (r >0.7) (Santamarina Cuneo 2000), this being consistent with the theory (e.g., Thorne et al. 1991). The concentrations and fluxes under calm weather conditions (March 1997) are illustrated in Figs 4A and 4B, respectively. Concentrations of the mud fraction reached 50 mg 14 during the peak ebb and 60 mg 14 during the peak flood current, the contribution of sand being negligible in both cases. During slack water the concentrations dropped to about 10 mg 1-1,indicating that a substantial portion of the finegrained suspended matter had large enough settling velocities to settle out. However, total fluxes did not exceed 300 kg s-1 during either tide. This demonstrates that the mass transport induced by tidal flow alone (i.e. without wave action) is relatively small. Nevertheless, the transport of suspended matter was higher in the flood phase (2988 tonnes) than in the ebb phase (2003 tonnes). As a result, a net import of 985 tonnes was recorded in this case.
.-.
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Figure 4. Concentration and flux of SPM under calm weather conditions over a complete tidal cycle. A: Concentrations of suspended sand and mud as a function of time. B: Flow volume and mass transport as a function of time. Negative values: export (ebb tide); positive values: import (flood tide).
Suspended particulate matter flux
47
In Figs 5A and 5B the calm weather situation of March 1997 is contrasted with the concentrations and fluxes observed under moderate wind conditions in May 1997. In this case, a northerly wind of 6 Bft was associated with 1 m high waves along the open coast. Wave-induced sediment resuspension in the nearshore zone explains the higher concentration of sand in May. Surprisingly, the concentration of mud remained at the same level as that recorded in the calm weather situation, indicating that waves 1 m in height are unable to resuspend mud known to occur in deeper waters further offshore (Figge 1981). Furthermore, the concentrations of both fractions were higher during the flood tide (positive velocities) than during the ebb tide (negative velocities). Whereas the sand fraction settled out during both slack tides, the higher wave-generated turbulence kept up to 20 mg 1-1 of mud in suspension.
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Figure 5. Concentration and flux of SPM over a complete tidal cycle under windy weather conditions (6 Bft wind speed and 1 m high waves along the open coast). A: Concentrations of suspended sand and mud as a function of time. B: Flow volume and mass transport as a function of time. Negative values: export (ebb tide); positive values: import (flood tide).
48
Santamarina Cuneo and Flemming
At 7557 tonnes, the mass transport was substantially higher during the flood phase as compared to the 4607 tonnes transported during the ebb phase. The much higher transport values in this case are evidently related to wave-induced resuspension along the open coast. The observed transport asymmetry thus resulted in a net import of 1640 tonnes of sand and 1310 tonnes of mud. In comparison to the calm weather situation of March 1997, the mass transport of SPM in the presence of 1 m waves along the open coast was about three times as high, thus emphasising the importance of wave action for the resuspension and transport of suspended matter in the region. Nevertheless, the data also demonstrate that, even under more windy conditions, overall concentrations of SPM in this part of the Wadden Sea remain low compared to values recorded in other environments, for example, the Severn Estuary, U.K. (Kirby & Parker 1983), the Changjiang Estuary, China (Shi et al. 1999), and the Amazon shelf (Kineke & Sternberg 1992).
5. DISCUSSION AND CONCLUSIONS This study has demonstrated that, if adequately calibrated, the intensity of the backscattered signals of ADCPs can be used effectively to infer suspended matter concentrations and fluxes. The method is fast and efficient, at the same time being sufficiently accurate to be as good as or superior to much more elaborate procedures such as round-the-clock profiling and pumping at fixed anchor stations (e.g., Jones et al. 1989). It was shown that the efficiency and accuracy of the approach was dependent on the calibration procedure, the method described in this paper having proved the most effective to date. Whereas stationary ADCPs will provide continuous data through the water column at a study site, mobile instruments will integrate over entire survey transects, for example, channel cross-sections. In addition, velocity measurements acquired simultaneously over the same spatial and temporal scales allow the calculation of SPM fluxes, and hence provide good estimates of sediment budgets. In this respect, ADCPs are superior to any other instrumentation currently available. It was further demonstrated that separate calibrations for individual size fractions increase the overall accuracy of the SPM estimates, the results complying with theoretical expectations and suggesting that there is room for further improvement. In particular, it would be of interest to investigate a larger number of size fractions, for example, by distinguishing between the flocculated and/or aggregated mud and the non-flocculated silt fractions as well as between fractions of different petrographic and geochemical compositions. The good results achieved in this study should also encourage ADCP manufacturers to upgrade the resolution of the analogto-digital conversion of the backscattered signal intensity. At the moment, this signal is still being regarded as a methodological by-product rather than a feature of high scientific value in its own right. The application of an accurately calibrated high-resolution 1.2 MHz ADCP to study SPM concentrations and fluxes into and out of a mesotidal back-barrier basin
Suspended particulatematterflux
49
has shown that the method is suitable for obtaining quantitative time-integrated estimates of material fluxes (excluding bedload transport) under different weather conditions. Wave-induced nearshore resuspension processes at wind speeds around 6 Bft. resulted in a substantial material import over a single tide, with an unexpectedly high proportion of suspended sand. This reflects the relatively low concentrations of SPM in southern North Sea waters (e.g., Eisma 1993), and implies that I m waves are ineffective in resuspending offshore muds. It also emphasises the necessity of calibrating the ADCP backscattering signals for different size fractions, including sands. Significantly, no net export was observed under the weather conditions covered in this study which ranged from calm weather to wind speeds up to about 6 Bft. A fragmentary data set collected under storm conditions (>9 Bft) indicated high SPM concentrations during the ebb phase. However, since the measurement programme had to be terminated prematurely because of technical problems, it was not possible to calculate net fluxes in this case. By implication, the evidence that a net long-term export of fine-grained material in the course of continued sea-level rise is linked to strong wind events and/or episodic storm action has remained inconclusive and awaits verification in future studies. A critical test of the net export hypothesis would obviously be a mass balancing of SPM fluxes recorded under weather conditions rougher than the range covered in this study (wind speeds >7 Bft). In particular, it would be necessary to develop ADCP-based survey techniques capable of handling severe storms. Other studies have shown that under such conditions flood currents can be reduced to almost zero by the backflow of dammed-up water masses. As a result there is no import of suspended material during the flood phase, although strong wave action in the backbarrier basin will keep remobilized muds in suspension. This material is then flushed out during the subsequent ebb surge which has been shown to reach velocities up to 65% higher than those of ebb currents under more benign conditions (Koch & Niemeyer 1978). As recently shown by Bartholdy & Anthony (1998), such episodic flushing events are evidently capable of exporting most of the material accumulated in calmer interim periods, much like the dramatically elevated sediment discharges associated with severe river floods.
ACKNOWLEDGEMENTS
We wish to extend our thanks to the captain and crew of the research vessel Senckenberg for their assistance during the station work and the surveys with the motorboat. The help of Astrid Raschke in carrying out much of the laboratory work was highly valued. The first author was supported by a bursary from the Deutscher Akademischer Austauschdienst (DAAD), whereas the Senckenbergische Naturforschende Gesellschaft provided running money and ship time. The generous support of both organisations is gratefully acknowledged.
50
Santamarina Cuneo and Flemming
REFERENCES
Austen, G., Fanger, H.-U., Kappenberg, J., Mfiller, A., Pejrup, M., Ricklefs, K., Ross, J. & Witte, G. (1998) Schwebstofftransport im Sylt-Romo Tidebecken: Messungen und Modellierung. In: G~itje, C. & Reise, K. (eds), Okosystem Wattenmeer: Austausch-, Transport und Stoffumwandlungsprozesse. Springer, Berlin, pp. 185214. Bartholdy, J. & Anthony, D. (1998) Tidal dynamics and seasonal dependent import and export of fine-grained sediment through a backbarrier tidal channel of the Danish Wadden Sea. In: Alexander, C.R., Davis, R.A. & Henry, V.J. (eds), Tidalites: Processes and Products. SEPM Spec. Publ. 61: 43-52. Bartholom~i, A., Flemming, B.W. & Delafontaine, M.T. (2000) Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Bunt, J.A.C., Larcombe, P. & Jago, C.F. (1999) Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter. Cont. Shelf Res. 19: 1199-1220. Davis, R.A. Jr. & Flemming, B.W. (1991) Time-series study of mesotidal bedforms, Martens Plate, Wadden Sea, Germany. In: Smith, D.G., Reinson, G.E., Zaitlin, B.A. & Rahmani, R.A. (eds), Clastic Tidal Sedimentology. Can. Soc. Petrol. Geol. Mem. 16: 275-282. Dijkema, K.S. (ed.) (1989) Habitats of the Netherlands, German and Danish Wadden Sea. Research Institute for Nature Management, Texel, and Veth Foundation, Leiden, 30 p. Eisma, D. (1993) Suspended Matter in the Aquatic Environment. Springer-Verlag, Heidelberg, 315 p. Figge, K. (1981) Sedimentverteilung in der Deutschen Bucht, Blatt Nr. 2900. Deutsches Hydrographisches Institut, Hamburg. Flemming, B.W. & Bartholom~i, A. (1997) Response of the Wadden Sea to a rising sea level: a predictive empirical model. Ger. J. Hydrogr. 49: 343-353. Flemming, B.W. & Davis, R.A. Jr. (1994) Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). Senckenbergiana marit. 24: 117-155. Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth. J. Aquat. Ecol. 28: 299-307. Flemming, B.W., Schubert, H., Hertweck, G. & Mfiller, K. (1992) Bioclastic tidalchannel lag deposits: a genetic model. Senckenbergiana marit. 22: 109-129. Groen, P. (1967) On the residual transport of suspended matter by an alternating tidal current. Neth. J. Sea Res. 3: 564-574. Hales, L. (1995) Accomplishments of the Corps of Engineers dredging research program. J. Coast. Res. 11: 68-88.
Suspended particulate matter flux
51
Hanes, D.M., Vincent, C.E., Huntley, D.A. & Clarke, T.L. (1988) Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island. Mar. Geol. 81: 185-196. Holdaway, G.P., Thorne, P.D., Flatt, D., Jones, S.E. & Prandle, D. (1999) Comparison between ADCP and transmissometer measurements of suspended sediment concentration. Cont. Shelf Res. 19: 421-441. Jay, D.A., Uncles, R.J., Largier, J., Geyer, W.R., Vallino, J. & Boynton, W.R. (1997) A review of recent developments in estuarine scalar flux estimation. Estuaries 20: 262-280. Jones, P.D., Head, P.C. & Whitelaw, K. (1989) A data recording station to measure water and solids fluxes through the Mersey Narrows. In: McManus, J. & Elliott, M. (eds), Developments in Estuarine and Coastal Study Techniques. Olsen & Olsen, Fredensborg, pp. 91-100. Kineke, G.C. & Sternberg, R.W. (1992) Measurements of high concentration suspended sediments using the optical backscatterance sensor. Mar. Geol. 108: 253-258. Kirby, R. & Parker, W.R. (1983) Distribution and behaviour of fine sediment in the Severn Estuary and Inner Bristol Channel, U.K. Can. J. Fish. Aquat. Sci. 40 (Suppl. 1): 83-95. Koch, M. & Niemeyer, H.D. (1978) Sturmtiden-Strommessungen im Bereich des Norderneyer Seegats. Forschungsstelle Norderney Jber. 29: 91-108. Kr6gel, F. (1997) Einflut~ von Viskosit~it und Dichte des Seewassers auf Transport und Ablagerung von Wattsedimenten (Langeooger R/ickseitenwatt, s/idliche Nordsee). Ber. Fachber. Geowissens. Universit~it Bremen 102, 168 p. Kr6gel, F. & Flemming, B.W. (1998) Evidence for temperature-adjusted sediment distributions in the back-barrier tidal flats of the East Frisian Wadden Sea (southern North Sea). In: Alexander, C.R., Davis, R.A. & Henry, V.J. (eds), Tidalites: Processes and Products. SEPM Spec. Publ. 61: 31-41. Kr6gel, F., Flemming, B.W. & Delafontaine, M.T. (2000) High-resolution sediment distribution patterns and dynamics in the Accumer Ee tidal basin: subtle effects of Europipe. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Osborne, P.D., Vincent, C.E. & Greenwood, B. (1994) Measurement of suspended sand concentrations in the nearshore: field intercomparison of optical and acoustic backscatter sensors. Cont. Shelf Res. 14: 159-174. Postma, H. (1961) Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res. 1: 148-190. Santamarina Cuneo, P. (2000) Fluxes of suspended particulate matter through a tidal inlet of the East Frisian Wadden Sea (southern North Sea). Berichte, Fachbereich Geowissenschaften, Universit~it Bremen (in press). Shi, Z., Ren, L.F. & Hamilton, L.J. (1999) Acoustic profiling of fine suspension concentration in the Changjiang Estuary. Estuaries 22: 648-656.
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Santamarina Cuneo and Flemming
Streif, H.J. (1989) Barrier islands, tidal flats, and coastal marshes resulting from a relative rise of sea level in East Frisia and the German North Sea coast. Proc. KNGMG Symp. Coastal Lowlands: Geology and Geotechnology, pp. 213-223. Suk, N.S., Guo, Q. & Psuty, N.P. (1998) Feasibility of using a turbidimeter to quantify suspended solids concentration in a tidal saltmarsh. Estuar. Coast. Shelf Sci. 46: 383-391. Thorne, P.D., Vincent, C.E., Hardcastle, P.J., Rehman, S. & Pearson, N. (1991) Measuring suspended sediment concentrations using acoustic backscatter devices. Mar. Geol. 98: 7-16. van Straaten, L.M.J.U. & Kuenen, P.H. (1957) Accumulation of fine grained sediment in the Dutch Wadden Sea. Geol. Mijnb. 19: 329-354.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
55
Surface erosion of fine-grained sediment revisited A. J. Mehta a* and T. M. Parchure b
aCoastaland OceanographicEngineering Department, University of Florida, Gainesville, FL 32611, U.S.A. bCoastaland Hydraulics Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS 39180, U.S.A. ABSTRACT
For applications in waters with low to moderate concentrations of suspended finegrained sediments, the formula of Kandiah (1974) for the rate of bed surface erosion remains a convenient model for simulating scour due to steady or quasi-steady flows. Arulanandan et al. (1980) show that the two parameters characterizing this formula, namely the erosion rate constant and the bed shear strength with respect to erosion, seem to be related in such a way that the rate constant decreases with increasing shear strength. Other studies have shown that the shear strength correlates with bed density. We have used these findings to develop a formula for estimating the rate of erosion from bed density for sediments which are largely inorganic. While this formula cannot replace the need for laboratory or prototype testing of sediment beds for an accurate determination of erosion rate, it may be used to obtain "first cut" values of the rate characterizing parameters in situations where they are unavailable from measurements. Recent experimental results suggest that the same formula may also be useful for estimating the rate of erosion of organic-rich sediments.
1. INTRODUCTION Modeling the erosion of fine-grained sediment beds continues to pose problems largely due to a lack of clear understanding of the way in which the bed-water interface responds to a flow-induced stress. For steady or quasi-steady (e.g., tidal) flows, numerous formulae relating the rate of surface erosion to the bed shear stress have been proposed. In this mode of erosion, particles or particulate aggregates at the bed surface are detached and entrained in the flow, thus causing bed scour. Some of the earlier formulae have been summarized by Mehta et al. (1982). These stress* Corresponding author: A. Mehta e-mail:
[email protected]
56
Mehta and Parchure
based formulae are generally applicable to cases of low to moderate suspended sediment concentrations. At high concentrations exceeding 4-20 g 1-1, settling of sediment is hindered, being controlled by the rate of upward seepage of interstitial water. Under these conditions, a layer of fluid mud may form over the bed due to the deposition of suspended sediment. The mechanism by which this layer erodes is not modeled well by stress-based formulations. In any event, to various degrees all such formulae are empirical-phenomenological approximations of very complex flowparticle interactions which ultimately cause bed particles and aggregates to dislodge, rupture and entrain. The formula proposed by Kandiah (1974) is
~;--gM
'L'b- "~s / Ts
(1)
in which ~ is the erosion rate or mass flux (mass eroded per unit bed area per unit time), % is the bed shear stress, "rs is the bed shear strength with respect to erosion, and the erosion rate constant ~Mis the value of r when zb = 2zs. Equation 1 is characteristically applicable to homogenous, uniform density, uniform shear strength beds, and indicates that ~ varies with the excess shear stress %-~s. Thus, a plot of r versus ZD-~s ideally appears as a straight line, as shown by, among others, Kandiah (1974) through careful laboratory experimentation on the erosion of clay and clay/silt mixtures of uniform density. This is exemplified in Fig. 1, in which the erosion rate and the shear strength (as determined by the intercept of a given line with the horizontal axis) is seen to depend on the percentage (by weight) of montmorillonite in the Yolo loam + montmorillonite mixture. Also observe that the effect of the highly cohesive montmorillonite was to decrease ~M(line slope) due to an increase in the shear strength of the mixture. 9
Yolo I.oam
+ Montmonllonlte (
3.5 10% =
3
~2.5 =' =r =
2 1.5
..,
,-.
1
0.5 0
i
0
2
4 6 Shear Stre~s, "~b(Pa)
8
Figure 1. Erosion rate versus bed shear stress for mixtures of Yolo loam and montmorillonite (percentage values indicate montmorillonite by weight; adapted from Kandiah 1974).
Fine-grained sediment erosion
57
For beds which are stratified with respect to density and shear strength, formulae which account for the variation in ~s with depth have been developed, e.g., by Parchure & Mehta (1985). Although these formulae differ from Eq. 1, in all of them the erosion rate varies with the excess shear stress. This similarity, as well as experience from modeling applications, suggest that Eq. 1 can also be used for stratified beds with a reasonable degree of accuracy by allowing ~s to vary with depth, i.e. by replacing zs by ~(z) where z denotes the vertical coordinate (Hayter & Mehta 1986). Vinzon (1997) used measured time-series of near-bed velocities and suspended sediment concentrations at sites on the Amazon Shelf off Brazil to develop the linear plot shown in Fig. 2. The observed relationship is akin to the lines in Fig. 1, and therefore conforms to Eq. 1 but with a considerably greater scatter of data points, as would be expected in field data. The shear strength ~s was obtained from a formula discussed below. Finally, with reference to Eq. 1 it is also interesting to note that a compilation of erosion rate formulae for wind- as well as mechanically-generated waves in laboratory flumes indicates the validity of the functional form of Eq. 1 (Mehta 1996). This information is summarized in Table 1, in which characteristic parameters are given for the expression (2)
'rb - ~ s ~s
E =E M
For 6 = 1, Eq. 2 reduces to Eq. 1. As seen from Table 1, experimental data at times have yielded values of 6 close to unity. In Eq. 2 % is the peak value of the bed shear stress during the wave cycle, and ~s can differ from that associated with currentinduced erosion due to the effect of cyclic loading on the soil matrix (Maa & Mehta 1987; Mimura 1993).
3 ..............
i. . . . . . . . . . . . . .
io
~1.5 "=.
!~ ...........
o
o o~ !
11 ........
os[ "
~
...................
.....~---;o ............ i ..............
..A'o..o ~ o
0
o
:
.
.
.
.
............................. i
0.05 0.1 0,15 Excess Shear Stress, ;b-~, (Pa)
0,2
Figure 2. Erosion rate versus excess shear stress based on the field data analysis of Vinzon (1997).
58
Mehta and Parchure
Table 1. Parameters for Equation 2 for w a v e - i n d u c e d erosion. Source
Mode of wave generation
Sediment
Parameter a ranges a (cm); co(rad s-1 ); k (cm1)
Parameter values in Eq. 2 EM
Ts
(Pa)
8
(g m -2 s -1)
Alishahi & Krone (1964)
Wind
Bay mud
0.9 < a _<3.4
Test 1:0.48 Test 2:11.2
0.29 0.39
1.72 1.15
Thimakorn (1984)
Mechanical
Rivermud
3.1 _<e0< 12.6 0.16 < ak < 1.60
_ 08b/2qTs b
Var.
1.00
Maa & Mehta (1987)
M e c h a n i c a l Kaolinite; bay mud
1.4 < a < 3.7 3.3 < 0) < 6.3
Kaolinite: 131 Mud: 30
DV DV
1.15 0.95
Mimura (1993)
Mechanical
0.6 < a < 6.9 4.8 < co< 8.2
0.27
0.15
1.82
Clays; bay mud
a = wave amplitude; co= wave frequency; k = wave number. amplitude of bottom orbital velocity; ~b = wave boundary layer thickness. Abbreviations: Var. = variable; DV = depth-variable. a
b0 =
For a p p l y i n g Eq. 1 to erosion b y steady or quasi-steady flows, it is essential that ~s a n d ~M be d e t e r m i n e d for every site-specific situation. In general this process tends to be tedious b o t h in the laboratory, and in the field (Lee & Mehta 1994). It is therefore natural to ask if a generalized, albeit approximate, formula can be d e v e l o p e d to assist in an initial estimation of ~s and eM, particularly for those situations in w h i c h no data other than bed density are available. It a p p e a r s that past studies can be helpful in this context, and in the following the question of estimating ~s and ~Mhas been a d d r e s s e d on an exploratory basis, using p r e v i o u s l y d e v e l o p e d concepts a n d correlations.
2. EROSION SHEAR STRENGTH A N D RATE C O N S T A N T The quantities 9 and ~M d e p e n d on s e d i m e n t composition, the stress history of the bed, a n d the chemistry and t e m p e r a t u r e of the pore and eroding fluids. A n extensive review of influential factors has revealed that over one h u n d r e d such factors have been e x a m i n e d (Lee & Mehta 1994; Lee et al. 1994). Despite this effort, significant site-specificity of conditions for erosion makes it impractical to develop multivariate expressions relating these or fewer factors to ~ a n d eM. For coastal a n d brackish w a t e r bodies, Berlamont et al. (1993) r e d u c e d these factors to a total of twenty-eight. Mehta & Li (1997) r e c o m m e n d e d six m e a s u r e m e n t s : particle size distri-bution of (dispersed) sediment, settling velocity of (non-dispersed) sediment, mineralogical composition,
59
Fine-grained sediment erosion
o r g a n i c content, cation e x c h a n g e capacity a n d salinity. These are p r i m a r i l y m e a n t to c h a r a c t e r i z e the b e d a n d the fluid e n v i r o n m e n t , r a t h e r t h a n z~ a n d EMas such.
2.1. Shear strength A l t h o u g h shear s t r e n g t h a n d b e d d e n s i t y are n e i t h e r u n i q u e l y r e l a t e d in the p h y sical sense n o r d i m e n s i o n a l l y h o m o g e n e o u s , a t t e m p t s h a v e b e e n m a d e to correlate these t w o p a r a m e t e r s e m p i r i c a l l y , r e c o g n i z i n g that the d e n s e r the soil, the h a r d e r it is likely to erode. G e n e r a l l y , g i v e n ~ as a m e a s u r e of soil s h e a r s t r e n g t h , r e l a t i o n s h i p s of the f o r m -c - ~(qo- qo~.)~
(3)
h a v e b e e n u s e d (Table 2) w h e r e ~ is the solids w e i g h t fraction, ~)~ is a l i m i t i n g or m i n i m u m v a l u e of ~) b e l o w w h i c h "r = 0, a n d ~, ~ are s e d i m e n t - s p e c i f i c coefficients. Table 2. E x p r e s s i o n s r e l a t i n g a characteristic shear stress to solids w e i g h t fraction. Source
Sediment
Shear strength ('c) (Pa)
~);~a
~
~
(~Range a
Krone (1963)
Estuary muds
Upper Bingham yield ('CB)
N.D.; Assume 0
466
2.55
0.008- 0.57
Migniot (1968)
Marine muds
Upper Bingham yield ('cB)
N.D.; Assume 0
Var.
4.00
0.094- 0.19
Owen (1970)
Estuary muds
Upper Bingham yield ('CB)
N.D.; Assume 0
1110
2.33
0.042- 0.11
Vinzon (1997)
Shelf muds
Upper Bingham yield ('r
N.D.; Assume 0
2024
2.62
0.021 -0.19
Hwang (1989)
Lake muds
Vane "r
0.06
22.6
1.00
0.060 - 0.26
Thorn & Parsons (1980)
Estuary muds
Surface erosion (,r
N.D.; Assume 0
37.5
2.28
0.014- 0.12
Kusuda et al. (1984)
Estuary muds
Surface erosion (~)
N.D.; Assume 0
6.50
1.60
0.032 - 0.11
Villaret & Paulic (1986)
Bay muds
Surface erosion (~)
N.D.; Assume 0
1.65
1.00
0.10 - 0.38
Black (1991)
Estuary muds
Surface erosion ('c)
N.D.; Assume 0
1.88
2.30
0.13 - 0.25
Berlamont et al. (1993)
Marine muds
Surface erosion (~s)
N.D.; Assume 0
5.41
0.90
0.02 - 0.07
Use ps = 2650 kg m 3 for converting ~) to density, p, except for the relationship of Hwang (1989) for which p~ = 2140 kg m -3. Abbreviations: N. D. = not defined. Var. = variable.
60
Mehta and Parchure
Thus, according to Eq. 3, x depends on the excess solids weight fraction. Note that the upper Bingham yield strength (XB),the vane shear strength (%), and xs have all been used, although only the last is of direct interest to the present analysis. Among these, ~Band ~v are representative of the bulk physical properties of the soil. ~Bis associated with soil rheology, and has been used, for example, to determine the bottom slope required to generate mud underflows (Einstein 1941). x is a measure of the bulk strength of the soil and has been used, for instance, in geotechnical evaluations of cohesive soil consistency. Thus, Annandale (1995) has suggested the following classification.
Soil Consistency
Identification
Vane Shear Strength, r (KPa)
Very soft
Easily moulded by fingers.
0-80
Soft
Moulded by fingers with some pressure.
80-140
Firm
Very difficult to mould. Penetrated by hand spade.
140-210
Stiff
Requires hand pick for excavation.
210-350
Very stiff
Requires power tool for excavation.
350-750
Most studies on the erosion of submerged soils in estuarine and marine environments are limited to very soft cohesive materials. This is so because waveand current-induced bed shear stresses in these environments are usually not large enough to erode stiffer soils. On the other hand, in rivers with high flow velocities, even firm soils can erode significantly over months and years. Thus, the vane shear strength is a convenient and commonly used parameter to assess the erosion potential of cohesive soils in a given flow environment, even though it is not highly accurate (Lee 1985). In contrast to % and ~, ~ is related to the strength of surface aggregates, and is a transport characterizing quantity. Referring to the results in Table 2, the characteristic difference between % and ~s is reflected by the values of the proportionality coefficient ~ which is considerably higher for % (mean ~ = 1200, excluding the data of Migniot 1968) than for ~s (mean ~ = 10.6). Likewise, the exponent ~ is higher for ~B (mean values of 2.88 and 1.62 for ~B and "r respectively). With respect to the sole correlation for ~, observe that ~ = 1. It should be noted, however, that this low value may not be representative of beds which are predominantly inorganic, because this relationship of Hwang (1989) was developed for a lacustrine mud which contained a high amount (39% by weight) of organic material.
Fine-grained sediment erosion
61
In a strict sense, ~ should represent the volume fraction of sediment aggregates rather than sediment weight fraction, because the inter-particle bond strength depends primarily on the degree of space-filling by the soil matrix. The use of solids weight in lieu of aggregate volume is an approximation which obviates the usual difficulty in estimating aggregate volume. In any event, conceptually the minimum value of ~, namely ~x, is analogous to the space-filling weight fraction at which the sediment matrix begins to exhibit a measurable shear modulus of elasticity, which increases with increasing ~ (>~x) (James et al. 1988). The same "threshold" condition may apply to the development of normal effective stress in the soil (Ross & Mehta 1990). Hwang (1989) determined ~;~ = 0.06 from a plot of measured % versus ~ by extrapolating the linear relationship (~ - 1) to xv = 0 when ~ = ~x. This author further showed that this value of ~x (0.06) was commensurate with the sediment density below which the mud was in a fluid-like state, and therefore was devoid of measurable vane shear strength. As seen from Table 2, others did not report ~x which has therefore been assumed to be zero in applying Eq. 3 to their data. An example of data conforming to Eq. 3 is shown in Fig. 3, based on the work of Kusuda et al. (1984) who used mud from the Chikugo River estuary in Japan. The relationship of Vinzon (1997) from Table 2 was used for calculating "r for the data presented in Fig. 2. Water Content (%) 0.2
1000 500 200 ................................................. Chikugo
el
~'0.1
III
~0,05
1:, = 6 . 5 ~ ' 6
r r~
0.02 .............................................................. 0.03 0.05 0.1 Solids Weight Fraction,
0.2
Figure 3. Bed shear stress versus solids weight fraction and water content for mud in the Chikugo estuary in Japan (after Kusuda et al. 1984).
Two significant factors on which ~, %and ~x depend are bed sediment composition and fluid chemistry. This dependence is reflected in the variability in ~ and ~ associated with ~s in Table 2. The relative importance of composition and chemistry cannot be sorted out easily in these cases, because the shear strength of a soil of given mineral composition can be vastly influenced by the chemical composition of water.
62
Mehta and Parchure
Furthermore, even though salinity is reported in most investigations on marine muds, other chemical factors can also affect the soil fabric, thereby influencing soil erodibility. For example, Fig. 4 shows the results of Kandiah (1974) for a montmorillonite in terms of the sodium adsorption ratio (SAR), the ratio of sodium ions to the sum of calcium and magnesium ions in the bed pore fluid. The plot shows that the state of this montmorillonite could be altered between dispersed and coagulated (or flocculated) merely by changing the pH of the pore fluid either by holding SAR constant or by holding constant the total cation concentration in the pore fluid. Since a dispersed clay bed can erode with considerably greater ease than a coagulated bed of the same clay, Fig. 4 demonstrates that sediment composition alone cannot be a unique, or even dominant, determinant of bed erosion potential.
5
: ~ + +-, . . . . . . . . . . . . . . . . . . . . .
I
,
<
......
i
Montmoriilonite
opH4-5
~= 30
+ pH 6 . 5 - 7.5
r~ ~
t ...........
g 25
9 pH 1 0 - 11
= 20 C , N
+
L. 15 o
"~ 10 r~ . . . . . . . . . .
,
0
i
...................
= ....
. . . . . . . . . .
I .
.
.
.
.
.
.
.
10 20 30 40 Total Cation Concentration (meq L +t)
.
.
50
Figure 4. Coagulation-dispersion boundary curves for a montmorillonite at three pH ranges (after Kandiah 1974).
2.2. R a t e c o n s t a n t
The erosion rate constant generally depends on the same factors which influence ~s. A noteworthy effect studied in the laboratory is the variation of cM with fluid temperature. In that context, surface erosion of cohesive beds has been treated as a mechanism which is phenomenologically akin to the rate process for chemical reactions. Conceptually, erosion occurs when a threshold "energy of activation" is exceeded, and inter-particle chemical bonds are broken. Following this concept it can be shown that cM, hence the rate of erosion ~, should increase with increasing temperature in such a way that logr would vary linearly with l / T , where T is the absolute temperature. This behaviour can be represented by ~ M
Fine-grained sediment erosion
63
A A----
= e
T
(4)
T This relationship was shown to hold for the erosion of a bed of grundite by Kelly & Gularte (1981; Fig. 5). The coefficients A = 34.7 and A = 10145 defining the line are specific to the sediment-fluid mixture used, and were obtained at a constant eroding flow velocity (0.18 m s-l). Their magnitudes conform to the units used for s and T in Fig. 5. Whereas A is an erosion rate scaling parameter, A characterizes the rate of decay of the erosion rate with decreasing temperature. In general, these coefficients can be expected to depend on the physicochemical properties of the sediment and fluid, on the solids weight fraction ~) and, especially with respect to A, on the applied bed shear stress. Figure 5 highlights the difference in the rate of erosion as might exist between temperate waters and cooler waters at higher latitudes. To illustrate this difference, consider water at 5~ (278~ and at 30~ (303~ From Fig. 5 we obtain r = 33 and 730 g m -2 s -1, respectively, indicating a 22-fold increase in the rate of erosion due to a 25~ rise in temperature. As Lau (1994) has noted, an increase in temperature affects the van der Waals attractive force at the particle surface only marginally, but the inter-particle repulsive force significantly. As a result, particleparticle bonds rupture more easily at higher temperatures, thereby leading to enhanced erosion.
,
.
.
.
t~/'l" = exp(A-A/T)
~o
Grundite
0.1 0.0032
0.0033 0.0034 0.0035 Inverse T e m p e r a t u r e I/T ( ~ ~)
0.0036
Figure 5. Erosion rate-temperature plot for a grundite (after Kelly & Gularte 1981).
Based on the observation that the rate of erosion decreases as the shear strength increases, Arulanandan et al. (1980) defined [~N = ~M/'~s ' and plotted it against "r derived from erosion tests on a large number of soil samples. Introducing this modified rate constant conveniently redefines Eq. 1 in terms of ~N and ~s, i.e. ~ = aN(%-~). The investigators found that, despite evident data scatter, in the mean [~N decreased
64
Mehta and Parchure
with ~ monotonically. This finding naturally raises the question as to w h e t h e r such a correlation can be established, even t h o u g h in an approximate way, for a w i d e r set of erosion data. Such a correlation w o u l d essentially mean that, on account of Eq. 3, the sediment density (hence the solids weight fraction) w o u l d be the principal physical parameter characterizing Eq. 1. Given the convenience in modeling erosion which the use of this correlation w o u l d entail, the question is addressed below. Table 3 lists selected laboratory erosion studies, the apparatus used and the sediment sources. Important features of the apparatus have been s u m m a r i z e d elsewhere (Lee & Mehta 1994). An examination of the erosion rate data s h o w e d that the ~N versus "cs relationship of A r u l a n a n d a n et al. (1980) could be sorted into seven groups, as s h o w n in Figs 6, 7 and 8. The groups are arranged such that, for a given value of ~s, ~N decreases with group n u m b e r increasing from 1 to 7. The principal characteristics considered to relate potentially to bed erosion for each g r o u p are s u m m a r i z e d in Table 4.
Table 3. S u m m a r y of selected erosion experiments. Source
Apparatus
Sediment
Espey (1963)
Rotating cylinder
Marl
Partheniades (1965)
Straight flume
Bay mud
Christensen & Das (1973)
Drill-hole device
Kaolinite
Raudkivi & Hutchison (1974)
Closed conduit
Kaolinite
Kandiah (1974)
Rotating cylinder
Loam; clays; bay mud; mixtures
Arulanandan et al. (1973)
Rotating cylinder
Loam
Arulanandan et al. (1975)
Rotating cylinder
Loam
Gularte et al. (1977)
Closed conduit
River mud
Fukuda (1978)
Annular flume
Lake mud
Thorn & Parsons (1980)
Straight flume
River and bay muds
Arulanandan et al. (1980)
Straight flume and rotating cylinder
Various soils
Gularte et al. (1980)
Closed conduit
Grundite and silt
Villaret & Paulic (1986)
Annular flume
Kaolinite; estuary mud
Hwang (1989)
Annular flume
Lake mud
Winterwerp et al. (1993)
Straight flume and annular flume
Kaolinite; estuary mud
Fine-grained sediment erosion
65
Table 4 provides information on bed density, clay content, total salt concentration and cation exchange capacity (CEC). While in most studies the salt concentration was the same in the bed pore fluid and the ambient eroding fluid, in many of the tests of Kandiah (1974) as well as Arulanandan et al. (1973, 1975, 1980) salts were confined to the pore fluid, the eroding fluid being salt-free water. Also, unlike most other studies, these investigators chose to impregnate pore water with monovalent as well as divalent cations (e.g., sodium and calcium). This meant that, in many cases, they used salts other than sodium chloride. In order to evaluate the data from the various studies in units common to all, salt concentration is expressed in milliequivalents of salt per liter of solution. Table 4. Characteristics of selected data groups. Group no.
Bed density, p (kg m -3) Range
Mean +_S.D.
Clay content
(%)
d
Range
Mean
Total salt concentration (meq L-') b Range
+_S.D.
Mean
Cation exchange capacity (meq 100 g-')c Range
_+S.D.
Mean +- S. D.
1 (7) a
1440-2270
1910+460
5-53
24+_16
1-205
54+_87
9-20
13+_4
2 (16)
1420-2080
1730_+170
12-46
27+-12
2-145
27_+42
7-28
18+6
3 (34)
1480-1860
1670+160
6-50
28+_10
4-40
19+7
9-23
15+5
4 (20)
1270-1990
1820_+210
12-42
23_+10
1-205
31+-64
8-30
15+_5
5 (26)
1350-2090
1820+_200
11-53
27+_11
2-205
22+_42
8-26
15+5
6 (23)
1070-2240
1740+310
6-80
33_+26
2-33
7_+9
8-25
16+6
7 (26)
1100-2400
1730+-380
6-94
34+-19
1-6
3+-2
5-100
23+23
a Values within parentheses are n u m b e r of data points. b milliequivalents per liter. c milliequivalents per 100 g r a m s of d r y sediment. d Standard deviation.
Since mean bed density did not vary substantially or systematically between the 7 groups, the influence of density o n ~N cannot be examined, apart from the effect of density o n [~N via ~. Clay content, a measure of bed cohesion in tandem with the CEC, increased from 24% for Group 1 to 34% for Group 7. This trend, therefore, correlates with a decrease in [~N in the expected way because of an increasing contribution of cohesion to erosion resistance. The mean CEC increased from 13 to 23 from Group 1 to Group 7 in a systematic way with the exception of Group 2. This general trend too is consistent with the observed decrease in CN" Finally, the (mean) total salt concentration showed a noteworthy effect. Thus, observe in Table 4 that the total salt concentration decreased in the mean from 54 meq L-1 for Group 1 to 3 meq L-1 for
66
Mehta and Parchure
Group 7. The influence of pore-fluid salt on bed stability is not independent of the composition of the sediment, hence a general statement concerning any interdependence of erosion rate and salt concentration cannot be made at this stage. Thus, on the basis of the data presented (Table 3), the influence of salt concentration o n E N must, if at all, be viewed in tandem with concurrent changes in clay content and CEC. However, we refrain from examining these interrelationships any further here. We contend that such an approach would be speculative and premature because various factors such as organic content, microbial effects, etc., which could be important in controlling erosion were overlooked or ignored in many earlier stvdies. Also, since the salt used was not sodium chloride in all of these cases, a c o m m o n basis for comparison in terms of salt concentration does not exist. Secondary physical factors which may have influenced erosion rate measured in earlier experimental works include fluid temperature and flow characteristics of the apparatus used (Rohan & Lefebvre 1991). In addition, Lee & Mehta (1994) suggested tentatively that the degree to which the sediment was disturbed during bed preparation by, for example, remoulding could affect subsequent measurements of erosion rate, although these effects remain to be accurately quantified. The curves in Figs 6, 7 and 8 are based on the relationship
~N--ENOe
~s
(5)
for which the value of ~N0conveniently chosen was 200 g N -~s -I, the coefficients Z and )~ for each group being given in Table 5. Figure 9 shows ENvalues for all 7 groups for xs<10 Pa. Each curve is essentially applicable only over the range of xs covered in Figs 6 through 8. These curves highlight the wide range of ~Nvalues which can occur for a given x~. 2x10 2 *.~" 10 2
".,.~roup 1 =
0o
o
10 "1 0
10
20 30 40 50 60 Bed Shear Strength, 1:, (Pa)
Figure 6. Erosion rate constant versus bed shear strength for Group 1.
Fine-grained sediment erosion
67
,.~xlO z
.~.10 2
~10 +
100
~1o-~ 10 ~ 0
2
4 6 8 ]Bed S h e a r S t r e n g t h , 1:, ( P a )
10
Figure 7. Erosion rate constant versus bed shear strength for Groups 2, 3, 4 and 5.
++;+op t ......... +" |~.
..........
,...
~101 a tm
= 100 r~
"
~
9
9, 1 0 .~ @
1 0 "z . .
.
:.
0
.
.
.
0.5
1
Bed
1.5
Shear Strength, x, (Pa)
Figure 8. Erosion rate constant versus bed shear strength for Groups 7 and 8. 2x102,
C-O ~ -z
........
,
,
,
, .........
. . . . -:=~-!~!!!!!!!!~!!!!!!!!!!!-!!~!!iii!!!!!i~!!!!!!!!!!! . . . . . . . :. . . . . . . . . . .
+. . . . . . . . . . .
.10
: ........... ........
:::
:~=~!Z!!!!!!!i!!!!!!!:!::!i!!::!!!!!!!!i::!!!!::!!:!! : .... ~. . . . . . . . : ............ 10 0 R
~
~
!
-
-
~
I
.....
.~ 1 0
---: ..........
1 0 -2
Bed
Shear Strength,
z,
0Pa)
Figure 9. Erosion rate constant versus bed shear strength for all groups.
68
Mehta and Parchure
Table 5. Coefficient values for Eq. 5 and main characteristics of Groups 1-7. Group no.
a
Coefficients in Eq. 5 a
Mean clay content
Mean total salt concentration
Mean cation exchange capacity
X
k
(%)
(meq L 4)
(meq 100 g-~)
1
1.345
0.368
24
54
13
2
2.892
0.372
27
27
18
3
3.905
0.356
28
19
15
4
4.938
0.355
23
31
15
5
6.594
0.382
27
22
15
6
9.011
0.386
33
7
16
7
10.582
0.252
19
3
These values of X and X apply when "r is in Pa, and
8N is in g N-1 s
23 -1
.
2.3. Erosion rate Combining the equations 1, 3, 4 and 5 results in A A--]x 8-SNoTe Te -z[((q~-q)x)r [l:b-~(q)-q)X) ~]
(6)
The application of Eq. 6 can be illustrated by the following example. Assume that the bed and fluid conditions for which r is to be calculated conform to those of Group 2 (Tables 4 and 5), and that the shear strength can be obtained from the relationship of Villaret & Paulic (1986) in Table 2. Accordingly, values of the coefficients required for solving Eq. 6 will be: ~N0 200 g N 1 s-l; Z - 2.892; k = 0.372; ~ = 1.65; ~ - 1.00; (~x = 0; A = 27.0; A - 10145. Consider a bed of density p - 1545 kg m -3 subjected to a flowinduced bed shear stress ZB-- 1 Pa at a water temperature T - 27~ (300~ The solids weight fraction is obtained from ~)- ( p - p w ) / ( p s - P w ) where Pw is water density. --
Thus, with Pw = 1000 kg m -3 and Ps - 2650 kg m -3 (Table 2), we obtain ~) = 0.33. Equation 6 then yields a - 2.98 g m 2 s -~. This value is comparable to those m e a s u r e d in the laboratory (e.g., Fig. 1) and in the field (Fig. 2).
3. ORGANIC-RICH SEDIMENTS
The incorporation of organic material into inorganic fine-grained sediment characteristically reduces the granular density of the mixture and, in the case of clays, the overall effect of inter-particle cohesion is reduced, even though organic particles
69
Fine-grained sediment erosion
in aggregates are m u t u a l l y b o u n d t h r o u g h intertwining and by biogenic adhesives such as mucopolysaccharides. In Table 6, erosion rate p a r a m e t e r s extracted from three studies are s u m m a r i z e d . In all cases the e x p e r i m e n t s w e r e c o n d u c t e d in the laboratory using m u d s from w a t e r bodies in Florida. These waters receive organic sediments from a variety of sources, but most of the material appears to be locally g e n e r a t e d (Mehta et al. 1997). Observe that the e x p o n e n t ~ of Eq. 3 is 0.2 for s e d i m e n t from Lake Okeechobee with 39% organic matter (by weight). This m a y be c o m p a r e d w i t h the m e a n value of 1.62 from Table 2 for largely inorganic sediment beds. The R o d m a n Reservoir (45% sediment organic content) and the Lower Kissimmee River basin (50% sediment organic content) samples s h o w e d practically no d e p e n d e n c e of the shear strength on density. This is illustrated in Fig. 10, in which it is also seen that the m e a n of all data points is 0.1 Pa, a v e r y low shear strength in relation to its range in Fig. 9. M e h t a et al. (1994) and Rodriguez et al. (1997) further observed that the erosion rate constant s h o w e d no systematic variation with shear strength of organic sediment, even t h o u g h the rates w e r e high (mean ~N 4.95 g N -1 s -1 a n d 2.02 g N -1 s -1, respectively) in relation to the range in Fig. 9. "
-
Table 6. Erosion p a r a m e t e r s for organic-rich sediments. Location/ Source
Lake Okeechobee (Hwang 1989)
Rodman Reservoir (Mehta et al. 1994)
Lower Kissimmee River Basin (Rodriguez et al. 1997)
Organic fraction (wt%)
39
45
50
Mixtur granular density, 9s (kg m -3) 2140
1914
1586
~);~
0.06
N. D.; Assume 0
N. D.; Assume 0
1.0
0.105
0.099
0.2
0
0
N.I.
4.95_+8.0
2.02_+10.1
0.06 - 0.17
0.02 - 0.28
0.08 - 0.38
Erosion rate constant, ~N
(mean_+S.D.) (g N "1s-1) (~Range
Abbreviations: N .I. = not included in this table; see Table 3. N. D. = not defined. S. D. = standard deviation.
70
Mehta and Parchure
0.t4
0.t2
o
O
O
§
§
§
~
+o
~ ~ean or aU datJ
o 9~ '
0.1
~ o.os
+ §
t~
4-
=
0.06
"~ 0 . 0 4
o : R o d m a n Reservoir + : L o w e r Kissimmee River
0.02 0
...............
0
i
0.1
,
,,
I
,
* .
0.2 0.3 Solids W e i g h t F r a c t i o n ,
.
.
.
.
|
0.4
0.5
Figure 10. Erosion shear strength vs solids weight fraction for organic-rich sediments.
The high erosion potential of organic-rich sediments is no doubt due to the comparatively light and weakly bound nature of the aggregates. The lack of significant dependence of erosion on bed density may be explained by the following scenario. Unlike clayey beds whose interface with water can be reasonably well defined, especially for dense beds, the organic-rich bed-water interface tends to develop a layer of "fluff" consisting of aggregates released from the bed with a thickness of a few aggregate diameters. When fluid stress is applied, it is this layer of weakly interconnected particles, with a low negative buoyancy, which is entrained. Further, as the layer erodes it is replenished by continual generation of aggregates from within the bed which is disturbed by flow-induced deformations. Then, since the density of the fluff layer is determined by the "released" aggregates rather than the bed, the erosion rate is largely unaffected by bed density.
4. CONCLUDING COMMENTS Equation 6 must be used with great caution because of the inherent and possibly unquantifiable uncertainties arising from experimental measurements, as well as confounding effects of numerous physicochemical and biological factors associated with the sediment, pore water and eroding fluids in the natural environment. The observation that remoulded beds may erode differently from undisturbed ones in laboratory setups (for example, Lee & Mehta 1994) has led to increasing reliance on in situ devices in recent times. A case in point is the "Sea Carousel" underwater flume developed by Maa (1993). Such devices, which generate their own flow field over the natural bed, tend to measure the shear strength and erosion rate of only the top few millimeters of the seabed. This is usually adequate in comparatively low-energy
Fine-grained sediment erosion
71
environments where scour is limited to an upper layer which is a few centimeters thick. In high-scour situations such as in rivers, or where wave action can intensely disturb the bottom, it becomes necessary to determine the erodibility of thicker layers as a function of depth. In one approach, McNeil et al. (1996) and Jepsen et al. (1997) reported erosion rates measured in a ducted flume ("Sedflume") using sediment cores collected from field sites. The core is gradually "fed" to the flume from beneath at a rate which compensates erosion at the top. Since the flow velocity, and hence the bed shear stress in the flume, can be varied over a wide range, it is possible to analyze fine-grained sediments ranging in consistency from soft to stiff. Knowing the density profile of the core material, the parameters which characterize Eq. 6 can be determined. A limitation which cannot be easily obviated is that the coring procedure itself may affect the in situ shear strength (Lee 1985).
REFERENCES
Alishahi, M.R. & Krone, R.B. (1964) Suspension of cohesive sediments by windgenerated waves. Tech. Rep. HEL-2-9, Hydr. Engrg. Lab., Univ. California, 24 p. Annandale, G.W. (1995) Erodibility. J. Hydr. Res. 33(4): 471-493. Arulanandan, K., Sargunam, A., Loganathan, P. & Krone, R.B. (1973) Application of chemical and electrical parameters to prediction of erodibility. In: Gray, D.H. (ed.) Soil Erosion: Causes and Mechanisms, Prevention and Control. Spec. Rep. 135, Highway Res. Board, Washington, DC, pp. 42-51. Arulanandan, K., Loganathan, P. & Krone, R.B. (1975) Pore and eroding fluid influences on surface erosion of soil. J. Geotech. Engrg. Div. Am. Soc. Civ. Engs. 101(1): 51-66. Arulanandan, K., Gillogly, E. & Tully, R. (1980) Development of a quantitative method to predict critical shear stress and rate of erosion of natural undisturbed cohesive soils. Tech. Rep. GL-80-5, U.S. Army Eng. Watwys. Expt. Sta., Vicksburg (variously paginated). Berlamont, J., Ockenden, M., Toorman, E. & Winterwerp, J. (1993) The characterisation of cohesive sediment properties. Coast. Engrg. 21: 105-128. Black, K.S. (1991) The erosion characteristics of cohesive estuarine sediments: some in situ experiments and observations. Ph.D. Dissertation, University of Wales, 313 p. Christensen, R.W. & Das, B.M. (1973) Hydraulic erosion of remolded cohesive soils. In: Gray, D.H. (ed.) Soil Erosion: Causes and Mechanisms, Prevention and Control. Spec. Rep. 135, Highway Res. Board, Washington, DC, pp. 8-19. Einstein, H.A. (1941) The viscosity of highly concentrated underflows and its influence on mixing. Trans. 22nd Ann. Meet. Am. Geophys. Union, Part I, Hydrol. Papers. Nat. Res. Coun., Washington, DC, pp. 597-603. Espey, W.H. (1963) A new test to measure the scour of cohesive sediment. Tech. Rep. UYD-01-6301, Hydr. Engrg. Lab., Univ. Texas, 45 p. Fukuda, M.K. (1978) The entrainment of cohesive sediments in freshwater. Ph.D. Dissertation, Case Western Reserve University, Cleveland, 210 p.
72
Mehta and Parchure
Gularte, R.C., Kelly, W.E. & Nacci, V.A. (1977) The threshold erosional velocities and rates of erosion for redeposited estuarine dredged material. Proc. 2nd Int. Symp. Dredg. Technol. Brit. Hydr. Res. Assoc., Cranfield, Paper H3. Gularte, R.C., Kelly, W.E. & Nacci, V.A. (1980) Erosion of cohesive sediments as a rate process. Ocean Engrg. 7: 539-551. Hayter, E.J. & Mehta, A.J. (1986) Modelling cohesive sediment transport in estuarial waters. Appl. Math. Model. 10: 294-303. Hwang, K.-N. (1989). Erodibility of fine sediment in wave-dominated environments. M.Sc. Thesis, University of Florida, 159 p. James, A.E., Williams, D.J.A. & Williams, P.R. (1988) Small strain, low shear rate rheometry. In: Dronkers, J. & Van Leussen, W. (eds) Physical Processes in Estuaries. Springer-Verlag, Berlin, pp. 488-500. Jepsen, R., Roberts, J. & Lick, W. (1997) Effects of bulk density on sediment erosion rates. Water, Air Soil Poll. 99: 21-31. Kandiah, A. (1974) Fundamental aspects of surface erosion of cohesive soils. Ph.D. Dissertation, University of California, 261 p. Kelly, W.E. & Gularte, R.C. (1981) Erosion resistance of cohesive soils. J. Hydr. Div. Am. Soc. Civ. Engrs. 107(10): 1211-1224. Krone, R.B. (1963) A study of rheological properties of estuarial sediments. Tech. Bull. 7, Comm. Tidal Hydr., U.S. Army Eng. Watwys. Expt. Sta., Vicksburg, 105 p. Kusuda, T., Umita, T., Koga, T., Futawatari, T. & Awaya, Y. (1984) Erosional process of cohesive sediments. Water Sci. Technol. 17: 891-901. Lau, Y.L. (1994) Temperature effect on settling velocity and deposition of cohesive sediments. J. Hydr. Res. 32(1): 41-51. Lee, H.J. (1985) State of the art: laboratory determination of the strength of marine soils. In: Chaney, R.C. & Demars, K.R. (eds) Strength Testing of Marine Sediments and In-situ Measurements. ASTM STP883, Am. Soc. Test. Mat., Philadelphia, pp. 181-250. Lee, S.-C. & Mehta, A.J. (1994) Cohesive sediment erosion. Rep. DRP-9406, U.S. Army Eng. Watwys. Expt. Sta., Vicksburg (variously paginated). Lee, S.-C., Mehta, A.J. & Parchure, T.M. (1994) Cohesive sediment erosion: Part 1. Test devices and field instrument assemblies. Part 2. Relationship between the erosion rate constant and bed shear strength. Rep. UFL/COEL-MP/94/02, Coast. Oceanogr. Engrg. Dept., Univ. Florida (variously paginated). Maa, P.-Y. (1993) VIMS Sea Carousel: its hydrodynamic characteristics. In: Mehta, A.J. (ed.) Nearshore and estuarine cohesive sediment transport. Am. Geophys. Union, Washington, DC, pp. 265-280. Maa, P.-Y. & Mehta, A.J. (1987) Mud erosion by waves: a laboratory study. Cont. Shelf Res. 7(11/12): 1269-1284. McNeil, J., Taylor, C. & Lick, W. (1996) Measurements of erosion of undisturbed bottom sediments with depth. J. Hydr. Engrg. 122(6): 316-324. Mehta, A.J. (1996) Interaction between fluid mud and water waves. In: Singh, V.P. & Hager, W.H. (eds) Environmental Hydraulics. Kluwer, Dordrecht, pp. 153-187.
Fine-grained sediment erosion
73
Mehta, A.J. & Li, Y. (1997) A PC-based short course on fine-grained sediment transport engineering. Rep. Coast. Oceanogr. Engrg. Dept., Univ. Florida, 85 p. Mehta, A.J., Parchure, T.M., Dixit, J.G. & Ariathurai, R. (1982) Resuspension potential of deposited cohesive sediment beds. In: Kennedy, V.S. (ed.) Estuarine Comparisons. Academic Press, New York, pp. 591-609. Mehta, A.J., Lee, S.-C., Li, Y., Vinzon, S.B. & Abreu, M.G. (1994) Analysis of some sedimentary properties and erodibility characteristics of bottom sediments from the Rodman Reservoir, Florida. Rep. UFL/COEL-MP/94/03, Coast. Oceanogr. Engrg. Dept., Univ. Florida (variously paginated). Mehta, A.J., Kirby. R., Stuck, J.D., Jiang, J. & Parchure, T.M. (1997) Erodibility of organic-rich sediments: a Florida perspective. Rep. UFL/COEL-MP/97/01, Coast. Oceanogr. Engrg. Dept., Univ. Florida (variously paginated). Migniot, P.C. (1968) A study of the physical properties of different very fine sediments and their behavior under hydrodynamic action. La Houille Blanche 7: 591-620 (in French, with English abstract). Mimura, N. (1993) Rates of erosion and deposition of cohesive sediments under wave action. In: Mehta, A.J. (ed.) Nearshore and Estuarine Cohesive Sediment Transport. Am. Geophys. Union, Washington, DC, pp. 247-264. Owen, M.W. (1970) Properties of a consolidating mud. Rep. INT 83, HR Wallingford (variously paginated). Parchure, T.M. & Mehta, A.J. (1985) Erosion of soft cohesive sediment deposits. J. Hydr. Engrg. 111(10): 1308-1326. Partheniades, E. (1965) Erosion and deposition of cohesive soils. J. Hydr. Div. Am. Soc. Civ. Engrs. 91(1): 105-138. Raudkivi, A.J. & Hutchison, D.L. (1974) Erosion of kaolinite clay by flowing water. Proc. Roy. Soc. London, A337, 537-554. Rodriguez, H.N., Jianhua, J. & Mehta, A.J. (1997) Determination of selected sedimentary properties and erodibility of bottom sediments from the lower Kissimmee river and Taylor Creek-Nubbin Slough basins, Florida. Rep. UFL/COEL-97/09, Coast. Oceanogr. Engrg. Dept., Univ. Florida (variously paginated). Rohan, K. & Lefebvre, G. (1991) Hydrodynamic aspects in the rotating cylinder erosivity test. Geotech. Testing J. 14(2): 166-170. Ross, M.A. & Mehta, A.J. (1990) Fluidization of soft estuarine mud by waves. In: Bennett, R.H., Bryant, W.R. & Hulbert M.H. (eds) The Microstructure of Finegrained Sediments: from Mud to Shale. Springer-Verlag, New York, pp. 185-191. Thimakorn, P. (1984) Resuspension of clays under waves. In: Denness, B. (ed.) Seabed Mechanics. Graham and Trotman, London, pp. 191-196. Thorn, M.F.C. & Parsons, J.G. (1980) Erosion of cohesive sediments in estuaries: an engineering guide. Proc. 3rd Int. Symp. Dredg. Technol. Brit. Hydr. Res. Assoc., Cranfield, pp. 349-358. Villaret, C. & Paulic, M. (1986) Experiments on the erosion of deposited and placed cohesive sediments in an annular flume and a rocking flume. Rep. UFL/COEL86/07, Coast. Oceanogr. Engrg. Dept., Univ. Florida, 61 p.
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Vinzon, S.B. (1997) A contribution to understanding fine sediment dynamics at the Amazon shelf. Ph.D. Dissertation, Federal University of Rio de Janeiro, 168 p. Winterwerp, J.C., Kuijper, C., De Wit, P., Huysentruyt, H., Berlamont, J., Toorman, E.A., Ockenden, M. & Kranenburg, C. (1993) On the methodology and accuracy of measuring physico-chemical properties to characterise cohesive sediments. Rep. MAST-1 G6-M Cohes. Sed. Proj. Grp. to Comm. Euro. Commun., Dir. Gen. XII, Delft Hydraulics, Delft (variously paginated).
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
75
Comparison of the erosion shear stress of oxic and anoxic sediments in the East Frisian Wadden Sea I. Austen* and G. Witte
GKSS Research Centre, Institute of Hydrophysics, D-21502 Geesthacht, Germany
ABSTRACT
In June 1996, large anoxic areas appeared on the intertidal sediment surface of the East Frisian Wadden Sea. These conditions caused large-scale mortality of benthic flora and fauna, and may thus have produced widespread changes in sediment stability. In this study potential differences in the erodibility of largely comparable oxic and anoxic sediment surfaces were investigated. At sites on the Baltrum backbarrier tidal flats, anoxic sediment samples showed systematically lower erosion shear stresses than their oxic counterparts. This observation, however, could not be assigned to a disappearance of sediment-stabilising microorganisms since anoxic surfaces did not always show significantly lower chlorophyll-a concentrations. On the other hand, the observed differences in erosion shear stress are well within the range of variability observed on oxic sediment surfaces for a larger data set incorporating the Sylt-Romo tidal flats of the North Frisian Wadden Sea. Thus, a slight decrease in sediment stability at sites with anoxic surfaces is evident from the data for the East Frisian Wadden Sea but the morphodynamic effect of this phenomenon does not appear to be significant.
1. INTRODUCTION Since the early 1980s closer attention has been given to so-called 'black spots' in the East Frisian Wadden Sea (e.g., H6pner & Michaelis 1994). Black spots are localised anoxic patches on the sediment surface which have long been documented but it is only recently that an increase in this phenomenon has been claimed for the region (H6pner & Michaelis 1994). Generally, the upper millimeter to decimeter-thick sediment layer is oxidised on the tidal flats, varying in thickness depending on sediment type. Below this oxic horizon, the anaerobic decomposition of organic material produces H2S and FeS which then causes a black discoloration of the underlying sediment. An increased chemical sediment activity resulting from, amongst other things, the decay of algal * Corresponding author: I. Austen e-mail:
[email protected]
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Austen and Witte
biomass, may result in such high oxygen consumption that anoxic conditions reach the sediment surface to produce black spots (Neira & Rackmann 1996). Limited aeration as well as excessive growth of macroalgae and microalgae resulting from eutrophication are presumed to be the main factors causing the appearance of anoxic patches on the tidal flats (Kolbe 1991; H6pner & Michaelis 1994). In June 1996 a catastrophic bloom of the diatom Coscinodiscus concinnus was documented along the East Frisian coast, and this phenomenon can largely explain the sudden appearance of large anoxic areas in the adjoining tidal flats (Delafontaine & Flemming 1997). As a result of these large-scale anaerobic conditions, a mass mortality of fauna and flora was documented at these sites. In the present study it is hypothesised that the absence of organisms may result in a higher mobility of the surface sediments since these are stabilised mainly by the binding effects of extracellular polymeric substances (e.g., Paterson & Darborn 1991). These 'glues' are produced by a wide range of organisms, including epipelic diatoms (Paterson 1989, 1994; Paterson et al. 1994). The erosion thresholds of oxic and anoxic sediment surfaces were compared in order to ascertain whether a decrease in microphytobenthic standing stock in oxygen-deficient black areas was associated with a higher erodibility of intertidal sediments.
2. S T U D Y AREA A N D M E T H O D S
The study was carried out at three intertidal locations, one being situated in the immediate rear of Baltrum Island (site Ba II) whereas the other two were closer to the mainland in the vicinity of Dornumer Siel (sites Ba I and Ba III; Fig. 1). At each site, 3 to 5 cores (diameter 10 cm) were taken from both oxic and anoxic sediments, resulting in 24 cores in all (see Table 1). The oxic and anoxic sample series showed a similar sediment composition (mud content) in each case. In a laboratory container installed nearby on land, erosion experiments were performed using the EROMES apparatus (Schtinemann & Ktihl 1991; Witte & Kfihl 1996). The EROMES system is a patented development of the GKSS Research Centre, and serves to measure erosion of naturally formed, muddy sediments. The system uses artificially induced turbulence to erode an undisturbed sediment surface and to keep the eroded material in suspension. After sampling, the sediment core is kept in a 10-cm diameter Perspex sampling tube with a 30-cm column of water above the sample. The turbulence and hence erosion is induced by a propeller positioned 5 cm above the sediment surface. Special mounting baffles prevent rotation of the water column and vertical balancing currents. The applied bottom shear stress is a function of the propeller speed and is calibrated by quartz sands of specified grain size and known critical shear stress. The suspension in the sampling tube is continuously pumped through a bypass to monitor its concentration by measuring the attenuation of a collimated light beam. For each experiment the attenuation is calibrated by repeated sampling of the suspension. The erosion rate as a function of time is calculated by differentiation of
77
Erosion shear stress in oxic and anoxic sediments
the measured concentration. During the erosion experiment, the shear stress is increased in uniform steps, and the critical erosion shear stress of the sediment is calculated from the increase in the rate of erosion. In order to analyse the large number of samples collected during the field campaign, the maximum shear stress measurements were limited to 1 N / m 2, although 3 N / m 2 can be generated with the EROMES apparatus. Additional sediment cores (diameter 5 cm) were collected to determine the chlorophyll-a content of the upper 1-mm sediment layer by means of HPLC (Wright et al. 1991). Chlorophyll a (Chl a) was used as an indicator of algal biomass (Admiraal 1984). In each case, a dried parallel sample was analysed for particulate organic carbon (POC) using a commonly employed infra-red detecting technique (Ernst 1975). Surficial sediment samples (ca. 10-mm upper layer) were wet sieved to determine mud content (grain-size fractions <63 pm) after measuring sediment wet bulk density and water content. Parallel samples were analysed for total organic matter (TOM) as loss of ignition at 550~ for 2 hours.
-,,
10km N
German Bight
7 ~ 30'E
45'N
Baltrum
"
Ba II
Ba III
Figure 1. Map of the study area showing the intertidal flats outlined by the lowest low-tide level. Sampling locations are indicated by annotated closed circles.
78
Austen and Witte
3. R E S U L T S
In oxic and anoxic sediments at site Ba III, mud contents, water contents, TOM contents and POC contents were all higher than values recorded at the other 2 sites (Table 1). Oxic and anoxic sediments showed similar values for these parameters at each site.
Table 1. Physicochemical characteristics of oxic and anoxic sediments on the Baltrum (Ba) and K6nigshafen (K6) tidal flats (K6 data extracted from Riethmiiller et al. 1998). The values for chlorophyll-a concentration and POC content are for the upper 1-mm surface sediment layer (SSL). Sample position
Sample number
Ba III anoxic Ba III oxic Ba II anoxic Ba II oxic Ba I anoxic Ba I oxic K6 anoxic K6 oxic n.a. not available
3 3 5 5 4 4 4 4
Mud content (%)
Water content (%)
TOM content (%)
88 86 46 43 19 19 80 80
75 67 38 49 25 23 70 57
11.1 10.7 1.2 1.1 0.3 0.4 n.a. n.a.
Mean Mean Mean POC Chl a per shear content m 2 SSL stress (%) (mg) ( N / m 2) 4.55 4.88 1.56 1.00 0.38 0.22 4.83 2.87
27.60 96.00 13.38 17.11 5.35 9.27 6.69 8.72
0.63 1.00 0.17 0.38 0.13 0.80 0.19 0.26
At all 3 sites, and despite considerable variations, oxic surfaces showed generally higher mean erosion thresholds than their anoxic counterparts with similar m u d contents (Fig. 2). This trend was observed over a wide range of m u d contents (ca. 2090%; Table 1). No erosion was observed for the cores of the oxic m u d d y Ba III site (mud content >85%) and, therefore, the experimental limit of 1 N / m 2 is shown as a m i n i m u m value. Except for the m u d d y Ba III site, chlorophyll-a concentrations were less than 30 m g / m 2 in the surface sediment layer (SSL = upper 1-mm sediment layer), the anoxic samples showing only slightly lower values (Fig. 3; Table 1). At site Ba III, chlorophyll-a concentrations varied in the range 57-141 m g / m 2 in the SSL. In oxic sediments at this site, the values were significantly higher than those recorded in anoxic sediments of comparable m u d contents (mean chlorophyll-a concentrations ca. 96 and 28 m g / m 2 SSL, respectively; Table 1).
79
Erosion shear stress in oxic and anoxic sediments
I' 1.0
---
Ba III G
E Z
Ba r
Bal I!
0.5 Bal
K5
O
,,,.,,..,
O
0
50
100
mud content [%] Figure circles) zontal (North
2. Mean critical erosion thresholds of oxic (open circles) and anoxic (closed sediments versus m u d contents for the 3 sites behind Baltrum Island (horibars indicate m i n i m u m and m a x i m u m values). Triangles = K6nigshafen Frisian Wadden Sea; data extracted from Riethmfiller et al. 1998). 200
--
E ..I r N
E
Bal 100
......
r
Bal 0
i
Ba II
- I ...............I 0
K6 i
Ba III
......i. .......................I
50
100
mud content [%] Figure 3. Mean chlorophyll-a concentrations in the 1-mm surface sediment layer (SSL) of oxic (open circles) and anoxic (closed circles) sediments versus m u d contents (horizontal bars indicate m i n i m u m and m a x i m u m values). Triangles = K6nigshafen (North Frisian Wadden Sea; data extracted from Riethmfiller et al. 1998).
Austen and Witte
80
A multiple Pearson correlation analysis of the combined data set (n = 36) for all oxic and anoxic sediment samples reveals that, of the 4 parameters investigated in the present study, erosion shear stress was best correlated to chlorophyll-a concentration in the SSL (Table 2). Considerably weaker correlations were observed in the case of POC content, mud content and water content.
Table 2. Pearson correlation matrix of chlorophyll-a concentrations, POC contents, mud contents and water contents for oxic and anoxic data sets combined. POC content (1-mm layer) 0.53
Mud content
H20 content
0.47 0.95
0.44 0.85 0.96
Erosion shear stress 0.73 0.30 0.22 0.30
Chl a (1-mm layer) POC (1-mm layer) Mud content H20 content
4. D I S C U S S I O N
On the tidal flats of the East Frisian Wadden Sea, anoxic surficial sediments were characterised by lower erosion thresholds than largely comparable oxic sediments. Thus, this trend appeared to be essentially independent of sediment mud content, water content, and organic matter content. These findings are consistent with the observations of Riethmiiller et al. (1998) for the K6nigshafen tidal flats on the island of Sylt in the North Frisian Wadden Sea (Table 1; Figs 2, 3). The results of the present study also demonstrated a good positive correlation between chlorophyll-a concentration in the SSL and erosion shear stress. However, this finding is based on only a single high value in chlorophyll-a concentration which was documented in the muddy oxic sediments of site Ba III in the vicinity of the mainland dike. In order to establish whether a reduction in microphytobenthic abundance was the main cause of lower erosion thresholds in the anoxic sediments on the tidal flats behind Baltrum Island, these data were compared with a larger data set from the K6nigshafen tidal flat in the Sylt-Romo Bight of the North Frisian Wadden Sea (Bayerl et al. 1998; Riethmtiller et al. 1998). Here, a strong dependence of erosion shear stress on chlorophyll-a concentration was demonstrated for clayey mud (>85% mud contents; Fig. 4c). For sediments with lower mud contents (25-85%; Fig. 4b), this relationship became weaker, and it vanished for sand and sandy mud (mud contents below 25%; Fig. 4a).
Erosion shear stress in oxic and anoxic sediments
P-...,i
3.0
m
2.0
""
1,0
""
81
E Z t,O U)
a) sand and sandy mud
03
J:: 03
,4t.
C O 03 O l=.,
o.o
§ 4-
Illt~ ..... F
+:~+
i
......
0
I
I
100
200
Chl a per m = SSL [mg]
3.0
-----
2.0
""
1.0
""
+.-
+
E Z r r (9 t,.
b) m u d d y sand
r
J:::: 03 r
+$ +
++
O r O 0.0
.................
I
, . . . . .
0
'
100
i 200
Chl a per m 2 SSL [rag]
3,0
+
-'-
E
+
4-
Z
c) clayey mud
03 I,,,,,
2,0
"-"
+
i,..,.
1'
03
9
1,0--+
O ,,..,,. O I...
0.0
--
|
i
.
.
.
.
.
.
100
i 200
Chl a per m 2SSL [mg]
Figure 4. Erosion shear stresses versus chlorophyll-a concentrations of the SSL for the Sylt-Rorno and Baltrum tidal flats, a) <25% mud contents; b) 25-85% mud contents; c) >85% mud contents. Crosses = Sylt-R~n~ (extracted from Riethmfiller et al. 1998). Open circles = oxic, and closed circles = anoxic surfaces for Baltrum.
82
Austen and Witte
The Baltrum data compare well with these observations (Fig. 4). For clayey mud (Fig. 4c), this suggests that the lower erosion shear stress of anoxic surface sediments was indeed due to a reduction in diatom biomass in this case, too. Nevertheless, the differences between the anoxic and oxic surface sediments documented for Baltrum (albeit significant) lie within the range of inherent variability for the region as a whole. Despite a considerable scatter in the data for the muddy sand flats (25-85% mud contents) of K6nigshafen and Baltrum (Fig. 4b), the general pattern certainly does not contradict this hypothesis. On the sandy mud flats (<25% mud contents; Fig. 4a), erosion shear stress is largely independent of chlorophyll-a concentration in both the Baltrum and Sylt-R~mo data sets. Other authors have also observed a reduction in the chlorophyll-a contents of anoxic surfaces, and have discussed a decrease in microphytobenthos as a possible reason for higher erodibility (Kolbe 1991; Neira & Rackmann 1996). Combining these results with our own measurements of erosion shear stress, a decrease in sediment stability indeed appears to be indicated. This is in accordance with R6hring & Ragutzki (1995) who documented that, on anoxic surfaces of both mixed flats and sand flats in the back-barrier area of the neighbouring island of Norderney (Fig. 1), erosion thresholds were lower than values recorded on oxic surfaces. These authors related this reduced sediment stability to a decrease in biogenic stabilisation, as indicated by the lower organic matter content of the anoxic surface sediments. However, they also observed lower mud contents and a better sorting in these sediments, and argued that this was the main reason for the lowering of erosion thresholds in the anoxic sediments in this case.
5. CONCLUSIONS Reduced sediment stability at anoxic sites suggests a higher mobility of such sediments (Kolbe 1991; R6hring & Ragutzki 1995). A comparison of the critical erosion shear stress data for the Baltrum anoxic areas with those of Riethmi~ller et al. (1998) for K6nigshafen shows that, although consistent, the lower erosion thresholds are nevertheless within the range of variability for oxidised sediments in the region. It is concluded, therefore, that morphodynamic effects of reduced stability in anoxic sediments are probable but not substantial.
ACKNOWLEDGEMENTS
We thank H. K~ihl and B. Peters for their help in the field and with the erosion experiments, K. Heyman and M. Heineke for the analyses of Chl a and POC, and our entire working group for stimulating discussions. Our appreciation also goes to R. Riethmi~ller for valuable criticism of the manuscript.
Erosion shear stress in oxic and anoxic sediments
83
REFERENCES
Admiraal, W. (1984) The ecology of estuarine sediment inhabiting diatoms. In: Round, F.E. & Chapman, G. (eds) Progress in Physiological Research 3. Biopress Ltd., Bristol, pp. 269-322. Bayerl, K., Austen, I., K6ster, R., Pejrup, M. & Witte, G. (1998) Sediment dynamics in the List Tidal Basin. In: G~itje, Ch. & Reise, K. (eds) Okosystem WattenmeerAustausch-, Transport- und Stoffumwandlungsprozesse. Springer, Berlin, pp. 127-159. Delafontaine, M.T. & Flemming, B.W. (1997) Large scale sedimentary anoxia and faunal mortality in the German Wadden Sea (southern North Sea) in June 1996: a man-made catastrophe or a natural black tide? German J. Hydrogr. Suppl. 7: 21-27. Ernst, W. (1975) Eine neue Verbrennungs-IR-Methode zur Mikrobestimmung von organischem Kohlenstoff in marinen Sedimenten. Ver6ff. Inst. Meeresforsch. Bremerhaven 15: 269-281. H6pner, T. & Michaelis, H. (1994) Sogenannte 'schwarze F l e c k e n ' - ein Eutrophierungssymptom des Wattenmeeres. In: Lozfin, J.L., Rachor, E., Reise, K., Westernhagen, v. H. & Lenz, W. (eds) Warnsignale aus dem Wattenmeer. Blackwell Wissenschaftsverlag, Berlin, pp. 153-159. Kolbe, K. (1991) Zum Auftreten 'schwarzer Flecken', oberfl~ichlich anstehender, reduzierter Sedimente, im ostfriesischen Wattenmeer. Ber. Nieders~ich. Landesamt Wasser Abfall, Arbeit. Forschungsstelle Ktiste, pp. 1-26. Neira, C. & Rackmann M. (1996) Black spots produced by buried macroalgae in intertidal sandy sediments of the Wadden Sea: effects on meiobenthos. J. Sea Res. 36(3/4): 153-170. Paterson, D.M. (1989) Short-term changes in the erodibility of intertidal cohesive sediments related to the migratory behavior of epipelic diatoms. Limnol. Oceanogr. 34 (1): 223-234. Paterson, D.M. (1994) Microbiological mediation of sediment structure and behaviour. NATO ASI Ser. G 35: 97-109. Paterson, D.M. & Darborn G.R. (1991) Sediment stabilisation by biological action: significance for coastal engineering. In: Peregrine D.H. & Loveless J.H. (eds) Developments in Coastal Engineering. University of Bristol Press, Bristol, pp. 111-119. Paterson, D.M., Yallop, M.L. & George, Ch. (1994) Spatial variability in sediment erodibility on the island of Texel. In: Krumbein, W.E., Paterson, D.M. & Stal, L.J. (eds) Biostabilization of Sediments. Bibliotheks- u. Informationssystem der Carl von Ossietzky Universit/it Oldenburg, Oldenburg, pp. 107-120. Riethmtiller, R., Hakvoort, H., Heineke, M., Heymann, K., Ktihl, H. & Witte, G. (1998) Relating erosion shear stress to tidal flat surface colour. In: Black, K.S., Paterson, D.M. & Cramp, A. (eds) Sedimentary Processes in the Intertidal Zone. Geol. Soc. London Spec. Publ. 139, pp. 283-293. R6hring T. & Ragutzki, G. (1995) Bestimmung der Erosionsfestigkeit und zugeordneter bodenphysikalischer Parameter im Bereich anoxischer und oxischer
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Sedimentoberfl~ichen. Forschungsber. 10802085/21 Okosystemforschung Wattenmeer, Teilvorh. Nieders~ich. Wattenmeer, 81 p. Sch~inemann, M. & K~ihl, H. (1991) A device for erosion measurements on naturally formed, muddy sediments: the EROMES System. Rep. GKSS Research Centre GKSS 91/E/18, 28 p. Witte, G. & K~ihl, H. (1996) Facilities for sedimentation and erosion measurements. Arch. Hydrobiol. Spec. Iss. Adv. Limnol. 47: 121-125. Wright, S.W., Jeffery, S.W., Mantoura, R.F.C., Llewellyn, C.A., Bjornland, T., Repeta, D. & Welschmeyer, N. (1991) Improved HPLC method for analyses of chlorophylls and carotenoids from marine phytoplankton. Mar. Ecol. Prog. Ser. 77: 183-196.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
85
Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea) A. Bartholom/i,* B. W. Flemming and M. T. Delafontaine
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
In a mixed Mytilus edulis/Lanice conchilega community on the Swinnplate sand flat behind Spiekeroog Island (southern North Sea), net seasonal fluxes of sand and mud were determined by combining measurements of topographic elevations, sediment dry bulk density, and m u d contents for 7 to 8-month time intervals from March 1995 to October 1996. These data were collected in a 0.7 km2-grid consisting of 90 sampling/measuring points at 100-m intervals. A concurrent census of M. edulis showed that mussel areal coverage varied between ca. 10 and 19% at the grid site at the time, the lowest coverage having been recorded in the aftermath of a severe winter in 1995-1996 when an ice sheet strongly decimated the mussel banks and interspersed lawns of the cold-intolerant, tube-dwelling polychaete L. conchilega. When present, mussel beds occurred always in a SW to NE-oriented 'corridor' on the Swinnplate. Despite a general deepening (erosion) reaching 0.5 m in places over the severe winter (September 1995-April 1996; cf. negative volume budget of-170,103 m 3 for the whole area), spatial patterns in m u d enrichment showed a similar SW-NE constancy. These data suggest the preservation of mussel 'signatures' by burial. Under more benign weather conditions after the ice winter, erosion continued in large areas of the grid (cf. volume budget of-78,103 m 3 in the period April-October 1996). This can at least in part be explained by a strong impairment of the stabilising effect of the L. conchilega tube lawns at the time. Sediment mass budgets were negative for the severe winter as well as for the summer thereafter (-196,103 , and -97,103 tonnes dry sediment, respectively). Over the preceding summer (March-September 1995), in contrast, positive volume and sediment mass budgets (86,103 m 3 and 99,103 tonnes dry sediment, respectively) were documented, accompanied by net increases of up to 0.3 m in elevation. At all times, net fluxes in m u d mass did not exceed ca. 12% of the total sediment budgets. In addition, m u d contents generally did not exceed ca. 20%. Even under the calmer hydrodynamic conditions reigning in summer, the reworking of sandy sediments evidently results in much higher fluctuations in elevation, and involves much larger material fluxes than is commonly assumed, both in tidal-flat sedimentology and ecology. * Corresponding author: A. Bartholom/i e-mail:
[email protected]
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1. INTRODUCTION The study of sediment dynamic processes on tidal flats has in recent years received increasing attention worldwide (e.g., Flemming & Bartholom/i 1995; Perillo 1995; Black et al. 1998). This interest reflects the recognition of complex interactions between physical and biological processes in the intertidal zone as well as the growing need for cost-efficient coastal management in the wake of large-scale industrial development (e.g., Vollmer & Grann 1999). Early classic studies focussing on the Wadden Sea region had intimated such interactions but lacked the tools to quantify the effects (e.g., H/intzschel 1939; Postma 1961; van Straaten 1961). In the East Frisian Wadden Sea (southern North Sea), sediment distribution patterns reflect a pronounced cross-shore energy gradient resulting in a progressive landward fining of the back-barrier sediments (e.g., Flemming & Ziegler 1995). Thus, it has been argued that depositional processes largely reflect the integrated effects of hydrodynamic factors and water temperature (e.g., Flemming & Delafontaine 1994; Flemming & Bartholom/i 1997; Kr6gel & Flemming 1998), one exception being the biologically-induced accumulation of fine particles in intertidal banks of the mussel Mytilus edulis. Because of the man-induced paucity of mud flats resulting from many centuries of land reclamation (Flemming & Nyandwi 1994), it has been postulated that the cycling of these biogenic muds has since become an increasingly important component of benthic-pelagic coupling in the region (Delafontaine et al. 1996). On the back-barrier tidal flats of Spiekeroog Island, Flemming and Delafontaine (1994) documented that even minor storms (winds <4 Bft) can result in the displacement of mussel aggregates as well as net sediment erosion of ca. 0.6 mm per day, coinciding to a net loss of ca. 0.5 kg sediment per m 2 per day which more than doubled after a 3-week spell of strong winds in mid-winter 1993. At the time, the mass balancing of sediment fluxes did not differentiate between the mud and sand components. Thus, the generally low mud contents (<20 dry weight-%) of sediments in the mussel banks (e.g., Delafontaine et al. 1996) can be explained by the remobilisation of the biodeposits by tidal currents (velocities commonly reach 35 cm per second over the banks; Flemming & Delafontaine 1994) as well as by mixing with fine sands (125-180 lJm; Flemming & Ziegler 1995) during stormy spells. Apart from the effect of the mussels themselves, the degree to which other biological factors are also involved in sediment erosion and accretion in the banks is still largely unknown. Biostabilisation of sediments has been investigated intensively since the 1980s (cf. Meadows & Meadows 1991; Snelgrove & Butman 1994), and a number of recent studies has demonstrated that a wide suite of organisms, ranging from diatoms over polychaetes to mussels (e.g., Macoma balthica, Lanice conchilega), can stem or promote erosion in nearby sediments (e.g., Paterson 1997; Widdows et al. 1998; Friedrichs et al. 2000). However, there is still a paucity of field evidence which convincingly demonstrates the significance of these processes at large temporal and spatial scales. Paradoxically, this applies to many highly dynamic environments, including the Wadden Sea which experiences large-scale disturbances such as frequent and severe storm action as well as episodic ice coverage. For example, in the detailed study of
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sediment fluxes in a juvenile mussel bed conducted by Flemming and Delafontaine (1994), values recorded at 20-cm intervals were integrated in a composite grid measuring only 12 m 2, a n area manageable over a single low-tide period but not large enough to concurrently assess any effects of the polychaete L. conchilega which occurred in dense but patchily distributed lawns of various sizes in the vicinity. In the present study we have investigated seasonal export and import fluxes of mud and sand fractions at larger spatial (700,103 m 2) and temporal (seasonal) scales on an intertidal sand flat, choosing a site which incorporated a series of M. edulis beds interspersed with communities dominated by the polychaetes Arenicola marina or L. conchilega. Roughly in the middle of the study period, there was a prolonged spell of ice coverage which, depending on the locality along the coast, lasted ca. 50-85 days from December 1995 to February 1996 (e.g., Anonymous 1996). This chance constellation of conditions allowed a better assessment of interactions between, and the relative contributions of physical and biological processes in sediment turnover.
2. MATERIALS AND METHODS 2.1. Study area, study grid, mussel beds The fieldwork was carried out on the Swinnplate tidal flat in the rear of the barrier island of Spiekeroog in the East Frisian Wadden Sea, southern North Sea (Fig. 1). The site represents a typical sand flat (mud contents usually <2 dry weight-%) dominated by the polychaete A. marina. Included was a mixed community of the mussel M. edulis and the tube-dwelling polychaete L. conchilega with associated biogenic mud accumulations (Kurmis 1995; Delafontaine et al. 1996; Bergfeld 1999).
Figure 1. Locality map (dotted lines in inset indicate watersheds).
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A grid which remained fixed for the duration of the main study period (March 1995-October 1996) consisted of 90 sampling/measuring points at 100-m intervals spanning several mussel beds over a rectangular area of 1400,500 m (0.7 km2; Fig. 2). Survey points were fixed by laser theodolite with an accuracy of <1 m, elevations being determined to ca. I cm relative to topographic chart datum (see below). The perimeters of the mussel banks were mapped on foot by means of a portable satellite navigator (differential GPS) with an accuracy of <3 m. Mapping campaigns were carried out at irregular intervals (3-13 months) over ca. 4 years (in October 1993, January, March, July and September 1994, March and July 1995, and August 1996). s3o4s,2o.w,
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Figure 2. Location of survey and sampling grid (bold dots) on the Swinnplate tidal flat in the rear of Spiekeroog Island (cf. Fig. 1). Depth contours are metres below topographic chart datum.
2.2. Mass balancing Mass balances of export/import fluxes for mud and sand were determined for three consecutive seasons in the time intervals March-September 1995 (7 months, summer), September 1995-April 1996 (8 months, winter), and April-October 1996 (7 months, summer). These 4 main field campaigns were preceded by a pilot survey conducted in July 1994. 2.2.1. Terminology In the following we define some key terms as used in the present study. Seasonal: a time interval spanning winter or summer, and lasting 7-8 months. Sand: sediment size fractions 0.063-2 mm. Mud: sediment size fractions <0.063 mm. Sediment: unfractionated sand+mud. Dry bulk density: dry mass of sediment (desalinated) in a unit volume of water-saturated sediment. Mud content (dry weight-%): (dry mass m u d / d r y mass sediment)*100. Export: flux from the study area (loss), reflecting net erosion in a given time interval. Import: flux to the study area (gain), reflecting net
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deposition in a given time interval. Total flux: export+import. Volume and mass 3 budgets (net fluxes): import volume (or mass)-export volume (or mass). Units" m wet sediment or tonnes dry sediment (sand, mud) per unit area per season. Tonne: metric ton (1000 kg). 2.2.2. Procedure
Export/import fluxes of sand and mud were determined by calculating the differences between the topographic elevations recorded at the beginning and end of a given 7 to 8-month study interval (difference in elevation: value at end minus value at beginning of interval), and then generating corresponding changes in wet sediment volume (Fig. 3). These data were used in combination with concurrent measurements of sediment composition (mud content), and a site-specific relationship between dry bulk density and mud content (see below) to convert volumetric changes into gains or losses of sand and mud masses. Positive and negative differences in elevations (volumes) were coupled with mud contents recorded at the end and at the beginning of each study interval, respectively. During the pilot survey of July 1994 the sediments were characterised by measuring mud content.
to determine BULK SEDIMENT FLUX erosion -- loss - export deposition -- gain - import
measure VOLUME CHANGE by repeated precision levelling by applying site-specific DRY BULK DENSITY, convert into BULK SEDIMENT MASS (gain or loss) calculate SUBSTRATE-SPECIFIC CONTRIBUTION from sand/mud contents SAND MASS (gain or loss)
MUD MASS (gain or loss)
Figure 3. Flow diagram of the procedure followed to measure export/import fluxes of mud and sand on the Swinnplate sand flat.
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2.2.3. Topography Topography was measured by means of precision levelling at all 90 grid points, using a laser theodolite (see above) and relating the elevation readings to the German topographic chart datum (NN = normal null). Measurements were carried out every 7-8 months in the period March 1995-October 1996 (see above). 2.2.4. Sediments Concurrently to the elevation measurements, surficial sediments were collected to depths of up to 4 cm at all 90 grid points, at least 5 subsamples being pooled in each case in order to reduce the probable sampling error by at least 50% (e.g., Krumbein & Pettijohn 1938). The samples were desalinated in the laboratory, and then wet sieved through a 0.063-mm mesh. The resulting sand and mud fractions were oven dried at 65~ for 24 hours, and weighed. The relationship between the dry bulk density and mud content of the sediments used in the present study was extracted from Flemming & Delafontaine (2000), i.e. ( X/125 8292772) y - -0.7955892+2.3863045e- " where y = dry bulk density (grams desalinated dry sediment per cm 3 of water-saturated sediment), x = mud dry weight-% content of the sediment, n = 337, r = -0.9847. 2.3. Data plotting Contour plots were computer generated by means of the software package SURFER (Windows Golden Software Inc. RT) using the distance interpolation procedure (e.g., Cressie 1991).
3. RESULTS
Repeated mapping of mussel occurrence shows that the distribution of M. edulis varied strongly between 1990 and 1996 in the 0.7-km 2 grid area (Fig. 4). Areal coverage was highest in the early 1990s when ca. 45% of the area was covered with mussel beds (Fig. 4a, b). Coverage had decreased to ca. 7% by summer 1994 (Fig. 4c-e), followed shortly thereafter by a marked increase (26% coverage) resulting from a large spatfall in the western sector of the grid in autumn (September) that year (Fig. 4f). The older M. edulis banks were strongly damaged by ice in the 1995-1996 winter and, by summer 1996, the mussels occupied only about 10% of the study site, mostly in the southwestern sector alongside the channel bordering the Swinnplate tidal flat to the south (Fig. 4i).
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Figure 4. Location of Mytilus edulis beds on the Swinnplate tidal flat in the period 1990-1996 (1990 data extracted from Hertweck 1995). Percentage values denote the proportion of the grid (cf. rectangle) which was occupied by mussels at any particular time. Depth contours are metres below topographic chart datum.
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In spring (March) 1995, much of the western half of the grid area lay more than 0.8 m below topographic chart datum (CD) whereas generally higher elevations were recorded to the east (0.5-0.8 m below CD; Fig. 5a). Elevations were by and large high (0.4-0.8 m below CD) the following autumn (September 1995), one exception being a centrally situated depression acting as a drainage zone (Fig. 5b). Thus, most of the grid area experienced a net increase of up to 0.3 m in elevation during the 1995 summer (Fig. 6a), resulting in a positive volume budget of ca. 86,103 m 3 in this case (Table 1).
Figure 5. Topographic contour maps of the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996. Elevations are metres below topographic chart datum.
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Seven months later after a severe winter (April 1996), a negative volume budget nearly twice as high was recorded for the whole site (-170.103 mS; Table 1), reflecting a general deepening which reached 0.5 m in places, except in the vicinity of the central drainage area which gained up to 0.3 m over the same period (Fig. 6b). In the western section of the grid, these losses in elevations had been largely compensated by the end of summer 1996 (cf. net gains of up to 0.2 m in October; Fig. 6c). More to the east, by contrast, deepening continued to reach 0.7 m at some sites (Fig. 6c). Thus, roughly 6 months after ice coverage, a negative volume budget was again documented for the grid area as a whole (-78,103 m3; Table 1), though this time over summer.
Figure 6. Contour maps showing the changes in topography in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate accretion, and negative values indicate erosion in a given time interval (net increases and decreases in elevation, respectively, in metres per time interval). In the summer (July) of 1994, mud contents of 10-20 dry weight-% were recorded over large areas of the grid (Fig. 7a). Higher values of 20-40% were found at some
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localities in the southwest and northeast, the latter coinciding with the spatial distribution of M . edulis which was restricted to 2 adjoining beds in the northeast grid sector at the time (Fig. 4e). By the end of the 1994-1995 winter, mud contents had decreased to 5-15% over much of the grid (Fig. 7b). Nevertheless, in March 1995 the spatial distribution of mud contents showed a pattern similar to that recorded the previous summer. In this case, however, localised areas of higher mud enrichment (15-20% contents) showed a stronger overlap with the mussel aggregates, in particular as a new mussel bed had established itself in the western grid sector (Fig. 4f, g).
Figure 7. Contour maps of the mud dry weight-% contents of the surficial (0-4 cm) sediments in the survey area in a) July 1994, b) March 1995, c) September 1995, d) April 1996, and e) October 1996.
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The net gains in elevation and sediment volume, recorded over the 1995 summer at most localities in the study area, were accompanied by a general increase in the mud contents of the sediment. Thus, values rose from <10% in March 1995 to 10-20% by September 1995 (Fig. 7b, c), resulting in net gains of 10-20% mud at many sites, the highest gains being recorded mostly in the southwestern and also northeastern sectors (Fig. 8a). At the time M. edulis was found in a well-delimited bed in the southwest of the study area as well as in a series of smaller beds to the northeast (Fig. 4h).
Figure 8. Contour maps showing the changes in mud dry weight-% contents of the surficial (0-4 cm) sediments in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate increases, and negative values indicate decreases in mud dry wt.-% contents in a given time interval. Corresponding to the seasonal variations in mud content the distribution of sand concentrations (volume-specific sand masses) follow an inverse trend (Fig. 9). High values after the winter months (Fig. 9a, 9c) reflect strong sediment reworking with concurrent mud export. At the end of summer the surficial sediments are enriched in mud due to bioturbation as a result of which sand concentrations decrease (Fig. 9b, 9d).
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Figure 9. Contour maps showing the sand mass in the surficial sediment maps of the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996. Masses are in kilogram per 0.5 dm 3 Combining these data sets reveals a net import of 10-40 kg mud m -2 for much of the grid, a net export of <10 kg mud m ~ being largely limited to the central drainage area as well as two sites eastward thereof (Fig. 10a). Sand import/export showed analogous trends over the 1995 summer but, compared to the values for mud, sand fluxes involved much higher masses, i.e. import values were ca. 100-300 kg m 2, and export values reached at least 100 kg m -2 in this case (Fig. 11a). As a result, the net gain in mud mass constituted only ca. 12% of the positive sediment mass budget (+99,103 t) in the summer of 1995 (Table 1).
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The large-scale decreases in elevation (and sediment volume) documented after the severe 1995-1996 winter were associated with generally lower mud contents (cf. values had decreased by up to 10%; Figs 7c, 7d, 8b). Nevertheless, the spatial distribution patterns in mud content were similar before and after ice coverage (cf. September 1995 and April 1996, respectively; Fig. 7c, d), in that higher values (10-15%) were largely restricted to southwestern and northeastern sites. A negative sediment budget (-196,103 t) was recorded for the grid area over this period, the net loss of mud again making up only about 12% of the total budget (Table 1). Thus, at most sites the export mass of sand (<400 kg m -2) exceeded that of mud (<40 kg m 2) by an order of magnitude (Figs 10b, 11b).
Figure 10. Contour maps showing the spatial variations in the gains and losses in dry mud mass in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate gains (import), and negative values indicate losses (export) of mud per unit area in a given time interval (kg m -2 season-I).
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Similarly, mud contents increased at most grid sites over summer in 1996. Values rose by up to 10% to reach 10-25% at most sites in October 1996, higher mud contents of 20-30% once again being largely localised in a strip stretching from the southwestern sector of the grid in a northeasterly direction (Figs 7d, 7e, 8c). In contrast to the summer of 1995, however, the continuing deepening of the eastern sector of the study area as well as the ensuing negative budgets for sediment volume and mass meant that there was no large-scale gain in mud mass that summer (Table 1). Thus, export balanced import of mud in the study grid, and net sediment loss involved only sand (-97.103 t; Figs 10c, 11c; Table 1), corresponding to at l~ast half of the amount exported the preceding ice winter.
Figure 11. Contour maps showing the spatial variations in the gains and losses in dry sand mass in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate gains (import), and negative values indicate losses (export) of sand per unit area in a given time interval (kg m -~seasonI).
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Table 1. Mass/volume budgets for the seasonal fluxes of mud and sand in the survey area on the Swinnplate tidal flat in the time intervals March-September 1995, September 1995-April 1996, and April-October 1996 (positive values are gains, and negative values are losses for the survey area in a given time interval, in tonnes dry mass*103 o r m 3 wet sediment,103 per 0.7 k m 2 per season; sediment - mud+sand). March 95September 95September 95 April 96 Export Import Export Import Sediment volume (m~,lO ~) 23 109 209 39 Sand mass (t*lO3) 28 119 227 47 Mud mass (t*lO3) 1 9 18 2 Sediment mass (t* 103) 29 128 245 49 Total fluxes (export+import) Sediment volume (m 3,10 ~) 132 248 Sand mass (t,103) 147 274 Mud mass (t,103) 10 20 Sediment mass (t, 103) 157 294 Net fluxes (impori :-export) Sediment volume (m ~, 10~) +86 170 Sand mass (t*103) +91 180 Mud mass (t,103) +8 16 Sediment mass (t* 103) +99 196
April 96 October 96 Export Import 134 56 160 63 7 7 167 70
........
190 223 14 237 -78 -97 0 -97
4. D I S C U S S I O N A N D IMPLICATIONS The long-term occurrence (>10 years) of M . edulis is ubiquitous in the Wadden Sea but the spatial distribution and size of the mussel banks can show marked seasonal and interannual variations (e.g., Dankers & Koelemaij 1989; Nehls & Thiel 1993). Likewise, these mussels have occurred intermittently on the Swinnplate tidal flat since the 1960s at least (Flemming & Ziegler 1995; Hertweck 1995; Millat & Herlyn 1999), and the results of the present study demonstrate that, between 1990 and 1996, the location and areal coverage of individual beds in the 0.7-kin 2 study grid varied considerably. Nevertheless, the large-scale (100s m) distribution pattern remained stable during this time, i.e. when present, the mussels occurred only within a broad belt (albeit patchily) curving from the southwest grid sector in a northeasterly direction, the southeastern and northwestern sectors remaining mussel-free during this 7-year 'mussel-watch' campaign. Despite seasonal fluctuations in mud enrichment levels (in summer mud contents were generally higher than in winter), spatial patterns in mud enrichment closely reflected this long-term SW-NE trend in mussel distribution. This means that 1) biogenic muds can persist at the surface of intertidal sediments for several years
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after the mussels die, and 2) these muds are worked deeper into the sediment with time. Such fingerprints left behind by older mussel banks present in the early 1990s (and possibly even before) would explain the SW-NE mud distribution pattern documented in the summer of 1994 when the western grid sector was largely free of mussels. Likewise, the persistence of the same 'signature' even after considerable erosion as well as damage and/or displacement of the mussels during several weeks of ice coverage in January-February 1996 (ca. 50 days in the Spiekeroog tidal flats; Gtinther & Niesel 1999) would be largely explained by the surfacing of older biogenic mud horizons induced by ice scour. Independent evidence of facies structure with depth, gained by taking box cores at spot localities on the Swinnplate tidal flats (Hertweck & Liebezeit 1996), corroborate the postulated existence of such large-scale fingerprints. Additional evidence includes fluctuations in the energy content of the mud organic matter which showed generally lower values after the severe winter, consistent with the presence of older organic matter originating at least partly from the pseudofaeces and faeces left over by former mussel banks (Delafontaine, unpubl. data). Field and flume experiments have shown that live specimens of M . edulis can anchor the underlying bed sediment (Meadows et al. 1998). However, this stabilising effect does not automatically mean that sediment turnover should be low within an actively growing bank. Thus, while the banks can act as barriers against erosion, they cannot avoid being covered by sediment, as was documented over the summer of 1995 when large-scale net increases in topography and a net influx of sediment were recorded over the Swinnplate study grid. This imported material was composed largely of sand in this case, presumably transported as bedload a n d / o r intermittent bottom suspension from adjoining tidal flats as a result of wave action during common autumn and winter storms (e.g., Fach 1996). In addition to producing pseudofaeces and faeces, the banks will also trap fine particles in suspension because their 3-D structure increases surface roughness. Consequently, it would be incorrect to consider that sediment turnover is 'high' on the Swinnplate sandflat but 'low-medium' in intact mussel beds at the site, as surmised by Villbrandt et al. (1999). By implication, it is necessary to consider the effects of sediment mixing and accretion in the ecology of M . edulis communities in the region, an aspect which has been ignored by these and other authors in their interpretation of interactions between biogeochemical sedimentary processes and macrobenthic characteristics on the Swinnplate (e.g., Kr6ncke 1996). In this context, it is widely accepted that accretion rate can strongly influence benthic biology and chemistry by influencing fluxes of, amongst other things, nutrients in the sediment (Boudreau 1994; Ruddy et al. 1998). For example, the admixture of sand increases drainage (and thereby flux rates) in muddy sediments (Delo 1988). The admixture of substantial amounts of sand to the sediments at the Swinnplate study grid can at least partly explain the low mud contents documented in the vicinity of the mussel banks. Thus, values rarely exceeded 20%, even in the middle of summer (e.g., in July 1994) when active mussel filtration, calmer weather and higher
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water temperatures would promote the accumulation of finer particles (e.g., Kr6gel & Flemming 1998). In addition, Ten Brinke et al. (1995) have reported that M. edulis biogenic muds are easily resuspended under the hydrodynamic conditions prevailing in the region, particularly in winter when we measured substantial export fluxes of mud from the study grid, and even lower mud contents. Some of this fine material will be mixed with the coarser sediment of the adjacent sand flat, resulting in characteristic M. edulis 'mud halos' (e.g., Flemming & Davis 1994; Oost 1995) in which, once again, levels of mud enrichment are generally low (e.g., Flemming & Ziegler 1995; Delafontaine et al. 1996). It should be noted that Hild (1999) has misassessed these data by reporting that sediments at the Swinnplate study site commonly had mud contents exceeding 50%. Using our regional relationship between bulk density and mud content (see Materials and Methods) we calculated that, in a given volume of an 80:20 sand-mud mixture in the mussel banks (i.e. sediments with the commonly observed 20% mud content; see above), the dry mass of sand would outweigh that of mud by a factor of ca. 4 in the present case. Note that these relationship can, and usually will vary between regions, i.e. the inter-relationships between mud content, sand concentration and mud concentration are commonly site-specific (Flemming & Delafontaine 2000). In the East Frisian Wadden Sea, therefore, 'contamination' by the sand fraction should be taken into consideration when processing whole sediment samples with the aim of characterising the biogenic mud component as such. This aspect has been overlooked in studies dealing with, amongst other things, organic compounds and biomarkers such as amino acids, fatty acids and sterols at the study site (e.g., Rohjans et al. 1999; Villbrandt et al. 1999). The results indicate that accurate and detailed measurements of sediment bulk density (or a proxy thereof such as water content; e.g., Flemming & Delafontaine 2000) are necessary to calculate sediment import/export fluxes on tidal flats. Coupled with measurements of sediment mud content, it becomes possible to quantify the relative importance of sand and mud fluxes. During the 1995-1996 study period at the Swinnplate study site, total seasonal fluxes (i.e. export+import) of sand always exceeded those of mud by an order of magnitude (note that Hild (1999) overlooked these significant sand fluxes in reporting these data). In comparison to the fine fraction, sand transport could therefore involve much higher amounts of sedimentary constituents than is commonly assumed for reasons of low enrichment levels. During the summer of 1995, for example, the mean organic carbon content of the mud fraction exceeded that of sand by a factor of ca. 18 at the Swinnplate grid site, but the total mass flux of mud organic carbon was only 1.3 times higher than that calculated for the sand because the flux of sand was ca. 15 times as high (this study; Delafontaine et al. 2000a, this volume). Total fluxes (i.e. export+import) of sand and mud were particularly high over the severe winter of 1995-1996 in the Swinnplate study grid when substantial erosion presumably involved ice rafting of the surface sediments (e.g., Dionne 1988, 1998; own obser.). However, fluxes were nearly as high (factor of 0.8) over the following summer, due largely to continued erosion in the eastern sector of the grid.
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Combining the 3 seasonal budgets spanning the 1.5 years of study, we calculate a net loss of about 200,103 tonnes of sediment (sand:mud mass ratio - ca. 23) for the study area during the period March 1995-October 1996. Thus, the claim of Hild (1999, p. 32), viz. "the morphology was re-established again approx, within 6 months after the ice winter", is evidently unfounded. Unfortunately, this mis-representation has been incorporated into a multi-faceted assessment of ecosystem stability for the region (Dittmann 1999). The cold-intolerant, tube-dwelling polychaete Lanice conchilega suffered large-scale mortality in the Wadden Sea during the severe 1995-1996 winter (e.g., Gtinther & Niesel 1999). In addition, it is well known that these tube-lawns can stabilise sediments in their immediate vicinity (e.g., F6ral 1989; Ziihlke et al. 1998), the effect becoming evident when abundance exceeds ca. 4000 tubes m -2 (Friedrichs et al. 2000). Such densities were observed locally in the eastern sector of the study site, values being generally lower more to the west (Bergfeld 1996). Therefore, we postulate that the ice-induced destruction of L. conchilega tube-lawns in late winter 1996 can at least partly explain why large-scale erosion persisted well into summer that year at some localities on the Swinnplate tidal flat. In view of the probable compounding effects of ensuing autumn-winter storms, and seeing that this worm species has a low recolonisation potential, it is likely that the negative sediment balance documented over the period March 1995-October 1996 will not have been re-adjusted before the summer of 1997 at the earliest. This would mean a one-year 'recovery' phase for the site as a whole, twice as long as that surmised by Hild (1999). We conclude that, even under calmer hydrodynamic conditions reigning in summer, the reworking of sandy sediments results in much higher fluctuations in elevation, and involves much higher material fluxes than is commonly assumed in studies dealing with intertidal sedimentology and ecology. Marked seasonal and episodic variations in elevations and sediment budgets resulting from, for example, prolonged ice coverage and storms, imply that accretion expected in the wake of mean sea-level rise (in this case ca. 18 cm per century) can be confidently identified only on the basis of long-term (decadal) data sets (e.g., Christiansen & Kristensen 1986; Flemming 1990; Ke & Collins 2000; Leatherman et al. 2000).
ACKNOWLEDGEMENTS
Our hearty thanks go to the captain, motorboat driver and crew of the research vessel Senckenberg for their expertise and unfailing good spirits during the fieldwork, oft carried out under difficult weather conditions. We acknowledge the laboratory assistance of A. Rascke and numerous students, and H. Lammers lent a welcomed helping hand in evaluating mussel coverage. The work was sponsored partly by the Senckenbergische Naturforschende Gesellschaft, Frankfurt, and partly by the Federal Ministry of Education and Research, Bonn (Grant No. 03F0112A).
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REFERENCES
Anonymous (1996) L/ingster Eiswinter seit 33 Jahren. Deutsche Seeschiffahrt 5/1996: 16. Bergfeld, C. (1996) Die Makrofaunabesiedlung der Swinnplate im Rfickenseitenwatt der Insel Spiekeroog, Ostfriesisches Wattenmeer. Diploma thesis, Universit/it G6ttingen, 144 p. Black, K.S., Paterson, D.M. & Cramp, A. (eds) (1998) Sedimentary Processes in the Intertidal Zone. Geol. Soc. London Spec. Publs 139, 409 p. Boudreau, B.P. (1994) Is burial velocity a master parameter for bioturbation? Geochim. Cosmochim. Acta 58: 1243-1249. Christiansen, C. & Kristensen, S.D. (1986) Two time scales of micro-tidal flat erosion and accumulation. II: Topographic changes. Aarhus University, GeoSkrifter No. 24: 103-112. Cressie, N.A.C. (1991) Statistics for Spatial Data. John Wiley & Sons, New York, 900 p. Dankers, N. & Koelemaij, K. (1989) Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring of other ecological parameters. Helgol. Meeresunters. 43: 529-535. Delafontaine, M.T., Bartholom/i, A., Flemming, B.W. & Kurmis, R. (1996) Volumespecific dry POC mass in surficial intertidal sediments: a comparison between biogenic muds and adjacent sand flats. Senckenbergiana marit. 26 (3/6): 167-178. Delafontaine, M.T., Flemming, B.W. & Bartholom/i, A. (2000) Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Delo, E.A. (1988) Estuarine Muds Manual. Hydraulics Research, Wallingford (UK), Repts SR164. Dionne, J.C. (1988) Characteristic features of modern tidal flats in cold regions. In: De Boer, P.L., van Gelder, A. & Nio, S.D. (eds), Tide-influenced Sedimentary Environments and Facies. Reidel, Dordrecht, pp. 301-332. Dionne, J.C. (1998) Sedimentary structures made by shore ice in muddy tidal-flat deposits, St. Lawrence estuary, Quebec. Sediment. Geol. 116: 261-274. Dittmann, S. (ed.) (1999) The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, 307 p. Fach, B. (1996) Die Entwicklung der Windverh/iltnisse in der Deutschen Bucht seit 1965 als wichtiger Umwelteinflut~ auf die Ktistenzone. Diplome thesis, Fachhochschule Wilhelmshaven (Germany), 75 p. F6ral, P. (1989) Influence des populations de Lanice conchilega (Pallas) (ann61ide polych6te) sur la s6dimentation sableuse intertidale de deux plages bas-normandes (France). Bull. Soc. G6ol. France 8, 5(6): 1193-1200. Flemming, B.W. (1990) Zur holoz/inen Entwicklung, Morphodynamik und faziellen Gliederung der mesotidalen Diineninsel Spiekeroog (si,idliche Nordsee). Ber. Fachber. Geowissen. Univ. Bremen 10: 13-73.
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Flemming, B.W. & Bartholom~i, A. (eds) (1995) Tidal Signatures in Modern and Ancient Sediments. Blackwell, Oxford, IAS Spec. Publ. 24, 358 p. Flemming, B.W. & Bartholom~i, A. (1997) Response of the Wadden Sea to a rising sea level: a predictive empirical model. Ger. J. Hydrogr. 49: 343-353. Flemming, B.W. & Davis, R.A. Jr. (1994) Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). Senckenbergiana marit. 24: 117-155. Flemming, B.W. & Delafontaine, M.T. (1994) Biodeposition in a juvenile mussel bed of the East Frisian Wadden Sea (southern North Sea). Neth. J. Aquat. Ecol. 28: 289-297. Flemming, B.W. & Delafontaine, M.T. (2000) Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications. Cont. Shelf Res. (in press). Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth. J. Aquat. Ecol. 28: 299-307. Flemming, B.W. & Ziegler, K. (1995) High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog Island (southern North Sea). Senckenbergiana marit. 26: 1-24. Friedrichs, M., Graf, G. & Springer, B. (2000) Skimming flow induced over a simulated polychaete tube lawn at low population densities. Mar. Ecol. Prog. Ser. 192: 219-228. Gtinther, C.-P. & Niesel, V. (1999) Effects of the ice winter 1995/96. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 193-205. H~intzschel, W. (1939) Tidal flat deposits (wattenschlick). In: Trask, P.D. (ed.), Recent Marine Sediments. Dover Publs, Dover, pp. 195-206. Hertweck, G. (1995) Verteilung charakteristischer Sedimentk6rper und Benthossiedlungen im Rtickseitenwatt der Insel Spiekeroog, s~dliche Nordsee. I. Ergebnisse der Wattkartierung 1988-92. Senckenbergiana marit. 26 (4/6): 81-94. Hertweck, G. & Liebezeit, G. (1996) Biogenic and geochemical properties of intertidal biosedimentary deposits related to Mytilus beds. Proc. 29th EMBS, Vienna, August 1994. Mar. Ecol. (PSZNI) 17: 131-144. Hild, A. (1999) Morphology and sedimentology of the Spiekeroog backbarrier system. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 31-35. Ke, X. & Collins, M. (2000) Tidal characteristics of an accretional tidal flat (The Wash, U.K.). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Kr6gel, F. & Flemming, B.W. (1998) Evidence for temperature-adjusted sediment distributions in the back-barrier tidal flats of the East Frisian Wadden Sea (southern North Sea). SEPM Spec. Publ. 61: 31-41.
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Kr6ncke, I. (1996) Impact of biodeposition on macrofaunal communities in intertidal sandflats. In: Dworschak, P.C., Stachowitsch, M. & Ott, J.A. (eds), Influences of organisms on their environment. The role of episodic events. Proc. 29th EMBS, Vienna, August 1994. Mar. Ecol. (PSZNI) 17: 159-174. Krumbein, W.C. & Pettijohn, F.J. (1938) Manual of Sedimentary Petrography. D. Appleton-Century, New York, 549 p. Kurmis, R. (1995) Quart~irgeologische Detailkartierung der Swinnplate im Spiekerooger Rtickseitenwatt, stidliche Nordsee. Diplome thesis, Geoscience Department, Bremen University, 98 p. Leatherman, S.P., Zhang, K. & Douglas, B.C. (2000) Sea level rise shown to drive coastal erosion. EOS 81: 55-57. Mai, S. & Bartholom/i, A. (2000) The missing mud flats of the Wadden Sea: a reconstruction of the accommodation space for mud lost in the wake of land reclamation. In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Meadows, P.S. & Meadows, A. (1991) The geotechnical and geochemical implications of bioturbation in marine sedimentary ecosystems. Symp. Zool. Soc. London 63: 157-181. Meadows, P.S., Meadows, A., West, F.J.C., Shand, P.S. & Shaikh, M.A. (1998) Mussels and mussel beds (Mytilus edulis) as stabilizers of sedimentary environments in the intertidal zone. In: Black, K.S., Paterson, D.M. & Cramp, A. (eds), Sedimentary Processes in the Intertidal Zone. Geol. Soc. London Spec. Publs 139: 331-347. Millat, G. & Herlyn, M. (1999) Documentation of intertidal mussel bed (Mytilus edulis) sites at the coast of Lower Saxony. Senckenbergiana marit. 29 (Suppl.): 83-93. Nehls, G. & Thiel, M. (1993) Large-scale distribution patterns of the mussel Mytilus edulis in the Wadden sea of Schleswig-Holstein: do storms structure the ecosystem? Neth. J. Sea Res. 31(2): 181-187. Oost, A.P. (1995) The influence of biodeposits of the blue mussel Mytilus edulis on fine-grained sedimentation in the temperate-climate Dutch Wadden Sea. Geol. Ultra. 126: 359-400. Paterson, D.M. (1997) Biological mediation of sediment erodibility: ecology and physical dynamics. In: Burt, N., Parker, R. & Watts, J. (eds), Cohesive Sediments. Proc. 4th Nearshore and Estuarine Sediment Transport Conf., Wallingford (England), July 1994. John Wiley & Sons, New York, pp. 215-229. Perillo, G.M.E. (ed.) (1995) Geomorphology and Sedimentology of Estuaries. Develop. Sedimentol. 53. Elsevier, Amsterdam, 471 p. Postma, H. (1961) Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res. 1: 148-190. Rohjans, D., Brocks, P., Scholz-B6ttcher, B.M. & Rullk6tter, J. (1999) Effect of ice rafting on surface sediments in the Wadden Sea traced by organic geochemical methods. Senckenbergiana marit. 29 (Suppl.): 135-139.
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Rohjans, D., Brocks, P., Scholz-B6ttcher, B.M. & Rullk6tter, J. (1999) Effect of ice rafting on surface sediments in the Wadden Sea traced by organic geochemical methods. Senckenbergiana marit. 29 (Suppl.): 135-139. Ruddy, G., Turley, C.M. & Jones, T.E.R. (1998) Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment-water interface. In: Black, K.S., Paterson, D.M. & Cramp, A. (eds), Sedimentary Processes in the Intertidal Zone. Geol. Soc. London Spec. Publs 139: 135-148. Snelgrove, P.V.R. & Butman, C.A. (1994) Animal-sediment relationships revisited: cause versus effect. Oceanogr. Mar. Biol. Ann. Rev. 32: 111-177. Ten Brinke, W.B.M., Augustinus, P.G.E.F. & Berger, G.W. (1995) Fine-grained sediment deposition on mussel beds in the Oosterschelde (The Netherlands), determined from echosoundings, radio-isotopes and biodeposition field experiments. Estuar. Coast. Shelf Sci. 40: 195-217. van Straaten, L.M.J.U. (1961) Sedimentation in tidal flat areas. J. Alberta Soc. Petrol. Geol. 9: 203-226. Villbrandt, M., Hild, A. & Dittmann, S. (1999) Biogeochemical processes in tidal flat sediments and mutual interactions with macrobenthos. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 95-132. Vollmer, M. & Grann, H. (eds) (1999) Large-scale Constructions in Coastal Environments. Springer, Berlin, 194 p. Widdows, J., Brinsley, M. & Elliott, M. (1998) Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna. In: Black, K.S., Paterson, D.M. & Cramp, A. (eds), Sedimentary Processes in the Intertidal Zone. Geol. Soc. London Spec. Publs 139: 85-97. Zfihlke, R., Blome, D., van Bernem, K.-H. & Dittmann, S. (1998) Effects of the tube building polychaete Lanice conchilega (Pallas) on benthic macrofauna and nematodes in an intertidal sandflat. Senckenbergiana marit. 29: 131-138.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea) M. T. Delafontaine,* B. W. Flemming and A. Bartholom/i
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
From March 1995 to October 1996, seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat (area: 1400,500 m - 0.7 km 2) were investigated in the vicinity of a series of M. edulis banks behind Spiekeroog Island (East Frisian Wadden Sea). In January/February 1996 the region experienced a severe ice winter, and together with typical a u t u m n / w i n t e r storms, the budget assessments presented in this paper are thus representative of major physical and biological disturbances common in the Wadden Sea. The contents and concentrations of mud in the sediments of the survey area varied considerably, being generally higher in late summer and lower at the end of winter, although almost the same basic pattern was retained over the 19 month survey period. The average POC content of the local sands was 0.106%, whereas that of the mud fractions varied in the range 1.5-4.0 wt%, lower values being associated with high mud contents, higher ones with low mud contents. Overall, POC concentrations in the mud fractions varied from <50-150 g m 2, although peak values of 300 g m 2 occurred locally. Between March and September 1995 a net import of 262 t POC was recorded. In the period September 1995 to April 1996, which included the severe ice winter, a net export of 477 t POC took place. In contrast to the previous year, the summer following the ice winter, i.e. the period from April to October 1996, was characterised by a net export of 83 t POC. In all cases, 40-44% of the total POC fluxes were associated with the sand fraction, in spite of the generally low POC contents of sand. This observation reveals an important and hitherto unrecognised role of sand in total POC fluxes in the Wadden Sea, a feature evidently linked to the large turnover of sand which contributed >90% to the total sediment flux. Contrary to widespread belief, sand does not necessarily dilute substances attached to the m u d fraction in sand-mud mixtures but can actually act as a concentration mechanism because it induces substantial changes in sediment bulk density. This feature will, amongst others, have to be taken into account in the feeding ecology of benthic marine organisms, deposit feeders in particular.
* Corresponding author: M.T. Delafontaine e-mail:
[email protected]
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1. INTRODUCTION It has long been known that a significant proportion of the pelagic particulate organic pool can be channelled to the benthic realm, creating a tight coupling between these 2 compartments in both coastal and deep-sea ecosystems (e.g., Hargrave 1980; Graf 1989; Dame 1993). In many deep-water environments, hydrodynamic conditions are weak enough to facilitate the deposition of particles originating in the productive upper layers. Shallow coastal waters, by contrast, are generally more turbulent and, consequently, the bottom sediments are often impoverished in fine-grained material. On the back-barrier tidal flats of the Wadden Sea (southern North Sea), mud flats have generally disappeared as a result of land reclamation (Flemming & Nyandwi 1994). Because of this loss in accommodation space, the physically controlled deposition of fine particles is today largely restricted to a few remaining embayments which are sheltered enough. By implication, the exchange of particulate matter between the water column and the sediments has become increasingly dependent on biodeposition, i.e. the condensation by filtering organisms of suspended organic and inorganic material in pseudofaeces and faeces. In this respect, the mussel Mytilus edulis is arguably the most important species in the region (e.g., Smaal et al. 1986), its filtering activity resulting in the formation of characteristic 'mud halos' in otherwise sandy areas where hydrodynamic conditions usually preclude the deposition of fine particles (e.g., Flemming & Ziegler 1995). The extent to which biogenic muds can be used as, amongst other things, a food source by benthic organisms depends largely on the size and turnover rate of this mud reservoir. In the Wadden Sea, fine-grained sediments are easily resuspended under existing hydrodynamic energy conditions, particularly during the more stormy autumn and winter months from about September to March (e.g., Flemming & Delafontaine 1994; Ten Brinke et al. 1995; Bartholom~i et al. 2000, this volume). In addition, a substantial mixing of biogenic muds with coarser grained sediments occurs on these tidal flats (e.g., Oost 1995), and sediments in the mussel banks mostly contain less than about 20% mud on a dry weight basis (e.g., Delafontaine et al. 1996; Bartholom/i et al. 2000, this volume). For the East Frisian Wadden Sea, seasonal mass balances have, in fact, demonstrated that total fluxes of mud (i.e. import+export) generally constitute only about 6-7% of total sediment fluxes in M. edulis banks (Bartholom/i et al. 2000, this volume). Higher enrichment levels in fine-grained material would tend to counterbalance this apparent dominance of coarser material in, for example, the cycling of particulate organic carbon (POC). However, the findings of Delafontaine et al. (1996, 2000) imply that such POC budgets are neither simple nor straightforward for the region. Thus, monthly POC enrichment levels varied strongly in the mud fractions of sandy back-barrier tidal flats behind the islands of Baltrum and Langeoog in the East Frisian Wadden Sea. Furthermore, in the mud fractions of sandy sites devoid of mussels, POC contents were generally higher (3.5-5.5 wt%) than those recorded in the vicinity of M. edulis banks where values fluctuated in the range 1.5-3.0%. Mussel
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occurrence also undergoes strong seasonal and annual fluctuations in the Wadden Sea (e.g., Dankers & Koelemaij 1989; Nehls & Thiel 1993; Flemming & Delafontaine 1994). Depending on factors affecting mussel survival and/or establishment, sandy sediments would therefore contain variable propor-tions of differently enriched mud pools. From March 1995 to October 1996 we investigated large-scale (700~103 m 2) seasonal export and import fluxes of POC in the mud and sand fractions of an intertidal sand flat behind Spiekeroog Island (East Frisian Wadden Sea), choosing a site which incorporated a series of M. edulis banks. POC data were collected concurrently to those for topographic elevation and sediment composition (cf. Bartholom/i et al. 2000, this volume). Roughly in the middle of the study period, a prolonged spell of ice coverage (ca. 50 days in January-February 1996; Anonymous 1996; Giinther & Niesel 1999) resulted in extensive damage and displacement of the mussels as well as considerable erosion (Bartholom/i et al. 2000, this volume). Together with typical autumn/winter storms, these budget assessments are therefore representative of major physical and biological disturbances common in the region.
2. MATERIALS AND METHODS 2.1. Study area, study grid The fieldwork was carried out on the Swinnplate tidal flat in the rear of the barrier island of Spiekeroog in the East Frisian Wadden Sea (southern North Sea; Fig. 1). The site represents a typical sand flat (mud contents usually <2 dry weight%) dominated by the polychaete Arenicola marina. A mixed community of the mussel M. edulis and the tube-dwelling polychaete Lanice conchilega with associated biogenic mud accumulations was present at the time of study (Kurmis 1995; Delafontaine et al. 1996).
Figure 1. Locality map (dotted lines in inset indicate watersheds).
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A grid which remained fixed for the duration of the study period (March 1995October 1996) consisted of 90 sampling/measuring points at 100-m intervals spanning several mussel beds over a rectangular area of 1400,500 m (0.7 km2; Fig. 2). The positions of the 90 grid points were fixed by laser theodolite with an accuracy of
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Figure 2. Location of sampling grid (bold dots) on the Swinnplate sand flat in the rear of Spiekeroog Island (cf. Fig. 1). Depth contours are metres below German topographic chart datum.
2.2. Mass balancing Mass balances of export/import fluxes for mud, sand, and POC were determined for three consecutive seasons in the time intervals March-September 1995 (7 months, summer), September 1995-April 1996 (8 months, winter), and April-October 1996 (7 months, summer). 2.2.1. Terminology In the following we define some key terms as used in the present study. Seasonal: a time interval spanning winter or summer, and lasting 7-8 months. Sand: sediment size fractions 0.063-2 mm. Mud: sediment size fractions <0.063 mm. Sediment: unfractionated sand+mud. Dry bulk density: dry mass of sediment (desalinated) in a unit volume of water-saturated sediment. Content of a substance in the sediment: dry mass of the substance in a unit mass of water-saturated sediment. Concentration of a substance in the sediment: dry mass of the substance (desalinated) in a unit volume of water-saturated sediment. Dry weight% content of a substance in the sediment: (dry mass of the substance/dry mass sediment)*100. Export: flux from the study area (loss), reflecting net erosion in a given time interval. Import: flux to the study area (gain), reflecting net deposition in a given time interval. Total flux: export+import. Mass budgets (net fluxes): import mass-export mass. Flux units" tonnes dry sediment (sand, mud) or POC per unit area per season. Tonne or t: metric ton (= 1000 kg).
Turnover of POC in mud and sand
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2.2.2. Procedure
Topography was measured by means of precision levelling at all 90 grid points, using a laser theodolite (see above) to determine elevations to ca. 1 cm relative to the German topographic chart datum (NN = normal null). Measurements were carried out every 7-8 months in the period March 1995-October 1996 (Bartholom~i et al. 2000, this volume). Export/import fluxes of sand and mud were determined by calculating the differences between the topographic elevations recorded at the beginning and end of a given 7 to 8-month study interval, and then generating corresponding changes in wet sediment volume (Fig. 3). These data were used in combination with concurrent measurements of sediment composition (mud content), and a site-specific relationship between dry bulk density and mud content (see below) to convert volumetric changes into gains or losses of sand and mud masses (Bartholom~i et al. 2000, this volume). Gains and losses in sand masses were multiplied by the POC content of local sands in order to calculate corresponding changes in sand POC masses. In order to calculate the mud POC masses, gains and losses in mud masses were coupled with the site-specific mud POC contents measured at the end and at the beginning of each study interval, respectively. to determine BULK SEDIMENT FLUX erosion = loss = export deposition = gain = import
measure VOLUME CHANGE by repeated precision levelling by applying site-specific DRY BULK DENSITY, convert into BULK SEDIMENT MASS (gain or loss)
calculate SUBSTRATE-SPECIFIC CONTRIBUTION from sand/mud contents SAND MASS
(gain or loss)
Z
MUD MASS (gain or loss)
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via site-specific
POC CONTENT of SAND
POC CONTENT of MUD
calculate
calculate
MASS of POC in SAND (gain or loss)
MASS of POC in MUD (gain or loss)
add to obtain MASS of POC in SEDIMENT (gain or loss)
Figure 3. Flow diagram of the procedure followed to measure export/import fluxes of mud, sand, and POC on the Swinnplate sand flat.
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2.2.3. Sediments Concurrently to the elevation measurements, surficial sediments were collected to depths of up to 5 cm at all 90 grid points, at least 5 subsamples being pooled in each case in order to reduce the probable sampling error by at least 50% (e.g., Krumbein & Pettijohn 1938). The samples were desalinated in the laboratory, and then wet sieved through a 0.063-mm mesh. The resulting sand and m u d fractions were oven dried at 65~ for 24 hours, weighed, and these values were then used to calculate the m u d contents of the sediments (e.g., Lewis & McConchie 1994). The empirical relationship between the dry bulk density and m u d content of the sediments used in the present study was extracted from Flemming & Delafontaine (2000), i.e. y = -0.7955892+2.3863045e ~-~1~5829277~where y - dry bulk density of the surficial sediment (upper 5 cm; mass (g) of desalinated dry sediment per cm 3 of water-saturated sediment), x = weight% of dry m u d content of the sediment, n = 337, r = -0.9847. 2.2.4. POC POC contents were determined in all mud fractions (mud POC content = [dry mass POC in m u d / d r y mass mud] * 100). Aliquots were ground by means of a pestle and mortar and, after acid leaching with HC1 fumes for 15-24 hours, the organic carbon was measured by means of an Hereaus CHNS analyser in each case (standard: acetanilide; reproducibility: +4.8%, n = 10; cf. Delafontaine et al. 1996). POC contents of whole (unfractionated) sediments were measured in additional samples collected on the Swinnplate and in nearby Jade Bay. In this case each sample was halved before desalting, resulting in 2 groups of complementary subsamples. One group served for the POC analyses after grounding in a mechanical mill, the other group for the determination of mud contents. This data set was used to estimate the mean POC content of local sands by means of regression analysis. 2.4. Data plotting Contour plots were computer generated by means of the software package SURFER (Windows Golden Software Inc. RT) using the distance interpolation procedure (e.g., Cressie 1991).
3. RESULTS
The variability of mud concentrations (i.e. volume-specific m u d masses) in the survey area are illustrated in Fig. 4. It is clearly evident that almost the same basic pattern was retained over the 19 month survey period, in spite of the fact that m u d concentrations varied considerably, being generally higher in late summer and lower at the end of winter. The similarity of the pattern was all the more surprising as there was also considerable variation in sediment turnover as documented by substantial local elevation changes (cf. Bartholom~i et al., this volume). Mud is thus either
Turnover of POC in mud and sand
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worked into the sediment to depths of up to several decimeters, or older m u d banks essentially occupying the same area are exhumed in the course of erosion.
Figure 4. Mud concentrations (volume-specific m u d masses) in the survey area over a period of 19 months (a: March 1195; b: September 1995; c: April 1996; and d: October 1996). Note the strong variability from survey to survey.
Regression analysis of POC contents in sediments of varying sand-mud proportions (ca. 0.5-65 wt% mud) shows that the POC content of local sands is 0.106%. Thus, y - 0.106+0.023x where y - POC content of sediment, x - m u d content of sediment, n - 55, r - 0.86.
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In spring (March) 1995, the POC contents of the mud fractions varied in the range 1.5-2.0 wt% in large areas of the Swinnplate grid (Fig. 5a). Higher values of 2.0-3.0% POC were recorded in the north-western and more centrally situated southern sectors. Contents had generally risen to 1.5-2.5% by the end of summer (September) that year, and higher values of 3.0-4.0% were again found at the north-west sites as well as more to the south-east near the centre of the grid (Fig. 5b).
Figure 5. Contour maps of the POC dry weight% contents of the mud fractions in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Seven months later (April 1996), i.e. after the severe ice winter, the same basic pattern in POC enrichment was still evident (Fig. 5c). Thus, a SW-NE corridor of lower POC contents (1.5-2.0%) was juxtaposed between higher values (3.0--4.0%) to the north-west and south-east. However, by the end of the following summer (October 1996) the pattern had largely disappeared, most of the 90 grid sites having POC enrichment levels of 2.5-3.0% except for higher values of 3.0-3.5% at some western localities (Fig. 5d). In the spring of both 1995 (March) and 1996 (April), the POC concentrations for the mud fractions varied from <50 to 150 g m 2 in the upper 5 cm of the sediments in the grid area as a whole, the higher values of 100-150 g m -2 being largely restricted to a SW-NE-oriented corridor in both cases (Fig. 6a, c). A similar spatial pattern was documented towards the end of summer (September 1995, October 1996) in both years when POC concentrations had risen to 150-300 g m -2 in the corridor, values of <100 g m 2 being largely limited to the north-west and central south-east sectors of the grid (Fig. 6b, d). During the four study campaigns, the spatial distributions in the POC concentrations for the sand fractions showed patterns which were the reverse of those documented for the mud fractions. Compared to the NW and SE corners of the grid, where sand POC measured 60-70 g m -2 in the upper 5-cm sediment layer, values were generally lower (<60 g m -2) in the SW-NE-oriented corridor at all times (Fig. 7). Over the summer of 1995 (March-September), a net import of POC was recorded in the sediments (mud+sand fractions) over much of the grid area, values varying in the range 20-80 g POC m -2 (Fig. 8a). A net export of <40 g POC m 2 w a s only recorded in a small central drainage area and 2 sites eastward thereof. This resulted in a positive POC budget of +262 t for this time interval, with nearly 44% (161 t) of the total POC flux (import+export) being associated with the sand fractions (Table 1). For the period spanning the severe winter (September 1995-April 1996), a negative POC budget of -477 t was documented for the grid area as a whole, the fluxes in sand POC (290 t) again making up over 40% of the total sediment budget (Table 1). Most grid localities experienced net exports of 40-160 g POC m -2 (Fig. 8b). Similarly to this severe winter but in contrast to the summer before, a net export of POC (-83 t) was observed over the summer of 1996 (April-October; Table 1). Once again, nearly 40% of the total POC flux (595 t) was linked to the sand fractions. The sites which experienced the highest export (80-160 g POC m ~ w e r e situated largely in the eastern grid sector (Fig. 8c). Compared to the 1995 summer period when a total flux of nearly 203 t POC was recorded for the mud fractions in the area as a whole, values were nearly twice as high (393 t POC) the following winter, a trend which continued (albeit less markedly) the summer thereafter (359 t POC; Table 1).
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Figure 6. Contour maps of the mud POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 7. Contour maps of the sand POC concentrations in the surficial (0-5 cm) sediments in the survey area in a) March 1995, b) September 1995, c) April 1996, and d) October 1996.
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Figure 8. Contour maps showing the gains and losses in the sediment (mud+sand) POC masses in the survey area in the time intervals a) March-September 1995, b) September 1995-April 1996, and c) April-October 1996. Positive values indicate gains (import), and negative values indicate losses (export) of sediment POC per unit area in a given time interval (g m -2 seasonS).
119
Turnover of POC in mud and sand
Table 1. Mass budgets for the seasonal fluxes of mud, sand, and POC in the survey area on the Swinnplate tidal flat in the time intervals March-September 1995, September 1995-April 1996, and April--October 1996 (positive values are gains, and negative values are losses for the survey area in a given time interval, in tonnes*103 or tonnes dry mass per 0.7 km 2 per season; sediment = mud+sand; data extracted partly from Bartholom/i et al. 2000, this volume).
Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO 3) Mud mass (t*lO 3) Sediment mass (t*lO 3) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t) Sand mass (t*lO ~) Mud mass (t*lO 3) Sediment mass (t* 103) Sand POC mass (t) Mud POC mass (t) Sediment POC mass (t)
March 95September 95September 95 April 96 Export Import Export Import 28 119 227 47 1 9 18 2 29 128 245 49 30 131 240 50 21 182 340 53 51 313 580 103 Total fluxes (import+export) 147 274 10 20 157 294 161 290 203 393 364 683 Net fluxes (import-export) +91 -180 +8 -16 +99 -196 +101 -190 +161 -287 +262 -477
April 96October 96 Export Import 160 63 7 7 167 70 169 67 170 189 339 256 223 14 237 236 359 595 -97 0 -97 -102 +19 -83
4. D I S C U S S I O N A N D I M P L I C A T I O N S
In the East Frisian Wadden Sea, assessments of the organic matter (OM) content of sand (based on measurements of loss-on-ignition at 450~ yielded a mean value of 0.24 wt% OM (Delafontaine et al. 1996). The value of ca. 0.11 wt% POC recorded in the present study implies that the commonly used conversion factor of 1.8 (e.g., Trask 1939; Morgans 1956; Birch 1977) would underestimate OM enrichment by about 18% in this case. Rather, a conversion factor of 2.3 would be more appropriate for sands in the back-barrier tidal flats of the southern North Sea. In the m u d fractions on the Swinnplate, the seasonal assessments of POC enrichment carried out in the present study for the period March 1995-October 1996
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showed levels consistent with those documented by Delafontaine et al. (1996) in the same study grid in July/September 1994 (ca. 2-3 wt% POC). In the vicinity of M. edulis banks, therefore, mud organic carbon enrichment is generally about 20 times that recorded in sand. Spatial patterns in mud organic enrichment were not always homogenous in the study grid, lower values of 1.5-2.5 wt% POC having sometimes been recorded in a SW-NE corridor, whereas POC contents reached 4 wt% at some of the other sites. A 7-year 'mussel-watch' campaign, carried out from 1990 to 1996 on the Swinnplate, demonstrated that the mussels were also largely confined to this corridor, and mud accumulation was higher here than at other grid sites (cf. 10-30 wt% and <10 wt% mud, respectively; Bartholom~i et al. 2000, this volume). In other words, muds had higher levels of organic enrichment at sandier sites at some distance from the mussel banks, the reverse being the case at muddier sites close by. These large-scale patterns persisted despite substantial erosion of the tidal flats and decimation of the banks at the time of ice coverage early in 1996 (see below). These findings are consistent with 1) the basin-wide occurrence of increased OM enrichment documented for finegrained sediments on sandier tidal flats behind the islands of Spiekeroog, Langeoog and Baltrum, probably reflecting relatively higher clay and fine-silt contents in the muds at more exposed localities (Xu 2000; Delafontaine et al. 2000), and 2) the preservation of biogenic mud 'signatures' by burial (Bartholom~i et al. 2000, this volume). They also argue against the widespread assumption that biodeposits as such are particularly enriched in organic matter (e.g., Villbrandt et al. 1999). The value of 1.5-2.5 wt% POC documented in the biogenic muds integrates different pools of fine particles, each probably having a distinct POC signal. Thus, fine particles transported in suspension are trapped in the mussel banks because the 3-D structure of the banks increases surface roughness. In addition, the mussels produce faeces and pseudofaeces. Despite the fact that the psedofaeces possibly contain undigested algal cells, they have high contents of inorganic material (Clausen & Riisg~rd 1996; Jorgensen 1996). Compared to the faeces, therefore, OM enrichment is markedly lower in the pseudofaeces of many filter feeders (e.g., Cerastoderma edule; Navarro & Widdows 1997). However, it is well known that the production of pseudofaeces far outweighs that of faeces in turbid coastal environments (e.g., Navarro & Widdows 1997), and also in the Dutch Wadden Sea (Dankers et al. 1989). Such comparative data are lacking for M. edulis feeding under natural conditions in the German Wadden Sea. For example, Dittmann (1987) measured only total C (i.e. organic and inorganic C) in faeces. Nevertheless, it is evident that strong 'contamination' from, amongst other things, pseudofaeces has to be accounted for (at least conceptually) when attempting to link levels of, for example, degradation products in bulk sediments to the digestive activity of the mussels, an aspect which has been overlooked in other studies dealing with the Swinnplate M. edulis community (e.g., Villbrandt et al. 1997). Indeed, the results of the present study provide quantitative assessments of another important source of "contamination" in such studies, namely the sand component of the sediments.
Turnover of POC in mud and sand
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For the periods March-September 1995, September 1995-April 1996, and AprilOctober 1996, net fluxes (import-export) of sand POC constituted at least 40% of those for the sand and mud fractions combined. Furthermore, total fluxes (import +export) of sand POC made up nearly half (ca. 40%) of the total sediment fluxes at all times. Seeing that OM enrichment was comparatively low in the sands (see above), these high values are better explained by the large amounts of sand transported across the tidal flats, presumably as bed load and/or intermittent bottom suspension as a result of wave action during common autumn and winter storms (e.g., Fach 1996). Thus, net fluxes of sand always outweighed those of mud by an order of magnitude, and total fluxes of mud constituted only ca. 6-7% of the total sediment fluxes at all times in the Swinnplate grid area (Table 1; Bartholom~i et al. 2000, this volume). The admixture of substantial amounts of sand to the biogenic muds can lead to a dilution (decrease in concentration) of the OM pool in the sediments. For sand contents varying from ca. 80 wt% in muddier sediments in the mussel banks to >95 wt% on the adjoining sand flat (Bartholom~i et al. 2000, this volume), the POC concentrations decreased from ca. 300 to <100 g m -2 in the upper 5 cm of the sediments. In the Wadden Sea, therefore, sand reworking probably decreases the efficiency of material transfer to benthic organisms via biodeposition because consumers would have to forage in larger volumes of sediment for a given amount of food in this case. Depending on the relative proportions of sand and mud, however, Flemming and Delafontaine (2000) have demonstrated that, contrary to widespread opinion (for example, cf. Tyson 1995), sand admixture does not automatically dilute but can also condense fine particles in sediments. In this context, we contend that some views about the significance of sediment mixing in the feeding ecology of deposit feeders in mussel banks and, for that matter, in tidal-flat environments in general need to be revised (cf., for example, Snelgrove & Butman 1994). In the Swinnplate study grid, total fluxes of mud POC always made up over half (ca. 60%) of the mud and sand fluxes combined, also in the aftermath of the severe 1995-1996 winter when prolonged ice coverage resulted in the large-scale decimation of the mussel banks (Bartholom~i et al. 2000, this volume). Furthermore, in the summer of 1996 total fluxes of mud POC outweighed those documented the previous summer by a factor of nearly 2, despite mussel coverage having decreased by nearly 50% over this time interval. By implication, the cycling of organic matter in the fine fraction of sandy tidal flats could involve considerable amounts of buried material, particularly when ice scour and ensuing erosion excavate the remains of former mussel bank muds. Indeed, marked erosion reaching 0.7 m at places was recorded at the study site at the time (Bartholom~i et al. 2000, this volume). Independent evidence of facies structure with depth, gained by taking box cores at spot localities at the Swinnplate study site (Hertweck & Liebezeit 1996), also corroborate the importance of excavated mud horizons in the cycling of OM on Wadden Sea tidal flats. We conclude that, under the strong hydrodynamic conditions prevailing in the Wadden Sea, the turnover of sedimentary organic matter in intertidal M. edulis banks evidently involves larger amounts of sand than is commonly assumed. Frequent
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storm events and episodic ice coverage result in a substantial reworking of sandy sediments, thereby also promoting the admixture of OM sequestered in buried biogenic mud reservoirs. These results demonstrate the hitherto little recognised importance of also considering the effects of elevation changes and sediment mass physical properties when assessing and interpreting material fluxes in such highly dynamic environments as the Wadden Sea.
ACKNOWLEDGEMENTS
Our hearty thanks go to the captain, motorboat driver and crew of the research vessel Senckenberg for their expertise and unfailing good spirits during the fieldwork, oft carried out under difficult weather conditions. We also acknowledge the laboratory assistance of A. Rascke and numerous students. The work was sponsored partly by the Senckenbergische Naturforschende Gesellschaft, Frankfurt, and partly by the Federal Ministry of Education and Research, Bonn (Grant No. 03F0112A).
REFERENCES
Anonymous (1996) L~ingster Eiswinter seit 33 Jahren. Deutsche Seeschiffahrt 5/1996: 16. Bartholom~i, A., Flemming, B.W. & Delafontaine, M.T. (2000) Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Birch, G.F. (1977) Surficial sediments on the continental margin off the west coast of South Africa. Mar. Geol. 23: 305-337. Clausen, I. & Riisgard, H.U. (1996) Growth, filtration and respiration in the mussel Mytilus edulis: no evidence for physiological regulation of the filter-pump to nutritional needs. Mar. Ecol. Prog. Ser. 141: 37-45. Cressie, N.A.C. (1991) Statistics for Spatial Data. John Wiley & Sons, New York, 900 p. Dame, R.F. (ed.) (1993) Bivalve Filter Feeders in Estuarine and Coastal Ecosystem Processes. Springer, Berlin, NATO ASI Ser. 33. Dankers, N. & Koelemaij, K. (1989) Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring of other ecological parameters. Helgol. Meeresunters. 43: 529-535. Dankers, N. Koelemaij, K. & Zegers, J. (1989) De rol van de mossel en de mosselcultuur in het ecosysteem van de Waddenzee. RIN Rep. 89/9, 66 p. Delafontaine, M.T., Bartholom~i, A., Flemming, B.W. & Kurmis, R. (1996) Volumespecific dry POC mass in surficial intertidal sediments: a comparison between biogenic muds and adjacent sand flats. Senckenbergiana marit. 26 (3/6): 167-178.
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Delafontaine, M.T., Flemming, B.W. & Kr6gel, F. (2000) Organic enrichment in backbarrier sediments of the Wadden Sea: a five-year environmental impact study spanning the Europipe landfall. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Dittmann, S. (1987) Die Bedeutung der Biodeposite fiir die Benthosgemeinschaft der Wattsedimente, unter besonderer Beriicksichtigung der Miesmuschel Mytilus edulis (L.). Dissertation, Universit/it G6ttingen, 182 p. Fach, B. (1996) Die Entwicklung der Windverh~iltnisse in der Deutschen Bucht seit 1965 als wichtiger UmwelteinfluB auf die Kiistenzone. Diplom, Fachhochschule Wilhelmshaven (Germany), 75 p. Flemming, B.W. & Delafontaine, M.T. (1994) Biodeposition in a juvenile mussel bed of the East Frisian Wadden Sea (southern North Sea). Neth. J. Aquat. Ecol. 28: 289-297. Flemming, B.W. & Delafontaine, M.T. (2000) Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications. Cont. Shelf Res. (in press). Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth J. Aquat. Ecol. 28: 299-307. Flemming, B.W. & Ziegler, K. (1995) High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog Island (southern North Sea). Senckenbergiana marit. 26: 1-24. Graf, G. (1989) Benthic-pelagic coupling in a deep-sea benthic community. Nature 341: 437-439. Giinther, C.-P. & Niesel, V. (1999) Effects of the ice winter 1995/96. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 193-205. Hargrave, B.T. (1980) Factors affecting the flux of organic matter to sediments in a marine bay. In: Tenore, K.R. & Coull, B.C. (eds), Marine Benthic Dynamics. University South Carolina Press, Columbia, pp. 243-263. Hertweck, G. & Liebezeit, G. (1996) Biogenic and geochemical properties of intertidal biosedimentary deposits related to Mytilus beds. Proc. 29th EMBS, Vienna, August 1994. Mar. Ecol. (PSZNI) 17: 131-144. Jorgensen, C.B. (1996) Bivalve filter feeding revisited. Mar. Ecol. Prog. Ser. 142: 287-302. Krumbein, W.C. & Pettijohn, F.J. (1938) Manual of Sedimentary Petrography. D. Appleton-Century, New York, 549 p. Kurmis, R. (1995) Quart~irgeologische Detailkartierung der Swinnplate im Spiekerooger Rfickseitenwatt, siidliche Nordsee. Diplom, Universit~it Bremen, 98 p. Lewis, D.W. & McConchie, D. (1994) Analytical Sedimentology. Chapman & Hall, London, 197 p. Morgans, J.F.C. (1956) Notes on the analysis of shallow water soft substrates. J. Anim. Ecol. 25: 367-387.
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Navarro, J.M. & Widdows, J. (1997) Feeding physiology of Cerastoderma edule in response to a wide range of seston concentrations. Mar. Ecol. Prog. Ser. 152: 175-186. Nehls, G. & Thiel, M. (1993) Large-scale distribution patterns of the mussel Mytilus edulis in the Wadden sea of Schleswig-Holstein: do storms structure the ecosystem? Neth. J. Sea Res. 31(2): 181-187. Oost, A.P. (1995) Dynamics and sedimentary development of the Dutch Wadden Sea with emphasis on the Frisian inlet. Doctoral thesis, Universiteit Utrecht, Geol. Ultra. 126, 455 p. Smaal, A.C., Verhagen, J.H.G., Coosen, J. & Haas, H.A. (1986) Interaction between seston quantity and quality and benthic suspension feeders in the Oosterschelde, The Netherlands. Ophelia 26: 385-399. Snelgrove, P.V.R. & Butman, C.A. (1994) Animal-sediment relationships revisited: cause versus effect. Oceanogr. Mar. Biol. Ann. Rev. 32: 111-177. Tyson, R.V. (1995) Sedimentary Organic Matter. Chapman & Hall, London, 615 p. Ten Brinke, W.B.M., Augustinus, P.G.E.F. & Berger, G.W. (1995) Fine-grained sediment deposition on mussel beds in the Oosterschelde (The Netherlands), determined from echosoundings, radio-isotopes and biodeposition field experiments. Estuar. Coast. Shelf Sci. 40: 195-217. Trask, P.D. (1939) Organic content of recent marine sediments. In: Trask, P.D. (ed.) Recent Marine Sediments. Dover, New York, pp. 428-453. Villbrandt, M., Hild, A. & Dittmann, S. (1999) Biogeochemical processes in tidal flat sediments and mutual interactions with macrobenthos. In: Dittmann, S. (ed.), The Wadden Sea Ecosystem: Stability Properties and Mechanisms. Springer, Berlin, pp. 95-132. Xu, W. (2000) Mass physical sediment properties and trends in a Wadden Sea tidal basin. Berichte, Fachbereich Geowissenschaften, Univ. Bremen, No. 157, 127 p.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Significance of microphytobenthic primary production in the Bodden (southern Baltic Sea) S. Gerbersdorf,* H. J. Black, J. Meyercordt, L.-A. Meyer-Reil, T. Rieling and I. Stodian
Institut fiir Okologie, Ernst-Moritz-Arndt-Universit~t Greifswald, 18565 Kloster, Germany
ABSTRACT
Investigations of phytoplankton and microphytobenthic primary production were carried out at two estuarine study sites located in the southern Baltic Sea (Germany). Special attention was given to the balance between pelagic and benthic primary production which is widely considered to have become disturbed by eutrophication in the region, as well as to the importance of the flocculent sediment surface layer for primary production. Study site A, the Rassower Strom, is situated in the NordRi~gensche Boddenkette whereas study site B, the Kirr-Bucht, is located in the central part of the Dart~-Zingster-Boddenkette. At both sites, pelagic production dominated total primary production (85%). The microphytobenthos contributed sunstantially to total production, with gross production rates measuring up to 2 mmol C m -2h q. Despite site-specific differences in physical conditions, the contribution of benthic primary production to total primary production was comparable at both sampling sites. The combination of water depth (high at study site A, and low at study site B) and light attenuation coefficient (low at study site A, and high at study site B) led to similar light regimes at the sediment surface in both cases. Calculated P-E curves (photosynthesis versus irradiance) based on primary production measurements in the laboratory showed highest Pma• and E k values in summer. At all seasons, light saturation of the microphytobenthos occurred at rather low irradiance (about 70 ~E m -2 s1) which might indicate a physiological adaptation of the microalgae to low light conditions. The more eutrophic and the deeper the water, the more irradiance will limit photosynthesis of microphytobenthic organisms. In a flocculent layer consisting of algae and detrital aggregates on top of the sediments, photoautotrophic processes dominated due to low respiration rates. The removal of the surface layer enhanced the primary production of the sediment below, presumably because buried cells became photosynthetically active again. Especially in the rather shallow Bodden where wind-induced currents and waves cause resuspension of bottom sediments, previously buried algae may thus contribute to primary production. * Corresponding author: S. Gerbersdorf e-mail:
[email protected]
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1. INTRODUCTION Primary production by microphytobenthic organisms is presumed to be significant in the shallow non-tidal waters of the Baltic due to corresponding high light levels (Wasmund 1986; Sundb~ick & J6nsson 1988). In the last decades, eutrophication has disturbed the balance between pelagic and benthic primary production in the region (B6rner & Kell 1982). The phytoplankton benefits from higher nutrient levels by increasing its biomass, thereby reducing light (Smayda 1990; Radach et al. 1990). This leads to light limitation at the sediment surface, which may significantly affect microphytobenthic production because light intensity is considered to be the primary controlling factor in benthic photosynthesis (Colijn 1982; Pickney & Zingmark 1993). The microphytobenthic community not only stabilizes the sediment (Miller et al. 1996) but, through its photosynthetic activity, it also oxygenates the sediment surface layers, thereby controlling chemical exchange processes near the sediment/water interface (Sundb/ick & Graneli 1988; Rizzo 1990; Wiltshire 1992). Therefore, a reduction of microphytobenthic activity might enhance anoxic conditions in the surface layers, with subsequent release of ammonia and phosphate to the water column intensifying nutrient fluxes (Koop et al. 1990). A flocculent layer is commonly present at the top of the sediment, consisting of bioseston, detritus and mineral particles which tend to aggregate (Kies 1995). Due to the shallowness of the Bodden, wind-induced resuspension of these particles and aggregates occurs frequently (Schnese 1973; Eisma 1993). Due to an improved light regime, primary production may be even enhanced after resuspension of benthic and / or buried microalgae. The present study deals with primary production processes for the phytoplankton and microphytobenthos at two estuarine sites in the German southern Baltic Sea, focussing on the balance between pelagic and benthic photosynthesis. In addition, the significance of the flocculent sediment surface layer for primary production processes was investigated. Our approach was based on field and laboratory experiments, using the oxygen exchange method as well as oxygen microelectrodes.
2. STUDY SITES
Study site A, the Rassower Strom, forms part of the Nord-R~igensche Boddenkette (Fig. 1) and is strongly influenced by the Baltic Sea, as indicated by relatively high salinities (8.2-9.4 psu). The water column is mesotrophic, with low chlorophyll a concentrations (1.3-4.5 1Jg 1~) leading to a low light attenuation coefficient (0.66 m~). Water depth is around 4 m at this locality, and the sediments consist of sandy mud with organic carbon contents of up to 4% at the sediment surface. Study site B, the Kirr-Bucht, is centrally situated in the Dart~-Zingster-Boddenkette (Fig. 1), and it is almost completely separated from the open Baltic Sea. This location is eutrophic, with limited water exchange. The chlorophyll a concentrations in the water column (12-33 pg 1-~) and the light attenuation coefficient (3.0 m -~) are high. Water depth is
129
Microphytobenthic production in the Bodden
0.6 m, and the salinity ranges from 5.0 to 6.6 psu. The sediments are sandy, with an organic carbon content of about 1%.
..
Baltic Sea
.~iq~:.{Y:::d~i~~[;.~
~
'6...!:).i.:..k~
Figure 1. Study sites in the estuarine areas along the German coast, southern Baltic Sea. Study site A" Rassower Strom; study site B" Kirr-Bucht (see text for further details).
3. MATERIAL A N D M E T H O D S
Sediment cores were collected by means of a multiple coring device or by hand, depending on water depth (see above). Water samples were obtained using a plankton sampler. Photosynthesis and respiration rates were determined using the oxygen exchange method and oxygen microelectrodes (see below). Sediment cores and water samples were taken once in a given season at each study site, and they were subsequently incubated without stirring under in situ conditions as well as in the laboratory. Water samples were incubated in situ in 8 dark and 8 transparent bottles over a period of about 10 hours at three different water depths (below surface, above bottom, and halfway in-between). Sediment cores collected in 3 dark and 3 transparent tubes were incubated on the seafloor for the same period of time. Variations in oxygen concentrations were determined after 5 and 10 hours, using the Winkler-method to calculate net photosynthesis and respiration. Over the period of incubation, the atmospheric photosynthetically active radiation (PAR: 400-700 nm) was measured daily by means of a planar LiCor sensor. Moreover, a light profile was run through the whole water column every hour, using a spherical underwater sensor to calculate the light attenuation coefficient. These data were used
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to determine the maximum, mean and minimum irradiance at the sediment surface in each case. In the laboratory, sediment cores were incubated in a constant-temperature chamber at in situ temperatures, and exposed to the maximum, mean and minimum PAR levels occurring in situ at the sediment surface at the time, using a light source with a solar-like spectrum in the PAR range (Saalmann; 1.2 kW). A 12:12-hour lightdark rhythm was applied. The flocculent surface layer was removed carefully in the laboratory and allowed to settle on a sterile layer of sand. For comparitive purposes, the isolated flocculent layer, sediment cores without surface layer as well as intact sediment cores were incubated under the same conditions of irradiance and temperature. Additionally, sediment cores were investigated in the laboratory using oxygen and light microsensors. Using the oxygen exchange method, photosynthetic activity was determined for the total area of an incubated sediment core (78 cm2). Microsensors were used for spot assessments of respiration and net photosynthesis at high temporal and spatial resolution (Revsbech & Jorgensen 1986; Lassen et al. 1992). Oxygen and light profiles were measured in a sediment core from each study site. Ten oxygen profiles were determined under dark conditions while six oxygen as well as three irradiance profiles were measured at each irradiance level. For each profile, oxygen concentration and light intensity were measured at depth increments of 100 lJm by lowering the sensor tip into the sediment until no oxygen or light were detectable anymore. Dark profiles of oxygen allow the calculation of respiration, while profiles of oxygen during illumination provide information about net photosynthesis. By applying different irradiances, the light dependency of net photosynthesis was investigated for the microphytobenthos. Gross primary production was calculated by summing respiration and net primary production, assuming that respiration is comparable for the dark and light periods. Gross primary production rates were expressed as C-equivalents by using a conversion factor of 1.2, and normalized to chlorophyll a. P-E curves (photosynthesis versus irradiance) were constructed based on gross primary production rates, using the model of Webb et al. (1974). Pmax values (maximum gross primary production related to sediment area) and pbmax values (maximum gross primary production normalized to chlorophyll a) provide information about photosynthetic capacity. The a value is a measure of the photosynthetic efficiency, while the Ek value is an indicator for photosynthetic acclimatization to prevailing light conditions. The determinations of chlorophyll a and phaeopigment a were carried out following the guidelines of the Baltic Marine Environment Protection Commission (Anonymous 1988). For both sediment and water samples, pigments were extracted with ethanol and the absorption of the extract was measured at 665 nm. The samples were then acidified to transfer the active chlorophyll to phaeopigment. From the ratio of absorption before and after acidification, concentrations of active and inactive chlorophyll can be calculated.
Microphytobenthic production in the Bodden
4. R E S U L T S
AND
131
DISCUSSION
Pelagic production clearly dominated primary production during all seasons at both study sites, with up to 85% of total production by pelagic and benthic microalgae. At both sites, differences in volume-specific plankton primary production were recorded. In July 1996, net primary production rates (integrated over the water column) were significantly higher at site B (Kirr-Bucht, 19.6 ~mol C 11 h 1) than at site A (Rassower Strom, 2.1 ~mol C 1-1 h 1) with corresponding concentrations of chlorophyll a in the water column showing values of 34.5 and 4.5 ~g 1-1, respectively. Phytoplankton production per m 2 varied in the same range at both study sites, despite the fact that water depth was about 6 times greater at site A than at site B (Fig. 2). Gross primary production rates of the microphytobenthos fluctuated from 0.5 to 2.2 mmol C m -2h -1 for the different seasons and stations (mean values over the entire incubation period of 10 hours), confirming the results of other authors (Colijn 1982; Wasmund 1986; Cahoon et al. 1993). The ratio of benthic to total production was comparable at both study sites in spring, summer and autumn (Fig. 2). This was caused by the similarity in light availability at the sediment surface, although water depth differed between the sites. Light acclimatization of the microphytobenthos was assessed by calculating P-E curves (photosynthesis versus irradiance), based on primary production measurements carried out in the laboratory. The model predicted that the benthic contribution to primary production would be significantly higher in shallower areas of study site A (Rassower Strom) than at comparable depths at study site B (Kirr-Bucht). Furthermore, in deeper areas (water depths >2 m) of the eutrophic Kirr-Bucht site, benthic primary production would no longer be detectable. This indicates the overriding significance of light for the photosynthesis of the microphytobenthos in the Bodden. Although there are indications of nutrient limitations in sandy, non-tidal sediments (Nilsson et al. 1991), Admiraal et al. (1982) pointed out that nutrient input plays only a minor role for the microphytobenthos in this respect, because of high rates of remineralization in the sediments. April 1997 16~--
--
July 1996 ~16i
October 1996 16
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14 ~r e- 12 10
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112 10
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.~
6
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2
0
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B
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Figure 2. Pelagic and benthic gross primary production rates (dark and light bars, respectively) at various seasons in 1996-1997, based on the oxygen exchange method and expressed as mmol C m "2 h "1 as well as percentages of total production (n = 6). KS: study site A, Rassower Strom; KB: study site B, Kirr-Bucht.
132
Gerbersdorf et al.
Light acclimatization of the microbenthic community was also studied by fitting P-E curves to data obtained from microsensor measurements of primary production (data presented only for site A, Rassower Strom; Fig. 3). P-E curves provide information about photosynthetic capacity (P~x) under the control of environmental factors such as temperature (Epping 1996). In contrast, the initial slope (a) is considered to be temperature independent (Epping 1996).
0,06
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PAR [pE m2s1] Figure 3. Benthic gross primary production rates [mmol C (mg Chl a)-I h -1] versus irradiance for different seasons in 1996 and 1997 at the Rassower Strom site (based on microelectrode measurements.
With increasing irradiance at the sediment surface, light penetration is enhanced, thereby activating photosynthesis in deeper layers. In July 1996, light profiles obtained by fibre optic measurements showed an illuminated zone of 1.5 mm thickness for sandy mud sediments at site A under minimal in situ irradiance (Table 1). This zone was expanded by one third under maximum in situ irradiance. At site A in July 1996, rates of net oxygen production increased with rising irradiance at the sediment surface, up to 0.74 mmol 02 m -2h ~ when light saturation was reached (Table 1). Accordingly, the oxygen concentration and oxygen penetration depth in the sediment increased as well. The oxygen penetration depth is an important parameter for aerobic metabolic processes. Therefore, the thickness of the oxic zone is of great significance for the distribution of organisms in the sediments. Measurements by microelectrodes in sediment cores from site A showed an expansion of the oxic layer from 1.6 mm in the dark to twice as much under in situ light conditions (Table 1). In this case, anaerobic processes like denitrification or sulfate reduction are displaced to deeper sediment layers (Nielsen et al. 1990). Our
133
Microphytobenthic production in the Bodden
seasonal investigations of microphytobenthic production confirmed the positive correlation between oxygen penetration depth and light availability.
Table 1. Light and oxygen penetration depths as well as net primary production rates at different irradiances for the Rassower Strom site in July 1996 (based on microelectrode measurements). The irradiance levels correspond to the minimum, intermediate and maximum natural light conditions which prevailed at the site at the time. Irradiance [laE m "2S"l] 0 45 117 180
Light penetration [Pm]
Oxygen penetration [pm]
Net primary production [mmol m -~h -~1
1500 1800 1900
1600 3000 3500 3600
0.32 0.74 0.59
pbmax values of gross primary production show a positive correlation with water temperature. Rasmussen et al. (1983) found an increase of 5 to 9% in primary production when raising the temperature by 1 K. In the present case, seasonal fluctuations in water temperature (17, 12, and 6~ in July 1996,, October 1996 and April 1997, respectively) can partly explain the different pbm~x values recorded (4.4"10-2, 3.4"10 .2 and 4.1"10 .2 [ m m o l C (mg Chl a)l h 1] for July 1996, October 1996 and April 1997, respectively; Fig. 3). The seasonal variations in ct values (a measure of photosynthetic efficiency) showed a similar trend to that recorded for the pbmaxvalues (6.54"10", 3.22"10" and 4.72"10" [mmol C (mg Chl a)-I h -1] for July 1996, October 1996 and April 1997, respectively). Like the pbmax values, the Ek values (indicators of photosynthetic acclimatization to prevailing light conditions) were highest in summer (80 pE m -2 s-1 in July 1996, compared to 50 gE m -2 s-~ in October 1996, and 65 pE m -2 s-1 in April 1997). Compared to other seasons, therefore, light saturation was achieved at higher light intensities in summer. At all seasons, light saturation occurred at rather low light intensities compared to the maximum irradiance which can be reached at the sediment surface under natural conditions (Table 1), probably indicating a physiological adaptation of the algae to low light conditions. The more eutrophic and the deeper the water is, the more irradiance will become a limiting factor for microphytobenthic production. In the Bodden, a flocculent layer consisting of algae and sedimented aggregates is commonly present on top of the sediments. To investigate any effects on primary production processes, this surface layer was analysed separately, and the primary production rate compared to those of sediment cores with and without surface layers. Compared to sediments without surface layer as well as to intact sediments,
Gerbersdorf et al.
134
the flocculent layer showed higher net production rates, largely due to low respiration rates (Fig. 4). Photoautotrophic processes therefore seem to dominate in the flocculent layer. Chlorophyll a values indicated only a slightly higher algal biomass in the surface layer (16+2.4 lag cm -3) compared to the underlying sediment (13.4_+1.0 lJg cm-3). Nevertheless, after normalization to chlorophyll, the net production rates of the flocculant surface layer were still higher than the values documented in the underlying sediment and in intact sediment cores. 1.5 "~
1.0
w J::
E 0.s f
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I I intact sediment 0 flocculent layer
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Figure 4. Net primary production rates [mmol C m 2 h -1] for different benthic compartments at the Rassower Strom site (based on the oxygen exchange method).
Thus, the removal of the surface layer did indeed enhance the production rates of the sediment below, indicating the potential of buried organisms to become photosynthetically active again. In the shallow Bodden and, for that matter, in other shallow estuarine sites as well (e.g., Schreiber & Pennock 1995) where strong water movement and resuspension are also common, the photosynthetic potential of the underlying sediment is evidently of importance in primary production budgets.
ACKNOWLEDGEMENTS
The investigations presented here are part of the interdisciplinary research project OKOBOD (Okosystem Boddengew~isser - Organismen und Stoffhaushalt) supported by the German BMBF (Bundesministerium fiir Bildung, Wissenschaft, Forschung und Technologie). The authors are especially grateful to Ingrid Kreuzer, Frank Neud6rfer, Georg Schubert, Joachim Timm and Wolfgang Zenke for excellent technical support.
Microphytobenthic production in the Bodden
135
REFERENCES
Admiraal, W., Peletier, H. & Zomer, H. (1982) Observations and experiments on the population dynamics of epipelic diatoms of an estuarine mudflat. Estuar. Coast. Shelf Sci. 14: 471-487. Anonymous (1988) Guidelines for the Baltic Monitoring Programme for the Third Stage. Baltic Mar. Environ. Prot. Comm., Baltic Sea Environ. Proc. 27D: 16-23. B6rner, R. & Kell, V. (1982) EinfluB von N/ihrstoffanreicherungen auf die Biomasse, Artensequenz und Prim/irproduktion des Phytoplanktons w/ihrend einer Komplex-analyse im Zingster Strom (Juni 1981). Wissenschaftl. Zeitschr. WilhelmPieck-Universit/it Rostock 31: 53-56. Cahoon, L.B., Beretich, G.R. Jr., Thomas, C.J. & McDonald, A.M. (1993) Benthic microalgal production at Stellwagen Bank, Massachusetts Bay, USA. Mar. Ecol. Prog. Ser. 102: 179-185. Colijn, F. (1982) Light absorption in the waters of the Ems-Dollard estuary and its consequences for the growth of phytoplankton and microphytobenthos. Neth. J. Sea Res. 15:196-216. Eisma, D. (1993) Suspended Matter in the Aquatic Environment. Springer-Verlag, Berlin, 315 p. Epping, E.H.G. (1996) Benthic phototrophic communities and the sediment-water exchange of oxygen, Mn(II), Fe(II), and silicic acid. Doctoral Thesis, University of Groningen, 215 p. Kies, L. (1995) Algal snow and the contribution of algae to suspended particulate matter in the Elbe estuary. Algae, Environment and Human Affairs, 93-121. Koop, K., Boynton, W.R., Wulff, F. & Carman, R. (1990) Sediment-water oxygen and nutrient exchanges along a depth gradient in the Baltic Sea. Mar. Ecol. Prog. Ser. 63: 65-77. Lassen, C., Plough, H. & Jorgensen, B.B. (1992) A fibre-optic scalar irradiance microsensor: application for spectral light measurements in sediments. FEMS Microbiol. Ecol. 86: 247-254. Miller, D.C., Geider, R.J. & MacIntyre, H.L. (1996) Microphytobenthos: the ecological role of the "Secret Garden" of unvegetated, shallow-water marine habitats. II. Role in sediment stability and shallow-water food webs. Estuaries 19: 202-212. Nielsen, L.P., Christensen, P.B., Revsbech, N.P. & Sorensen, J. (1990) Denitrification and photosynthesis in stream sediment studied with microsensor and whole-core techniques. Limnol. Oceanogr. 35: 1135-1144. Nilsson, P., J6nsson, B., Lindstr6m-Swanberg, I. & Sundb/ick, K. (1991) Response of a marine shallow-water sediment system to an increased load of inorganic nutrients. Mar. Ecol. Prog. Ser. 71: 275-290. Pickney, J. & Zingmark, R.G. (1993) Photophysiological responses of intertidal benthic microalgal communities to in situ light environments: methodological considerations. Limnol. Oceanogr. 38: 1373-1383.
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Radach, G., Berg, J. & Hagmeier, E. (1990) Long-term changes of the annual cycles of meteorological, hydrographic, nutrient and phytoplankton time series at Helgoland and at LV ELBE in the German Bight. Cont. Shelf Res. 10: 305-328. Rasmussen, M.B., Henriksen, K. & Jensen, A. (1983) Possible causes of temporal fluctuations in primary production of the microphytobenthos in the Danish Wadden Sea. Mar. Biol. 73: 109-114. Revsbech, N.P. & Jorgensen, B.B. (1986) Microelectrodes: their use in microbial ecology. Adv. Microb. Ecol. 9: 293-352. Rizzo, W.M. (1990) Nutrient exchanges between the water column and a subtidal benthic microalgal community. Estuaries 13: 219-226. Schnese, W. (1973) Untersuchungen zur Produktionsbiologie des Greifswalder Boddens (s~idliche Ostsee) I. Die Hydrographie: Salzgehalt, Sauerstoffgehalt, Temperatur und Sestongehalt. Wissenschaftl. Zeitschr. Universit~it Rostock 22: 629-639. Schreiber, R.A. & Pennock, J.R. (1995) The relative contribution of benthic microalgae to total microalgal production in a shallow sub-tidal estuarine environment. Ophelia 42: 335-352. Smayda, T.J. (1990) Novel and nuisance phytoplankton blooms in the sea: evidence for a global epidemic. In Gran61i, E., Sundstr6m, B., Edler, L., Anderson, D.M. (eds) Toxic Marine Phytoplankton. Elsevier, Amsterdam, pp. 29-40. Sundb~ick, K. & Gran61i, W. (1988) Influence of microphytobenthos on the nutrient flux between sediment and water: a laboratory study. Mar. Ecol. Prog. Ser. 43: 6369. Sundb~ick, K. & J6nsson, B. (1988) Microphytobenthic productivity and biomass in sublittoral sediments of a stratified bay, SE Kattegatt. J. Exp. Mar. Biol. Ecol. 122: 63-81. Wasmund, N. (1986) Die Gr6Be der Prim~irproduktion im Barther Bodden (s~idliche Ostsee) unter besonderer Ber~icksichtigung des Mikrophytobenthos. Wissenschaftl. Zeitschr. Wilhelm-Pieck-Universit~it Rostock, Naturwissenschaftl. Reihe 5: 22-26. Webb, W.L., Newton, M. & Starr, D. (1974) Carbon dioxide exchange of A l n u s rubra: a mathematical model. Oecologia 17: 281-291. Wiltshire, K.H. (1992) The influence of microphytobenthos on oxygen and nutrient fluxes between eulittoral sediments and associated water phases in the Elbe Estuary. In: Colombo, G., Ferrari, I., Ceccherelli, V.U. & Rossi, R. (eds) Marine Eutrophication and Populations Dynamics. Proc. 25th EMBS. Olsen & Olsen, Fredensborg, pp. 63-70.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 Elsevier Science B.V. All rights reserved.
137
Bodden waters (southern Baltic Sea) as a source of methane and nitrous oxide S. Dahlke, a* C. Wolff," L.-A. Meyer-Reil, a H. W. Bange, b R. Ramesh, b S. Rapsomanikis b and M. O. Andreae b
alnstitut fiir Okologie der Ernst-Moritz-Arndt-Universit~t Greifswald, 18565 Insel Hiddensee, Germany bMax Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020 Mainz, Germany
ABSTRACT
A first assessment of N20 and CH 4 emissions from Bodden waters (southern Baltic Sea) was carried out with the aim of investigating the seasonal and spatial variability of the emissions as well as characterizing the corresponding biological processes (i.e. denitrification, nitrification, and methanogenesis) along pronounced gradients in trophic status and salinity. Saturation values of CH4 varied from 102 to 16,300%. Supply of organic carbon to the sediments, temperature, and input of riverine CH 4 can be considered as important controls on methane concentration. The influence of salinity seems to be insignificant in this respect. On average, 86% of sedimentary methane is oxidized within the sediment. In contrast, methane oxidation is restricted to the water column w h e n stagnation of water bodies occurs. In addition to the seasonal pattern, a diurnal rhythm of methane concentrations was detected. The N20 saturation values of the Bodden waters were low (90 to 309%), and showed a significant correlation with nitrate concentrations (r = 0.83). The Peene River was identified as a strong source for allochthonous N20. From the present data it is hypothesized that, in the inner Bodden waters, high riverine loads of nitrate (which fuels denitrification) and allochthonous N20 might cause enhanced concentrations of N20. In the outer Bodden waters, nitrification may be the prevailing source of nitrous oxide, especially in summer. A preliminary estimate gives an annual emission of 58 tonnes N20 , and 960 tonnes CH 4 for the whole Bodden area. Thus, Bodden waters constitute an important source of methane, and a modest source of nitrous oxide in the Baltic region.
* Corresponding author: S. Dahlke e-mail:
[email protected]
138
Dahlke et al.
1. INTRODUCTION Nitrous oxide (N20) and methane (CH4) are atmospheric trace gases. Both N20 and CH 4 contribute to the greenhouse effect, with N20 being indirectly involved in the depletion of the stratospheric ozone layer. N20 is a natural product of biological denitrification and nitrification. These processes may have increased in coastal marine systems because of enhanced eutrophication due to higher nutrient input, especially by rivers. Estuaries contribute significantly to the global oceanic emissions of nitrous oxide (Bange et al. 1996a). Methane is a product of organic matter decomposition via methanogenesis. Estuarine systems constitute an important element of marine methane sources (Bange et al. 1994). High river loads of nutrients and enhanced eutrophication rates induced by anthropogenic activities support this contention. Methane production in estuarine sediments should be lower than in freshwater sediments, because of competition between methanogenic and desulfuric bacteria. Although globally covering only 0.4% of coastal areas, estuaries emit 33% of the N20, and 7.5% of the CH 4 in these areas (Bange et al. 1994, 1996a). Estuarine N20 and CH 4 emissions usually show marked seasonal and spatial variability due to the influence of various hydrographical and biological factors. However, an accurate assessment of these emissions is still lacking. The aim of the present study was to determine the seasonal estuarine emissions of N20 and CH4, and to characterize the related biological processes in shallow coastal lagoons along the southern Baltic coast, so-called Boddens.
2. S T U D Y AREA
The south coast of the Baltic Sea is characterized by semi-enclosed Boddens, some of which have a chain-like arrangement with only one narrow outlet to the sea (Fig. 1). The outer parts are strongly influenced by the open Baltic Sea, whereas the inner parts are more affected by terrestrial processes, including anthropogenic influences. Thus, from the outer to the inner parts, a pronounced salinity gradient (range 0.7-9%o) is accompanied by an increasing gradient in eutrophication (e.g., K6ster et al. 1997). Bodden waters are comparatively shallow (mean depth 3.8 m) and polymictic. Vertical density gradients (i.e. haloclines and thermoclines) are rare and of short persistence. In recent years there has been some discussion about the efficiency with which Bodden waters act as filter for natural and anthropogenic inputs. Indeed, it has been claimed that inputs over the last decades may have exhausted this capacity (Lampe 1994). The Darss-Zingster Bodden, the Nordriigensche Bodden, the Greifswalder Bodden and the western Oder River estuary are all major components of the Bodden complex (Fig. 1). Bodden waters cover a total area of 1542 k m 2 (Lampe 1994), and are influenced by river inputs of 7000 t N a -1 and 252 t P a -1 from the German mainland
Methane and nitrous oxide in Bodden waters
139
(H. Behrendt, Institut ftir Gew/isser6kologie und Binnenfischerei, pers. comm. ). Additionally, the western Oder estuary is loaded with 3960 t N a-' and 365 t P a -~from the Oder River (H. Meyer, University of Greifswald, pers. comm.). Depending upon hydrographical and morphological conditions, up to 80% of the Bodden bottom are covered with muddy sediments. In the present study, measurements were performed in the western Oder River estuary (at Peenestrom, Achterwasser, and Kleines Haft) and in the Greifswalder Bodden, the choice of study sites reflecting the importance of these areas in the drainage of nutrients before these reach the southern Baltic Sea. Additional measurements were carried out in the Nordriigensche Bodden and the DarssZingster Bodden (Fig. 1). Data were collected in various seasons in the period 19941997.
Figure 1. Map showing the locations of the sampling stations (filled circles) in the Bodden waters and in the western Oder estuary.
140
Dahlke et al.
3. MATERIALS A N D M E T H O D S
CH, and N20 were determined with a dual-column gas chromatograph equipped with a flame ionization detector and an electron capture detector (cf. Bange et al. (1996b). Seawater was pumped continuously from water depths of I m or I m above the bottom into an equilibrator installed on board ship. Equilibration of seawater with sample air took place in a seawater/air equilibrator developed by R.F. Weiss (Scripps Institution of Oceanography, La Jolla, CA, USA). Saturation values (expressed in %, i.e. 100% = equilibrium) were calculated as the ratio of the dissolved gas and the expected equilibrium value derived from the ambient air dry mole fraction (Bange et al. 1996b). Emissions were computed with the equations of Liss and Merlivat (1986). Meteorological and hydrographic data were obtained from ship records. Salinity values are reported according to the Practical Salinity Scale 1978. Undisturbed sediment samples were collected using a multiple core sampler (Barnett et al. 1984) modified for application in shallow waters. Denitrification and methane emissions of the sediments were determined in whole cores. Estimation of denitrification was carried out with the acetylene block technique (Klemedtsson et al. 1990). Methane emission of the sediment was measured according to King and Wiebe (1978). Autotrophic nitrification was quantified indirectly in slurries of the top 0 to 0.5-cm sediment layer with the technique of Billen (1976).
4. RESULTS A N D D I S C U S S I O N 4.1. Methane
The C H 4 saturation values of surface waters (1-m water depth) showed large spatial variability ranging from 102 to 16,300%. The highest saturation was observed near the mouth of the Peene River. Saturation decreased seawards along the salinity gradient. In contrast to a conservative mixing behavior, the observed methane distribution was not a result of dilution with Baltic Sea water, as shown by the plots of CH 4 concentration versus salinity (Fig. 2). Thus, we contend that the distribution of dissolved methane can be interpreted as a result of sedimentary processes in the Boddens. This is supported by the observation of a significant vertical concentration gradient of methane which was documented in the Kleines Haft (western Oder estuary) during a period of water-column stratification in summer 1994. Below a steep temperature gradient, methane saturation showed values reaching 19,300%. After the wind speed had increased from 2.5 to 8 m s-1, CH 4 concentrations showed similar values at the surface and near the bottom (1200-1400%), reflecting mixing of the water column. Aerobic incubation of sediment cores showed emission of methane. The water column is usually oxygenated, and on average 86% (range 50-100%) of the methane formed in the sediment is oxidized at the sediment surface, as suggested by aerobic and anaerobic incubations of sediment cores. Therefore, methane is largely not lost
141
Methane and nitrous oxide in Bodden waters
by diffusion. Highest rates of methane formation (range 1000-10,000 l~mol m -2 h -1) were detected in interface areas such as the mouth of the Peene River and the outlet of the western Oder estuary. This may be a consequence of high loads of fresh organic carbon supplied by river runoff as well as by the lysis of organisms resulting from shifts in salinity following mixing processes. In contrast, in the Kleines Haff only 557 lamol m -2 h -1were detected on average (range 1-2750 ~mol m -2 h - l ) . Similarly, Heyer and Berger (1996) reported methane production rates of 20-5700 ~mol m ~ h -1 in the sediments of Bodden waters, with 3-96% of the methane formed being oxidized in the sediment.
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However, the results of our incubation experiments cannot be directly compared with the field measurements of dissolved methane because the data sets were not obtained at the same date. In addition, the accumulation of methane in the water column may be counteracted by dilution caused by water mixing processes. Therefore, active sedimentary CH~ release does not necessarily cause high methane enrichment in the water column. High CH 4 saturation levels (mean 1700%, range 200-14,000%) were observed in the Rassower Strom. With a mean salinity of 8.5%~ the Rassower Strom has the most
142
Dahlke et al.
saline Bodden waters, caused by intensive water exchange with the Baltic Sea. Thus, we speculate that the reason for these high CH 4 concentrations lies in an abundant supply of organic carbon to the sediment caused by the accumulation and subsequent lysis of drifting algae, especially Pilayella littoralis. Ephemeral macroalgae were indeed observed to accumulate at this sampling station. Decomposition of algal mats favours the coexistence of sulfate reduction and methane production in estuarine sediments, and stimulates methane release from the sediments (J. Heyer, Fraunhofer-Institute for Atmospheric Environmental Research, pers. comm.). Enrichment of labile organic matter in sediments of brackish waters generates only minor competition for substrates (e.g., acetate) between sulfate-reducing and methane-producing bacteria, a finding also documented for Danish fjords (Holmer & Kristensen 1994). Methane saturation of the water varied seasonally. Maximum values were measured in late summer, corresponding to maximum CH4 release rates by the sediments. Higher microbial activity favoured by seasonally controlled carbon supply to the sediments and warmer water temperatures (16-17~ may have been responsible for this pattern. Furthermore, high CH 4 saturation values (range 4700-11,000%) were observed in the plume of the Peene River in spring (Fig. 3). This could be explained by high riverine loads of organic carbon and allochthonous methane in this season. DZB
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Figure 3. Methane saturation values for all cruises combined in June/July 1994, September 1996, December 1996 and March/April 1997. DZB: Darss-Zingster Bodden, NRB: Nordrtigensche Bodden; GB: Greifswalder Bodden; PS: Peenestrom; KH: Kleines Haft.
143
Methane and nitrous oxide in Bodden waters
Methane saturation in the Rassower Strom underwent a diurnal rhythm, whereby concentrations increased twofold at night (Fig. 4). This might be due to different rates of CH 4 oxidation in the sediment, resulting from the absence of oxygen production at the sediment surface by microphytes at night. Microphytobenthic photosynthesis controls the thickness of the oxic sediment layer in the Rassower Strom (Gerbersdorf et al. 2000).
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date, local time Figure 4. Diurnal rhythms of saturation concentrations of methane, nitrous oxide and oxygen as well as water temperature and salinity for the Rassower Strom station in the period 01-02 October 1996 at 0.5-1.0 m above the bottom (3.0-3.5 m water depths).
Diurnal variations of methane concentrations may also be caused by corresponding shifts of water temperature. In this case, fluctuations in water temperature parallel shifts of methane release from the sediments, as was documented by Heyer and Berger (1996) at shallow sites in Bodden waters. This pattern was, however, restricted to near-shore areas with high methanogenic activity favoured by the accumulation of macrophytal biomass. Because methane release was mainly by ebullition, these authors suggested that the decrease of methane solubility in porewaters at higher temperatures was responsible for the higher methane release. At sites with
144
Dahlke et al.
lower methanogenic activity, the same authors detected increased methane emissions at night (J. Heyer, Fraunhofer-Institute for Atmospheric Environmental Research, pers. comm.). Similarly to our own interpretation for the Rassower Strom, Heyer and Berger (1996) assumed that microphytobenthic oxygen production controls sedimentary methane oxidation at these sites. In the Rassower Strom, diurnal variations of methane saturation were observed both in spring and late summer, i.e. independently of the different CH 4 saturation related to the season. The biologically determined shift of methane concentrations was obscured by water renewal, as indicated by the alteration in salinity (Fig. 4). A preliminary estimate of the methane emissions to the atmosphere gives an annual CH 4 flux of 960 metric tons, approximately 13% of the annual emission for the entire Baltic Sea (Table 1). Thus, we conclude that the Bodden waters act as a significant source of methane in the region.
Table 1. Estimated annual emissions of CH 4 and N20 from the Bodden waters. Locality
Nordr~igensche Bod. Greifswalder Bod. Peenestrom Kleines Haff Sum Bodden waters (total) Baltic Sea
Area a
Mean flux density
[km2l
[pmol s -1 m -2]
159 510 164 277 1110 1542 384700
CH4
N20
2426 358 2406 1486
4
22 46 37
Emissions
b
[metric tons year -1] CH4 194 92 199 208 693 960 7336 r
N20 0.8 16 11 14 42 58 -2693 c
a Data extracted from Lampe (1994) Emissions - flux density multiplied by area c Bange et al. (unpubl.), February and J u l y / A u g u s t 1992
b
4.2. Nitrous oxide
The N20 saturation in the surface waters ranged from 90-309%. Maximum N20 saturation was observed in the plume of the Peene River (Fig. 5). N20 concentrations decreased seawards along the salinity gradient. However, no correlation was found between salinity and N20 concentration in this case (Fig. 6). The sediments were identified as being both a source and a sink for nitrous oxide. M u d d y sediments become a sink when stagnation of the water column prevents aeration of bottom waters. Generally, we observed no marked difference between saturation at the surface (1 m) and at the bottom. However, slightly higher saturation levels were sometimes documented in the bottom waters, this occurring when stratification of the water column was not coupled with oxygen depletion at the bottom (Table 2).
Methane and nitrous oxide in Bodden waters
DZB
3s~1
NRB
GB
145
PS
KI. Haft
3OO ,---, 200
C
0 (~
150
H
L_
El
B
E!
H
D
I
u~ 100 0
"1
= ml I
B
~
! []
~ H i
=
lol
" O4
Z 50
month
Figure 5. Nitrous oxide saturation values for all cruises combined in June/July 1994, September 1996, December 1996 and March/April 1997. DZB: Darss-Zingster Bodden; NRB: Nordr~igensche Bodden; GB: Greifswalder Bodden; PS: Peenestrom; KH: Kleines Haft. 50-
4O O
E c
,_._,
30 []
O z "o ~t > i o W ,i
20
10
"o
I
0
'
I
2
'
I
4
'
I
'
6
I
8
'
I
10
'
I
12
salinity [PSU]
Figure 6. Relationship between nitrous oxide concentration and salinity for the cruises in March and April 1997. Spearman rank order correlation coefficient r = -0.0969, n = 171, p = 0.207 (not significant).
146
Dahlke et al.
Table 2. Concentrations of N20 during stratification at Kleines Haft, March 13 1997. Depth
I m below surface I m above bottom
Salinity [PSU]
Water temperature [~
N20 concentration [nmol 1-']
2.3 5.3
5.6 4.3
19.3 25.8
Incubation of sediment cores showed both consumption (0.2 ~mol N m "2 h -1) and release (up to 0.7 ~mol N m -2 h -1) of N20. Surprisingly, no correlation was found between nitrification or denitrification in the sediment cores and N20 formation. This might be caused by the high spatial variability of the data, associated with the fact that these measurements were carried out in different sediment cores. Highest N~O formation occurred in sediments of the outer Bodden waters, namely in the Greifswalder Bodden and not in the inner Oder estuary. A significant correlation was found (Spearman rank order correlation coefficient r = 0.83, n = 94, p<0.0001) between N20 saturation and nitrate concentration in the water. Both nitrate concentration and N20 saturation showed the same seasonal variations, characterized by concentration maxima in early spring caused by river runoff. On the basis of these data, we hypothesize that, in the inner Bodden waters, high riverine loads of nitrate (which fuels denitrification) and allochthonous N20 might cause enhanced concentrations of N20. As denitrification rates are lower in the outer Bodden waters, nitrification may be the prevailing source of nitrous oxide, especially in the summer. Nitrification is not restricted to the sediments, with 20-70% of the activity being located in the water column. Following a storm event, the enhanced N20 concentrations (up to 131%) which were observed in July 1994 in the Greifswalder Bodden and the Peenestrom were paralleled by particularly high nitrification activities in the water column. There is, however, no direct indication in the literature that nitrification may be a significant source for N20 in estuarine waters. Svensson (1998) did not find a correlation between denitrification and N20 release in limnetic sediments, which led him to assume that nitrification was the N20-forming process. In other studies, denitrification is associated with N~O release in estuarine waters. Robinson et al. (1998) and Olgivie et al. (1997) found a correlation between the rate of N20 release and denitrification activity which was related to nitrate concentrations in the water. However, direct evidence identifying either nitrification or denitrification as the main N20 forming process in estuarine waters has to date not been presented. This is due to a difficult methodological problem arising from the potential association of the two processes. The use of inhibitors (usually acetylene and 2-chlor6(trichlormethyl)-pyridine) or even the addition of nitrogen tracers (lSNO3-, ~SNH4§ may result in artifacts if sedimentary nitrification is the main nitrate source for
Methane and nitrous oxide in Bodden waters
147
denitrification. It is almost impossible to produce a specific effect on only one process without influencing the other. N20 saturation of the Bodden waters was low compared to other estuaries (Bange et al. 1996a; Robinson et al. 1998). However, it must be pointed out that riverine nitrogen loads are strongly influenced by precipitation. Consequently, spring nitrate concentrations can vary markedly from year to year. N20 formation rates and emissions of nitrous oxide might vary in the same manner. In contrast to the open Baltic Sea, the Bodden waters generally act as a source of N20 to the atmosphere (Table 1). A preliminary estimate gives an annual emission of 58 metric tons.
5. CONCLUSIONS The Bodden waters in the coastal zone of the southern Baltic represent an important source of atmospheric methane for the region. CHa concentrations showed a marked spatial and seasonal variability. Both internal processes, i.e. methanogenesis and methane oxidation, and external sources such as riverine loads determine methane concentrations in these waters. It is suggested that methane formation in other eutrophic lagoons of the Baltic Sea may constitute an important source of atmospheric methane in these regions as well. The Bodden waters are also a source of atmospheric N20 , especially in spring. The present data identified external sources, e.g., the nutrient-rich Peene River, as "hot spots' for N20. More research is necessary to quantify the overall riverine load of N20 in the Bodden waters, and to distinguish between the two internal sources of N20 formation, i.e. nitrification and denitrification.
ACKNOWLEDGEMENTS
We thank the crew of the R/V Prof. Fritz Gessner for their helpful assistance. The study formed part of the interdisciplinary project Greifswalder Bodden und Oder~istuar-Austauschprozesse (GOAP-Greifswalder Bodden and Oder Estuary Exchange Processes). It was supported financially by the German Bundesministerium ffir Bildung, Wissenschaft, Forschung und Technologie (BMBF-Federal Ministry for Education, Science, Research, and Technology, Grant No. 03F0095C), and by the Max Planck Society.
REFERENCES
Bange, H.W., Bartell, U.H., Rapsomanikis, S. & Andreae, M.O. (1994) Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Glob. Biogeochem. Cycles 8: 465-480.
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Bange, H.W., Rapsomanikis, S. & Andreae, M.O. (1996a) Nitrous oxide in coastal waters. Glob. Biogeochem. Cycles 10: 197-207. Bange, H.W., Rapsomanikis, S. & Andreae, M.O. (1996b) The Aegean Sea as a source of atmospheric nitrous oxide and methane. Mar. Chem. 53: 41-49. Barnett, P.R.O., Watson, J. & Cornelly, D. (1984) A multiple corer for taking virtually undisturbed samples from shell bathyal and abyssal sediments. Oceanol. Acta 7: 339-409. Billen, G. (1976) Evaluation of nitrifying activity in sediments by dark ~4C-bicarbonate incorporation. Water Res. 10: 51-57. Gerbersdorf, S., Black, H.J., Meyercordt, J., Meyer-Reil, L.-A., Rieling, T. & Stodian, I. (2000) Significance of microphytobenthic primary production in the Bodden (southern Baltic Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Heyer, & Berger, (1996) Methane cycling in a brackish water ecosystem. In: Fischer, U. & Grieshaber, M.K. (eds) Processes and Structures in Marine Sulfide Biotopes. Shaker Verlag, Aachen, pp. 11-13. Holmer, M. & Kristensen, E. (1994) Coexistence of sulfate reduction and methane production in an organic-rich sediment. Mar. Ecol. Prog. Ser. 107: 177-184. King, G.M. & Wiebe, W.J. (1978) Methane release from soils of a Georgia salt marsh. Geochim. Cosmochim. Acta 42: 343-348. Klemedtsson, L., Hansson, G. &. Mosier, A (1990) The use of acetylene for the quantification of N 2 and N20 production from biological processes in soil. In: Revsbech, N.P. & Sorensen, J. (eds) Denitrification in Soil and Sediment. Plenum Press, New York, pp. 167-180. KOster, M., Dahlke, S. &. Meyer-Reil, L.A (1997) Microbiological studies along a gradient of eutrophication in a shallow coastal inlet in the southern Baltic Sea (Nordr~igensche Bodden). Mar. Ecol. Prog. Ser. 152: 27-39. Lampe, R. (1994). Die vorpommerschen K~istengew~isser- Hydrographie, Bodenablagerungen und Kfistendynamik. Die K~iste 56: 25-49. Liss, P.S. & Merlivat, L. (1986) Air-sea exchange rates: Introduction and synthesis. In: Buat-M6nard, P. (ed.) The Role of Air-Sea Exchange in Geochemical Cycling. D. Reidel, Norwall, Mass., pp. 113-127. Olgivie, B., Nedwell, D.B., Harrison, R.M, Robinson, A. & Sage, A. (1997) High nitrate, muddy estuaries as nitrogen sinks: the nitrogen budget of the River Colne estuary (United Kingdom). Mar. Ecol. Prog. Ser. 150: 217-228. Robinson, A.D., Nedwell, D.B., Harrison, R.M. & Ogilvie, B.G. (1998) Hypernutrified estuaries as sources of N20 emission to the atmosphere: the estuary of the River Colne, Essex, UK. Mar. Ecol. Prog. Ser. 164: 59-71. Svensson, J.M. (1998) Emission of N20, nitrification and denitrification in a eutrophic lake sediment bioturbated by Chironomus plumosus. Aquat. Microb. Ecol. 14: 289-299.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
149
Geochemical investigations of iron and manganese in coastal sediments of the southern Baltic Sea I. Stodian,* H. J. Black, S. Gerbersdorf, M. K6ster, L.-A. Meyer-Reil and T. Rieling
Institut fiir Okologie, Ernst-Moritz-Arndt-Universitdt Greifswald, 18565 Kloster, Germany ABSTRACT
Seasonal fluxes of iron and manganese were investigated in sediments from the Rassower Strom (Nordrfgensche Bodden, Baltic Sea, Germany). For both iron and manganese, the effective fluxes measured in incubation experiments were up to 46 times lower than the diffusive fluxes calculated from pore-water profiles, this being due to the reoxidation of Fe 2§ and Mn 2§ The effective manganese fluxes were approximately one order of magnitude higher than the effective iron fluxes. Highest fluxes of manganese were observed in a u t u m n 1996, whereas fluxes in winter and spring in 1997 were up to 18 times lower. Calculated turnover times for pore-water manganese also varied seasonally. The shortest turnover times were calculated for summer 1996 (<2 hours). If the flux of dissolved manganese is not balanced by reoxidation of Mn 2§ the manganese oxide pool is depleted in a short time (120 hours). Total solid iron content at the sediment surface was 50 times higher than the total manganese content. The same ratio was found between the reducible phases of iron and manganese.
1. Introduction
The degradation of organic matter in sediments comprises many complex processes. Sedimentary organic carbon is oxidized sequentially by 0 2, NO3-, MnO 2, Fe203, and SO42-. It is well known that this sequence is manifested by characteristic patterns in the concentrations of 0 2, NO3-, Mn 2§ Fe 2§ and SO42-in pore-water profiles (e.g., Bender et al. 1989). In contrast, carbon oxidation and the production of reduced chemical compounds by iron and manganese oxide reduction are poorly investigated. Solid reducible phase profiles allow rough calculation of iron and manganese oxide reduction in organic matter oxidation. However, the role of, amongst other things, bioturbation has to be considered in the interpretation of solid reducible profiles. Thus, Canfield et al. (1993b) demonstrated that rates of iron and manganese * Corresponding author: I. Stodian e-mail:
[email protected]
150
Stodian et al.
reduction based on the calculation of chemical gradients in pore-water profiles underestimate the actual rates of reduction. Metal oxide reduction in sediments may be coupled to the oxidation of reduced species other than carbon (e.g., sulfide). In addition, the manganese oxide might be reduced by Fe 2§ produced by iron oxide reduction (Canfield et al. 1993a; Wang & Van Capellen 1996). The distribution of different binding forms of iron and manganese is important in this context, especially the fraction available for microbial oxidation processes. The main objective of the present study was to investigate the significance of carbon oxidation by iron and manganese oxide reduction as well as the interactions between iron and manganese cycles in the sediments of shallow-water coastal inlets in the southern Baltic Sea. For this purpose, we 1) measured the concentrations of dissolved iron and manganese in the pore water, and the contents of these elements in the sediments, 2) determined the different chemical binding types of iron and manganese in the sediments, and 3) measured the fluxes of iron and manganese at the sedimentwater interface.
2. STUDY SITES The main study site was located in the Rassower Strom (water depth 4.0 m) in the outer part of the Nordrtigensche Bodden, Germany (Fig. 1). This site has sandy mud sediments with organic carbon contents of 2-4% at the sediment surface (Fig. 2). Due to severe ice coverage, the Rassower Strom site could not be reached in the winter of 1997. Therefore, this winter sampling campaign took place at a comparable location in the Kloster Loch (water depth 3 m).
iil ....... B-.~. N.
:~{, .
'g
;'~#
Baltic Sea
Figure 1. Location of the sampling sites in the Rassower Strom (A) and Kloster Loch (B) in the Nordrfigensche Bodden (Germany).
151
Iron and manganese in coastal sediments
20 i, O
.u.
40 I
60 I
[%]
TOC
fraction < 63 pm [%] 80 i
100
0
1
2
3
4
5
I
I
....I,
l
I
1
6 _
.....&
_
2-
s 8 .....
10
- - o - - s u m m e r 1996 - - o - - autumn 1996
winter 1996197 spring 1997
Figure 2. Downcore variations in the contents of sediment fractions <63 p m and total organic carbon (TOC) in the sediments in various seasons.
3. MATERIALS A N D M E T H O D S Sampling campaigns took place in summer (July) and autumn (September) in 1996, and in winter (January) and spring (April) in 1997. Undisturbed sediment cores were collected in Plexiglas tubes of 10-cm inner diameter by means of a multiple corer. In a glove box, five sediment cores were sectioned under argon atmosphere into horizons at 0-0.2, 0.2-0.5, 0.5-1.0, 1.0-2.0, 2.0-3.0, 4.0-5.0, 6.0-7.0, and 9.0-10.0 cm. Iron and manganese contents were determined in the sediment fractions <63 p m under anaerobic conditions according to the BCR procedure (Community Bureau of Reference, Fig. 3) as described by Peula-Lopez (1995). After centrifugation (4.000 rpm, 30 min), the pore water was decanted, filtered through 0.45-pm cellulose acetate filters, and acidified with H N O 3 for preservation. This was carried out in a glove box to avoid oxidation of dissolved iron and manganese. The molecular diffusive fluxes were calculated from the pore-water concentration gradients by using Fick's first law of diffusion, i.e. Diffusive flux = - ~ D dC dx -~ where 9 = porosity, dC dx 1 = concentration gradient with depth, and D s = sediment diffusion coefficient according to Ullman and Aller (1982), in which D S= ~lSD where D = temperature and salinity-corrected diffusion coefficient (Li & Gregory 1974). The concentration gradients of dissolved iron and manganese in the uppermost sediment
152
Stodian et al.
layer were calculated by linear interpolation of the concentration in the bottom water (taken as sediment surface), and in the pore water of the uppermost sediment layer.
wet sediment, i
lg
]
BCR procedure
I 40 ml 0.11 moli -1HOAe 20~ shake, 16 h
I
extraction solution 1[
residue
I 40 ml 0.1 mol I-I NH2OH HC! (pH:2, HNOs) 20 ~ shake, 16 h ] ......... [extraction solution 2 ] residue
I 10 m130 % H2O 2 (pH:2-3, HNO3) 1 h, 20 ~ 1 h 85 oC, addition of 10 m130 % HzOz (pH:2-3, !~O3) 1 h, 85 ~ addition of 50 ml 1 mol !-~NH40Ac (pH: 2, HNOs)
20 ~
shake, 16 h ]
i
I extracti~ s~176
3]
residue
I determination of the remaining Fe and Mn content
Figure 3. Extraction procedure to determine different binding forms of iron and manganese in aquatic sediments.
For the measurement of Fe 2§ and M n 2§ effective fluxes, intact sediments were incubated under in situ temperatures in gas-tight Plexiglas tubes (1 liter of overlying water). The fluxes were monitored for an aerobic light-dark shift of 12 hours. After an aerobic light-dark period of 36 or 48 hours, an anaerobic period of up to 138 hours followed. The cores were kept anaerobic by flushing with nitrogen. Fluxes were calculated from variations in Fe 2+and M n 2§ concentrations in the overlying water of the sediment cores. The overlying water was stirred during sampling. The light intensity was chosen according to values measured concurrently at the sediment surface
Iron and manganese in coastal sediments
153
under in situ conditions (between 35 and 117 laE m -2 s 1 in winter 1996-1997 and summer 1997, respectively). The concentrations of iron and manganese were measured with inductively coupled p l a s m a - atomic emission spectroscopy (ICP-AES; +3% S.D.).
4. RESULTS A N D D I S C U S S I O N 4.1. Iron Total iron contents decreased slightly with increasing sediment depth (Fig. 4). In summer 1996, average total iron content was 2.6% in the sediment fractions <63 lum at the sediment surface, this value decreasing to 2.2% at a depth of 10 cm. iron [% Fe] 0
1
2
3
4
5
6
I
I
I
I
I
I
I
0.06
m a n g a n e s e [% 0.00
MnO 2]
0.01
0.02
0.03
0.04
0.05
I
I
I
I
I
0 -
2 i--i
E J= e~ 0 "0
4
e~ 0
.E_
6-
"o o
--e--
8 -
--
manganese iron
10
Figure 4. Downcore variations in the contents of total solid iron (summer 1996) and manganese (autumn 1996) in the sediments.
In the pore water, the highest concentrations (up to 45 lJmol 1-~ in autumn 1996) of dissolved Fe 2§ were recorded at 0.5 to 2.0-cm sediment depths (Fig. 5). Outside this zone, only 3.6 pmol 1~ F e 2§ w a s detected. In the pore water of the sediment incubated for about 138 hours under anaerobic conditions, the highest value of 57.3 lamol 1-1Fe2§ was measured 1 m m below the sediment surface. According to Thamdrup et al. (1994b), the diffusive flux from pore-water profiles describes the m i n i m u m transport
Stodian et al.
154
of dissolved iron or manganese into the oxic surface layer, taking bioirrigation, spatial resolution of pore-water profiles, and adsorption to the solid phase into consideration. F e 2§ gradients in the pore water implied insignificant reduction of solid reducible iron phases. It is possible that precipitation of reduced iron is coupled with manganese oxide reduction. If this occurs, it should happen rather quickly. Canfield et al. (1993b) described a mechanism whereby F e 2§ is continuously reoxidized, thus becoming available for further reduction. Iron precipitates mainly as iron oxyhydroxide after oxidation. In this case, the reduction rates calculated from pore-water F e 2§ profiles would underestimate the real values.
Fe 2+ [IJmol I "1] 0
10
20
30
4O
50
I
I
I
I
I
water sediment
~'r
2
i,..i J~ r
4 r-
E
.... "0 u)
6
10 =
s u m m e r 1996
- - o - - winter 1996197
=
autumn 1996
- - o - - spring 1997
Figure 5. Downcore variations of dissolved F e 2§ in the pore water in various seasons. The values represent discrete horizons pooled for five sediment cores in each case.
The transport of dissolved iron or manganese from the surface sediment into the overlying water (measured during an incubation experiment) was calculated as an effective flux. During incubation under aerobic light and dark conditions, iron fluxes into the sediment showed values of up to 1.4 1Jmol m -2 h -~, and fluxes into the overlying water were up to 5.0 lJmol m -2h 1. The daily fluxes (accounting for seasonal differences in the duration of light and dark phases) showed that the release rates of
155
Iron and manganese in coastal sediments
dissolved iron into the overlying water were low compared to the fluxes of dissolved manganese (Table 1). After incubation under anaerobic conditions (up to 138 hours), a release of Fe 2. into the water column was detected (maximum: 10 pmol m -2 h -1 in autumn 1996). During all seasons, the calculated diffusive fluxes of iron were up to 29 times higher than the effective fluxes.
Table 1. Oxygen penetration depths, calculated diffusive and effective fluxes of dissolved iron and manganese, and turnover times of dissolved manganese for various seasons. Positive values of effective fluxes indicate fluxes into the overlying water. Summer - July 1996; autumn - September 1996; winter - January 1997; spring = April 1997. n.d. - not determined. Season
depth Light
Effective flux
Diffusive flux
0 2 penetration
Dark
(cm)
Daily
Daily
(pmol -2 m d ~)
m -2 d - )
Light
(pmol
Turnover time Dark
(pmol -2 m h ~)
(hours)
Summer
0.35
0.18
Fe
92.4
28.6
-0.5
5.0
n.d.
Autumn
0.23
0.22
Fe
405.6
13.9
-1.4
2.5
n.d.
Winter
0.35
0.20
Fe
69.6
40.8
-1.4
1.9
n.d.
Spring
0.45
0.22
Fe
39.8
-11.0
-0.6
-0.3
n.d.
Summer
0.35
0.18
Mn
1164.0
25.2
-5.1
7.9
1.7
Autumn
0.23
0.22
Mn
3259.2
694.8
18.5
39.0
1.9
Winter
0.35
0.20
Mn
368.2
154.8
5.6
6.9
4.6
Spring
0.45
0.22
Mn
182.4
41.0
1.3
2.3
12.5
The enrichment of reducible solid iron phases in the uppermost sediment layers is a result of u p w a r d diffusive transport through the pore water and following oxidation. The BCR method allowed the extraction of, on average, 60-70% of the total iron at all seasons (Fig. 6). However, it is u n k n o w n how much of the total iron is available for microbial carbon oxidation. Considering the total iron content, 15.9% at the surface, and 2.2% at a depth of 10 cm were mobilized in the first step of the extraction procedure. In this first step, mainly carbonates, phosphates and acid volatile sulfides are extracted. The reducible part of iron (mainly iron oxyhydroxide), extracted in the second step, also decreased with increasing depth. Between 14.1% (at the sediment surface) and 0.7% (at 10-cm sediment depth) of the total iron was bound
156
Stodian et al.
to the reducible part, with a maximum at depths of 0.2 to 0.5 cm. During this season (summer 1996), the maximum value of dissolved iron in the pore water was detected at sediment depths between 1 and 2 cm. The oxidizable phase of total iron in the sediment (e.g., iron sulfide) was measured in another step of the extraction. In the uppermost layers of the sediment and at a depth of 10 cm, 38 and 56% of the total iron were detected, respectively. These data clearly demonstrate that Fe(III) reduction coincided with Fe(II) accumulation with depth in the solid phase.
Figure 6. Downcore variations in different binding forms of total solid iron (a) and manganese (b) in summer 1996 (see text for definition of different extraction steps).
4.2. Manganese Total contents of manganese decreased from the surface, where a value of 0.052% was recorded, to a sediment depth of 2 cm (Fig. 4). Below 2 cm, a small increase of total solid manganese (reaching 0.04% at a depth of 10 cm) was measured.
Iron and manganese in coastal sediments
157
The enrichment of solid manganese in the uppermost sediment layers is assumed to be the result of early diagenesis. Under anaerobic conditions, dissolved M n 2§ is released. The reduced M n 2§ diffuses to the uppermost layer of the reduced zone. Under aerobic conditions, manganese oxide is precipitated. In the pore water, the maximum value for dissolved M n 2§ w a s found in the uppermost layer of the sediment at all seasons, except in spring 1997 when the highest concentration was located at sediment depths of 0.2 to 0.5 cm (Fig. 7). The maximum seasonal values varied between 16.4 ~mol 1-1in winter 1996-1997 and 110 lumol 1-1in autumn 1996. According to Bender and Heggie (1984), Mn 2§ can diffuse up to 5 cm into the oxic zone. Release of dissolved manganese into the water column occurs when manganese oxidation in the uppermost sediment layer cannot keep up with manganese reduction supplying dissolved manganese from below (Thamdrup et al. 1994a). The concentration profiles for dissolved manganese indicate that oxidation was restricted to the uppermost 1-2 mm of the sediment. In the oxidized zone of the sediment, manganese oxides were reduced, coupled to dissolved F e 2§ oxidation. According to the model of Wang and Van Capellen (1996), the oxidation of dissolved F e 2§ by manganese oxides is responsible for the measured pore-water profiles of dissolved F e 2§ and M n 2§ The manganese fluxes are affected by oxygen concentrations in the bottom water, oxygen penetration depth, temperature, and the input of organic matter (Hunt 1983; Hunt & Kelly 1988). In addition, the effective flux of manganese is influenced by desorption of M n 2§ in the topmost sediment (Wang & Van Capellen 1996). Mn 2+ [IJmol I "1] 0
25
50 I
75 I
100 I
125 I
150
water sediment
i" 2
4
8
10 --e--
s u m m e r 1996
- - o - - winter 1996197
=
a u t u m n 1996
--o--
spring 1997
Figure 7. Downcore variations of dissolved M n 2§ in the pore water in various seasons. the values represent discrete horizons pooled for five sediment cores in each case.
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In our incubation experiments, the Mn 2§ release increased up to six times during the anaerobic phase, compared to the aerobic dark phase (maximum: 39 ~mol m -2 h 1 in autumn 1996; Table 1). The aerobic light phase showed either a decrease or a slight increase of Mn 2§ detected in the overlying water. In spring 1997, the daily flux was roughly 17 times lower than in autumn 1996. The net release of dissolved manganese from sediments with a thin oxic surface layer leads to precipitation of manganese oxides in the water column. Due to lateral transport of manganese oxides in the water column and subsequent sedimentation, an enrichment of manganese in more oxidized sediments is to be expected (Sundby et al. 1981; Canfield et al. 1993b). The calculations of manganese turnover times were based on pore-water concentration profiles, effective fluxes, and oxygen penetration depths in the sediments (after Thamdrup et al. 1994a). The results showed very short turnover times for pore-water manganese of less than two hours in summer 1996 and up to 12 hours in spring 1997 (Table 1). At the study site, oxygen penetration depths varied between 1.8 (summer 1996, dark) and 4.5 mm (spring 1997, light). Calculations have shown that the manganese oxide pool is depleted in a short time (120 hours) if the flux of dissolved manganese is not partly balanced by reoxidation of Mn 2§ Canfield et al. (1993b) contend that manganese can undergo hundreds of cycles of oxidized and reduced phases before being ultimately buried in the sediment, playing a role as an intermediate oxidant of the redox cycle of iron and oxygen (Aller 1994). Between 60 and 80% of total solid manganese was extracted by the BCR method (Fig. 6). In the first step of the extraction, mainly carbonates, phosphates and acid volatile sulfides of iron and manganese are extracted. In the extract solution, there is the possibility of reaction between the extracted dissolved iron by manganese oxides. However, between 34 (at the sediment surface) and 14.5% (at 10-cm depth) of total manganese were extracted in this first step. In the second step, the reducible phase was measured (between 7.6 and 9.5% of total manganese). It is not known how much of this manganese (first and second steps) is available for microbial reduction. In the third step of the extraction, the oxidizable phase was detected. Between 25 (in the uppermost sediment layers) and 57% (at a depth of 10 cm) of the total manganese was calculated as oxidizable phase. Bioturbation is the main transport mechanism for particulate iron and manganese (Aller 1990; Canfield et al. 1993b), and this process also shows seasonal fluctuations. The iron and manganese oxides, and the solid reduced forms of iron and manganese will be transported downwards and upwards into the sediment to be reduced and oxidized, respectively. The gradient of reactive manganese detected in winter 1996-1997 was 10 times shallower than the one recorded in autumn 1996, and 4 times shallower than the gradient in summer 1996 (Table 2). This indicates a seasonality of the manganese cycle at the Rassower Strom site, showing the highest manganese oxide reduction rates and the steepest gradient of reactive manganese in autumn 1996, as predicted by the assessment of the pore-water profiles (Fig. 7).
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Table 2. Mn oxides reduction rates for various seasons, n.d. = not determined. Mn reduction rate (mmol m -2d-')
Gradient (lamol cm -4)
Summer 1996
1.16
0.21
A u t u m n 1996
3.26
0.52
Winter 1996/1997
0.37
0.05
Spring 1997
0.18
n.d.
Season
5. CONCLUSIONS Incubation experiments for the Rassower Strom showed that the effective fluxes of dissolved manganese into the overlying water were clearly higher than the fluxes of dissolved iron (Fig 8). This can be explained by reduction of manganese oxides coupled to Fe 2§ oxidation. Manganese showed fast recurrent cycles of oxidized and reduced phases with fairly short turnover times in the pore water (<2 hours in summer 1997). In manganese oxide-rich sediments, manganese reduction is supposed to be the main anaerobic carbon oxidation pathway. Due to a low content of manganese oxide, the carbon oxidation by manganese oxide reduction was insignificant at our study site. The main part of manganese oxide reduction seems to be a result of oxidation of dissolved iron in the present case.
Figure 8. Relationships between the contents and fluxes of iron and manganese.
ACKNOWLEDGEMENTS This study forms part of the interdisciplinary project OKOBOD (Okosystem Boddengew/isser - Organismen und Stoffhaushalt), supported by the BMBF (Bundesministerium ffir Bildung und Forschung).
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REFERENCES
Aller , R.C. (1990) Bioturbation and manganese cycling in hemipelagic sediments. Phil. Trans. Roy. Soc. London A 331" 51-68. Aller, R.C. (1994) The sedimentary Mn cycle in Long Island Sound: its role as intermediate oxidant and the influence of bioturbation, 02, and Co~gflux on diagenetic reaction balances. J. Mar. Res. 52: 259-295. Bender, M. & Heggie, D.T. (1984) Fate of organic carbon reaching the deep sea floor: a status report. Geochim. Cosmochim. Acta 48: 977-986. Bender, M., Jahnke, R., Weiss, R., Martin, W., Heggie, D.T., Orchardo, J. & Sowers, T. (1989) Organic carbon oxidation and benthic nitrogen and silica dynamics in San Clemente Basin, a continental borderland site. Geochim. Cosmochim. Acta 53: 685-697. Canfield, D.E., Jorgensen, B.B., Fossing, H., Glud, R., Gundersen, J., Ramsing, N.B., Thamdrup, B., Hansen, J.W., Nielsen, L.P. & Hall, P.O.J. (1993a) Pathways of organic carbon oxidation in three continental margin sediments. Mar.Geol. 113: 27-40. Canfield, D.E., Thamdrup, B. & Hansen, J.W. (1993b) The anaerobic degradation of organic matter in Danish coastal sediments: iron reduction, manganese reduction, and sulfate reduction. Geochim. Cosmochim. Acta 57: 3867-3883. Hunt, C.D. (1983) Variability in the benthic Mn flux in marine ecosystems resulting from temperature primary production. Limnol. Oceanogr. 28: 913-923. Hunt, C.D. & Kelly, J.R. (1988) Manganese cycling in coastal regions: response to eutrophication. Estuar. Coast. Shelf Sci. 26: 527-558. Li, Y.-H. & Gregory, S. (1974) Diffusion of ions in sea water and deep-sea sediments. Geochim. Cosmochim. Acta 38: 703-714. Peula-Lopez, F.J. (1995) Bindungsformen von Schwermetallen in anoxischen FluBsedimenten: Ein Vergleich verschiedener Extraktionsverfahren. Heidelberger Beitr~ige zur Umwelt-Geochemie 3. Sundby, B., Silverberg, N. & Chesselet, R. (1981) Pathways of manganese in an open estuarine system. Geochim. Cosmochim. Acta 45" 293-307. Thamdrup, B., Glud, R.N. & Hansen, J.W. (1994a) Manganese oxidation and in situ manganese fluxes from a coastal sediment. Geochim. Cosmochim. Acta 58: 2563-2570. Thamdrup, B., Fossing, H. & Jorgensen, B.B. (1994b) Manganese, iron, and sulfur cycling in a coastal marine sed!ment, Aarhus Bay, Denmark. Geochim. Cosmochim. Acta 58: 5115-5129. Ullmann, W.J. & Aller, R.C. (1982) Diffusion coefficients in nearshore marine sediments. Limnol. Oceanogr. 27: 552-556. Wang, Y. & Van Capellen, P. (1996) A multicomponent reactive transport model of early diagenesis: application to redox cycling in coastal marine sediments. Geochim. Cosmochim. Acta 60: 2993-3014.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Phosphorus in sediments from coastal waters of MecklenburgVorpommem (southern Baltic Sea) S. Berghoff,* G. Schlungbaum and U. Selig
Institute of Applied Ecology, Department of Biology, Rostock University, D-18051 Rostock, Germany ABSTRACT
The aim of this study was to investigate the seasonal and spatial gradients in the contents of phosphorus and its bound forms in sediments of the Rassower Strom and the Kirr-Bucht (southern Baltic Sea). Sediment cores were collected in summer and autumn in 1996, and they were cut into layers I cm thick. With 9-11 g P / m 2, the KirrBucht showed a generally larger phosphorus pool. In overlaying waters, the effective phosphorus potential depends on the content of phosphorus as well as biochemical reactivity and sediment mobility. Biochemical reactivity was assessed by phosphorus fractionation in the present study. The contents of phosphorus easily available to algae were very low, with values of only 1% being recorded at both study sites. The phosphorus potentially available to algae in the Rassower Strom decreased from about 40% at the surface to 15-20% of total phosphorus at a depth of 10 cm. In the Kirr-Bucht, the values were 30-50% at the surface, and 7-14% of total phosphorus at a depth of 10 cm. Pore-water analyses show that, in the Rassower Strom, orthophosphate and total dissolved phosphorus concentrations were 10 times higher than values recorded in the Kirr-Bucht. The exchange between the pore water and the overlying water was characterized by sediment resuspension at these study sites.
1. INTRODUCTION Phosphorus is one of the main controlling nutrients for primary production in ecosystems. The structure and dynamics of sediments play an important role in the phosphorus cycle. Phosphorus availability to algae in the overlying water depends both on its accumulation in sediments and its release from sediments (Nixon 1981). As a result of accumulation, sediments of coastal waters are sinks of phosphorus (Balzer 1986; Sundby et al. 1992), determining its transport into the sea (Ruttenberg 1993). Thus, the evaluation of the levels of certain bound forms of phosphorus as well as of phosphorus reactive elements is of crucial importance in determining potential * Corresponding author: S. Berghoff Fax: ++3814982011
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phosphorus release under a given set of local environmental conditions (Ruttenberg 1992; Kim & Lee 1993; Jensen & Thamdrup 1993; Jensen et al. 1995). Besides chemical and biological processes, sediment resuspension can also play a major role in phosphorus release in shallow coastal ecosystems. Demers et al. (1987) and Arfi et al. (1993) have reported wind velocities >4 m s 1 as a cause of resuspension in estuaries and lagoons. This process leads to the release of nutrients from the pore water (Floderus & Hakanson 1989). In addition, an intensification of the turnover of microbial substances in sediments has been observed under such conditions (Sloth et al. 1996).
2. STUDY AREA
The Darss-Zingst Bodden and the North Ri,igen Bodden are two shallow coastal ecosystems along the coast of Mecklenburg-Vorpommern in the southern Baltic Sea. Kirr-Bucht is a shallow inner bay in the Darss-Zingst Bodden chain with an average depth of 0.7 m. This site is strongly influenced by nutrient inputs from the mainland. The Rassower Strom has a water depth of 4 m, and it is situated at the inlet of the North R~igen Bodden system. This site is therefore more influenced by the open Baltic Sea. Altogether, Bodden waters cover an area of 1542 km 2 (Lampe 1994), and receive a riverine input of 252 t P a 1 from the Mecklenburg-Vorpommern mainland (Behrendt 1996). There is a marked seaward-increasing salinity gradient in the North Ri~gen Bodden, values varying from I PSU in the inner sectors to 9 PSU in the inlet.
3. MATERIALS A N D METHODS 3.1. Sample collection Sediment cores were collected in summer 1996 (01.07.-12.07.) and in autumn 1996 (30.09.-11.10.). The cores were cut into the following horizons: 0-0.2 (surface layer), 0.2-0.5, 0.5-1, 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-8, 8-9, and 9-10 cm. The cores were taken at each site, and samples from given depths were mixed prior to analysis. 3.2. Sediment parameters The sediments were dried at a temperature of 105~ to determine dry weight (DW) as % of wet weight (WW; Schlungbaum 1979). Organic matter content (% DW) was determined as loss on ignition (LOI) after combustion at 550~ (Hirota & Szyper 1975). Sediments were wet-sieved through a 0.063 m m sieve to separate m u d from sand. Both size fractions were dried at 100~ for 4 hours and weighed. Total phosphorus (TP) was determined as the HCI-soluble proportion from the residues on combustion by using the method described by Andersen (1976). Concentrations of organic matter (and TP) were calculated using the equation
Phosphorus in sediments
163
SD = 1/[(1-%DW/100)+(%DW/100)/(2.6*(1-%LOI/100)+1.05*%LOI/100)] where SD = sediment density. Following complex formation with chromazurol-S in acetate buffer, the aluminium content was measured photometrically at 560 nm (Dunemann & Schwedt 1984). 3.3. Pore water
The pore water was obtained by centrifugation and filtration. The orthophosphate concentration of the pore water was determined photometrically by using the molybdenumblue method in a flow-through system at 660 nm (Malcolm-Lawes & Koon 1990). The determination of the total dissolved phosphorus was based on an acid hydrolysis under UV irradiation, with detection of resulting orthophosphate photometrically in a flow-through system at 660 nm after reaction with molybdenumblue (Nakamura et al. 1980). 3.4. Phosphorus fractionation
The extraction method of Psenner et al. (1984) was used to determine the different bound forms of phosphorus in the sediments (Table 1).
Table 1. The first four steps in the extraction procedure of Psenner et al. (1984), and the corresponding P-fractions (SRP - soluble reactive phosphorus; NRP = nonreactive phosphorus). Step Extraction solvent P-fraction
Bounding forms
1
H20 (distilled water)
H20-P
Soluble phosphorus in pore water, unstable at the surface, adsorbed phosphates, phosphates available to algae
BD (0.11M) (bicarbonatedithionite)
BD-SRP
Soluble phosphates under reducing conditions, iron-bound phosphorus
NaOH (1M) (natriumhydroxide)
NaOH-SRP
Sorptive-bound phosphorus
NaOH-NRP
Organic-bound phosphorus in microorganisms and detritus, phosphates bound to humic substances
HC1-SRP
Carbonate-bound phosphorus, apatitebound phosphorus
HC1-NRP
Acid-sensitive organic phosphorus
HC1 (0.5M) (hydrochloric acid)
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The extraction method of Psenner et al. (1984) has been applied in limnology, and has been proven to be suitable for m u d d y coastal waters, whereas the method for marine sediments (Ruttenberg 1992) is suited for sandy sediments. The method by Psenner et al. (1984) was also used for coastal waters in modified form by Kim & Lee (1993) and Jensen et al. (1995). Different solvents used in a certain sequence allow assessments of the binding sites of phosphorus (Table 1). The soluble reactive phosphorus (SRP) is determined in the extraction agent. The total phosphorus is determined by means of acid hydrolysis under UV irradiation. The non-reactive phosphorus (NRP) is defined as the difference between the total phosphorus and the soluble reactive phosphorus.
4. RESULTS A N D D I S C U S S I O N 4.1. Sediments
In the Rassower Strom, the sediments had dry weights of 18-40% WW in summer, and of 17-40% WW in autumn. At the Kirr-Bucht site, in contrast, dry weights were considerably higher, showing values of 58-74% WW in summer, and 47-75% WW in autumn. Organic matter contents of sediments were low in the Kirr-Bucht, varying from about 2% DW in summer to 1.2-3.9% DW in autumn. In comparison, organic matter contents were up to ten times higher in the Rassower Strom, the values ranging from 7-17% DW in summer to 8-14% DW in autumn. Comparison of organic matter concentrations (g LOI per m 2 in the 0.2-5 cm sediment layer; Table 2) showed that the difference between the 2 study sites was considerably smaller in this case. In summer in the Rassower Strom, organic matter concentration in the 0.2-5 cm horizon was about 50% higher than that recorded in the sediments of the Kirr-Bucht, and only 15% higher in autumn (Table 2). These results show that the sediments of the Rassower Strom accumulated more organic matter than did the sediments of the Kirr-Bucht.
Table 2. Organic matter concentrations and total phosphorus concentrations (mass per m 2 in the 0.2-5 cm layer) in sediments at both study sites (calculated by using site-specific sediment densities; LOI = loss on ignition, TP - total phosphorus). Season
Site
Sediment density [g/cm 3]
LOI [ g / m 2]
TP [ g / m 21
Summer 1996
Rassower Strom
1.15
1628.95
6.12
Kirr-Bucht
1.68
1101.49
9.10
Autumn 1996
Rassower Strom
1.15
1561.23
5.20
Kirr-Bucht
1.60
1370.28
11.18
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Mud contents (proportion of the size fractions <63 l~m in the total sediment = mud dry weight-%; e.g., Delafontaine et al. 1996) were determined at both study sites in summer. The data show that the surficial sediments of the Kirr-Bucht can be classified as sand, while those from the Rassower Strom consisted of sandy mud down to a depth of 5 cm. The next layer (5-10 cm) can be classified as m u d d y sand. In the KirrBucht, mud contents remained almost constant down to a depth of 10 cm, and varied from 5 to 9% DW. In the Rassower Strom, in contrast, the values decreased with increasing depth. Mud contents reached a peak value of 65% DW in the surface layer, and then decreased to 37% DW at a depth of 10 cm. In summer, the gradients of sediment dry weight and organic matter content showed similar trends. At Kirr-Bucht, these parameters also remained relatively constant down to a depth of 10 cm. However, the gradients for the Rassower Strom did not show such clear trends. Sediment dry weight reached 27% WW at the sediment surface, decreased to 18% WW at a depth of 6 cm, and then increased to 40% WW at a depth of 10 cm. The organic content increased from 10% DW to a peak value of 17% DW at a depth of 6 cm, and then decreased to 7% DW at 10-cm depth. In autumn, gradients were observed at both sites, an increase in sediment dry weight and a decrease in organic matter content being documented downcore. The organic matter sinking down from the overlying water column accumulated in the upper layers of the sediment. Therefore, the gradient of organic content showed a decreasing trend with increasing sediment depth. Sediment dry weight increased downcore because of compaction.
4.2. Total phosphorus At both sites, the variations in phosphorus contents did not show any marked trends down to depths of 10 cm. At the Rassower Strom site, total phosphorus contents varied from 0.32 to 0.72 mg P / g DW in summer, and from 0.23 to 0.71 mg P / g DW in autumn (Fig. la, b). In the Kirr-Bucht, the values did not show such an extreme range. They varied between 0.13 and 0.22 mg P / g DW in summer, reaching a peak value of 0.33 mg P / g DW in autumn (Fig. lc, d). There was no evidence of seasonal fluctuations at the Rassower Strom site. In the Kirr-Bucht, a very high total phosphorus content of approx. 1 mg P / g DW was documented in the sediment surface layer in summer, decreasing to less than 0.2 mg P / g DW in autumn. The value for the surface layer in autumn was similar to that recorded in the 0.2-0.5 cm horizon in summer. Resuspension due to wind events (e.g., Arfi et al. 1993) results in the destruction of the surface layer. Since the Kirr-Bucht site is considerably shallower than the Rassower Strom, resuspension events take place constantly, thereby accounting for the results obtained. At the Rassower Strom, resuspension is observed only during stormy weather, explaining the higher phosphorus accumulation found in the sediments at this site. Geochemically, the highest phosphorus accumulation was found in the sediments of the Rassower Strom, being 2-3 times higher than that documented in the Kirr-Bucht. Due to differences in sediment density between the 2 sites (Table 2), the Kirr-Bucht site showed a larger phosphorus pool compared to the Rassower
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Strom, with 9 and 11 g P / m 2 having been recorded in summer and autumn, respectively. Geochemically, the highest phosphorus accumulation was observed in muddy sediments. However, due to higher sediment densities, higher phosphorus concentrations were found in sandy sediments.
Figure 1. Total phosphorus contents of sediments collected in the Rassower Strom a) in summer 1996, and b) in autumn 1996, and in the Kirr-Bucht c) in summer 1996, and d) in autumn 1996.
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167
The effective phosphorus potential in the overlying water depends on the content of phosphorus, biochemical reactivity and the mobility of the sediment. Thus, evaluation of phosphorus concentration alone is insufficient for assessing phosphorus release from the sediments into the water column. The typical downward-decreasing phosphorus gradient described by Jensen et al. (1995) in Aarhus Bay, Denmark, Sundby et al. (1992) in the Laurentian Trough, Canada, and Balzer (1986) in the Kieler Bucht, Germany, was not observed in the present study. The phosphorus contents in the Kieler Bucht (0.5-1.0 mg P / g DW) are similar with the values documented in the present study. In contrast, contents of total phosphorus in Aarhus Bay and in the Laurentian Trough were between 1.6 mg P / g DW in the upper layers and 0.8 mg P / g DW in the lower layers. In comparison with our study sites, more phosphorus is accumulated in the sediments of these regions. This is not surprising, seeing that Jensen et al. (1995) investigated a site in 16 m water depth, and the site of Sundby et al. (1992) was in water depths of 300-500 m. The Kirr-Bucht and the Rassower Strom are evidently much shallower regions (water depths of 0.7 and 4 m, respectively) where resuspension is more important in controlling phosphorus enrichment. 4.3. Pore water
Pore-water analyses demonstrated that there was a marked downcore increase in orthophosphate concentrations at the Rassower Strom study site (Fig. 2a, b), whereas concentrations showed similar values in all sediment horizons in the Kirr-Bucht (Fig. 2c, d). At the Rassower Strom site, phosphorus concentrations in the pore water were higher than values recorded at the Kirr-Bucht site. Orthophosphate concentrations were low in the upper 2-3 cm of the sediments at both sites. This indicates the absence of a stable sediment-water contact zone at these sites. Evidently, exchange between the pore waters and the overlying water is not only by means of diffusion but it is influenced mainly by sediment resuspension in the present case. The deeper sediment layers showed an increase in phosphorus concentrations in the Rassower Strom. A similar trend was found in the Kirr-Bucht down to a depth of about 4 cm. At the same time, orthophosphate concentrations at the Rassower Strom site showed values which were tenfold higher than those recorded at the Kirr-Bucht site. At depths of 9-10 cm in the Rassower Strom, pore-water concentrations were 44 lumol o-PO4/1 in summer, and 42 lumol o-PO4/1 in autumn. While the phosphorus found at the Rassower Strom site was reactive at all times, the phosphorus in the Kirr-Bucht was mainly non-reactive. Approximately 50% of the phosphate could not be detected until a UV oxidative fraction had been made. This proportion must be considered as dissolved organic phosphorus, which has to be broken down by phosphatase enzymes before it is available to algae. This indicates that the sediments of the Rassower Strom have a considerably higher degree of degradation than the sediments in the Kirr-Bucht. K6ster et al. (1997) reported an increase of ~/c~-glucosidase activities with increasing sediment depth for the Rassower Strom. This trend was not found in the Kirr-Bucht, and seasonal variations were not detected here either.
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Figure 2. Concentrations of total dissolved phosphorus and orthophosphate in pore waters collected in the Rassower Strom a) in summer 1996, and b) in a u t u m n 1996, and in the Kirr-Bucht c) in summer 1996, and d) in autumn 1996.
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4.4. P-fractionation
At both sites in summer and autumn, fractionation showed that carbonate-bound phosphorus (e.g., apatite) was the most abundant of the bound forms in the sediments. According to Pettersson et al. (1988), this form of phosphorus can be considered as non-available. In the Rassower Strom, more than 50% of the phosphorus was bound to carbonates and apatite. At the Kirr-Bucht site, this value ranged from 30 to 40% in summer, increasing up to 70% in autumn. In addition to HCI-P, there is also the phosphorus which can be extracted in H20, BD and NaOH (see Table 1). These forms of phosphorus are considered more mobile and may be subject to release e.g., under anoxic conditions. The H20-P has been defined as easily available (Psenner et al. 1984). At the Rassower Strom and KirrBucht sites (Fig. 3), the amounts of phosphorus available to algae decreased with increasing depth both in summer and autumn. In the surface layer in the Kirr-Bucht in summer, a considerably higher phosphorus content amounting to 19.6 pg P / g DW was documented. In autumn, the values were higher than in summer. In general, it should be noted that, at both study sites, the easily available phosphorus made up just about 1% of the total extractable phosphorus in the sediments. BD-P and NaOH-P-fractions are defined as biologically available (Sonzogni et al. 1982; Olila et al. 1995). The BD-SRP-extract shows the phosphorus soluble after reduction from Fe-(III) to Fe-(II). The iron-bound phosphorus decreased with increasing depth at both sites in summer and autumn (Fig. 3). At the Rassower Strom site, the sediments were highly enriched in this bound form of phosphorus down to a depth of 1 cm. Under anoxic conditions, a high amount of phosphorus can be released from this bound form into the overlying water. In summer, this constituted 13-25%, and in autumn it made up 10-14% of the total phosphorus. Below a sediment depth of 1 cm, the iron-bound phosphorus became insignificant for releasing processes, as it showed levels of only 1-3% in the deeper sediment layers. In the KirrBucht, the iron-bound phosphorus did not exceed a level of 6%. The penetration depth of oxygen did not exceed 2 or 3 mm in both areas, with anaerobic conditions prevailing downcore. The NaOH-SRP-extract specifies the aluminum phosphorus and the exchangeable, sorptive-bound phosphorus (Psenner et al. 1984). Below 2 cm in summer and below 1 cm in autumn, the contents recorded in the Kirr-Bucht sediments remained constant (Fig. 3). The values found in the upper layers were higher than 50 lag P / g DW but they reached 300 pg P / g DW in the surface layer in summer. Since the sorption capacity is significantly lower under anaerobic conditions than under aerobic conditions (Schlungbaum 1982), it can be assumed that phosphorus is aluminum-bound in the deeper layers, and that a significant amount of phosphorus is sorptive-bound in the upper layers. Such bounding trends were not detected in the Rassower Strom. At this site, the NaOH-SRP was markedly higher than the level recorded in the Kirr-Bucht. The aluminium content at the Kirr-Bucht site (1-6 mg A1/g DW) was also lower than the value recorded in the Rassower Strom (3-13 mg A1/g DW). Due to this high aluminium content, it must be assumed that it was exclusively aluminium-bound phosphorus. It constituted about 10-20% of the total
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phosphorus during both study periods. At the Kirr-Bucht site in summer, the NaOHSRP-extract made up 13-28% of the total extractable phosphorus, and in autumn this level was 11-23%.
Figure 3. Downcore variations of phosphorus fractions in sediments from the Rassower Strom and Kirr-Bucht.
Phosphorus in sediments
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The NaOH-NRP-extract analyzes the main part of the organic phosphorus. In addition to the phosphorus found in microorganisms and detritus, by this method we also extracted the phosphorus bound to humic substances (Uhlmann & Bauer 1988). In the upper layers of the sediments collected in the Kirr-Bucht, more phosphorus in this bound form was found than in the lower layers (Fig. 3). Due to the high organic content of the surface layer in summer, the organic-bound phosphorus constituted 27% of total extractable phosphorus at the sediment surface. In the deeper layers, this value was only 15 to 19% in summer. However, in autumn this fraction was considerably less enriched (less than 10 ~g P / g DW), and the organic-bound phosphorus made up 3-7% of the total extractable phosphorus. At the Rassower Strom site, a slight gradient was documented. In comparison with the KirrBucht, the autumn values were considerably higher. Organic-bound phosphorus accounted for 1-9% of total extractable phosphorus in summer, and 5-13% in autumn. It must be pointed out that, in the sediments of the Rassower Strom, the total phosphorus was considerably higher than that recorded in the Kirr-Bucht. Comparing the sum of all fractions for the first three extracts, the phosphorus potentially available to algae amounted to 40% of the total phosphorus at a sediment depth of 1 cm in the Rassower Strom. Only 1.3% were easily available. In deeper layers, the phosphorus potentially available to algae decreased. At the Kirr-Bucht site, this phosphate pool reached levels of 30-50% in the upper layers, and 7-14% in the 9-10 cm layer.
5. S U M M A R Y
In the Rassower Strom, sandy mud sediments accumulated more organic matter than sandy sediments in the Kirr-Bucht. The organic matter accumulated in the upper layers. Therefore, contents of organic matter decreased with increasing sediment depth. The larger phosphorus pool (9-11 mg P / m 2) was found in sediments of the KirrBucht. Because of higher phosphorus concentrations and higher mobility (resuspension) of sediments at this site, the phosphorus release into the overlying water was higher in the Kirr-Bucht than in the Rassower Strom. There was no stable sediment-water contact zone at either site. The orthophosphate concentrations in the upper 3 cm were rather low. The sediments of the Rassower Strom showed a higher degree of degradation than the sediments of the Kirr-Bucht. Sediments in the Kirr-Bucht are more mobile due to resuspension, and no complete decomposition can be observed. Phosphorus fractionations showed that the phosphorus potentially available to algae was 30-50% in the upper layers, decreasing in the deeper layers. The phosphorus potentially available to algae accumulated in the upper layers. The potential biochemical reactivity to release phosphorus was similar at both sites, and it was high in the upper layers only.
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ACKNOWLEDGEMENTS
This study forms part of the subproject Investigations on the phosphorus cycle, in the project Ecology of the Baltic Sea Boddens - Balance of Materials and Organisms which is supported by the BMBF (Code No. 03F0162B).
REFERENCES
Andersen, J.M. (1976) An ignition method for determination of total phosphorus in lake sediments. Water Res. 16: 119-126. Arfi, R., Guiral, D. & Bouvy, M. (1993) Wind induced resuspension in a shallow tropical lagoon. Estuar. Coast. Shelf Sci. 36: 587-604. Balzer, W. (1986) Forms of phosphorus and its accumulation in coastal sediments of Kieler Bucht. Ophelia. 26: 19-35. Behrendt, H. (1996): Quantifizierung der N/ihrstoffeintr~ige aus Flut~gebieten des Landes Mecklenburg-Vorpommern. Studie im Auftrag des Landesamtes ffir Umwelt und Natur des Landes Mecklenburg-Vorpommern, 68 p. Delafontaine, M.T., Bartholom~i, A., Flemming, B.W. & Kurmis, R. (1996) Volumespecific dry POC mass in surficial intertidal sediments: a comparison between biogenic muds and adjacent sand flats. Senckenbergiana maritima. 26: 167-178. Demers, S., Therriault, J.-C., Bourget, E. & Bah, A. (1987) Resuspension of the shallow sublittorial zone of a macrotidal estuarine environment: wind influence. Limnol. Oceanogr. 32: 327-339. Dunemann, L. & Schwedt, G. (1984): Zur Analytik von Elementbindungsformen in Bodenl6sungen mit Gelchromatographie und chemischen Reaktionsdetektoren. Fres. Z. Anal. Chem. 317: 394-399. Floderus, S. & Hakanson, L. (1989): Resuspension, ephemeral mud blankets and nitrogen cycle in Laholmsbukten, south east Kattegat. Hydrobiologia 176/177: 6175. Hirota, J. & Szyper, J.P. (1975) Separation of total particulate carbon in inorganic and organic components. Limnol. Oceanogr. 20: 896-900. Jensen, H.S., Mortensen, P.B., Andersen, F.O., Rasmussen, E. & Jensen, A. (1995) Phosphorus cycling in a coastal marine sediment, Aarhus Bay, Denmark. Limnol. Oceanogr. 40(5): 908-917. Jensen, H.S. & Thamdrup, B. (1993) Iron-bound phosphorus in marine sediments as measured by bicarbonate-dithionite extraction. Hydrobiologia 253: 47-59. Kim, B.J. & Lee, C.W. (1993) Forms of phosphorus in the sediments of Masan Bay, Korea. Wat. Sci. Tech. 28 (8-9): 195-198. K6ster, M., Dahlke, S. & Meyer-Reil, L.-A. (1997) Microbial studies along a gradient of eutrophication in a shallow coastal inlet in the Southern Baltic Sea (Nordrfigensche Bodden). Mar. Ecol. Prog. Ser. 152: 27-39. Lampe, R. (1994): Die vorpommerschen K~stengew~isser- Hydrographie, Bodenablagerungen und K~stendynamik. Die K~iste 56: 25-49.
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Malcolm-Lawes, D.J. & Koon, H.W. (1990) Determination of orthophosphate in water and soil using a flow analyzer. Anal. Bd. 15: 65-67. Nakamura, T., Yamaguchi, H. & Ohashi, S. (1980) Problems on use of autoanalyzer for condensed phosphates. J. Occup. Environ. Health (Japan) 2/2: 199-205. Nixon, S.W. (1981) Remineralization and nutrient cycling in coastal marine ecosystems. In: Neilson, B.J. & Cronin, L.E. (eds), Estuaries and Nutrients. Humana, pp. 111-138. Olila, O.G., Reddy, K.R. & Harris, W.G. (1995) Forms and distribution of inorganic phosphorus in sediments of two shallow eutrophic lakes in Florida. Hydrobiologia 302: 147-161. Pettersson, K., Bostrom, B. & Jacobsen, O. (1988) Phosphorus in sediment- speciation and analysis. Hydrobiologia 170: 91-101. Psenner, R., Pucsko, R. & Sager, M. (1984) Die Fraktionierung organischer und anorganischer Phosphorbindungen von Sedimenten- Versuch einer Definition 6kologisch wichtiger Fraktionen. Arch. Hydrobiol. Beih. 30: 25-41. Ruttenberg, K.C. (1992) Development of a sequential extraction method for different forms of phosphorus in marine sediments. Limnol. Oceanogr. 37(7): 1460-1482. Ruttenberg, K.C. (1993) Reassessment of the oceanic residence time of phosphorus. Chem. Geol. 107: 405-409. Schlungbaum, G. (1979) Untersuchungen fiber die Sedimentqualit~it in den Gew~issern der Dart~-Zingster Boddenkette unter besonderer Berficksichtigung der Stoffaustauschprozesse zwischen Wasser und Sediment. Habilitationsschrift, Fachbereich Biologie, Universit~it Rostock. Schlungbaum, G. (1982) Sedimentchemische Untersuchungen in Ktistengew~issern der D D R - Teil 11: Phosphatsorptionsgleichgewichte zwischen Sediment und Wasser in flachen eutrophen Kiistengew~issern. Acta Hydrochim. Hydrobiol. 10/2: 135-152. Sloth, N.P., Riemann, B., Nielsen, L.P. & Blackburn, T.H. (1996) Resilience of pelagic and benthic microbial communities to sediment resuspension in a coastal ecosystem, Knebel Vig, Denmark. Estuar. Coast. Shelf Sci. 42: 405-415. Sonzogni, W.C., Chapra, S.C., Armstrong, D.E. & Logan, T.J. (1982) Bioavailability of phosphorus inputs to lakes. J. Environ. Qual. 11: 555-563. Sundby, B., Gobeil, C., Silverberg, N. & Mucci, A. (1992) The phosphorus cycle in coastal marine sediments. Limnol. Oceanogr. 37(6): 1129-1145. Uhlmann, D. & Bauer, H.D. (1988) A remark on microorganisms in lake sediments with emphasis on polyphosphate-accumulating bacteria. Int. Rev. Ges. Hydrobiol. 73: 703-708.
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Benthic microbial decomposition of organic matter and nutrient fluxes at the sediment-water interface in a shallow coastal inlet of the southern Baltic Sea (Nordr/igensche Bodden) T. Rieling,* S. Gerbersdorf, I. Stodian, H. J. Black, S. Dahlke, M. K6ster, J. Meyercordt, and L.-A. Meyer-Reil
Institut fiir Okologie, Ernst-Moritz-Arndt-Universiti~t Greifswald, Schwedenhagen 6, 18565 Kloster, Germany ABSTRACT
In summer and autumn in 1996, the concentration, chemical composition and microbial decomposition of organic matter in sediments were investigated in the Nordri,igenschen Bodden, shallow coastal lagoons in the southern Baltic Sea. Fluxes of DIN (dissolved inorganic nitrogen, NH 4, NO 3, NO2) and DIP (dissolved inorganic phosphorus, o-PO4) from the sediment into the overlying water were calculated based on laboratory incubations of undisturbed sediment cores. Microbial decomposition activity generally increased with increasing organic carbon concentrations, while there was a negative relationship between activity and C / N ratios. Decreasing microbial availability of organic matter with increasing sediment depth was reflected by a shift from the hydrolysis of (x-D-glycosidic bounds at the sediment surface to that of 13-D-glycosidic bounds in deeper sediment horizons. Microbial remineralization led to DIN and DIP liberation from the sediment into the overlying water. In October (autumn), nutrient release from the sediment supplied 6% of the nitrogen demand, and 9% of the phosphorus demand in the water column while in July (summer), the corresponding values were 8 and 20%. Diel variability of nutrient liberation could be explained by the activities of the benthic photoautotrophic communities.
1. INTRODUCTION The Nordri,igenschen Bodden consist of a chain of sheltered shallow-water basins. The outer parts of the Bodden are strongly influenced by their connection with the open Baltic Sea, whereas the inner parts are influenced mainly by terrestrial processes. This results in marked land-sea gradients in chemical and biological parameters in the water column. Nutrient concentrations, primary production, and * Corresponding author: T. Rieling Fax: ++3830064444
176
Rieling et al.
phytoplankton biomass all decrease from the inner to the outer parts of the Bodden, whereas salinity increases seawards. Investigations of surface sediments in the Bodden have confirmed this gradient in eutrophication. Generally, organic carbon and nitrogen concentrations, prokaryotic biomass, and hydrolytic enzyme activities increased with increasing levels of eutrophication (K6ster et al. 1997). Due to the accumulation of organic matter, high amounts of nutrients are stored temporarily in the Bodden sediments. The microbial hydrolysis of this organic matter leads to a liberation of nutrients which then become available for other chemical and biological processes. Due to the shallowness of Bodden waters, the supply of inorganic nutrients by the sediments may have an important influence on phytoplankton primary production in the region. Especially during the last decade, internal nutrient sources have become relatively more important because of a reduction in external inorganic nutrient input. Besides geochemical and hydrophysical processes, the intensity of nutrient liberation depends on the concentration and chemical composition of the organic matter accumulated in the sediments as well as on microbial activity. Organic matter quality is modified by progressive microbial degradation, leading to a shift in specific extracellular enzymatic activities which control the material available for microbial metabolism. Therefore, ratios of specific enzymatic activities are suitable indicators for the availability and composition of organic matter in sediments. The main objective of the present study was to assess the relationship between the amount and quality of organic matter as well as the microbial enzymatic activities of Bodden sediments. Furthermore, we examined the influence of the phytobenthos on nutrient exchange across the sediment-water interface, and assessed the potential importance of sediment nutrient fluxes for phytoplankton primary production.
2. M A T E R I A L S A N D M E T H O D S
The study site in the Rassower Strom has a water depth of 4 m and is strongly influenced by exchange processes with the open sea and the inner Bodden (Fig. 1). The location was chosen because of the availability of long-term data sets which allowed a detailed assessessment of local hydrographic conditions. The sediments were classified as sandy mud with organic carbon contents of up to 4% dry weight. In July (summer) and September (autumn) in 1996, sediment cores were taken using a multiple corer (Barnett et al. 1984) modified for application in shallow waters. Five cores were sectioned into sediment horizons of 0-0.2, 0.2-0.5, 0.51, 1-2, 2-3, 4-5, 6-7, and 9-10 cm. Total organic carbon (TOC) contents were determined using a Perkin Elmer CHN240C-analyser (Verado et al. 1990) after treatment with HC1 to remove inorganic carbon. The quality and chemical composition of the organic matter were assessed by determining ~/c~-D-glucosidase activity and C / N ratios. Decomposition activities of the microbial community were assessed by using fluorogenic model substrates
Organic matter decomposition and nutrient fluxes
177
(fluorescein diacetate, MUF-c~-D-glucoside, and MUF-~-D-glucoside; K6ster et al. 1997; K6ster & Meyer-Reil 1998). Sediment aliquots were centrifuged (2000 rpm) to obtain pore water for nutrient analyses following standard procedures according to Grasshoff et al. (1983) and HELCOM (1988). Nutrient fluxes were calculated by monitoring the concentrations of inorganic DIN and DIP in the overlying water of incubated sediment cores. Incubations were performed with intact sediment cores in Perspex tubes which were maintained under light and dark conditions (117 and 80 pE m -2s-1in July and October in 1996, respectively) with a periodicity of 12 hours. Experimental temperatures were set to 16 and 12~ for summer and autumn, respectively. In order to simulate field nutrient conditions, GF/F-filtered bottom water was collected from the study site for use in these experiments. Biological oxygen demand was calculated by determining the oxygen consumption of untreated and formaldehyde-treated (4% formaldehyde in the overlying water) sediment cores according to Boucher et al. (1994). Using the Redfield ratio (Redfield 1934) to calculate the phytoplankton nitrogen and phosphorus demand, the potential influence of nutrient fluxes on phytoplankton primary production was estimated for summer and autumn in 1996.
Figure 1. Locality map showing the study site (A) in the Rassower Strom on the ~outhern Baltic Sea coast of Germany (inset).
Rieling et al.
178
3. RESULTS A N D D I S C U S S I O N
In the summer (July) and autumn (September) of 1996, relatively high contents of organic carbon were recorded in the sediments of the Rassower Strom. In the upper sediment horizons, an accumulation of organic carbon of up to 4% of sediment dry weight was detected, The organic carbon content of the sediment decreased by 25% from July to September. This decline could not be explained by microbial decomposition processes alone (K6ster et al. 2000). Documentation of the study area using an underwater camera system showed that high amounts of organic particles and aggregates were being transported horizontally in the water column. Therefore, resuspension and horizontal transport processes might also be responsible for the seasonal fluctuations of organic carbon content in the surficial sediments of the Rassower Strom. In the deeper sediment horizons, refractory organic carbon accumulated, as shown by the C / N ratios which increased from the upper towards the deepest sediment horizons (Fig. 2). This interpretation was supported by high ~/~-D-glucosidase activity ratios in the deepest sediment horizons, indicating a shift from the predominant decomposition of ~-glucosidic bonded compounds at the sediment surface to that of [3-glucosidic bonded compounds in deeper sediment horizons (Table 1). Since [3-glucosidic compounds (e.g., cellulose) are generally more refractory than ~-glucosidic compounds, it seems reasonable that high [3/~-D-glucosidase ratios in the deeper sediment horizons are indicative for a more refractory organic carbon which might be more resistant against microbial attack. Likewise, Herndl (1992) interpreted high [3/c~-D-glucosidase ratios as indicator for refractory organic matter in aged, pelagic aggregates. C/N ratio 8
9
0
10 ,
I
11 ,
I
12 ,
-2
g ~"
-4
~ -6 -8
-10 July 1996
- - 0 - - October 1996
Figure 2. Downcore C / N ratios for the Rassower Strom in July and October in 1996 (error bars indicate standard deviations; n - 3).
179
Organic matter decomposition and nutrient fluxes
The Bodden sediment samples investigated by K6ster et al. (1997) also showed a dominance of carbohydrate-decomposing enzymes in deeper sediment horizons, while activity for the decomposition of easily degradable proteins was higher at the sediment surface. The high [3/o~-D-glucosidase ratio documented in the 0.5 to 1.0-cm sediment horizon in July 1996 might be explained by burrowing macrofauna disturbing the generally downward-increasing gradient in [~/o~-D-glucosidase ratios.
Table 1. Downcore variations in esterase activities and ratios of ~/(x-D-glucosidase activities in sediments of the Rassower Strom in July and October in 1996.
Sediment depth
Esterase activity [nmol cm -3h "1]
(x-D-glucosidase activity [nmol cm 3 h -1]
[3-D-glucosidase activity [nmol cm 3 h -1]
~/aD-glucosidase ratio
[cml
July 1996
Oct. 1996
July 1996
Oct. 1996
July 1996
Oct. 1996
July 1996
Oct. 1996
0.0-0.2
898.8
918.1
14.2
14.1
47.1
31.7
3.3
2.3
0.2-0.5
940.4
942.0
11.8
25.6
37.4
76.2
3.2
3.0
0.5-1.0
975.3
796.7
10.8
16.0
51.8
41.6
4.8
2.6
4.0-5.0
1130.9
557.1
6.2
2.8
26.3
14.1
4.2
5.0
9.0-10.0
106.4
283.6
2.6
2.9
12.9
14.8
5.0
5.1
Total microbial decomposition activities (assessed by hydrolysis of FDA) showed no pronounced seasonal trend. Compilation of the summer and autumn data showed only a weak relationship between organic matter concentrations and hydrolytic activities in the sediment (Fig. 3a). Especially in summer 1996, the combination of low activities and high carbon concentrations was particularly noticeable in sediment horizon E (9-10 cm). Seeing that the sediments of this horizon were characterized by high clay and low water contents, the low activity levels could be due to unfavourable diffusion conditions for electron acceptors in this case. However, a positive relationship between amounts of organic matter and levels of microbial activity is generally accepted in the literature, and it has been suggested also for our study area by K6ster et al. (1997). Boschker and Cappenberg (1998) found a strong correlation between organic matter contents and enzymatic activities, comparing own and literature data for various types of sediments. Furthermore, the negative relationship (r = -0.879, p <0.05) between microbial decomposition activities and organic matter quality (C/N ratios) recorded in deeper sediment horizons in the present study (Fig. 3b) indicates a limitation of microbial decomposition activities imposed by the availability and quality of organic matter. This limitation has also been shown by Boetius and Lochte (1996) for deep-sea sediments. These authors found increasing bacterial activity and biomass in sediments cores enriched with easily available,
180
Rieling et al.
nitrogen-rich organic matter (albumin, chitin, glycine), whereas the addition of pure carbon sources (cellulose, lipids) led to only minor increases of microbial activity. 1200
"7
.E: (?
E
DO
DO B
1000 -
o E r"
800
9~-
600 -
2 ~o x:
400 -
< a LL
200 -
-
C
BO
CA~A)B OC
CO
,__.,
>.,
DO
DO
EO
EO (a) 0
6
Ee
EQ
(b)
I
I
I
I
8
10
12
14
16
8
I
i
9
10
TOC [mg cm3] Q
July 1996
11
CIN O
October1996
Figure 3. Relationships between a) microbial decomposition activities (FDA hydrolysis) and total organic carbon concentrations (TOC), and b) microbial decomposition activities (FDA hydrolysis) and C / N ratios for selected depth horizons in sediments of the Rassower Strom in July and October in 1996 (A = 0.0 to 0.2, B = 0.2 to 0.5, C = 0.5 to 1.0, D = 4.0 to 5.0, and E = 9.0 to 10.0-cm depth horizon).
In summer, the decomposition of organic matter resulted in steep gradients of ammonium and phosphate concentrations in the pore water of the sediments in the Rassower Strom, leading to high nutrient fluxes into the overlying water (Figs 4, 5). The amounts of nutrients liberated agreed well with the findings of earlier studies in the Baltic Sea and adjacent areas (cf. Koop et al. 1990). Ammonium fluxes reached a mean value of 68 lJmol m 2 h -1 in the dark, and 23 lamol m -2 h -1 in the light, corresponding to a reduction of ammonium fluxes of 65% during the light period. No liberation of nitrate or nitrite from the sediment was detected in either July or October. Phosphate fluxes accounted for 17 lamol m -2 h -1 in the dark. Compared to these values, only 10% of the phosphate flux occurred during the light period. To explain the reduction of nutrient fluxes observed during the light period, we used the Redfield ratio (Redfield 1934) to estimate the nitrogen and phosphorus demand of the microphytobenthic community at the study site. Benthic gross primary production was calculated by concurrent measurements of oxygen fluxes in incubation experiments (for more details, see Gerbersdorf et al. 2000). The results show that the decrease in ammonium flux during the light period was of the same order of magnitude as the estimated demand, implying nutrient assimilation by the microphytobenthos (cf. Bruns & Meyer-Reil 1998). In contrast, the phosphorus demand was not sufficient to explain the decrease in phosphate flux under light
Organic matter decomposition and nutrient fluxes
181
conditions, indicating other processes to be also involved in the reduction of this flux. One such process could be the deeper penetration of oxygen into the sediment under light conditions, and the chemical immobilization of phosphate (Carlton & Wetzel 1988; Rizzo et al. 1992; James et al. 1996). NH 4 [pmol ml ~ p~] 0
200 I
i
0
400 ~
9
.=
600 ..... I
800 ,
_
)
)
-2
N
-4
c
.E_
-a
if)
-8
-10
). ---0-
.
.
.
July 1996
..-. -0--
October 1996
Figure 4. Downcore NH 4 concentrations in the pore water of the Rassower Strom.
Figure 5. Fluxes of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) as well as biological oxygen demand (BOD) in the Rassower Strom (values include both dark and light incubations; error bars indicate standard deviations).
182
Rieling et al.
No significant seasonal trend was found for fluctuations in the magnitude of DIN fluxes out of the sediment but the stochiometric relationship between the oxygen and nitrogen fluxes suggests that different processes are involved in the liberation of DIN in summer and autumn. During aerobic decomposition of organic matter, approximately 6.6 Mol 02 are consumed for every atom of nitrogen released as ammonium (Redfield 1934; Koop et al. 1990). During autumn, the m e a n O 2 / N H 4 flux ratio was close to the Redfield ratio (12.8:1), explaining the DIN fluxes as the result of aerobic decomposition. During summer, the O 2 / N H a flux ratio was much lower (1.5:1), indicating that ammonium release was mainly the result of anaerobic decomposition. This interpretation is supported by the high concentrations of ammonium recorded in the pore water of the upper anoxic sediment layers in summer. During autumn, no gradients of ammonium or phosphate concentrations were found in the pore water, implying a liberation of ammonium by the heterotrophic aerobic community at the sediment-water interface. To estimate the influence of the DIN and DIP fluxes on phytoplankton primary production, we calculated the daily nitrogen and phosphorus demand of the phytoplankton, again using the Redfield ratio as well as the concurrent measurements of primary production carried out by Gerbersdorf et al. (2000) at the same study site. Sediment DIN and DIP contributions to nutrient demand in the water column were higher in summer than in autumn. In summer, nutrient fluxes from the sediment could support 8% of the phytoplankton N demand, and 20% of the phytoplankton P demand. During autumn, sediment nutrient release covered only 6% of the N demand, and 9% of the P demand in the water column. Earlier studies have found a significant coupling between benthic decomposition processes and pelagic production processes in several shallow-water systems worldwide (e.g., Koop et al. 1990; Reay et al. 1995), showing that sediments are temporarily important sources of inorganic nutrients. The present work demonstrates that sediment-water exchange processes play a major role in the nutrient and carbon budgets of the Bodden ecosytem, too.
ACKNOWLEDGEMENTS
This study forms part of the interdisciplinary project OKOBOD (Okosystem Boddengew~isser - Organismen und Stoffhaushalt) of the BMBF (Bundesministerium fiir Bildung und Forschung). The authors are especially grateful to T. Br~iggmann, M. Gau, S. Kl~iber and I. Kreuzer for their excellent technical assistance.
REFERENCES
Barnett, P.R.O., Watson, J. & Conelly, D. (1984) A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments. Oceanol. Acta 7: 339-409.
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Boetius, A. & Lochte, K. (1996) Effect of organic enrichments on hydrolytic potentials and growth of bacteria in deep-sea sediments. Mar. Ecol. Prog. Ser. 140: 239-250. Boschker, H.T.S. & Cappenberg, T.E. (1998) Patterns of extracellular enzyme activities in littoral sediments of Lake Gooimeer, The Netherlands. FEMS Microbiol. Ecol. 25: 79-86. Boucher, G., Clavier, J. & Garrigue, C. (1994) Oxygen and carbon dioxide fluxes at the sediment-water interface of a tropical lagoon. Mar. Ecol. Prog. Ser. 107: 185-193. Bruns, R. & Meyer-Reil, L.-R. (1998) Benthic nitrogen turnover and implications for the budget of dissolved inorganic nitrogen compounds in the Sylt-Romo Wadden Sea. In: G~itje, C. & Reise, K. (eds) Okosystem Wattenmeer - Austausch-, Transportund Stoffumwandlungsprozesse. Springer Verlag, Berlin, pp. 219-232. Carlton, R.G. & Wetzel, R.G. (1988) Phosphorus flux from lake sediments: effect of epipelic algal oxygen production. Limnol. Oceanogr. 33: 562-570. Gerbersdorf, S., Black, H.J., Meyercordt, J., Meyer-Reil, L.-A., Rieling, T. & Stodian, I. (2000) Significance of microphytobenthic primary production in the Bodden (southern Baltic Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam, (this volume). Grasshoff, K., Ehrhardt, M. & Kremling, K. (1983) Methods of Seawater Analysis. Verlag Chemie, Weinheim, 419 p. HELCOM (1988) Guidelines for the Baltic Sea Monitoring Programme for the Third Stage, Part 27D. Baltic Marine Environmental Protection Commission, Helsinki. Herndl, G.J. (1992) Marine snow in the northern Adriatic Sea: possible causes and consequences for a shallow ecosystem. Mar. Microb. Food Webs 6: 149-172. James, W.F., Barko, J.W. & Field, S.J. (1996) Phosphorus mobilization from littoral sediments of an inlet region in Lake Delavan, Wisconsin. Arch. Hydrobiol. 138: 245-257. Koop, K., Boynton, W.R., Wulff, F. & Carman, R. (1990) Sediment-water oxygen and nutrient exchanges along a depth gradient in the Baltic Sea. Mar. Ecol. Prog. Ser. 63: 65-77. K6ster, M. & Meyer-Reil, L.A. (1998) Enzymatischer Abbau von organischem Material in Sedimenten - Standardisierung und Anwendungsbeispiele. VAAMMethodenhandbuch-Mikrobiologische Charakterisierung aquatischer Sedimente. Oldenbourg Verlag, M~inchen, pp. 74-86. K6ster, M., Dahlke, S. & Meyer-Reil, L.-A. (1997) Microbial studies along a gradient of eutrophication in a shallow coastal inlet in the Southern Baltic Sea (Nordrtigensche Bodden). Mar. Ecol. Prog. Ser. 152: 27-39. K6ster, M., Babenzien, H.-D., Black, H.J., Dahlke, S., Gerbersdorf, S., Meyercordt, J., Meyer-Reil, L.-A., Rieling, T., Stodian, I. and A. Voigt (2000) Significance of aerobic and anaerobic mineralization processes of organic carbon in sediments of a shallow coastal inlet in the southern Baltic Sea. In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam, (this volume).
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Reay, W.G., Gallagher, D.L. & Simmons, G.M. Jr. (1995) Sediment-water column oxygen and nutrient fluxes in nearshore environments of the lower Delmarva Peninsula, USA. Mar. Ecol. Prog. Ser. 118: 215-227. Redfield, A.C. (1934) On the proportions of organic derivatives in seawater and their relation to the composition of plankton. James Johnstone Mem. Vol., University Press, Liverpool, pp. 176-192. Rizzo, W.M., Lackey, G.J. & Christian, R.R. (1992) Significance of euphotic, subtidal sediments to oxygen and nutrient cycling in a temperate estuary. Mar. Ecol. Prog. Ser. 86: 51-61. Verado, D.J., Froelich, P.N. & McIntyre, A. (1990) Determination of organic carbon and nitrogen in marine sediments using the Carlo-Erba NA-1500 analyzer. DeepSea Res. 37: 157-165.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
185
Significance of aerobic and anaerobic mineralization processes of organic carbon in sediments of a shallow coastal inlet in the southern Baltic Sea M. K6ster, a* H.-D. Babenzien, b H. J. Black, ~S. Dahlke, ~S. Gerbersdorf, ~J. Meyercordt, ~ L.-A. Meyer-Reil, a T. Rieling, ~I. Stodian, a and A. Voigt b
alnstitut fidr Okologie der Ernst-Moritz-Arndt-Universit~t Greifswald, Schwedenhagen 6, 18565 Kloster/Hiddensee, Germany blnstitut fidr Gew~sser&ologie und Binnenfischerei, Alte Fischerhidtte 2, 16775 Neuglobsow, Germany ABSTRACT
The significance of aerobic and anaerobic organic carbon mineralization was investigated in sandy m u d surface sediments of the Nordri,igensche Bodden (southern Baltic Sea) in different seasons in 1996 and 1997. By measuring oxygen penetration depths, rates of oxygen uptake, the release of manganese and iron as well as denitrification and sulfate reduction, it could be shown that sulfate reduction was the dominant respiration process in sediments sampled in summer, autumn, and winter. The ratio of the relative percentage of anaerobic to aerobic organic carbon oxidation varied between 3:1 and 15:1. In spring 1997, however, oxygen respiration was dominant. Carbon oxidation by manganese, nitrate and iron was insignificant at all times. A total benthic respiration of 0.8 to 2.3 mmol C m -2 h -1 was estimated. Between 20 and 89% of the total gross pelagic and benthic primary production was respired by heterotrophic organisms.
1. INTRODUCTION In shallow coastal waters sediments play an important role in the carbon cycle for the modification and remineralization of organic matter. Up to 50% of the organic matter production may be degraded in the bottom sediments of such areas (Jorgensen 1982, 1983). The driving forces of organic matter degradation are microbially mediated electron transfer processes which are controlled by the input of organic carbon (electron donors), and the availability of electron acceptors for organic carbon oxidation (Froelich et al. 1979). For the microbial degradation processes, a sequence of electron acceptors is used which follows increasing sediment * Corresponding author: M. K6ster e-mail:
[email protected]
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depth, decreasing redox potential, and availability of energy. When oxygen is depleted by aerobic respiration, manganese, nitrate, iron, sulfate, and carbon dioxide may serve subsequently as secondary electron acceptors for anaerobic degradation processes (Canfield et al. 1993). In coastal marine sediments, the decomposition of organic material via oxygen and sulfate respiration has been studied intensively whereas the role of other mineralization processes, in particular manganese, nitrate and iron respiration, has been given less attention (see Stodian et al. 2000). In organic-rich coastal sediments, aerobic microbial respiration processes are restricted to a narrow zone in the uppermost surface sediments. Below this oxic zone only a few millimeters thick (Mackin & Swider 1989), anaerobic degradation processes occur. In the case that oxygen consumption by microbial respiration exceeds the diffusion of oxygen from the bottom water, anaerobic decomposition of organic material may even occur directly at the sediment/water interface. In fine-grained coastal sediments, much of the sediment oxygen uptake is used to reoxidize the reduced products of anaerobic respiration (Canfield et al. 1993). Aerobic and anaerobic degradation processes lead to the release of mineralization products which diffuse into the overlying water where they become available for primary producers. Within the framework of the interdisciplinary research project OKOBOD (Bodden Ecosystem), major aerobic and anaerobic degradation processes were studied in sandy mud sediments of a shallow coastal inlet in the Nordrtigensche Bodden (southern Baltic Sea, Germany) in 1996 and 1997. The Nordrtigensche Bodden are located on the coast of Mecklenburg-Vorpommern, and form a transition zone between the open sea and the mainland. They act as filters for natural and anthropogenic terrestrial inputs. Because of the shallow water depths, the sediments play a significant role in the turnover of organic matter in these coastal lagoons.
2. MATERIALS AND METHODS 2.1. Sampling sites and sampling period Sampling campaigns took place in summer (July) and autumn (September) in 1996, and in winter (January) and spring (April) in 1997. The main sampling site, the Rassower Strom (water depth of 4 m), is located in the outer part of the Nordrfigensche Bodden and is strongly influenced by exchange processes between the inner part of the Bodden and the open sea. In January 1997 the Rassower Strom could not be accessed because of extensive ice coverage. A more readily accessible neighbouring site at Klosterloch (water depth of 3 m) was sampled instead by drilling a hole into the ice. With respect to inorganic nutrient loads, both locations are regarded as mesotrophic to eutrophic. Salinity ranged between 8.2 and 9.1 PSU at the sampling sites. Water temperatures were highest in July and September in 1996 (16 and 12~ respectively), and lowest in January and April in 1997 (0 and 6~ respectively).
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2.2. Biological and chemical measurements Undisturbed sediments were collected by means of a modified multiple corer (Barnett et al. 1984) or a hand corer in 10-cm diameter plexiglas tubes. Sediment cores were sectioned into intervals of 0.2-1.0 cm (0-0.2, 0.2-0.5, 0.5-1.0, 1-2, 2-3, 4-5, 6-7, and 9-10 cm) in a glove box flushed with argon. Sediments were classified by determining the percentage by weight of the sediment fractions <63 lum (Figge et al. 1980). For the determination of organic carbon and nitrogen contents, sediments were dried at 60~ acidified with concentrated HC1, and combusted at 950~ in a Heraeus vario-el CHN analyzer (cf. K6ster 1992). Total sediment oxygen uptake was determined from changes in oxygen concentrations in the overlying water of intact sediment cores incubated at in-situ temperature in the dark (Rieling et al. 1997). The chemical oxygen uptake rate was analysed in sediment cores to which formaldehyde had been added (Boucher et al. 1994). Respiration rates were calculated from the difference between total and chemical oxygen uptake (Canfield et al. 1993). In additional sediment cores, the oxygen penetration depth was determined at high spatial resolution by using oxygen microelectrodes in the dark (Gerbersdorf & Meyercordt 1998; Neud6rfer & MeyerReil 1998). The net release of dissolved manganese and iron into the overlying water was calculated from changes in concentrations of both elements during dark incubation of intact sediment cores (Rieling et al. 1997). Additionally, manganese and iron fluxes were calculated from pore-water profiles using Fick's first law (Thamdrup et al. 1994). Concentrations of manganese and iron in the samples were analyzed by using ICP-AES (inductively coupled plasma - atomic emission spectroscopy; Stodian et al. 2O0O). Denitrification was measured in intact sediment cores by the acetylene inhibition technique according to Klemedtsson et al. (1990; see also Louis et al. 1998). The isotope pairing technique was used to determine the source of nitrate for denitrification (Nielsen 1992). Nitrate and sulfate concentrations in the pore water were analyzed by ion chromatography (Dahlke & Htibel 1994; Voigt & Babenzien 1997). Sulfate reduction rates were measured directly by radiotracer injections of 358042-into undisturbed sediment cores (Fossing & Jorgensen 1989; Voigt & Babenzien 1997). Numbers of sulfate reducing bacteria were determined in microtiter plates using the most probable number technique (Piker 1995).
3. RESULTS A N D D I S C U S S I O N 3.1. Sediment characteristics The surface sediments of the Rassower Strom and Klosterloch sites were generally characterized by sandy mud with a relatively high organic carbon content (3.6-4.7%). Ratios of organic carbon to nitrogen varied between 7.4 and 8.9 in the uppermost surface sediments (0-0.2 cm). In the absence of bioturbation, diffusion of oxygen into the sediments was limited to the upper 1-2 mm. In July 1996, the black colour of the
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surface sediments suggested that anaerobic respiration had occurred already at the sediment surface. In April 1997, the sediments collected in the Rassower Strom showed different sediment properties. Sandy sediments with a low organic carbon content (1.3%) prevailed, and an enhanced number of macrofaunal organisms was observed at the time. The dominant organisms were polychaetes (Marenzelleria viridis), small snails (Hydrobia ulvae), and cockles (Cerastoderma lamarcki). The presence of infauna and the sandy character of the sediment might explain oxygen penetration into deeper sediment horizons (3-4 mm).
3.2. Aerobic respiration At the Rassower Strom and Klosterloch sites, total sediment oxygen consumption rates were low in summer 1996 and winter 1997 (0.3 mmol 02 m -2 h -1 during both seasons). A twofold increase in this value was measured in autumn 1996 (0.6 mmol O 2 m -2 hI), and maximum oxygen consumption rates were reached in spring 1997 (1.6 mmol O 2 m -2 hI). Generally, biological oxygen uptake (0.08-0.49 mmol 0 2 m 2 h -I) accounted for about one to two thirds of total oxygen uptake in the sandy mud sediments. In autumn 1996 the biological oxygen consumption was approximately 5 times higher than that recorded in summer and winter. In April 1997 in the sandy sediments of the Rassower Strom, respiration rates were even 20 times higher compared to values measured in summer and winter. This strong increase in benthic aerobic respiration was probably due to the presence of macrofauna. The chemical oxygen uptake contributed sometimes substantially to total oxygen consumption in the sandy mud sediments of the Rassower Strom and Klosterloch. In summer, autumn and winter, 73, 20, and 63%, respectively, of total oxygen consumption was attributed to chemical oxygen uptake. The extremely low contribution of chemical oxygen uptake rates to total oxygen consumption in April 1997 might be explained by the different sediment properties prevailing in the Rassower Strom at the time (cf. above). 3.3. Anaerobic respiration Manganese fluxes (7.6-135.8 ~mol m -~h -1) calculated from pore-water profiles were generally one order of magnitude higher than iron fluxes (1.7-16.9 ~mol m -~h-~). High fluxes of manganese and iron were recorded in summer and autumn in 1996, coinciding with low oxygen penetration depths. In winter and spring in 1997, comparatively low values were recorded. The extremely low iron fluxes might have resulted from the rapid oxidation of dissolved iron by oxygen and the precipitation of iron oxides in the oxidized surface sediments (see Hines et al. 1991). As the oxidation of iron is closely coupled with the dissolution of manganese oxides (manganese reduction), this process might be responsible for the high manganese fluxes and the low iron fluxes at the sediment/water interface. The reaction of reduced iron with sulfide (FeS and FeS2 precipitation) in deeper anaerobic sediment horizons might be an additional explanation. In marine sediments, the precipitation of iron sulfide and the microbial reduction of ferric iron coinciding with the release of dissolved ferrous iron can occur simultaneously in distinct microenvironments (Canfield 1989).
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Net release rates of manganese and iron calculated from incubation experiments were a factor 50 lower than fluxes calculated from pore-water profiles of reduced manganese and iron. The specific oxidation capacity of the oxic sediment zone was calculated from the difference between net release rates and fluxes calculated from pore-water profiles in relation to the oxygen penetration depth (Thamdrup et al. 1994). In summer and autumn, oxidation rates were 1-2 orders of magnitude higher, and caused a higher turnover of manganese whereas at lower temperatures in winter and spring, the oxidation rate of manganese was drastically reduced (Rieling et al. 1998). In the sediments of the Rassower Strom and Klosterloch, nitrate concentrations were generally less than 0.1 pM. It was only in April 1997 when macrofauna was abundant at the site that nitrate (1.1-6.6 pM) was easily detectable even in deeper sediment horizons. Mean values of denitrification rates ranged from 0.5 to 8.6 pmol N m -2 h -1, and showed a similar seasonal trend as has been previously reported for fluxes of dissolved manganese and iron. High denitrification rates occurred in summer and autumn in 1996 (3.7 and 8.6 pmol N m -2 h 1, respectively), although nitrate concentrations were less than 1 pM near the sediment surface. Low nitrate concentrations in the surface sediments indicate a rapid turnover of nitrate by microbial activities. In winter when temperature decreased to 0~ denitrification activities were very low (<1 pmol N m -2 h-l). Low denitrification rates were also measured in the sandy sediments sampled in April 1997, although nitrate concentrations were relatively high (6.6 pM). The denitrification rates measured in the sediments of the Rassower Strom in the summer and autumn of 1996 were comparable to those observed in other coastal marine environments (e.g., Zimmermann & Benner 1994; Nielsen & Glud 1996), whereas the values measured in winter and spring in 1997 were extremely low. At all sampling dates, nitrate concentrations were low in the bottom water (<4 pM), and most of the denitrification activity (more than 98%) was coupled to nitrification in the sediment (coupled denitrification). Nielsen and Glud (1996) also reported a coupled nitrification/denitrification of 60 to 100% in coastal sediments of Aarhus Bight (Denmark). Cumulative sulfate reduction rates of sediments at the sampling sites were calculated down to a depth of 10 cm assuming that, in these relatively organic-rich sediments, most of the sulfate reduction activity takes place in the upper 10-cm layer (Voigt & Babenzien 1998). In sediments of the Rassower Strom and Klosterloch sites, integrated sulfate reduction rates decreased from 0.7 and 0.9 mmol SO 4 m -2 h 1 in summer and autumn 1996 to 0.3 and 0.2 mmol SO 4 m -~h -~ in winter and spring 1997. Measurements of sulfate reduction rates in two coastal marine sediments in Denmark during winter and summer were in the same order of magnitude, and showed similar seasonal variations (Sorensen et al. 1979). Spatial variations in sulfate reduction rates were generally correlated with the number of sulfate-reducing bacteria. In the uppermost oxic layer of the sediments, however, high bacterial abundance coincided with very low rates of sulfate reduction. It can be assumed that sulfate-reducing bacteria are oxygen stressed at the sediment/water interface but that they are still able to survive under these conditions (Voigt & Babenzien 1998).
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Sediment profiles of sulfate reduction rates in summer and autumn in 1996 showed distinct maximum values of 417 and 878 nmol cm 3 d -~, respectively, at sediment depths of 0.5-1.0 cm. Below this zone, sulfate reduction rates decreased rapidly. In contrast, profiles of sulfate reduction rates in winter and spring in 1997 were characterized by relatively low values (<100 nmol cm -3 d -1) extending down to a depth of 10 cm. Sulfate concentration in the pore water varied between 1.3 and 6.9 mM at all seasons, indicating that sulfate was not a limiting factor for sulfate reduction (Lovley & Klug 1986). Relatively low sulfate reduction rates in deeper sediment layers were probably due to a decreased abundance of sulfate-reducing bacteria and a low availability of metabolizable carbon sources. The decrease of sulfate reduction rates in the winter and spring of 1997 might be explained by low temperatures and the low abundance of sulfate-reducing bacteria. Low temperatures and low concentrations of degradable organic matter as well as input of oxygen by bioturbation might be essential factors responsible for the low sulfate respiration activity measured in the sandy sediments of the Rassower Strom in April 1997. The relationship between sulfate reduction rates of selected sediment horizons and organic carbon contents indicates that the availability of organic matter was an important factor controlling sulfate reduction at the study sites, especially in summer and autumn in 1996 when sulfate reduction was enhanced at higher temperatures (16 and 12~ respectively). The upper sediment horizons, characterized by higher sulfate reduction rates, showed higher organic carbon contents whereas sulfate respiration activity was lower in deeper horizons with lower organic carbon contents. At lower temperatures in winter and spring (0 and 6~ respectively), however, no significant relationship was found. 3.4. Carbon budget for the Rassower Strom In sediments of the Rassower Strom and Klosterloch sites, rates of different organic mineralization pathways were quantified on the basis of carbon equivalents using stochiometric conversion factors according to Froelich et al. (1979). Total benthic respiration was estimated as the sum of the various degradation processes investigated in the present study. In summer, autumn and winter, oxygen respiration accounted for 6-23% of the total organic carbon oxidation in the organic-rich sediments of the study sites. Manganese reduction, denitrification and iron reduction contributed to less than 3% of total benthic respiration. Sulfate respiration was the dominant organic carbon mineralization process (74-92% of organic carbon mineralization; see Table 1). Canfield et al. (1993) reported a similar proportion of oxygen respiration (3.6-17.4%) to total organic carbon oxidation in coastal sediments of Denmark and Norway. A comparable partitioning between aerobic and anaerobic respiration (3-14% and 65-85%, respectively) was documented by Mackin and Swider (1989) in the saltmarsh sediments of Long Island Sound. These authors stated that aerobic respiration accounted for more than 20% of total organic carbon oxidation only in benthic environments characterized by (1) low overall decomposition of organic matter a n d / o r (2) intense bioturbation. In our investigations, aerobic respiration contributed 81% to
191
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total benthic mineralization in the sandy sediments sampled in April 1997 which were characterized by a low content of organic material and high macrofaunal colonization. Summing up the different degradation processes, a total organic carbon oxidation of 0.8-2.3 mmol C m -2 h "1was estimated for the sediments of the Rassower Strom and Klosterloch (Table 1). The total benthic respiration might have been underestimated in the present case because the contribution of methane production and fermentative processes was not known at the time. Indeed, Holmer and Kristensen (1994) have shown that methane production can be a significant anaerobic mineralization process in organic-rich sediments.
Table 1. Seasonal variations in the mineralization of organic matter by aerobic and anaerobic respiration (July 1996" summer; September 1996: autumn; January 1997: winter; April 1997: spring).
Oxygen respiration Mn reduction* Denitrification Fe reduction* Sulfate respiration**
[pmol [pmol [pmol [pmol [pmol
0 2 m -2 h -~]
Oxygen respiration Mn reduction* Denitrification Fe reduction* Sulfate respiration** Total respiration
[pmol C m -2 h 1] [pmol C m -2 h ~] [pmol C m 2 h -1] [pmol C m -2 h 1] [pmol C m -2 h q] [mmol C m -2 h -1]
Mn 2§m -2 h -~] N m -2h-'] Fe 2+m -2h -1] SO 4 m -2 h -1]
Rassower Rassower Strom Strom July September 1996 1996 80.0 490.0 48.5 135.8 3.7 8.6 3.9 16.9 707.0 939.0
96.0 24.3 3.6 1.0 1414.0 1.54
Klosterloch January 1997 110.0 15.3 0.6 2.9 305.0
Carbon equivalents 588.0 132.0 67.9 7.7 8.3 0.6 4.2 0.7 1878.0 610.0 2.55 0.75
Rassower Strom April 1997 1550.0 7.6 0.5 1.7 211.0
1860.0 3.8 0.5 0.4 422.0 2.29
* Calculated from pore-water profiles. ** Integrated down to a sediment depth of 10 cm.
Between 20 and 89% of the gross pelagic and benthic primary production was mineralized by the activity of heterotrophic organisms in summer and autumn in 1996, and in spring 1997. During winter in 1997, gross primary production was extremely low (0.04 mmol C m -2 h -1) under the ice cover. As benthic mineralization exceeded primary production by an order of magnitude, it can be assumed that the organic carbon for benthic respiration was supplied mainly by organic material which had accumulated in the sediments.
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ACKNOWLEDGEMENTS
The work was supported by the BMBF (Bundesministerium f~ir Bildung und Forschung) within the framework of the interdisciplinary research project Okosystem Boddengew~isser - Organismen und Stoffhaushalt (OKOBOD). The authors are grateful to the crew of the RV Prof. F. Gessner for assistance during the sampling campaigns.
REFERENCES
Barnett, P.R.O., Watson, J. & Conelly, D. (1984) A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments. Oceanol. Acta 7: 339-409. Boucher, G., Clavier, J. & Garrigue, C. (1994) Oxygen and carbon dioxide fluxes at the water-sediment interface of a tropical lagoon. Mar. Ecol. Prog. Ser. 107: 185-193. Canfield, D.E. (1989) Reactive iron in marine sediments. Geochim. Cosmochim. Acta 53: 619-632. Canfield, D.E., Jf~rgensen, B.B., Fossing, H., Glud, R., Gundersen, J., Ramsing, N.B., Thamdrup, B., Hansen, J.W., Nielsen, L.P. & Hall, P.O.J. (1993) Pathways of organic carbon oxidation in three continental margin sediments. Mar. Geol. 113: 27-40. Dahlke, S. & H~ibel, H. (1994) Stoff- und Energietransfer in den flachen K~istengew~issern der deutschen Ost- und Nordsee und Entwicklung von dazugeh6rigen MeBtechnologien und Systemen. BMBF AbschluBber. 03F0047A. Figge, K., K6ster, R., Thiel, H. & Wieland, P. (1980) Schlickuntersuchungen im Wattenmeer der Deutschen Bucht. Zwischenber. KFKI. Fossing, H. & Jorgensen, B.B. (1989) Measurement of bacterial sulfate reduction in sediments: evaluation of a single-step chromium reduction method. Biogeochemistry 8: 205-222. Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. & Maynard, V. (1979) Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta 43: 1075-1091. Gerbersdorf, S. & Meyercordt, J. (1998) Prim~irproduzierende Prozesse in Pelagial und Benthal. Verbundproj. Okosystem Boddengew~isser- Organismen und Stoffhaushalt (OKOBOD), BMBF 2. Zwischenber. Hines, M.E., Bazylinski, D.A., Tugel, J.B. & Lyons, W.B. (1991) Anaerobic microbial biogeochemistry in sediments from two basins in the Gulf of Maine: evidence for iron and manganese reduction. Estuar. Coast. Shelf Sci. 32: 313-324. Holmer, M. & Kristensen, E. (1994) Coexistence of sulfate reduction and methane production in an organic-rich sediment. Mar. Ecol. Prog. Ser. 107: 177-184. Jorgensen, B.B. (1982) Mineralization of organic matter in the sea-bed - the role of sulphate reduction. Nature 296: 643-645.
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Jorgensen, B.B. (1983) Processes at the sediment-water interface. In: Bolin, B. & Cook, R.B. (eds) The Major Geochemical Cycles and their Interactions. John Wiley & Sons, Chichester, pp. 477-515. Klemedtsson, L., Hansson, G. & Moisier, A. (1990) The use of acetylene for quantification of N 2 and N20 production from biological processes in soil. In: Revsbech, N.P. & Sf~rensen, J. (eds) Denitrification in Soil and Sediment. Plenum Press, New York, pp. 167-180. K6ster, M. (1992) Mikrobieller Abbau von organischem Material an Grenzzonenerl~iutert an Beispielen von Sedimenten der Nordsee und des Europ~iischen Nordmeeres. Ber. Sonderforschungsber. 313, 35, Universit~it Kiel. Louis, A., Wolff, C., Dahlke, S. & Meyer-Reil, L.-A. (1998) Kopplung zwischen Nitrifikation und Denitrifikation. Verbundproj. Okosystem Boddengew~isserOrganismen und Stoffhaushalt (OKOBOD), BMBF 2. Zwischenber. Lovley, D.R. & Klug, M.J. (1986) Model for the distribution of sulfate reduction and methanogenesis in freshwater sediments. Geochim. Cosmochim. Acta 50: 11-18. Mackin, J.E. & Swider, K.T. (1989) Organic matter decomposition pathways and oxygen consumption in coastal marine sediments. J. Mar. Res. 47: 681-716. Neud6rfer, F. & Meyer-Reil, L.-A. (1998) Kleinr~iumige Bestimmung respiratorischer und photosynthetischer Prozesse mit Sauerstoffmikroelektroden in Sedimenten. In: Vereinigung fiir Allgemeine und Angewandte Mikrobiologie (VAAM): Mikrobiologische Charakterisierung aquatischer Sedimente. Oldenbourg Verlag, Miinchen, pp. 245-260. Nielsen, L.P. (1992) Denitrification in sediment determined from nitrogen isotope pairing. FEMS Microbiol. Ecol. 86: 357-362. Nielsen, L.P. & Glud, R.N. (1996) Denitrification in a coastal sediment measured in situ by the nitrogen isotope pairing technique applied to a benthic flux chamber. Mar. Ecol. Prog. Ser. 137: 181-186. Piker, L. (1995): Dynamik der Sulfatatmung und ihre Bedeutung fiir die KohlenstoffMineralisierung in Ostsee-Sedimenten. Ph.D. Thesis, University of Kiel. Rieling, T., Stodian, I., K6ster, M. & Meyer-Reil, L.-A. (1997) Aspekte des mikrobiellen Kohlenstoffkreislaufes. Verbundproj. Okosystem Boddengew~isserOrganismen und Stoffhaushalt (OKOBOD), BMBF 1. Zwischenber. Rieling, T., Stodian, I., Black, H.J., K6ster, M. & Meyer-Reil, L.-A. (1998) Aspekte des mikrobiellen Kohlenstoffkreislaufes. Verbundproj. Okosystem Boddengew~isserOrganismen und Stoffhaushalt (OKOBOD), BMBF 2. Zwischenber. Sorensen, J., Jorgensen, B.B. & Revsbech, N.P. (1979) A comparison of oxygen, nitrate, and sulfate respiration in coastal marine sediments. Microb. Ecol. 5: 105-115. Stodian, I., Black, H.J., Gerbersdorf, S., K6ster, M., Meyer-Reil, L.-A. & Rieling, T. (2000) Geochemical investigations of iron and manganese in coastal sediments of the southern Baltic Sea. In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds) Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam, (this volume).
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Thamdrup, B., Glud, R.N. & Hansen, J.W. (1994) Manganese oxidation and in situ manganese fluxes from a coastal sediment. Geochim. Cosmochim. Acta 58: 2563-2570. Voigt, A. & Babenzien, H.-D. (1997) Bedeutung der Sulfatreduktion. Verbundproj. Okosystem Boddengew~isser - Organismen und Stoffhaushalt (OKOBOD), BMBF 1. Zwischenber. Voigt, A. & Babenzien, H.-D. (1998) Bedeutung der Sulfatreduktion. Verbundproj. Okosystem Boddengew~isser - Organismen und Stoffhaushalt (OKOBOD), BMBF 2. Zwischenber. Zimmermann, A.R. & Benner, R. (1994) Denitrification, nutrient regeneration and carbon mineralization in sediments of Galveston Bay, Texas, USA. Mar. Ecol. Prog. Ser. 114: 275-288.
Muddy Coast Dynamics and Resource Management, B. W. Flemming,M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Effects of plant roots on salt-marsh sediment geochemistry M. I. Ca~ador, a* M. J. Madureira b and C. Vale b
alnstituto de Oceanografia, Departamento de Biologia Vegetal, Faculdade de Ci~ncias da Universidade de Lisboa, Bloco C2, Campo Grande, 1700 Lisboa, Portugal blnstituto de Investiga~o das Pescas e do Mar, IPIMAR, Av. Brasllia, 1400 Lisboa, Portugal
ABSTRACT
Sediment cores were collected in two salt marshes of the Tagus estuary in vegetated and non-vegetated areas. E, and pH were measured in loco whereas zinc, lead, sulphate and acid volatile sulphides were determined in the laboratory. The results show the direct influence of vascular plants on the sedimentary chemistry. Sediments containing higher root density are more oxidative and acid, enriched in zinc and lead, and sulphur is mainly in oxidised forms. The amount of oxidants released from the living roots seems to be sufficient to avoid the net consumption of sulphate. Zinc and Pb are in low reactive forms, which implies a temporary immobilisation in the vegetated sediments.
1. INTRODUCTION Salt marshes are often considered as subsystems of estuarine environments (Adam 1990). One major characteristic which contributes to this classification is that the organic matter of salt-marsh sediments results mainly from the decomposition of below-ground biomass, whereas subtidal and intertidal sediments receive organic matter from the water column. The underground input of organic matter, together with the delivery of oxygen and other oxidants by the roots to the surrounding sediments (Tinker & Barraclough 1988), creates a great range of micro-environments (Caetano et al. 1995; Doyle & Otte 1997) with high rates of organic matter decomposition in certain zones. The plants also interact with the sediments through the uptake of nutrients (Marschner 1988) and metals (Beeftink et al. 1982) which implies diffusive fluxes towards the rizosphere (Ernst 1990). Therefore, the salt-marsh vegetation plays an important role in metal recycling, and this helps to diminish the contamination effects in estuarine areas (Ca~ador et al. 1996a). This paper compares sulphur speciation as well as metal concentrations in non-vegetated and vegetated * Corresponding author: M.I. Ca~;ador e-mail:
[email protected]
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sediments of the Tagus salt marshes, and discusses the effect of plant roots on the sediment chemistry.
1.1. The Tagus estuary The estuary at the mouth of the Tagus River is one of the largest of its kind along the Atlantic coast of Europe, covering an area of 300 km 2 at low tide and 340 km 2 at extreme high tide (Fig. 1). The Tagus is a mesotidal estuary, and the southern and eastern sectors contain extensive intertidal mud flats (mainly silt and clay) harbouring salt-marsh plant communities dominated by Spartina maritima (Poales: Poaceae), Halimione portulacoides (Caryophyllales: Chenopodiaceae) and Arthrocnemum fruticosum (Caryophyllales: Chenopodiaceae). A typical zonation is visible, with homogeneous stands of the pioneer species S. maritima colonising the bare muds of low marshes and natural depressions, H. portulacoides occupying the banks of creeks, and A. fruticosum being found largely in the upper parts of the salt marshes. Due to the highly branched system of channels, the entire salt-marsh area is inundated daily by the tide. Contrary to many cases in Europe where pollutants from industrial regions are discharged into rivers which then contaminate the estuaries, in the Tagus most pollutants are discharged directly into the estuary. The site receives effluents from the ca. 2.5 million inhabitants of the greater Lisbon area, together with discharges from chemical, steelmaking, and shipbuilding industries. Thus, previous studies have shown that the Tagus salt marshes receive large quantities of anthropogenic metals which are subsequently incorporated into the sediments (Vale 1990; Ca~ador et al. 1996b), the sink capacity of the marshes being high for Pb and Zn (Ca~ador et al. 1993).
o':Y
..........
study sites in salt m a r s h e s
J e sTagus t~ [
,aTLANTIC ~,..,u,, u,u=, ~.~,,vf.~ ]. ........O CEAN ........, "~ , ~ "~
,=~,~
Figure 1. Locality map of the Tagus estuary.
Salt-marsh geochemistry
199
2. MATERIALS A N D METHODS The study was carried out in two salt marshes of the Tagus estuary, i.e. the Pancas and Corroios marshes (Fig. 1). Pancas is a large flat marsh which forms part of a nature reserve, whereas Corroios is smaller and close to an industrial zone. The sampling strategy was based on the collection of sediment cores at vegetated and nonvegetated sites in each marsh. For sulphur analysis, duplicate cores were collected from vegetated (S. maritima) and non-vegetated sites which were separated by less than 100 meters. For metals, one core was collected from each of three areas dominated by S. maritima, H. portulacoides or A. fruticosum, another three cores being taken at nearby non-vegetated sites. In each marsh, vegetated and non-vegetated sediments were sampled at the same inundation level, and the sediments showed similar grain-size distributions (Cagador et al. 1993). Sediment cores were sliced in loco into layers about 5 cm thick down to depths of 50-55 cm. These samples, used for sulphur analysis, were stored in polycarbonate tubes which were filled to the rim and sealed in order to avoid sulphide oxidation (Madureira et al. 1997). For metal analysis, the sediments were sliced downcore into the layers 0-5, 5-15, 15-25, 25-35, 35-45 and 45-55 cm, and the samples were stored in plastic vials. Potential redox and pH were measured in each sediment layer immediately after collection. In the laboratory, the sediment samples for sulphur analysis were centrifuged and the interstitial water was analysed for sulphate and chloride in each case. This procedure was carried out in a N 2 atmosphere. The solid phase was analysed for acid volatile sulphides (AVS). For metals, the samples were air dried, cleaned of roots by means of tweezers, passed through a 0.25-mm sieve, and subsequently ground and homogenised. The parameters determined were 1) organic matter content, by measuring loss on ignition (LOI) at 600~ for 2 hours; 2) sulphate concentration, by turbidimetry; 3) chlorinity, using an argentometric method (APHA 1975); 4) AVS contents, using voltametric methods according to Henneke et al. (1991) and Madureira et al. (1997); 5) nitrogen and carbon contents, by means of an elemental analyser; 6) metal contents, by extracting sediments with water, ammonium acetate solution, DTPA and HNO3/HC1 (3:1 v / v ) at 130~ (Otte 1991), followed by analyses using atomic absorption spectrometry.
3. RESULTS A N D DISCUSSION 3.1. pH, E. and organic matter There were clear differences of EH, pH, and C / N ratios between the non-vegetated and vegetated sediments of the Pancas and Corroios salt marshes. This is illustrated for the Pancas locality in Figs 2 and 3. Thus, in the vegetated sediments, the redox potential reached +360 mV and the pH varied from 6.0 to 7.0 whereas, in the absence of plants, the redox potential was always negative, and the pH fluctuated between 6.6 and 7.5 (Fig. 2). These results illustrate the effects of root activity on the basic
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properties of sediments. Generally, the pH and redox conditions of the rizosphere differ considerably from those recorded in the marsh sediments (Otte 1991). Presumably, the alterations recorded in the bulk vegetated sediments of the Tagus estuary were due to high root density (cf. this can reach 8 kg m -2 in the region; Ca~ador, unpublished data). 400
vegetated sed. 200 > v
E 1"
I.U
A
non-vegetated sed. -200
, 5.8
,
,
6.2
,
7
6.6
7.4
7.8
pH Figure 2. EH (mV) versus pH in the vegetated (~) and non-vegetated (~) sediments of the Pancas salt marsh.
10 E o
v
20
rc~ 13 r r
E
30
40
..D
13
50 vegetated sed. 60
'
10
i
11
'
non-vegetated sed. I
12
'
C/N
I
13
'
I
14
'
15
Figure 3. C / N ratios in vegetated (~) and non-vegetated (I) sediments of the Pancas salt marsh.
Salt-marsh geochemistry
201
Ranges of organic matter contents were comparable in non-vegetated and vegetated sediments (5-9%) but C / N ratios showed values higher than 13 in the non-vegetated sediments, and around 11 in the vegetated sediments (Fig. 3). The higher proportion of N in the vegetated sediments reflects differences in the nature of the organic matter in the two sediment types, and ultimately in the fractionation process during degradation.
3.2. Sulphur species Vertical profiles of sulphate concentrations (corrected for depth chlorinity) in pore waters obtained at the non-vegetated and vegetated sites of the Pancas marsh are presented in Fig. 4. Clearly, sulphate values in pore waters of vegetated sediments always exceeded those observed in non-vegetated sediments. Two down-core trends can be discerned: 1) in non-vegetated sediments, the SOa~-/C1- profile was relatively constant (0.06) at depths below 7.5 cm, and it showed much lower values than that recorded in the overlying water (0.14); 2) for the vegetated site, by contrast, concentrations in the pore water at first displayed a maximum value of 0.15 at 7.5-cm depth, sometimes exceeding the value recorded in the water column, and then decreased with depth. Depth variations in the AVS contents of the sediments are shown in Fig. 4. In accordance with the sulphate profiles, the AVS values differed between the nonvegetated and vegetated sites, too. In the upper 30 cm of vegetated sediments, the contents of this reduced form of sulphur were below 5 pmol g-1 (d.w.) while, in the non-vegetated sediments, the values were higher, ranging from 33 to 189 1Jmol g-1 (d.w.). In the absence of plants, the down-core variation in sediment AVS content was non-uniform and, consequently, no trend could be discerned with depth in this case.
AVS (pmol g~ d.w.)
S042/Cl 0
0.05 i
i
i
I
I
0.1 ,
,
,
,
I
0.15 ,
,
,
,
I
0.2 . . . .
0
50 ,
I
100 ,
I
200
150 ,
I
,
J
250 i
vegetated s e ~ E o 10tCL r 2013 re 30-
E
. i
r 40 ffl
/
vegetated sed.
/~non-vegetatedsed.
Figure 4. Vertical profiles of SO42/C1- and AVS contents (1Jmol vegetated (~) and non-vegetated (i) sites of the Pancas salt marsh.
g-1
d.w.) at the
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The pronounced differences in sulphur speciation between vegetated and nonvegetated salt-marsh sediments seem to be primarily due to the release of oxidants through the plant roots into the anoxic sediment enviroment (Tinker & Barraclough 1988). In the absence of oxygen, sulphate is extensively used in the mineralization of organic matter (Luther & Church 1992), as illustrated by the down-core decrease of sulphate at the non-vegetated site in the Tagus estuary. Otherwise, in the sediment layers with higher root density (5-15 cm depths), the sulphate concentrations reached or even exceeded the values documented in the overlying water. This means a less intensive sulphate reduction by organic matter mineralization in the oxidative environment, and probably the oxidation of reduced sulphur species as oxidants are released by the roots. In spite of the organic matter input below the surface sediments, the amount of oxidants released from the living roots seems to be sufficient to avoid sulphate reduction. The low levels of insoluble metal sulphides (AVS) in the vegetated sediments conform with the lower sulphate reduction. The absence of sulphate reduction implies little or no soluble sulphide production and, consequently, little or no AVS precipitation. 3.3. Zn and Pb
The Pancas and Corroios marshes showed a similar contrast in metal profiles between non-vegetated and vegetated sediments. Figure 5 shows these data for the Corroios marsh. In non-vegetated sediments, Zn and Pb contents decreased gradually with depth whereas the levels were generally higher in the vegetated sediments, presenting enrichment peaks in the subsurface layers with higher root densities. Higher contents recorded in the upper layers of the sediments reflect the environmental contamination of the estuarine system (Vale 1990). However, the subsurface maximum observed in the vegetated sediments of the Corroios marsh are not related to environmental contamination but rather to root activity which emerges as the major factor influencing metal enrichment in this case (Ca~ador et al. 1996b). The Zn and Pb peaks showed large standard deviations (cf. Fig. 5) because the values integrate the effects of three different salt-marsh communities, i.e. the S. maritima, H. portulacoides and A. fruticosum communities of the Tagus estuary. This variability suggests that root-sediment interactions vary between plant species, which seems reasonable seeing the different parenchyma and below-ground biomass of the three species investigated in the present study. Zn and Pb retained in the sediments are present in low reactivity forms (Table 1) which are not easily available to plants (Cagador et al. 1996a). This implies that Zn and Pb are temporarily immobilised in the vegetated sediments though, according to the sulphur results, not in insoluble metal sulphides. Their incorporation into iron oxides has been recorded mainly in thin tubular structures which are formed in the vicinity of the roots, and are designated as rhizoconcretions (Vale et al. 1990; Sundby et al. 1998)
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Salt-marsh geochemistry
-1 Pb(lagg d.w.) 200 400
Zn (lagg-1 d.w.) 0
200
400
600
0
800
0
0
10
10
X
20
20
"~
30
30
_
40
40
50
50
60
60
600
Figure 5. Vertical profiles of total Zn and Pb contents (lug g-l) in sediments between roots (I) and in intertidal sediments ([3) of the Corroios salt marsh.
Table 1. Percentages of Zn and Pb obtained with three different extraction procedures (NH4Ac, HNO 3 and DTPA) in relation to HNO3/HC1 digestion in the 5 to 15-cm layers of the vegetated and non-vegetated sediments. With vegetation
Without vegetation NH~Ac
HNO~
DTPA
NHaAc
HNO 3
DTPA
Zn (%)
63
43
41
18
4
3
Pb (%)
12
70
73
1
4
3
4. CONCLUSIONS By creating a subsurface layer in the sediments characterised by re-oxidation, recycling, and extensive consumption of diagenetically produced ions, in the Tagus estuary the salt-marsh vegetation directly influences sedimentary chemical reactions through its root system. Vegetated sediments are dominated by sulphur-oxidised species and enriched in zinc and lead. These findings are important in terms of aquatic ecosystem health because salt marshes can act as filters for anthropogenic metals in estuarine waters. Since metals become immobilised in vegetated sediments, the preservation of salt marshes as well as the creation of artificial wetlands would be an efficient and natural means of maintaining ecosystem health a n d / o r restoring ecosystem quality.
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REFERENCES
Adam, P. (1990) Salt Marsh Ecology. Cambridge University Press, Cambridge, 566 p. APHA (1975) Standard methods for the examination of water and wastewater. Washington, 1193 p. Beeftink, W.G., Nieuwenhuize, J., Stoeppler, M. & Mohl, C. (1982) Heavy metal accumulation in salt marshes from the Eastern and Western Scheldt. Sci. Total Environ. 25: 199-223. Ca~ador, I., Vale, C. & Catarino, F.M. (1993) Effects of plants on the accumulation of Zn, Pb, Cu and Cd in sediments of the Tagus estuary salt marshes, Portugal. In: J.-P. Vernet (ed.) Environmental Contamination. Elsevier, Amsterdam, pp. 355-365. Ca~ador, I., Vale, C. & Catarino, F.M. (1996a) Accumulation of Zn, Pb, Cu, Cr and Ni in sediments between roots of the Tagus Estuary salt marshes, Portugal. Estuar. Coast.Shelf Sci. 42: 393-403. Ca~ador, I., Vale, C. & Catarino, F.M. (1996b) The influence of plants on concentration and fractionation of Zn, Pb, Cu and Cd in salt marshes sediments (Tagus Estuary, Portugal). J. Aquat. Ecosyst. Health 5: 193-198. Caetano, M., Madureira, M.J., Vale, C., Bebianno, M.J. & Gon~alves, M.L. (1995) Variations of Mn, Fe and S concentrations in sediment pore waters of Ria Formosa at different time scales. Neth. J. Aquat. Ecol. 29: 275-281. Doyle, M.O. & Otte, M.L. (1997) Organism-induced accumulation of iron, zinc and arsenic in wetland soils. Environ. Pollut. 96: 1-11. Ernst, W.H.O. (1990) Ecophysiology of plants in waterlogged and flooded environments. Aquat. Bot. 38: 73-90. Henneke, E., Luther, G.W. & DeLange, G.J. (1991) Determination of inorganic sulphur speciation with polarographic techniques: some preliminary results for recent hypersaline anoxic sediments. Mar. Geol. 100: 115-123. Luther, G.W. & Church, T.M. (1992) An overview of the environmental chemistry of sulphur in wetland systems. In: Howarth, R.W., Stewart, J.W.B. & Ivanov, M.V. (eds) Sulphur Cycling on the Continents. John Wiley, New York, pp. 125-142. Madureira, M.J., Vale, C. & Sim6es-Gon~alves, M.L. (1997) Effect of plants on sulphur geochemistry in the Tagus salt-marshes sediments. Mar. Chem. 58: 27-37. Marschner, H. (1988) Mineral Nutrition in Higher Plants. Academic, London, 889 p. Otte, M. L. (1991) Heavy metals and arsenic in vegetation of salt marshes and floodplains. Ph.D. Thesis, Vrije Universiteit Amsterdam. Sundby, B., Vale, C., Ca~ador, I., Catarino, F., Madureira, M.J. & Caetano, M. (1998) Metal-rich concretions on the roots of salt-marsh plants: mechanism and rate of formation. Limnol. Oceanogr. 43: 245-252. Tinker, P.B. & Barraclough, P.B. (1988) Root-soil interactions. In: Hutzinger, O. (ed.) Reactions and Processes 2 (D). Springer-Verlag, Berlin, pp. 154-171. Vale, C. (1990) Temporal variations of particulate metals in the Tagus river estuary. Sci. Total Environ. 97/98: 137-154. Vale, C., Catarino, F. Cortes~o, C. & Ca~ador, M.I. (1990) Presence of metal-rich rhizoconcretions on the roots of Spartina maritima from the salt marshes of the Tagus estuary, Portugal. Sci. Total Environ. 97/98: 617-626.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier ScienceB.V. All rights reserved9
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Elemental composition of siderite grains in early-Holocene deposits of Youngjong Island (west coast of Korea), and its palaeoenvironmental implications 9
a~.
B.-K. Khlm, K.-S. Choi b and Y. A. Park b
aResearch Institute of Oceanography, Seoul National University, Seoul 151-742, Korea bDepartment of Oceanography, Seoul National University, Seoul 151-742, Korea
ABSTRACT
Macrotidal muddy coastal deposits around Youngjong Island (west coast of Korea, eastern Yellow Sea) comprise four lithostratigraphic units (unit I, unit II, unit III and unit IV). Overlain unconformably by unit I of modern muddy intertidal sediments, Unit II of early-Holocene age (ca. 8000 yr BP) is characterized by a distinct color, peat fragments, rootlets, and especially microscale (ca. 150 ~tm) siderite grains whose mineralogy was confirmed by X-ray diffraction. Siderite concretions occur as aggregated spherulitic forms with texturally well-developed rhombs on the grain surface, as verified by means of scanning electron microscopy. Electron microprobe analyses indicate that the siderite grains have low contents of Mg (<3 mol%) and Ca (<8 mol%). In contrast, high contents of Fe (ca. 70 mol%) and Mn (ca. 15 mol%) suggest that the siderite precipitated in a non-marine depositional environment such as freshwater bogs. The occurrence of such diagenetic and authigenic siderites in early-Holocene sediments highlights a potential for providing information on depositional environments for which unequivocal sedimentological evidence is otherwise lacking, such being the case for the muddy coastal deposits in the eastern Yellow Sea.
1. INTRODUCTION Recent research indicates that the precipitation of diagenetic siderite grains is influenced largely by microbially-mediated reactions (Coleman 1993; Mortimer et al. 1997). However, the geochemistry of diagenetic carbonates is still of importance for providing information on depositional environments (e.g., Curtis et al. 1986). In particular, attempts to discriminate between marine and non-marine deposits have been successfully carried out using the elemental compositions of siderite concretions * Corresponding author: B.-K.Khim e-mail:
[email protected]
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(Mozley 1989; Mozley & Carothers 1992; Baker et al. 1996). Thus, the occurrence of siderite grains is potentially useful in situations where the depositional environment cannot be determined clearly by traditional approaches based on sediment texture, sedimentary structures, etc. The Yellow Sea forms a shallow continental platform which exemplifies tidallydominated epicontinental shelf seas. During the last glacial maximum of sea-level low stand, the entire Yellow Sea basin experienced subaerial exposure, resulting in a continuous connection between the Chinese mainland and the Korean Peninsula (Bloom & Park 1985; H a n & Meng 1987; Park et al. 1994). With the subsequent onset of the Holocene transgression, the sea advanced rapidly across the low-gradient exposed shelf, generating rapid shoreline retreat. Concurrently, the middle to lateHolocene fine-grained sediments which were discharged from Chinese and Korean rivers accumulated over the sandy basal transgressive deposit (Keller & Prior 1986; Alexander et al. 1991; Khim et al. 1997; Lee & Yoon 1997). The west coast of Korea, fringing the easternmost Yellow Sea, is well known for its extensive tidal flats, most of which have formed during the late Holocene (e.g., Kim 1988). These muddy coastal regions are typical depositional sites with thicknesses reaching more than 20 m. Since the early 1990s, comprehensive studies of sedimentary sequences in these intertidal flats have been conducted to establish the evolutionary history of the region during the late Quaternary (Lee et al. 1994; Chang 1995; Park et al. 1995). Most of the tidal-flat sediments along the west coast of Korea comprise yellowish, oxidized and semi-consolidated muddy deposits of latePleistocene age, separated from the overlying Holocene fine-grained tidal deposits by a distinct unconformity. In the present study, we report the occurrence of siderite grains from the earlyHolocene sediments around Youngjong Island, west coast of Korea. We present data concerning the texture and geochemistry of these grains, with the aim of facilitating the interpretation of the local depositional environment, thereby contributing to the reconstruction of palaeoenvironmental events in the eastern Yellow Sea.
2. S T U D Y AREA A N D S T R A T I G R A P H Y
The study area is located at the eastern margin of the Yellow Sea where extensive non-barred tidal flats have formed in response to modern wave and tidal conditions (Fig. 1). The Youngjong Island intertidal flats have a macrotidal regime and lie in the northern part of Kyunggi Bay, west coast of Korea. The Han River, situated northeast of the study site, discharges fair amounts of clastic sediments to the study area (Chough 1983). On the basis of an extensive data set comprising information from hundreds of sedimentary boreholes (Park & Choi 1998), four major lithostratigraphic units have to date been identified for the approximately 40-m-thick muddy coastal deposits in the Youngjong Island tidal-flat area (Choi & Park 1996; Fig. 2).
Siderite grains of the west Korean coast
207
Figure 1. Youngjong Island tidal flats, regional geomorphology and bathymetry in the eastern Yellow Sea. Inset C shows the location of the deep-drilling sites. 0 m: mean sea level.
Unit I (approximately 6-10 m thick) consists of sandy silt to mud sediments of lateHolocene age. This unit is equivalent to the modern tidal-flat deposits which are well developed along the west coast of Korea (Kim 1988; Chang 1995; Park et al. 1995). Characteristic tidal laminations, glauconitic sands and coarsening-upward features are indicative of the marine transgression.
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Figure 2. Composite stratigraphic units of the late Quaternary deposits of Youngjong Island tidal flats, west coast of Korea (modified after Choi & Park 1996). M: mud, Z: silt, sZ: sandy silt, zS: silty sand, gS: gravelly sand.
Siderite grains of the west Korean coast
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Unit II (<2 m thick) is composed of stiff and very fine-grained sediments (>7 phi in mean grain size). This unit contains considerable amounts of authigenic siderite grains, pellets, and organic debris. Radiocarbon ageing (ca. 8000 yr BP) and the sealevel curve for the Yellow Sea indicate that unit II formed in a terrestrial environment during the early Holocene (Bloom & Park 1985; Han & Meng 1987; Chang 1995). Bounded unconformably to unit II, unit III is divided into two subunits in which unit III-1 (2-5 m thick) is composed of semi-consolidated muddy sediments characterized by cryogenic structures and organic clays with plant roots. The highlyoxidized nature of these sediments seems to reflect long-term subaerial exposure. In contrast, unit III-2 in the lower part of unit III is 10-15 m thick, consisting of oxidized and non-oxidized sandy silt, silty sand, and muddy sediments showing well-defined rhythmic tidal bedding of sand-mud couplets. Finally, unit IV also comprises two subunits. Unit IV-1 (2-5 m thick) comprises late-Pleistocene (radiocarbon dates range from 25,000 to 40,000 yr BP) peaty organicrich sediments, containing abundant plant stems and rootlets suggestive of freshwater marsh deposits. The lowermost litho-unit overlying the weathered soil and basement rock (dominantly Jurassic granites), unit IV-2 (3-7 m thick), is composed of gravelly sandy sediments dominated by pure quartz and feldspar.
3. MATERIALS A N D M E T H O D S
A total of 20 sediment samples were collected from unit II at ten geotechnical boreholes descending as deep as 40 m down to the basement rock. About 2 grams of carbonate concretions selected from the bulk sediments were pulverized for X-ray diffraction (XRD) analyses in order to confirm the presence of siderite. Randomly-oriented carbonate powders were scanned in the range 3-70~ at l~ on a rotating sample holder of a JEOL X-ray diffractometer under Nifiltered CuK(x radiation. Selected samples were vacuum-impregnated with Spurr low-viscosity epoxy resin to facilitate the preparation of thin sections. The chemical composition of single siderite grains was determined quantitatively by means of electron microprobe analysis (EMPA) with an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam diameter of 20 ~tm. A scanning electron microscope (SEM) was used to observe the crystal morphology and surface texture of the grains.
4. RESULTS A N D D I S C U S S I O N
Siderite concretions have often been documented in the geological records of both ancient and modern environments (Weber et al. 1964; Franks 1969; Curtis et al. 1975; Gill & Segnitt 1980; Gautier 1982; Carpenter et al. 1988; Pye et al. 1990; Hart et al. 1992; Browne & Kingston 1993; Baker et al. 1996). In the present case, the existence of
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siderite concretions in unit II was confirmed by XRD analyses which showed that the principal mineral of the concretions is almost pure siderite (Fig. 3). Primar~y peak intensity was measured at 31.78 ~ and corresponding~ cell dimension was 2.81A. Compared with the standard values of 32.08~ and 2.79A of essentially pure FeCO 3 siderites for primary peak intensity and cell dimension, respectively, the siderite grains of this study are thus slightly expanded. However, XRD analyses failed to detect measurable amounts of either calcite or aragonite in the grains. 6.00K
Qtz u~
EL 3.00
rO
9
, 2.00
1 10.00
,
I 20.00
1
I 30.00
~
I
~t
1 40.00
~"
,
i 50.00
,
I 60.00
, 70.00
2| Figure 3. X-ray diffractogram of carbonate concretions selected from sediments of unit II. The primary peaks (*) confirm the identification of the mineral siderite.
In the sediments of unit II, siderite occurs mainly as aggregates of spherulitic grains measuring 100-300 ~tm in diameter (Fig. 4a). These aggregates are easily distinguished from the siderite-hosting silt-sized sediments by their large size and distinct color (dark yellowish brown under open nicols). Such aggregates are preserved with very weak bonding and appear 'grape-like' in structure (Fig. 4b). However, these bunches are easily broken into individual spherulitic grains when they are brushed (Fig. 4c). Each siderite grain is well rounded and nearly spherical, with well-developed equant rhombs on the surface (Fig. 4d). In some cases, concretions display smoothed surfaces without distinct development of rhombs, suggesting possible dissolution after precipitation. Similar spherulitic siderite grains of comparable size have also been reported for lacustrine sediments (Zhang et al. 1996), paleosols (Browne & Kingston 1993), and fluvial mudrocks (Baker et al. 1996).
Siderite grains of the west Korean coast
211
Figure 4. Top left: Siderite grains (1), quartz (2) and pellets (3) examined by light microscopy (scale: 1 mm). Top right: Thin section of aggregation (dark red shade) of siderite grains examined by light microscopy (scale: 1 mm). The host sediments are primarily silt-sized. Bottom left: Typical spherulitic form of a siderite grain examined by SEM (scale: 100 gm; inset: see Fig. 4d). Bottom right: Texturally well-developed rhombs on the surface of a siderite grain examined by SEM (scale: 10 gm).
Clastic particles of silt-sized quartz and feldspar are commonly incorporated as inclusions in siderite concretions (Fig. 5). Within single spherulitic siderite grains, these clastic minerals show a random orientation but they decrease in number towards the rim. Backscattered images of single siderite concretions show that the rims are brighter in the outer than in the inner sections, implying an enrichment of heavier elements. This is attributed to ferro-oxide minerals such as hematite which coat the grain surfaces (Fig. 5). These clastics may therefore form nuclei for the precipitation of siderites. The chemical composition of carbonate concretions is strongly influenced by that of the ambient water or porewater at the time of precipitation (e.g., Veizer 1983). Unlike other carbonate concretions, siderites have been shown to effectively record
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early diagenetic conditions in a variety of depositional environments, without undergoing subsequent recrystallization and re-equilibration (Curtis et al. 1975; Matsumoto & Iijima 1981; Gautier 1982; Curtis & Coleman 1986; Carpenter et al. 1988). Consistent with these earlier findings, in the present study siderite grains show a distinctive crystal shape, and no evidence of secondary replacement (Fig. 5). Furthermore, the lack of any cracks within the grains, and the distinct boundary characteristics between the ambient sediments and the siderites are indicative of insignificant replacement by other minerals such as calcite. Thus, these siderite grains seem to be formed entirely by direct precipitation.
Figure 5. Backscattered image of SEM view of single siderite grain in thin section. Many clastic inclusions are situated in the inner sections. The brighter outer rim is likely to be a hematite coating.
Results of EMPA demonstrate that the siderites are generally quite pure, containing 64.8-80.6 mol% FeCO 3 (mean value 66.7 mo1%+3.8) but only 6.9-12.2 mol% CaCO 3 (mean value 7.6 mo1%+1.4) and 2.1-4.9 mol% MgCO 3 (mean value 2.5 mo1%+0.5), with limited Mg and Ca substitution. Mg substitution does not exceed 5.0 mol%. The mean ionic Fe/Ca/Mg ratios (relative values of the 3 components) are 86.8/3.3/9.9, which is reflected in a distinct cluster in the Fe-Ca-Mg ternary diagram (Fig. 6a). Mn contents vary in the range 6.5-23.8 mol% MnCO 3 (mean value 15.1 mo1%+3.2). Excepting for 4 outliers in the Mn-Ca-Mg (59.9/30.2/9.9) ternary diagram (Fig. 6b), the data are well grouped, suggesting that the siderite grains are relatively uniform in composition, with significant Fe substitution by Mn. Our EMPA
Siderite grains of the west Korean coast
213
results are very comparable to those of Mozley (1989), in which the non-marine siderites are characterized by the enriched Fe and Mn components.
Figure 6. EMPA elemental composition of siderite in the sediments of unit II (n = 66). a) Fe-Ca-Mg ternary diagram, b) Mn-Ca-Mg ternary diagram. The shaded areas of the left and right-hand diagrams represent the non-marine siderites from the Ivishak Formation and Tyonek Formation, respectively (after Mozley 1989)
Siderite-hosting sediments of unit II lack of microfossils and primary sedimentary structures which could otherwise provide information on this depositional environment along the west coast of Korea. However, abundant peat fragments, inorganic pellets and vascular plants which are concurrently preserved in unit II have been dated back to approximately 8000 yr BP. Consistent with the presence of plant materials, the siderite-hosting sediments are characterized by relatively higher total organic carbon contents (2--4 dry weight-% TOC) than those measured in the overlying Holocene tidal sediments (<1% TOC). According to the Holocene sea-level curves for the Yellow Sea, sea level was at least 10-15 m below the present-day mean sea level about 8000 yr BP (Bloom & Park 1985; Han & Meng 1987; Lee et al. 1994). Thus, considering the thickness of unit I, Unit II of siderite-host sediments might have formed up to 5 m above the palaeomean sea level at that time. Siderite precipitation presumably occurred in a freshwater setting where input of coarsegrained sediments was minimal. Otherwise, pyrite would have formed instead. Siderite concretions are frequently interpreted as having formed early in the host sediment's burial history from bacterially-generated CO 2 and cations derived from the sediments and porewater. In marine settings, the onset of siderite precipitation is delayed until porewater sulfate has been depleted (Gautier 1982; Carpenter et al. 1988). Mozley (1989) demonstrated that siderites from a variety of marine environments have higher M g / C a ratios, and contain less Fe and Mn than those formed in freshwater sediments. In other words, the very low Ca and Mg contents,
214
Khim et al.
and relatively high Mn contents of the siderites in the present study indicate possible precipitation in a non-marine or terrestrial environment. If the siderites had formed in marine/brackish or freshwater settings, considerable variation in Ca and Mg contents should be apparent, as suggested by Hart et al. (1992). In contrast, the elemental composition of the siderite grains from unit II of Youngjong Island tidalflat deposits (Fig. 6) indicates a freshwater depositional environment, comparable to that described by Mozley (1989). Indeed, considering other sedimentological properties of unit II, including abundant peat fragments and rootlets as well as the absence of microfossils and primary sedimentary structures, siderite precipitation might have occurred in freshwater bogs of an ephemeral lake in this case. Finally, it has been suggested that there is a relationship between the timing of siderite formation and the onset of methane accumulation (Curtis 1967). Siderite from freshwater environments is commonly quite pure as it often shows endmember composition (Mozley 1989). Under such conditions, the production of H2S in the sediments is usually controlled by the generally low availability of dissolved sulphate which is utilized by sulphate-reducing bacteria. This promotes methane production and concurrently the formation of siderite concretions at relatively shallow depths (Berner 1981; Gautier 1982; Moore et al. 1992). Thus, it is widely accepted that siderite forms only below the zone of sulphate reduction, equivalent to a greater depth in the diagenetic zone. The formation of siderite concretions in the zone of methane production can be assessed in terms of the carbon isotopic composition of the siderites (Curtis et al. 1986; Mozley and Carothers 1992). Unfortunately, such data are not available in the present case.
5. CONCLUSIONS AND IMPLICATIONS In siderite concretions of early-Holocene muddy coastal deposits (about 8000 yr BP) on Youngjong Island in western Korea, relatively high contents of Fe (ca. 70 mol%) and Mn (ca. 15 mol%), and relatively low contents of Mg (<3 mol%) and Ca (<8 mol%) suggest precipitation of the siderite grains in a freshwater setting. Carbon isotope analyses would, however, be needed for this interpretation to be substantiated. The results of the present study therefore clearly demonstrate that detailed information on the occurrence and elemental composition of siderites can be important in reconstructing depositional environments, and in interpreting the early geochemical history of ancient sediments. We contend that this approach represents a useful (albeit not new) but generally overlooked tool in palaeoenvironmental assessments, particularly for regions such as the eastern Yellow Sea for which unequivocal evidence of past depositional processes is scarce or even lacking.
Siderite grains of the west Korean coast
215
ACKNOWLEDGEMENTS
This study was carried out during the second author's (K.S.C.) dissertation research which was supported by a predoctoral fellowship of the Korea Research Foundation in 1997. We wish to thank J.T. Wells (University of North Carolina) and H.-J. Brumsack (University of Oldenburg) for constructive comments to an earlier version of the manuscript. We also appreciate the editorial and language corrections of M.T. Delafontaine and the reviewers.
REFERENCES
Alexander, C.R., DeMaster, D.J. & Nittrouer, C.A. (1991) Sediment accumulation in a modern epicontinental-shelf setting: the Yellow Sea. Mar. Geol. 98: 51-72. Baker, J.C., Kassan, J. & Hamilton, P.J. (1996) Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, southern Bowen Basin, eastern Australia. Sedimentology 43: 77-88. Berner, R.A. (1981) A new geochemical classification of sedimentary environments. J. Sediment. Petrol. 51: 359-365. Bloom, A.L. & Park, Y.A. (1985) Holocene sea-level history and tectonic movements, Republic of Korea. Japan. Quat. Res. 24: 77-84. Browne, G.H. & Kingston, D.M. (1993) Early diagenetic spherulitic siderites from Pennsylvanian paleosols in the Boss Point Formation, Maritime Canada. Sedimentology 40: 467-474. Carpenter, S.J., Erickson, J.M., Lohmann, K.C. & Owen, M.R. (1988) Diagenesis of fossiliferous concretions from the Upper Cretaceous Fox Hills Formation, North Dakota. J. Sediment. Petrol. 58: 706-723. Chang, J.H. (1995) Depositional processes in the Gomso Bay tidal flat, west coast of Korea. Ph. D thesis, Seoul National University, 192 p. Choi, K.S. & Park, Y.A. (1996) Lithostratigraphy and depositional environment of the coastal deposits in the Youngjong-do tidal flat, west coast of Korea. Abstr. Vol. Tidalites '96, Int. Conf. Tidal Sedimentology, March 1996, Savannah, pp. 18. Chough, S.K. (1983) Marine Geology of Korean Seas. International Human Resources Development Corporation, Boston, 157 p. Coleman, M.L. (1993) Microbial processes: controls on the shape and composition of carbonate concretions. Mar. Geol. 113: 127-140. Curtis, C.D. (1967) Diagenetic iron minerals in some British Carboniferous sediments. Geochim. Cosmochim. Acta 31: 2109-2123. Curtis, C.D. & Coleman, M.L. (1986) Controls on the precipitation of early diagenetic calcite, dolomite and siderite concretions in complex depositional sequences. In: Gautier, D.L. (ed.), Roles of Organic Matter in Sediment Diagenesis. Soc. Econ. Paleontol. Mineral. Spec. Publ. 38: 23-33.
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Curtis, C.D., Pearson, M.J. & Somogyi, V.A. (1975) Mineralogy, chemistry, and origin of a concretionary siderite sheet (clay-ironstone band) in the Westphalian of Yorkshire. Mineral. Mag. 40: 385-393. Curtis, C.D., Coleman, M.L. & Love, L.G. (1986) Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite, and siderite concretions. Geochim. Cosmochim. Acta 50: 2321-2334. Franks, P.C. (1969) Synaeresis features and genesis of siderite concretions, Kiowa Formation (Early Cretaceous), north-central Kansas. J. Sediment. Petrol. 39: 799803. Gautier, D.L. (1982) Siderite concretions: indicators of early diagenesis in the Gammon Shale (Cretaceous). J. Sediment. Petrol. 52: 859-871. Gill, E.D. & Segnitt, E.R. (1980) Siderite from the Otway coast of Victoria, S.E. Australia. Aust. Mineral. 31: 152-154. Han, Y. & Meng, G. (1987) On the sea-level changes along the eastern coast of China during the past 12,000 years. In: Qin, Y. & Zhao, S. (eds), Late Quaternary SeaLevel Changes. China Ocean Press, Beijing, pp. 119-136. Hart, B.S., Longstaffe, F.J. & Plint, A.G. (1992) Evidence for relative sea level change from isotopic and elemental composition of siderite in the Cardium Formation, Rocky Mountain Foothills. Bull. Can. Petrol. Geol. 40: 52-59. Keller, G.H. & Prior, D.B. (1986) Sediment dynamics of the Huanghe (Yellow River) delta and neighbouring Gulf of Bohai, People's Republic of China: project overview. Geo-Mar. Lett. 6: 63-66. Khim, B.K., Choi, J.H., Kim, S.Y. & Park, Y.A. (1997) Late Holocene fine-grained sediment deposition on the western and central Yellow Sea shelf: significance of clay mineral assemblages. In: Lee, C.B. & Zhao, Y.Y. (eds), Holocene and Late Pleistocene Environments in the Yellow Sea Basin. Seoul National University, pp. 135-149. Kim, Y.S. (1988) Sedimentary environments and evolution of intertidal deposits in Sajangpho coast, Cheonsu Bay. Ph. D thesis, Seoul National University, 169 p. Lee, H.J. & Yoon, S.H. (1997) Development of stratigraphy and sediment distribution in the northeastern Yellow Sea during Holocene sea-level rise. J. Sediment. Res. B67: 341-349. Lee, H.J., Chun, S.S., Chang, J.H. & Han, S.J. (1994) Landward migration of isolated shelly sand ridge (chenier) on the macrotidal flat of Gomso Bay, west coast of Korea: controls of storms and typhoon. J. Sediment. Res. A64: 886-893. Matsumoto, R. & Iijima, A. (1981) Origin and diagenetic evolution of Ca-Mg-Fe carbonates in some coalfields of Japan. Sedimentology 28: 239-259. Moore, S.E., Ferrell, R.E. Jr. & Aharon, P. (1992) Diagenetic siderite and other ferroan carbonates in a modern subsiding marsh sequence. J. Sediment. Petrol. 62: 357-366. Mortimer, R.J.G., Coleman, M.L. & Rae, J.E. (1997) Effect of bacteria on the elemental composition of early diagenetic siderite: implications for palaeoenvironmental interpretations. Sedimentology 44: 759-765. Mozley, P.S. (1989) Relation between depositional environment and the elemental composition of early diagenetic siderite. Geology 17: 704-706.
Siderite grains of the west Korean coast
217
Mozley, P.S. & Carothers, W.W. (1992) Elemental and isotopic composition of siderite in the Kuparuk Formation, Alaska: effect of microbial activity and water/sediment interaction on early pore-water chemistry. J. Sediment. Petrol. 62: 681-692. Park, Y.A. & Choi, K.S. (1998) Recognition of silty tidal rhythmite from the upper Pleistocene sedimentary sequence, western coast of Korea. J. Korean Soc. Oceanogr. 33: 71-79. Park, Y.A., Khim, B.K. & Zhao, S. (1994) Sea level fluctuation in the Yellow Sea basin. J. Korean Soc. Oceanogr. 29: 42-49. Park, Y.A., Choi, J.Y., Lim, D.I., Choi, K.W. & Lee, Y.G. (1995) Unconformity and stratigraphy of late Quaternary tidal deposits, Namyang Bay, west coast of Korea. J. Korean Soc. Oceanogr. 30: 332-340. Pye, K., Dickson, J.A.D., Schiavon, N., Coleman, M.L. & Cox, M. (1990) Formation of siderite-Mg-calcite-iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England. Sedimentology 37: 325-343. Veizer, J. (1983) Chemical diagenesis of carbonates: theory and application of trace element technique. In: Arthur, M.A. (ed.), Stable Isotopes in Sedimentary Geology. Soc. Econ. Paleontol. Mineral. Short Course Notes No. 10(3): 1-100. Weber, J.N., Williams, E.G. & Keith, M.L. (1964) Palaeoenvironmental significance of carbon isotopic composition of siderite nodules in some shales of Pennsylvanian age. J. Sediment. Petrol. 34: 814-818. Zhang, X., Wang, Y. & Lei, H. (1996) Authigenic mineralogy, depositional environments and evolution of fault-bounded lakes of the Yunnan Plateau, southwestern China. Sedimentology 43: 367-380.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
221
The impacts of sea-level rise on the coastal fisheries resources and aquatic ecosystems of Cameroon (Central Africa) C. E. Gabche,* T. J. Youmbi and C. A. Angwe
Research Centre for Fisheries and Oceanography, PMB 77 Limbe, South-West Province, Cameroon
ABSTRACT
Sea-level rise (SLR), in association with the topography of low-lying coastal areas, their hydrology, sedimentology, natural dynamics, and anthropogenic interactions, will profoundly affect the coastal zone of Cameroon in the new century. Anticipated impacts of SLR on species abundance, distribution, and diversity in the wetlands and aquatic ecosystems are highlighted, focusing on fish and plant species. Such impact assessments are based on time series (1970-1990) of fisheries resource production, projected demands (1990-2010), and economic value as well as the aquatic ecosystems at risks in case of SLR. As an example, the artisanal fisheries resources within three fishing camps highly vulnerable to SLR are evaluated with respect to several SLR scenarios. Adaptive measures and policies are suggested to combat these effects, thereby reducing possible socio-economic and cultural losses in the region.
1. I N T R O D U C T I O N Sea-level rise (SLR) is one of the more important and inevitable environmental consequences of global climate change in the wake of global warming, amongst others resulting from industrial green-house gas emissions into the atmosphere. Over the past 100 years, the earth's temperature has increased by 0.3-6.6~ and sea level has risen by 10-25 cm due to thermal expansion of the upper water layer of the world oceans as well as the large-scale melting of snowfields, ice sheets and glaciers (Barth & Titus 1984). Another factor to be considered in this context are the effects of vertical tectonic movements of the earth's crust, especially when acting in conjunction with coastal landforms. Estimates of current rates of SLR in different parts of the world are listed in Table 1, global mean values ranging from 1.5-3.4 m m per year. An evaluation of tide-gauge measurements in the 1980s showed that the majority of the 226 tide-gauge records which span >30 year indicate a SLR of about 1.2 m m per year (Pirazzoli 1986). This value was confirmed in the 1990s, a global * Corresponding author: C.E. Gabche Fax: ++237332376351357
222
Gabche et al.
relative sea-level change of <0.7_+1.0 m m per year having been predicted (Pirazzoli 1996). In 1990, the Intergovernmental Panel on Climate Change (IPCC 1990) estimated a global SLR of 15-30 cm by the year 2030, accelerating up to 1 m by the end of the 21st century. Long-term tide-gauge measurements are very limited along the African coast, no such records being available for the coast of Cameroon. The estimate of Abe and Kaba (1996) for the stable African plate, which incorporates data for Senegal (Karen et al. 1995) and Bony-Nigeria (Nwilo et al. 1995), ranges between 1 and 4.6 m m per year (Table 1). When viewing global SLR (1990-2100) in conjunction with the values for Takoradi, Ghana (1930-1969) and Santa-Cruz, Tenerife (1927-1974), it w o u l d appear reasonable to expect a similar order of magnitude for the low-lying coast of Cameroon (cf. Fig. 1). Table 1. Estimates of sea-level rise (SLR). SLR (mm year-') 1.5-3.4 (15-30 cm by 2030)
Region World Stable African plate 9 Ghana Takoradi 9 Nigeria 9 Chte d'Ivoire 9 Senegal 9 Bonny-Nigeria Others 9 Argentina 9 Uruguay 9 Tenerife (Santa Cruz)
Source IPCC (1990)
3.4_+0.6 4.6 1.5 1.4 1.0
Abe & Kaba (1996) Abe & Kaba (1996) Abe & Kaba (1996) Karen et al. (1995) Nwilo et al. (1995)
1.6 1.6 2.6_+0.5
Karen et al. (1995) Volont6 & Nicholls (1995) Abe & Kaba (1996)
725 -
) 9 Santa
Cruz
(Tenerife
-~ 7 ~ b -
___~ 7 0 5 -
d,
~-
69b-
~e5
'~",'"~'!, j i ~1 ~ ~ ' , , i , t i i t l i ~ [ ~ l ~ , l ~ t , , 1930 I B40 1950
, , i ~ , l'~ ] , ~ , , i +' , ' , , 1960 1970
i,
~'~ ~ ! 19130
year
Figure 1. Sea-level rise scenarios for Takoradi (Ghana) and Santa Cruz (Tenerife).
Sea-level rise and coastal resources in Cameroon
223
The overall objective of this paper is to elaborate on some aspects of Cameroon's low-lying coastal zone, including natural coastal processes and anthropogenic influences. Specifically, attention is focused on resources in aquatic ecosystems, notably in fishing grounds and mangroves. Assuming a no-protection scenario for fisheries, coastal ecosystems, and human population growth in the region, an assessment of the possible impacts of SLR is made. In addition, measures to combat these effects are suggested.
2. COASTAL ZONE CHARACTERISTICS Cameroon's low-lying coastal zone is located in the central Gulf of Guinea, extending approx. 20 nautical miles offshore (Fig. 2). It comprises the Ndian, Fako, Wouri, Sanaga Maritime and Ocean administrative divisions. +,.= '~.I. A k w a y a f e
.-.
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-
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d e l .Rey. -[•
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NDIAN d~v.
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,,,oi,,,, + I,o-X +,oo'
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,!'::::'.:':" ~ / 2 ~ ~'L .... ,.". " ,"'~ ..'" ~, :,.:,~~,:-.,:,,:":':" ,'.. ,.'. :, . .,::,,.,.,,...<: , .,..,...,.....,,.~.. I"-',_ + FAKO dlv. ~ ] .
T,k
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' ..... ~, ~
+-'+ U l m
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'~
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•
&
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A
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,-~
;~_L.,.. ;'SANAGA ,._-.-- MARITIME div.
:-::::~~ .~"
Sanaga r i v e r
,
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/
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&
Nyong r i v e r
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ATLANTIC OCEAN
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=~ ~
~
',\
Kribi
[20km,
fishing grounds for fish and s h r i m p
(P,vraf.,e,',aeop,c,/s at/ant/ca)
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fishing grounds for shrimp fishing grounds
mangroves + tropical
div. = c o a s t a l
rain
/
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t forest
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&
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Figure 2. Cameroon's fishing grounds, mangroves, and administrative divisions.
224
Gabche et al.
2.1. Hydrology The coastal river systems and hydrological zones are dominated by an extensive drainage network comprising 12 major rivers which discharge into the Atlantic Ocean (Table 2; Fig. 2). In the north-west, the Ndian and Meme rivers join shortly before reaching the Atlantic to form the Rio-del-Rey estuary. In the central region the Mungo, Dibamba, and Wouri rivers discharge jointly through the Cameroon estuary, whereas the Sanaga river has its own estuary, as do the Nyong, Lokoundje, Kienke, Lobe, and Ntem rivers of the southern coastal region. The Sanaga river has the largest catchment area (1,135,000 km2), followed by the Wouri river (82,000 km2). The total discharge (not considering the Rio-del-Rey estuary) is approximately 12,000 m 3 s -I, thus ranking second to the Zaire river along the Central/West African coast.
Table 2. Cameroon's coastal river systems and hydrological zones. Major rivers
Cross Ndian Meme Mungo Wouri Dibamba
Length (km)
,
,
,
Sanaga
160 . . -150 250 150 890
j
Nyong
i
800
Lokoundje 185 Kienke 80 Ntem 460 a Annual mean flow u Total dissolved solids |
i
Catchment Sediment area yield ( k m 2) (kg year -~) West river system 800 -. . . . 975 -2500 lx 109 82,000 -2400 -Sanaga river system 1,135,000 2.8x109 South river system 14,000 -. . 1900 31,000
.
. . --
.
|
. !
Flow rate (m3s -1)
TDS (mg 1-~)
171-7570 246 300 27-236 49-1425 480
38.8 --78.1 43.6 28.4
500-5700
96.3
26-376 (194 a)
19.1
. . . . . . . 50-764 (288 ~)
--
b
Sea-level rise and coastal resources in Cameroon
225
With a continental shelf of 14,000 km 2, the hydrography of Cameroon's coastal zone is controlled by the large-scale circulation in the Gulf of Guinea (Wauthy 1983) which, in turn, is influenced by the Benguela, South Equatorial and Guinea currents. It is also related to the dominant wind regimes of the Gulf of Guinea (Schneider 1992). The resulting alternation of oceanic fronts plays a major role in determining the productivity and biogeographical limits of fin and shellfish species. In Cameroon, the distribution, abundance, diversity, and productivity of fin and shellfish is strongly influenced by the input of freshwater from the many rivers, the effects being lower in the dry season and larger in the rainy season. This is unlike other countries along the African coast where productivity is influenced more by upwelling (cf. Binet 1997). The seasonal rainfall contributes much to the hydrology of the region, as much as about 5.4-6 9 106 m 3 o f rain water being discharged through the Cameroon and Rio-del-Rey estuaries alone. Other sources of freshwater are smaller streams and groundwater seepage. 2.2. Sedimentology The geology of the coastal zone reflects the Pleistocene history of the Gulf of Guinea. The coastal sedimentary formations and sandstone deposits stretching between the Akwayafe river in the west and the mouth of the Lokoundje river in the south are interrupted only by an outlier of volcanic rocks originating from an active volcano, the Mount Cameroon (4070 m high). The seabed sediments between the upper estuaries and the 100-m depth contour comprise a mixture of coastal hard grounds, coral, sand, muddy sand, sandy mud, and mud. Muddy deposits are found in the estuaries of all 12 major rivers, being particularly prominent in the Cameroon and Rio-del-Rey estuaries. The main sediment sources are the hinterlands of the Adamaoua Plateau where most rivers originate (Fig. 2). The coastal deposits are 6-16 km thick, the annual sedimentation rates ranging from 0.03-0.1 m (UNEP 1984). Terrigenous sands and silts contribute the largest proportion (90%), the remainder being made up of ferritic alluvial soils (Gabche & Angwe 1996). The high water discharge and sandy-silty sediment loads of the rivers are responsible for the formation of extensive coastal marshlands. These marshy areas serve as breeding grounds for fin and shellfish species, and promote the luxurious growth of mangroves, besides facilitating agricultural land use. By contrast, the thick mud deposits of the Cameroon and Rio-del-Rey estuaries have had largely negative impacts on the development of the towns of Douala, Limbe and Bamusso. 2.3. Coastal processes High river discharge (cf. Table 2) has important implications for the effects of SLR on Cameroon's muddy coast. The tides are semi-diurnal and, with spring tidal ranges reaching 3 m, tidal current velocities >1 m s-~are not uncommon (Keita et al. 1991). The tidal wave penetrates far up into the estuaries (Morin & Kuete 1989), travelling upstream by as much as 40 km in the case of the Mungo river. As a result, large intertidal flats and sand banks are exposed at low tide in the estuaries. This not only
226
Gabche et al.
obstructs boat traffic but, together with the high freshwater discharge, also inflicts a high level of disturbance onto the aquatic ecosystems, intertidal organisms being particularly strongly affected. In addition, the intermittently active volcano near Buea constantly threatens the stability of Cameroon's coastal zone (UNEP 1984), thereby making it even more vulnerable to SLR. Cameroon's coastal zone is exposed to monsoonal winds of the Guinea type. They blow predominantly from the south-west in the wet season, producing a humidity close to saturation. During the dry season the winds blow from the north-east. These so-called Hamattan winds reach average speeds of 0.5-2 m s-1 over most of the year, but can reach 18 m s1 in April. The coast is dominated by two types of climate, the so-called Guinean and Cameroon types. These can show local modifications due to the topography of the coast. The Guinean type extends from the Kribi region in the south up to the mouth of the Sanaga river to the north (Fig. 2). It is characterised by 4 distinct seasons (a long and a short rainy season, and a long and a short dry season). The Cameroon climate type is mainly found in the south-western coastal region, but it also occurs in the neighbourhood of Mount Cameroon. It is characterised by two seasons, a wet one lasting about 8 months, and a dry one of about 4 months duration. Maritime climate conditions extend from the west coast to the mouth of the Nyong river. They are characterised by constant high temperatures and humidity. For example, the seaward facing slopes of Mount Cameroon receive the full force of the south-west monsoon, as a result of which the town of Debundscha receives a rainfall of roughly 10,000 mm per year, making it the second wettest place on earth. 2.4. Anthropogenic influences The population of Cameroon's coastal zone (amounting to 1,399,870 for an area of only 27,064 km 2 in 1987) is concentrated in the urban centres where 60% of the country's manufacturing industries are found. The highly urbanised population around the Cameroon river estuary (890 km 2) in the Wouri Division was estimated at 83,4471 in 1987, whereas the census for the Rio-del-Rey estuary (6275 km 2) in the Ndian Division was 87,435, being mostly made up of fishing communities. These population groups are highly vulnerable to SLR. Anthropogenic impacts on the coastal zone include agricultural land use which represents 30% of the gross domestic product (GDP), 70% of which is income from export revenue, with 75% of the labour force being employed in this sector. This is followed by the fishing industry (about 25,000 fishermen in the artisanal sector), the manufacturing sector (17% of the GDP, 16% of the trade), transport, communication, a promising petroleum production (industrial activities represent 60% of the national production), and tourism which concentrates on ethnic and ecological diversity (e.g., sandy beaches, mud flats, and waterfalls in Kribi, Edea, Tiko and Limbe).
Sea-level rise and coastal resources in Cameroon
227
3. T R E N D S , P R E D I C T I O N S A N D A S S U M P T I O N S
In this study, trends have been estimated for Cameroon's main marine fisheries resources for the period 1970-1990, based on the production of fin and shellfish which make use of the m u d d y coastal regions as breeding grounds, nurseries and transition zones. The data used in this study were partly provided by the Ministry of Livestock, Fisheries and Animal Industries of Cameroon, and partly extracted from Seki and Bonzon (1993). The actual trends of the last few decades were then used to estimate projected fish demand for 1990-2010, and artisanal fisheries resources at risk in the wake of SLR. The diversity and distribution of faunal and floral components in muddy-coast aquatic ecosystems were, amongst others, chosen as criteria for gauging possible losses resulting from SLR. The proposed scenarios were based on the following assumptions: 9 SLR is the only factor causing major coastal changes. 9 SLR estimates of IPCC (1990), and Abe and Kaba (1996, for Takoradi in Ghana) are applicable to Cameroon's m u d d y coast as well. 9 The Cameroon and Rio-del-Rey estuaries experience subsidence as a result of natural compaction and anthropogenic influences such as crude oil extraction from offshore wells. 9 Because of the low topography, the m u d d y coastal regions of Cameroon will be flooded and eroded in the course of SLR. 3.1. Fish production trends and values at risk
Trends in the annual catches of Cameroon's marine fish industry (total catch, and fish and shrimp catches) show that the total production fluctuated between ca. 30 and 45 * 103 metric tons (t) in the time interval from 1970-1978. This was followed by a sudden increase to more than 70 * 103 t in the years from 1979-1981 (Fig. 3). 80-
70-
60o
50
~4o "~ r c~
3020-
10
i
0 --
./ I
19"/0
A ' ~ " A ~ A ~ = ' A " =
'
I
1915
'
s.r,mO I
1980
'
'
'
'
I
1985
'
'
~'"
"
I
1990
year
Figure 3. Marine fisheries production trends in Cameroon for the period 1970-1990.
228
Gabche et al.
A few years later, a less-marked but short-lived increase to ca. 75 9 10~ t occurred, followed by a general decrease which lasted up to the year 1990 at least. The general decrease observed in the total annual catches also applies to fish production. The annual shrimp catches, however, remained steady throughout this period. These decreasing trends can be attributed to increasing fishing effort, and to ecological degradation resulting from SLR. Table 3. Projected fish demand for Cameroon (1990-2010). Population
Year
(1000s)
Fish supply Per capita Total (kg year -I) (1000s t) 2.6 149.5
Projected demand (1000s t)
1989-1990
11,833
2000
14,787
.
.
.
.
187
2010
19,286
.
.
.
.
244
m
~
Table 4 gives the quantities and monetary values for the total export of marine fisheries, on the one hand, and that of crustaceans, on the other hand, for the period 1980-1990. Total exports had a mean annual monetary value of ca. 4.5 million US $. Although crustaceans made up only ca. 7% of the total mean annual export mass, their mean annual monetary export value amounted to 3.9 million US $, i.e. ca. 86% of the value of the total fisheries export. This emphasises the point that crustaceans, which thrive particularly well in the muddy coastal areas, are clearly Cameroon's most valuable export commodity (Fig. 4).
Table 4. Quantities and monetary values for the export of fish and crustaceans (shrimps and others). Total export Year 1980
US $ ( x 106) 4.0
mass (Mt) 4018
Export of crustaceans US $ mass ( x 106) (Mt) 3.4 609
Contribution by crustaceans (%) US $ mass ( x 106) (Mt) 85.0 15.2
1982
1.6
4981
1.1
291
68.8
5.8
1984
2.7
10,952
2.4
461
88.9
4.2
1986
3.2
18,609
2.1
306
65.6
1.6
1988
7.1
8661
6.0
752
84.5
8.7
1900
8.2
2484
8.1
1106
98.8
44.5
Total
26.8
49,705
23.1
3525
86.2
7.1
Mean
4.5
8284
3.9
588
86.1
7.1
229
Sea-level rise and coastal resources in Cameroon
The projected fish demand stands at 187 and 244 * 1 0 3 t for the years 2000 and 2010, respectively (Table 3). These give close estimates of expected losses if we consider that a SLR of 3.4 mm per year would result in a total sea-level rise of 1.02 and4.42 cm, respectively.
...
a
m
70 &
n
E
6-
v
rj~ 5 &
..m.
.C::::: 4" ~0 r
-~
/.
total shrimp
r
3-
_er___
e~ 2 -
fish XqX
0
-|--1970
I 1975
1980
1985
1990
year
Figure 4. Monetary values (US $) for Cameroon's marine fisheries production.
Artisanal fishery is practised in the estuaries, mangroves, coastal rivers and up to 2 nautical miles offshore. It is dominant in the Rio-del-Rey, Cameroon and Sanaga regions. Of all the coastal regions, the Wouri coastal division has by far the highest population density (ca. 940 per km 2, 1987 census), harbours about 27% of the artisanal fishermen, and maintains roughly 22% of the landing camps (Table 5). This region, in which 80% of Cameroon's industrial fisheries are established, will clearly incur the greatest losses in the wake of SLR. Although the Rio-del-Rey region has a low population density (ca. 14 per kin2), it has the largest number fishermen and the greatest potential for fisheries development (notably shrimps), besides its potential for offshore petroleum drilling and mining.
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Table 5. Population densities (1987 census), sizes of mangrove areas as well as the numbers of fishermen and their landing camps in Cameroon's coastal divisions. Landing camps (no.)
(no. km -2)
Mangrove area (ha)
Ndian
13.9
150,000
9387
33
Fako and Ocean
120.0
20,000
4908
40
Wouri
937.6
180,000
6484
46
Sanaga Maritime
14.7
180,000
2415
34
Whole coast
51.7
350,000
24,136
206
Coastal division
Population density
Fishermen
(no .)
Investigating the values at risk in the wake of SLR for 3 fishing camps in the Douala-Edea Reserve (Sanaga Maritime division), Gabche (1997) reported a total value of 1293.4 billion FCFA (approx. 2.4 billion US $), with a mean value of 430.8 billion FCFA (approx. 0.86 billion US $; Table 6). The total value for the 34 camps of the Sanaga Maritime division (Table 5) is thus estimated at ca. 29.2 billion US $.
Table 6. Estimated values at risk (xl03 FCFA) for 3 fishing camps along Cameroon's muddy coast (extracted from Gabche 1997). Mangrove Total values wood stocks 401,706 4365 14,744 224,630 577,923 2588 10,282 309,050 313,774 1500 7954 222,320 1,293,403 8453 32,980 756,000 430,811 2818 10,660 165,333 252,000 Mean a Fishing economic units for fishing gear, canoes, engines and accessories b Fish-processing (smoke-drying) units Fishing camp Mbiako Yoyo I Yoyo II Total
FEUs a
Kitchens and living houses 158,000 256,000 82,000 496,000
Bandas b
3.2. Muddy-coast aquatic ecosystems Most coastal rivers and estuaries in the region are dominated by a freshwater fauna including osteoglossiforms, perciforms and characiforms (Tengels et al. 1992). The estuarine zone, characterised by an accumulation of sediments and swamp organic matter, contributes to a high zooplankton production. This provides sustenance for stocks which spawn and use this zone as a nursery. Common organisms in this area are oysters, periwinkles, mullets and catfish (Table 7).
231
Sea-levelriseand coastalresourcesin Cameroon Table 7. Estuarine and mangrove fish species and their habitats. Species
Common name
Habitat Spat on aerial roots of mangroves and at intertidal levels
Crassostrea gasar
Oyster, bivalve
Tympanotonus fuscatus
Periwinkle
Mud in swamps
Callinectes marginatus
Crab
Brackish estuarine mangrove swamps
Periopthalmus hoelferi
Mud skipper
Mud in swamps
Mugil spp.
Mullet
Penaeus notialis
Pink shrimp
Nematopalaemon hastatus White shrimp
waters
and
Flats in swamps of Cameroon and Rio-del-Rey estuaries Juveniles in brackish waters and muddy deposits Juveniles in brackish waters and muddy deposits
Macrobrachium spp.
Giant river prawn
Ethmalosa fimbriata
Bonga
Sardinella maderensis
Strong kanda
Arius heudeloti
Catfish
Demersal, estuaries
Cynoglossus spp.
Sole
Demersal, muddy sediments at 15-100 m
Lutjanus spp.
Snapper
Demersal, estuaries
Polydactylus quadrifilis
Shrine nose
Demersal, estuaries
Sphyraena piscatorium
Barracuda
Demersal, estuaries
Pseudotolithus typus and Croaker P. elongatus
Riverine and brackish waters Pelagic, estuarine and mangrove transit/nursery zones Light sandy-muddy habitats at 6-30 m
Demersal, estuaries
Mud deposits in the estuarine zone are also beneficial for the shrimp fishing industry. The abundance and distribution of the fish and shrimp stocks in these muddy regions are largely determined by the physicochemical conditions resulting from freshwater input and tidal fluctuations within the mangrove forests. The continental shelf fisheries comprise artisanal fishing inshore and industrial fishing beyond the 2-nautical mile zone up to a water depth of 200 m. Species composition and catch estimates for both fisheries show that clupeids dominate in the artisanal, whereas sciaenids are more important for the industrial fisheries.
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Table 8. Zonation of floral species in mangroves. Intertidal Species Habitat .Rhizopora Range is 20-45 m, racemosa zones are (epiphytic, 4-8 min parasitic) back zones .Nypa fruticans Zones flooded at high tide .Hibiscus Occurs in mixed filaiceus com.Inu.Acrostichium nity aurium .Drepanocarpus lunatus .Dalbergia ecastaphyllum 9Carapa procera Forina.Chrysobalamus tion of rarely orbicularis flooded .Manilkara banks obovata .Crudia klainei .Cinometra manii .Oxystignia manii
Intermediate Species i Habitat ,Rhizophora Flooded, horrisonii muddy upper ,Pandamus stratum candelabrum .Dalbergia 1Flooded, ecastaphyllum 'muddy ,Drepanocarpus discontinuous lunatus .Oramacarpum 'middle stratum verrucosum .Conocarpus erectus .Fimbristhylis Flooded, muddy ferruginea lower .Eleocaris stratum geniculata '(grassy) .Paspalum vaginatum .Becope decumebs ,Acrostichium aureum .Utridulation spp. 9Xyris anceps
Mangroves Species Habitat .Antocleista Always flooded, vogetli common .Atlophyllum freshwater inophyllum 9Phoenix swamp rechinata species .Rhizophora horrisonii ,Drepanocarpus lunatus .Arcrostichum aureum .Ormacapum verracosum
The zonation of floral species within the mangrove ecosystem (Table 8) shows a high species diversity. Of these, 12 species occur in the intertidal, 13 in the intermediate, and 7 in the mangrove swamp areas. The fauna is dominated by 19 fish species, followed by 4 mollusc, 2 oyster, 1 cephalopod, and 1 mussel species. In addition, about 14 bird species and various monkey families are found. The human population is dominated by foreign nationals. Man-induced influences include the exploitation of mangroves for the construction of houses and canoes, as well as for medicinal and fish processing purposes such as providing wood for smoke-drying of fish. Further uncontrolled exploitation of the mangroves will result in a serious decline in biodiversity, changes in coastal morphology, and accelerated accretion. These negative impacts would be compounded by more frequent flooding and salt-water intrusions in the wake of SLR.
Sea-level rise and coastal resources in Cameroon
233
3.3. Impacts of sea-level rise Complex, multifaceted links exist between fisheries resources, aquatic ecosystem stability, agro-industrial activities, industries, sustenance of the population, and anthropogenic influences. The immediate impacts of SLR would come from flooding, causing the loss of land and livelihood for traditional fishing communities. The intrusion of saline waters into flood-plain soils will also have the effect of "pushing back" the habitats of salinityintolerant flora, e.g., the water hyacinth Eichornia crassipes, further upstream. On the positive side, this will result in a reduced disruption of coastal fishing. On the negative side, increasing saltwater intrusions will induce changes in the dynamics and chemical composition of groundwater and the water regimes at river mouths. As a result, water-logged soils will be converted into mud, necessitating extensive road construction and also the desalination of freshwater aquifers. Despite the fact that the use of contaminated waters for domestic purposes is prohibited, such bans are being widely ingnored. Since the improvement of water-processing techniques is very costly, an increase in the loss of lives through infectious diseases must be expected. The promotion of a greater utilisation of rainwater from roof catchment systems will result in exorbitant prices for roof sheeting. Coupled with reduced production from muddy-coast agriculture, serious economic losses must be anticipated for many sites. Salinity stress will induce a redistribution of species assemblages and result in the destabilisation of the ecological balance between estuarine and mangrove domains. The losses of ecological niches and nursery grounds for juvenile fishes will reduce production and export earnings, and lead to an impoverishment of the fauna and a decreased fish protein supply. Long-lasting floods will result in root decay and mass tree kills with recruitment failure due to anoxic conditions. Other possible effects include changes in sediment deposition patterns, increased storminess resulting in beach erosion, changes in ocean circulation and nutrient availability, higher risks at sea for fishermen, and a general decline in coastal property values. Pollution will increase in the fishing grounds and coastal ecosystems because of greater influxes of anthropogenic pollutants such as heavy metals and petroleum products. Land shortage will result in the use of the swamps as dumps and toilets, thereby promoting epidemics. Water dispersal will increase the virulence of water-borne diseases such as typhoid and cholera which would also spread into the hinterland. In addition, there will be a higher prevalence of malaria since the larvae will have more extensive breeding grounds. It is evident that SLR will cause the widespread destruction of coastal ecosystems, notably the mangroves. Since these serve as important breeding grounds, and also contribute effectively to coastal defence (e.g., Mazda et al. 1997), their decimation will seriously undermine the social and economic survival of the coastal population.
234
Gabche et at.
4. A D A P T I V E RESPONSES A N D POLICIES TO C O M B A T SEA-LEVEL RISE
In combination with intensified educational programs, adaptive measures to combat the negative effects of sea-level rise should include: 9 the installation of tide gauges by trained manpower; such data will promote the timely implementation of effective coastal management strategies; 9 the upgrading of agro-forestry practices, thereby limiting harmful CO2 emissions; 9 the promotion of local skills in coastal defence; for example, the use of sand bags to reinforce barrier weirs, and of higher gabions with additional diaphragms; 9 the relocation of fishing grounds, and the establishment of conservation zones, including nurseries; this will improve biodiversity and safeguard endemic species; 9 the relocation of roads and residential areas in less vulnerable regions; 9 the establishment of nurseries for mangrove trees and salt-tolerant coconut and oilpalm strains; 9 the promotion of public awareness about the negative effects of deforestation; 9 the promotion of more efficient fishing techniques, especially the use of fuel-saving fish-smoking kilns which conserve the mangrove fuel; 9 the introduction of mariculture and other farming techniques based on local materials; 9 the improvement of the existing mesh size of gears, and the strict definition of fishing-ground limits for artisanal/industrial fisheries; this will reduce competition and loss of gear in the wake of SLR; 9 the installation of domestic solar-driven desalination units for the production of drinking water; 9 the construction of permeable, non-concrete breakwaters and groynes using local materials for the purpose of increasing fisheries yield; 9 the mapping of fisheries resources, aquatic ecosystems, and related infrastructure, using advanced mapping techniques in combination with geographical information systems; 9 the support of short and long-term research programmes focused on fish conservation, mariculture, muddy coast nutrient cycling, and physical dynamic processes; 9 the organisation of public-awareness campaigns focused on sustainable ecosystem use, deforestation, coastal erosion, etc.; this should maintain the quality and improve the biomass of ecosystems as well as discourage the illegal exploitation of resources. In conclusion, a holistic approach to the impacts of SLR is proposed for Cameroon's fisheries resources and aquatic ecosystems, accounting for multifaceted aspects such as increased temperature, flooding, destabilization of fish-processing practices, and impaired human health. To assess and alleviate these impacts, an integrated coastal management plan would thus be required. In anticipation of continued SLR along Cameroon's coast, strategic policy trusts to promote technology as well as research programmes are needed, also focusing on the complex interactions between anthropogenic influences and natural factors. Because Cameroon's coastal zone characteristics are typical for other countries in the Gulf of
Sea-level rise and coastal resources in Cameroon
235
Guinea, it is suggested that such an approach would be meaningful for the region as a whole and, for that matter, also for similar regions worldwide (cf. IPCC 1996). ACKNOWLEDGEMENTS
We express our gratitude to the Ministry of Livestocks, Fisheries and Animal Industries for making their data sets available for this study, and to the organising committee of the international conference Muddy Coasts 97 for financial support to the main author during his stay in Wilhemshaven, Germany. REFERENCES
Abe, S. & Kaba, N. (1996) Problems and management strategies of the Ivorian coastal zone. In: Ibe, C., Kothias, J.B.A. & Ajayi, T.D. (eds), Perspectives in Coastal Areas Management in the Gulf of Guinea Region. Tech. Publs Series GOG-LME, 15 p. Barth, M.G. & Titus, J.G. (1984) Greenhouse Effect and Sea Level Rise. Van Bostrand Reinhold, New York, 325 p. Binet, D. (1997) Climate and pelagic fisheries in the canary and Guinea currents 1964-1993: the role of trade winds and the southern oscillation. Oceanol. Acta 20: 177-190. Gabche, C.E. (1997) An appraisal of fisheries activities and evaluation of economic potentials of the fish trade in the Douala-Edea reserve- Cameroon. Fish. Consult. Rep. Cameroon Wildlife and Conservation Society, Yaounde, 39 p. Gabche, C.E. & Angwe, C.A. (1996) Coastal erosion and sedimentation in Cameroon. Int. Sem. Coastal Zone of West Africa: Problems and Management. 25-29 March 1996, Accra, 18 p. IPCC (1990) Strategies for adaptation to sea-level rise. Rep. Coast. Zone Subgroup, IPCC Working Group III. Rijkswaterstaat, The Netherlands, 122 p. IPCC (1996) Second Assessment Report of Climate Change. Chapter 9: Impacts of climate change on coastal zones and small islands. Cambridge University Press, Cambridge, pp. 289-324. Karen, C.D., Niang-Diop, I. & Nicholls, R.J. (1995) Sea level rise and Senegal: potential impacts and consequences. J. Coast. Res. 14: 243-261. Keita, M.L., Johnson, R., Diallo, E.H.A. & Nzegge, E.J. (1991) La courantologie dans l'estuaire de la Bimbia (Cameroon). Atel. Rech. Conj. Product. Estuaires Mangroves Afrique de l'Ouest. UNESCO/COMARAF Rapp. Tech. Projet, pp. 5-9. Mazda, Y., Magi, M., Kogo, M. & Hong, P.N. (1997) Mangroves as a coastal protection from waves in the Tong King delta, Vietnam. Man. Salt Mar. 1: 127-135. Morin, S. & Kuete, M. (1989) Le littoral Cameroonais. Probl6mes morphologiques. Trav. Lab. G6ogr. Phys. Appl. Inst. G6ogr. Univ. Bordeaux III, II: 5-53. Nwilo, P.C., Onuoha, A.E. & Mike, P.T. (1995) Monitoring global sea-level rise/relative sea-level rise in a developing country. The Nigerian experience. Proc. Int. Conf. Coastal Change 95. Bordomer-IOC, Bordeaux, 1995, pp. 24-31.
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Pirazzoli, P.A. (1986) Secular trends of relative sea level changes indicated by tide gauge records. J. Coast. Res. Spec. Publ. 6: 11-5. Pirazzoli, P.A. (1996) Sea-Level Changes - the Last 20,000 Years. John Wiley & Sons, Chichester, 211 p. Schneider, W. (1992) Fiches FAO d'identification de la p~che. Guide de terrain des resources marines commerciales du Golfe de Guin6e. Collaboration Bur. R6gion. FAO Afrique. Rene FAO, 268 p. Seki, E. & Bonzon, A. (1993) Selected aspects of African fisheries: a continental overview. FAO Fish. Circ. 810, 158 p. Tengels, G.E., Reid, G. & King, R.P. (1992) Fishes of the Cross River Basin (Cameroon -Nigeria): Taxonomy, Zoogeography, Ecology and Conservation. Mus6e Royal de l'Afrique Cental, Tervuren. Ann. Sci. Zool. 226, 132 p. UNEP (1984) The marine and coastal environment of the West and Central African region and its state of pollution. UNEP Region. Seas Rep. Stud. 46. Volonte, C.R. & Nicholls, R.J. (1995) Uruguay and sea-level rise: potential impacts and responses. J. Coast. Res. 14: 262-284. Wauthy, B. (1983) Introduction a la climatologie du Golfe de Guin6e. Oc6aonogr. Trop. 18(2): 103-138.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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Impacts of sea-level rise and human activities on the evolution of the Pearl River delta, South China M.-K. Han, a* L. W U l a Y.-F. Liu a and N. Mimura b
aDepartment of Geography, Peking University, Beijing 100871, P.R. China bDepartment of Urban System Engineering, Ibaraki University, Hitachi 316, Japan ABSTRACT
Research based on Landsat image interpretation, GIS-topographic mapping, historical records, and ground truthing indicates that the evolution of the muddy coast and the expansion of the Pearl River delta have been strongly affected by human activities in historical times. In recent decades the region has experienced severe man-induced siltation coupled with rapid but premature reclamation of muddy tidal flats in the wake of economic development and population expansion. At present, there is practically no natural coastal landscape left, the shoreline being characterized by man-made dikes throughout. In addition, most of the delta plain is poorly protected, being situated below local high-tide and storm-surge levels. The delta region is thus exposed to natural disasters such as typhoon-driven storm surges and ground subsidence caused by local sediment compaction and regional tectonics. These effects are compounded by the threat of accelerated relative sea-level rise which has been estimated to reach 0.5 m within the next 50 years. Without massive protection works this would lead to the inundation of 96.5% of the delta region, and would include the destruction of even entire cities such as Guangzhou. We contend that the effects of human interventions in the Pearl River delta region have reached the same significance as those associated with geological processes. This important aspect has to be taken into account when studying the recent evolution of the delta, especially when seeking sustainable solutions for the economic development of the region.
1. INTRODUCTION The Pearl River (Zhujiang) delta (Fig. 1) is the third largest in China, being surpassed in size only by those of the Yangtze River (Changjiang) and Yellow River (Huanghe). The delta receives its sediment mainly from the west, the north and the * Corresponding author: M.-K. Han e-mail:
[email protected]
238
H a n et al.
east in association with the three large tributaries comprising the Xijiang (Western River), the Beijiang (Northern River), and the Dongjiang (Eastern River), respectively. Since the Holocene marine transgression peaked at about 6000 years B.P., the former shallow estuarine embayment, dotted with numerous rocky islands, has been completely infilled by deltaic deposits, in the process leading to the development of the Pearl River. Today it has an average runoff of 302,10 ~ m 3 year -1, and a sediment load of ca. 84,106 t year -1, with an average sediment discharge of 0.306 kg m -3. The delta has been utilized for agriculture as early as the Han Dynasty (206 B.C.-220 A.D.), and for aquaculture since the Song Dynasty (618-1279 A.D.; cf. Han et al. 1988; Li et al. 1991; Fig. 2).
Figure 1. Locality map of the study area.
2. HUMAN ACTIVITIES 2.1. Types of activities The Pearl River delta evolved by the southward expansion of its frontal margin. Since historical times the muddy coastal fringe has been increasingly affected by growing human interventions. These activities have in recent decades led to increased siltation in the wake of rapid land reclamation to combat fast urbanization and the resulting expansion of agriculture, aquaculture, and industrial development exerted by the policy of opening China to the outside world (Yan 1984, Han et al. 1995; Li et al. 1995).
Sea-level rise and human activities in the Pearl River delta
239
;igure 2. Historical evolution (after Li et al. 1991), and distribution of elevations in he Pearl River delta region based on GIS-topographic mapping. Shore line since I the Qing Dynasty, II) the Ming Dynasty, III) the Song Dynasty, IV) the Tang )ynasty, V) the Han Dynasty, VI) 4000 B.P., and VII) 6000 B.P. (elevations relative to ?cal datum at Pearl River mouth, i.e. 0.6 m above national datum or mean sea level ,f the Yellow Sea).
240
Han et al.
Most natural river deltas are characterized by extensive m u d d y fringes along the coastal margin, the estuaries, and the banks of distributary channels. Such lands are commonly reclaimed once they have aggraded above mean sea level (MSL). In the Pearl River delta, however, the local people developed innovative techniques by which the creation of new land along the m u d d y coast was speeded up. This new method of land reclamation was implemented in two stages. To reclaim an intertidal area, rubble dikes were at first constructed at low tide with their crestlines at the elevation of mean sea level. During the rising tide these dikes were overtopped, and the basins in their rear were filled with m u d d y tidal waters. During the following ebb tide the suspended matter in the trapped water body settled out to form a thin mud deposit. This process repeated itself with each tidal cycle until the level of the deposited mud reached the elevation of the mean sea level. The dikes were then progressively raised until the mud deposits reached the hightide level. The land reclaimed in this way was initially cultivated with salt-tolerant plants. Later, when the remaining salt in the soil had washed out, the cultivation of rice could begin. In a second stage, the separately diked areas were merged into larger reclaimed regions by upgrading the main dikes, and by building sluices and structures for the control of irrigation waters and river floods. The remaining diked but low-lying and frequently water-logged grounds were then converted into so-called mulberry (or fruit-tree) dike fish-pond systems. These were artificial ecosystems which promoted more efficient land use (Han et al. 1988). To accelerate the land-reclamation process, the former manual procedure was mechanized over the past three to four decades. Thus, the junks formerly used for the transport of rubble were replaced by motor vessels, and the construction of dikes is now conducted by elevator-ships. Also, the diked but water-logged low-lying grounds are being drained by pump ships, and then filled with silts dredged from nearby distributary channels. This procedure has led to a situation where the elevation of the tidal flat to be reclaimed is now as low as 0.3 m, and in some cases even 0.5 m below mean sea level. We consider this to be a premature land-reclamation process which may threaten the lives and livelihood of future generations in the wake of climate change and accelerated sea-level rise. Our evaluation of Landsat TM data for the years 1986, 1988, 1992, 1994, and 1996 as well as a comparison with the topography of 1966 show that the land reclaimed along the margin of the Pearl River delta over the past +30 years (1966-1996) has attained an area of 344 km 2, corresponding to a mean reclamation rate of 11 km 2 year -~(Figs 3, 4).
Sea-level rise and human activities in the Pearl River delta
241
~igure 3. Growth of the Pearl River delta resulting mainly from land reclamation in he last 100 years (based on Landsat imagery; see Fig. 4 for framed area).
.2. Consequences of human activities The accelerated land reclamation and the increased man-induced siltation has ;enerated a number of features which today characterize the muddy coasts of the 'earl River delta.
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Han et al.
A. Natural intertidal mudflats, which commonly line the coastal fringe of large deltas, have become a rare sight in the Pearl River delta. Instead, the coast displays a rather narrow stretch of embryonic tidal flat which either imbricates against the foot of the dikes along the frontal margin of the delta or may be expressed as a small patch at the apex of some rocky embayments. Any expansion of the tidal flat immediately triggers reclamation. As a result, most of the coastline along the Pearl River delta margin has an artificial, man-made appearance. B. The land-reclamation process in the Pearl River delta region lacks an integrated and comprehensive coastal management plan. All reclamation projects primarily pursue the purpose of creating new land either for agriculture and aquaculture or for urban and industrial development, without any effort to design and implement an ecologically sound environmental protection and preservation scheme. As a result, ecologically important habitats such as wetlands, especially the mangrove zones, have been completely eliminated. Similarly, in the development of aquaculture, interest is focussed only on building shrimp and fish-breeding pond systems without considering the maintenance of breeding grounds for shellfish, oysters, etc., on the tidal flats. The latter are not only much more profitable, but would also increase the biodiversity in the region. C. Because of the premature land-reclamation procedure, most of the delta region comprises very low-lying muddy ground (Fig. 2 and Table 1) which is very vulnerable to many kinds of natural disasters. The constant acceleration of land reclamation has extended the lengths of river outlets, thereby creating complicated distributary networks which have difficulty in discharging river flood waters into the sea. This greatly enhances the risk of flooding. In some cases, peak flood levels, being constrained within high levees on both river banks, have generated such large hydrostatic pressures that the groundwater has spurted out upwards from the low-lying farmland behind the levees. Clearly, such silt-laden gushing waters can easily develop into destructive floods, incurring huge economic losses. This has recently been experienced behind the eastern bank of the Beijiang River northwest of the city of Guangzhou. Furthermore, in many places the reclaimed delta plain is not sufficiently protected against the frequent typhoon-generated storm surges. This is because the dikes are neither high enough nor in good enough condition for the purpose. Consequently, serious economic losses have been suffered in such inadequately-protected regions. Areas on the delta underlain by 5 to 10-m-thick mud deposits experience continuous subsidence due to compaction of the sediment. This may bring about safety problems during construction if it is not considered already at the early planning stages. For example, the Huangpu Economic Technological Development Zone, located east of Guangzhou near the outlet of the Dongjiang River and built only in 1988, has already experienced such problems. In combination with crustal subsidence and sea-level rise, the lowering of the ground level caused by sediment compaction has resulted in the flooding of the container wharf (originally designed
Sea-level rise and human activities in the Pearl River delta
243
close to high-tide level) once in every two or three years. Some buildings have been deformed and even cracked due to the uneven subsidence. Here again, costly cleaning-up, repair, and maintenance operations are incurred by the authorities.
Figure 4. Growth of the Pearl River delta resulting from the reclamation of the muddy coast in the vicinity of the Moudaomen Outlet in the recent 100 years, especially in the period 1966-1996 (based on Landsat imagery).
3. SEA-LEVEL RISE
Against the background of the above discussion, it is clear that any sustained sealevel rise would create serious problems in the Pearl River delta environment, thereby exacerbating the already existing disaster risks. The eustatic rise in sea level due to global warming is estimated at 20-30 cm over the next 50 years (until 2050). However, the relative sea-level rise in the Pearl River region is expected to triple. This is mainly due to the additional impacts of crustal subsidence, man-made subsidence triggered by the extraction of groundwater, oil and gas, and sediment compaction in the reclaimed lands. Thus, for the Pearl River delta area the relative sea-level rise over the next 50 years has been estimated at
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0.42 m by the State Oceanic Information Center (reported by the China Environmental News, Chinese edition, 10 August 1996, page 1). From Table 1 it can be seen that, if the Pearl River delta were not protected by dikes, the area which would be flooded at times of maximum high-tide levels would today amount to ca. 65.3% of the whole deltaic plain. This would increase to ca. 89.0% when adding a 1-m elevation in water level during typhoon storm surges, and to ca. 96.5% when a relative sea-level rise of 0.5 m is considered in addition. In the latter case, all the cities on the delta, including Guangzhou, would be under water.
Table 1. Surfaces areas between individual elevation intervals of the Pearl River deltaic plain based on GIS-topographic mapping and calculations (the surface areas of rocky hills and islands are not included). Elevations are relative to MSL of the Yellow Sea (national datum of China) or MSL of the South China Sea (local datum) which lies 0.6 m above national datum. Mean tidal range: 0.86-1.64 m. Maximum tidal range: 2.53 m. Elevation local datum (m) <0.3 0.3-0.6 0.6-1.0 1.0-1.5 1.5-2.0 2.0-3.0 3.0-4.0 4.0-5.0
Elevation national datum (m) <-0.3 -0.3-0 0-0.4 0.4-0.9 0.9-1.4 1.4-2.4 2.4-3.4 3.4--4.4
Area (km2)
Percentage of total area
(%) 603.22 282.45 669.45 1715.28 805.98 1470.69 478.51 216.25 Total 6241.83
9.66 4.53 10.73 27.48 12.91 A 23.56 B 7.67 C 3.47 Total 100.00
Cumulative percentage
(%)
9.66 14.19 24.92 52.40 65.31 88.87 96.54 100.00
,,.
A: area flooded at the present maximum high-tide level (1.49 m). B: area flooded when adding a 1-m storm-surge level. C: area which would be flooded when adding a 0.5-m sea-level rise.
Of course, Chinese practice and experience based on cost-benefit analysis have indicated that all potential dangers can be effectively combated by taking timely counter measures through the implementation of well-prepared response strategies. The most important and urgent strategy would be to upgrade the present protective structures within the framework of an integrated and comprehensive management and disaster prevention plan for the whole delta region. This would reduce the risks, and possibly even prevent major disasters.
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It is emphasised that the Rearl River delta region is one of general economic prosperity. Consequently, the high costs of such a disaster prevention scheme are actually affordable to the local provincial authorities (Han et al. 1995).
4. CONCLUSIONS The situation in the Pearl River delta, as in the case of the Rhine delta in the Netherlands, is a striking example which demonstrates that the effects of human actions have reached the same proportions as those normally associated with natural geological processes. This profound human influence on the environment needs to be taken into consideration when reconstructing the morphodynamic evolution of the Pearl River delta. However, as emphasized in the international literature (e.g., Nicholls 1993; Hillen et al. 1993), such impacts of man must also be taken into account when designing coastal zone/area management plans for sustainable development.
ACKNOWLEDGEMENTS
The authors are obliged to the Global Environment Research Fund of the Japanese Government for supporting the project 'Impact Assessment of Sea-Level Rise due to Global Warming'. The present research is part of this project. The authors are also grateful to Li Ping-Ri of the Guangzhou Institute of Geography, Chinese Academy of Sciences for providing valuable data and information as well as helpful comments. REFERENCES
Han, M.-K., Zhao, S.-S. & Ge, R.-Q. (1988) China coastal environment, utilization and management. University of Chicago Press, Ocean Yearbook 7: 223-240. Han, M.-K., Hou, J.-J. & Wu, L. (1995) Potential impacts of sea level rise on China coastal environment and cities: a national assessment. J. Coast. Res. Spec. Issue 14: 79-95. Hillen, R., Smaal, A., van Huijssteeden, E.J. & Misdorp, R. (1993) The Dutch delta: aspects of coastal zone management. Proc. World Coast Conference 1993. Center Coastal Zone Management, The Netherlands, Vol. 2, pp. 653-658. Li, P.-R., Qiao, P.-N., Zheng, H.-H., Fang, G.-X. & Huang, G.-Q. (1991) The Environmental Evolution of the Zhujiang (Pearl River) Delta in the Last 10,000 Years. Ocean Press, Beijing, 154 p. Li, P.-R., Fan, G.-X. & Huang, G.-Q. (1995) Essential features of the Zhujiang (Pearl River) delta and effects of sea-level rise. In: Earth Science Department (Chinese Academy of Sciences), Impacts of Sea-Level Rise on China's Deltaic Areas and
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Response Strategies. Science Press, Beijing, pp. 315-324 (in Chinese with English abstract). Nicholls, R.J. (1993) Syntheses of vulnerability analysis studies. Proc. World Coast Conference 1993. Center Coastal Zone Management, The Netherlands, Vol. 1, pp. 181-216. Nicholls, R.J. & Leatherman, S.P. (1995) The implications of accelerated sea level rise for developing countries: a discussion. J. Coast. Res. Spec. Issue 14: 303-323. Yan, J.-Y. (1984) Features of tidal flats in the Pearl River delta. Acta Oceanol Sinica 6(4): 471-481 (in Chinese with English abstract).
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
247
The combined impacts of mean sea-level rise and secular trends in mean tidal range on the marine environment in the vicinity of the Huanghe River mouth J. Zhang* and J. Wang
National Marine Data and Information Service, State Oceanic Administration, 93 Liuwei Road, Tianjin 300171, P.R. China
ABSTRACT
In this paper the effects of topographic forcing on the hydrography of Laizhou Bay, imposed by the huge sediment supply of the Huanghe River, is investigated. Particular attention is given to the temporal evolution of various parameters characterizing the M2 tide, especially the mean largest tidal range (MLTR), the mean highest high water (MHHW), the mean lowest low water (MLLW), and the mean sea level (MSL). Numerical simulations and statistical analyses of tidal records show that water levels have increased in general, causing tidal ranges and mean sea level to increase, coupled with a delay in the tidal phase by as much as 1.3 hours. The rapidly changing hydrographic situation urgently calls for a reconsideration of engineering design standards coast in order to mitigate the effects of extreme events along the coast. Thus, the currently assumed 50-year return period should be replaced by a 20year return period.
1. I N T R O D U C T I O N In recent years a number of studies have focussed on the modelling of tidal characteristics in the Yellow Sea region, including its northern appendix, the Bohai Sea. Thus, a range of eddy viscosity closure models were used by Lee & Jung (1999) to examine the M 2 tidal elevations and currents, whereas Lee & Beardsley (1999) investigated the influence of stratification on residual tidal flow. Lefbvre et al. (2000), in turn, tested a global ocean tidal model with respect to its performance at a regional scale, using the Yellow and East China seas as an example. In contrast to these larger scale studies, Huang et al. (1999) modelled the seasonal thermal stratification and baroclinic circulation in the Bohai Sea. Bao et al. (2000) developed a threedimensional tidal model for the Bohai Sea on the basis of a boundary-fitted curvilinear grid in order to achieve a better approximation of the coastline and the * Corresponding author: J. Zhang e-mail:
[email protected]
248
Zhang and Wang
bathymetry than is the case for regular grid models. All these studies made use of the latest ground-truth data available at the time, and the results are therefore constrained by those specific situations. However, in view of the fact that about 1.6 billion tons of sediment are discharged per year into the Bohai Sea by the Huanghe River (Yellow River), it has to be anticipated that the resulting changes in bathymetry and coastal shape will continuously influence the tidal characteristics in the affected region. For example, the delta lobe of the Huanghe progrades into Laizhou Bay at a rate of 1.8-3.6 km per year (Fan & Guo 1992). At such rates, a typical lobe measuring 50 km in length and 20 km in width would develop in a time span of only ten years. As a result of this, the local amphidromic point of the semi-diurnal tide should gradually be pushed further into Laizhou Bay. This topographically forced hydrographic response is expected to change the local character of the tidal wave. To assess the effects which topographic changes in the southern part of Bohai Sea might have on the hydrography of the area, the temporal evolution of the main semidiurnal tidal constituent (M 2 tide) over past decades was investigated. The aim of the exercise is to demonstrate that the resulting changes are significant, that they will continue in the future, and that they can not be neglected when evaluating existing engineering design standards and coastal zone management principles currently applied in Laizhou Bay and around the Huanghe River mouth.
2. METHODS
The evolution of the main semi-diurnal tidal constituent was numerically simulated for the years 1934, 1959, and 1989. In each case the temporal development of tidal amplitudes (HM2) and phase lags (gM2) were considered. The results were validated by comparing them with linear regression analyses of the temporal trends observed in the tidal elevation data recorded since 1952 by the tide gauge in the town of Yangjiaogou, located along the south-west coast of Laizhou Bay. The computations included increases in tidal range ( H M 2 and HS2), the mean largest tidal range (MLTR), the mean highest high water (MHHW), the mean lowest low water (MLLW), and the mean sea level (MSL). In each case the regression equation, the correlation coefficient, and the residual variance are presented. In addition, the yearly highest water levels (YHW) for the period 1972-1994 were determined, including the confidence limits and the expected return periods. The coefficients characterising the general tidal situation were computed from the relationship (HO 1+ HK1)/HM 2.
3. RESULTS 3.1. Changes in the main semi-diurnal tidal constituents The numerical computations of the M 2 tidal regime in Bohai Sea for the years 1934, 1959, and 1989 are illustrated in the tidal charts of Figs 1, 2, and 3, respectively. They
Impacts of mean sea level and tidal range
249
clearly demonstrate that the ocean environment in Laizhou Bay has changed considerably since the early parts of the last century. The tidal situation for the year 1934 (Fig. 1) is illustrated by the distribution of tidal ranges (HM2, solid lines) and tidal phases (gM2, dashed lines). In 1934, the Huanghe River mouth was located at 38~ and 118~ According to the computation, the mean tidal ranges (HM2) in Laizhou Bay varied from 20-35 cm at that time, the tidal range increasing from NW-SE.
Figure 1. Numerically computed tidal chart of the M 2 w a v e in the Bohai Sea for the year 1934. Solid lines denote the mean tidal ranges in cm, dashed lines the phase lags in degrees.
250
Zhang and Wang
By 1959 (Fig. 2), the position of the river mouth had changed to 37~ and 118~ having thus been displaced slightly towards the south-east into Laizhou Bay. By that time, the mean tidal ranges (HM2) in the bay had increased to values of 35 cm in the north and about 55 cm in the south, the increases being of the order of 57-75% within the 25-year period. In contrast to 1934, the tidal range in Laizhou Bay increased from north to south, marking a clockwise rotation of almost 90 ~ This trend is also visible in the orientation of the lines marking the tidal phases in the bay (gM2).
Figure 2. Numerically computed tidal chart of the M 2 wave in the Bohai Sea for the year 1959. Solid lines denote the mean tidal ranges in cm, dashed lines the phase lags in degrees.
Impacts of mean sea level and tidal range
251
By 1989 (Fig. 3), the river mouth had migrated still further into Laizhou Bay, its position now being 37~ and 119~ By that time, the mean tidal ranges (HM2) in the bay had increased to 45 cm in the north and about 80 cm in the south-west. In the 30-year period since 1959, the increase in the tidal range was of the order of 29-45%. Relative to 1934, i.e. over a period of 55 years, the tidal ranges thus increased by 125-229%, the largest increases being observed along the south coast of Laizhou Bay. At the same time the range gradient rotated further to the west.
Figure 3. Numerically computed tidal chart of the M 2 wave in the Bohai Sea for the year 1989. Solid lines denote the mean tidal ranges in cm, dashed lines the phase lags in degrees.
Zhang and Wang
252
With respect to the tide gauge at Yangjiaogou, a port located along the south-west coast of Laizhou Bay, the tidal range increased from 30 cm in 1934 to about 55 cm in 1959, and to ca. 80 cm in 1989. This amounts to an increase of about 267% in the course of the 50-year period investigated in this study. The phase lag of the M+ tide (gM2) at the same location changed from about 285 ~ in 1934 to roughly 310 ~ by 1959, and to more than 325 ~ by 1989. In other words, the tidal phase of the semi-diurnal tide in 1989 was delayed by as much as 1.3 hours relative to 1934. The other main semi-diurnal constituent in Laizhou Bay, the HS 2 tide, has responded in similar manner. The regression analysis of hourly water-level data (Fang et al. 1986) recorded at the tide gauge of Yangjiaogou shows a similar trend in the period from 1952 to 1994. Thus, the tidal range (HM+) of the M 2 tide increased from 34.58 to 67.05 cm, marking an overall increase of almost 33 cm (Fig. 4A), whereas the range of the HS 2 tide increased by 8.6 cm from 12.21 to 20.82 cm (Fig. 4B). The rates of increase in the tidal range over this 42-year period thus amounted to 0.77 cm per year in the case of the HM 2 time series, and 0.21 cm per year in the case of the HS 2 time series. By contrast, the evolution of the tidal range at Longkou, a port situated along the north-east coast of Laizhou Bay (cf. Figs 1-3), showed a negative trend, the M 2 tide dropping by 9 cm, and the S2 tide by 3 cm in the 30-year period from 1965 to 1995.
70-
A 30
',,"
y = 0.205x + 1.546 r = 0.923 q=1.14
6O v
r..3
so
o,le~m
20, ./"
-r-
y = 0.773x - 5.616 r = 0.974 q = 2.37
40
,<.. 9
+0
.To
'. . . .
year
+"'
,.0
,iio
9
-r"
2/00
10
.
.... ............
Figure 4. Increases in the tidal ranges of the M 2 (A) and the and 1994 (r - correlation coefficient; q - residual variance).
.
.
.
,i7o 82
.
.
.
.
year
Im
,
2m
tide (B) between 1952
3.2. C h a n g e s in m e a n water l e v e l s
Corresponding to the evolution of the main tidal constituents, all the relevant water levels recorded at the tide gauge of Yangjiaogou inceased between 1954 and 1994, i.e. the mean largest tidal range (MLTR), the mean highest high water (MHHW), the mean lowest low water (MLLW), and the mean sea level (MSL). This is also reflected in the tidal coefficient which decreased from a value of 1.3 in 1952 to a value of only 0.69 in 1994. From 1954 to 1994, MLTR increased at a rate of 1.6 cm per
253
Impacts of mean sea level and tidal range
year (Fig. 5A), M H H W at 1.0 cm per year (Fig. 5B), MLLW at 0.6 cm per year (Fig. 6A), and MSL at 0.23 cm per year (Fig. 6B). 18o
A
4~-
/
B
170
" " , ~ - ' " "" .'/
y = 1,366x+48,337 r = 0,952
.._.. 1s0 E
""
E
O
r'r" 150' I--
9
9
9
390
-r-
..J
~E
y = 0.776x + 319.782 r = 0,907
400
-r- 380 140 370
130
120
10
19"/0
1980
1990
2i00
360
10 9~".....
10 6
.........
1980 '
1990 '
~
2000 '
............
year
year
Figure 5. Evolution of the mean largest tidal range, MLTR (A), and the mean highest high water, M H H W (B) between 1954 and 1994 (r - correlation coefficient; q residual variance).
A
250"
Y = -0.589x + 271.445 r = -0,784 q = 5.55
9
._.240
34o
B
y = 0.213x + 293.330 r = 0,562 q =4.I8
9
E
"~" 320
e
-.<
230
..
0 ..j
9
9
.-.J .-J
~; 300 220
9 ~ - ~ " . ~ . . . . _ t i - - ----~ 9 . .
oo
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' t960
' ~970
' ~9~0
~9~0
20'00
280
........... j ..............
~96o
year
~97o
i98o
year
~99o
...... ' 2000
Figure 6. Evolution of the mean lowest low water, MLLW (A), and the mean sea level, MSL (B) between 1954 and 1994 (r - correlation coefficient; q = residual variance).
3.3. F r e q u e n c i e s and return p e r i o d s of yearly h i g h e s t waters
The frequencies and return periods of yearly highest waters (YHW) recorded between 1972 and 1994 at the tide gauge of Yangjiaogou are illustrated in Fig. 7A (see Table 1). This is contrasted with the YHWs predicted purely on the basis of the observed increases in the tidal range. As illustrated in Fig. 7B (see Table 1), the predicted YHWs increase at a rate of 1 cm per year. This means that the predicted YHW for the 50-year return period is 20 cm higher than the observed height, whereas
Zhang and Wang
254
that for the 100-year return period is 23 cm higher. Clearly, if MSL rise and other effects are also considered, the predicted differences in height would be even larger. return period (years)
return period (years) 100
20 10
5
2
5
100 10 20
5
2
5
10 20
100
100
A
73.~..-~\\\
20 10
-
770I",,,,
B
-
.._.670 ",,\ -r
E
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" -'~,~....
530-
"
%
-
,,01
430 t
5 10 20 30
N.
9"'.~N~."-..
4901 1 2
I"-,
50
70
1 2
90 95 98 99
5 10 20 30
%
50
70
90 95 9899
Figure 7. Measured (A) and predicted (B) frequencies and return periods of yearly highest water (YHW) levels at Yangjioagou between 1972 and 1994.
Table 1. The distribution of yearly highest water at Yangjiaogou. Frequency
(%)
Observed data
Confidence limit
709 685
2 3
660 641
5
624
10
596
Confidence limit
Return period (years)
73 59 46 40
200 100
(cm)
(cm) 0.5 1
Simulated data
67 55 43 37 30 22
734 708 680 659 641 611
32
50 33 20
23
10
4. DISCUSSION, CONCLUSIONS AND IMPLICATIONS
The results of this study clearly show that, in the course of the last century, the ranges of the M Rtide (HM2) have become larger in the western part of Laizhou Bay, the tidal phase being postponed at the same time. In the south-eastern part of the
Impacts of mean sea level and tidal range
255
bay, by contrast, the tidal ranges have become slightly smaller. A plausible explanation for this progressive change of the tidal regime in Laizhou Bay is the large amount of sediment supplied by the Huanghe River, and the resulting deposition in the vicinity of the mouth. Such an influence of topographic forcing on the character of the tidal wave was first suggested by Fang & Young (1985). The delay in the computed tidal phases, in fact, proves that the amphidrome of the semidiurnal tide must have shifted progressively further to the east into Laizhou Bay, i.e. away from the coast, although the computed tidal charts do not show this as clearly as one might have expected. The changes in the depth and the shape of the bay have thus led to a pronounced increase in various measures of tidal elevation, e.g., the mean largest tidal range (MLTR), the mean highest high water (MHHW), the mean lowest low water (MLLW), and the mean sea level. In comparison to the British Isles (Woodwort et al. 1991), the rate of increase in MLTR at Yangjiaogou is generally higher. On the other hand, the trends in mean tidal ranges (MTR) are very low or even negative in Scotland (Aberdeen and Lerwick) as well as in the central and western Irish Sea (Holyhead, Douglas, and Dublin), probably due to the postglacial isostatic uplift. Along the coast of England, by contrast, the mean tidal range increases by 0.5-1.0 mm per year (Woodwort et al. 1991). Since the evolution of the tidal character in Laizhou Bay is still in progress, it must be anticipated that the increasing tidal range, coupled with the observed rise in sea level, will result in continuing coastal erosion and a concommitant intrusion of saline waters along the southern and south-western coast of Laizhou Bay. A positive effect has been that, as the water has deepened, vessels which formerly were unable to negotiate the local shipping lanes can today pass safely even during neap tides. In view of this, it is of fundamental importance that MSL rise, increases in the tidal range, and sedimentation trends in the Huanghe estuary are closely monitored in order to adjust coastal management principles in time to avoid or at least mitigate natural disasters. Low design standards have often been the main reason for severe disasters in the past. Thus, because of MSL rise and increases in MHHWs, the tropical storms No. 16 in 1992, and No. 11 in 1997 caused the destruction of many sea farms and dams along the west and south coasts of Laizhou Bay. The complex relationships between sediment supply, MSL rise, changing tides and current regimes need to be investigated further. The original coastline of Laizhou Bay has changed substantially because of sediment accumulation, coastal progradation, and the formation of intertidal mud flats. Tidal currents have changed direction and today flow at different speeds. As mud is eroded from the seabed, the water gets deeper, tidal ranges get larger, current speeds become faster, coastal erosion increases, and salinisation of the coastal lowlands becomes more severe. Especially during storm surges and cold air influxes, the coast is eroded by huge waves and strong storm-induced currents. In view of this dramatic situation, engineering design standards currently applied in the area around the mouth of the Huanghe River will have to be adjusted in accordance with the rapid changes affecting the environment. The current practice of
256
Zhang and Wang
merely using the observed highest or lowest annual water levels for the estimation of extreme water levels is quite inappropriate within a scenario in which MSL is rising, and yearly highest water levels are changing continuously. The concept of a 50-year return period of estimated highest water levels should be replaced by a 20-year return period, because MHHWs and MSL have risen by more than 35 cm and 9 cm since the end of the 1950s, respectively.
ACKNOWLEDGEMENTS
Guohong Fang and Deming Cao kindly provided some recent results on numerical simulations of the M 2 tidal constituent. Funian Chen helped with the drawing of the figures.
REFERENCES
Bao, X.W. (2000) A three-dimensional tidal model in boundary-fitted curvilinear grids. Estuar. Coast. Shelf Sci. 50: 775-788. Fan, Z. & Guo, Y. (1992) Analysis charts of remote sensing trends along the Yellow River delta. China Ocean Press, Beijing. Fang, G. (1986) Tide and tidal current chart for the marginal seas adjacent to China. Chin. J. Oceanol. Limnol. 4: 1-16. Fang, G. & Yang, J. (1985) A two-dimensional numerical model of the tidal motions in the Bohai Sea. Chin. J. Oceanol. Limnol. 3(2): 135-152. Fang, G., Zheng, W., Chen, Z. & Wang, J. (1986) The analysis and prediction of tides and tidal currents. China Ocean Press, Beijing. Huang, D., Su, J. & Backhaus, J.O. (1999) Modelling the seasonal thermal stratification and baroclinic circulation in the Bohai Sea. Cont. Shelf Res. 19: 1485-1505. Lee, J.C. & Jung, K.T. (1999) Application of eddy viscosity closure models for the M2 tide and tidal currents in the Yellow Sea and the east China Sea. Cont. Shelf Res. 19: 445-475. Lee, S.-H. & Beardsley, R.C. (1999) Influence of stratification on residual tidal currents in the Yellow Sea. J. Geophys. Res. 104(C7): 15,679-15,701. Lef6vre, F., Le Provost, C. & Lyard, F.H. (2000) How can we improve a global ocean tide model at a regional scale? A test on the Yellow Sea and the East China Sea. J. Geophys. Res. 105(C4): 8707-8725. Woodwort, P.L., Shaw, S.M. & Blackman, D.L. (1991) Secular trends in mean tidal range around the British Isles and along the adjacent European coastline. Int. J. Geophys. 104: 593-609.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine, M.T. and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
257
The missing m u d flats of the W a d d e n Sea: a reconstruction of sediments and accommodation space lost in the wake of land reclamation S. Mai* and A. Bartholom~i
Senckenberg Institute, Schleusenstr. 39a, D-26382 Wilhelmshaven, Germany
ABSTRACT
Over past centuries, the combined effect of land reclamation and sea-level rise has resulted in higher average energy levels along the mainland coast of the West and East Frisian Wadden Sea because catchment areas have been greatly reduced in size. This is reflected in a conspicuous depletion of fine-grained sediments in modern back-barrier tidal flats, pure mud flats being found only in a few sheltered coastal embayments. However, the widespread occurrence of clay-rich soils on the reclaimed lands suggests that large mud flats and salt marshes must have existed in the past along most of the southern North Sea coast. In this study, the results of a modelling exercise are presented in which the composition and the accommodation space of fine-grained sediments lost in the course of land reclamation have been reconstructed for two East Frisian tidal basins. This has been achieved by extending the shorenormal gradients documented in the mean settling velocities of Wadden Sea sediments landwards into the reclaimed areas by numerical procedures. The modelling results confirm that large intertidal flats occupied these areas, and that the sediments were predominantly composed of grain sizes <63 gm. The results suggest that land reclamation is the main reason for the missing mud flats in the Wadden Sea. Since sea-level rise is accompanied by increasing energy levels in back-barrier tidal basins, fine-grained sediments will continue to be eliminated from the landward margin of the Wadden Sea. Any acceleration in sea-level rise will therefore aggravate the situation.
1. INTRODUCTION Natural mud flats and salt marshes are a surprisingly rare sight in the Wadden Sea of today (Dijkema 1987, 1989). The apparent deficiency of fine-grained sediments along the diked shoreline was initially explained by invoking natural processes (cf. Hansen 1951; Figge et al. 1980). Hansen (1951), for example, suggested that mud could only deposit in the vicinity of mussel banks and salt marshes, implying that * Corresponding author: S. Mai e-mail:
[email protected]
258
Mai and Bartholomii
fine sediment deposition required the presence of suitable trapping mechanisms. Others argued that in the wake of land reclamation sufficiently sheltered conditions were created along the foot of the dikes to allow suspended sediments to settle out (e.g., van Straaten & Kuenen 1957), the influx being controlled by the so-called settling lag/scour lag mechanism (cf. also Postma 1961, 1967). The question as to why only narrow belts of mixed flats and hardly any natural salt marshes developed under such circumstances was evidently never asked. By contrast, more recent investigations have identified land reclamation and dike construction as the main cause for fine-grained sediment depletion in the Wadden Sea (e.g., Flemming & Nyandwi 1994). By gradually shifting the dike line seawards, tidal catchments and tidal prisms were progressively reduced. This led to an overall decrease in the tidal energy flux through the inlets and main channels. However, over the tidal flats the rate of energy dissipation remained almost unchanged because the perimeters of the back-barrier tidal basins decreased in proportion to the tidal prism. This is clearly reflected by the fact that the shore-normal grain-size gradient did not adjust to maintain the complete, even though narrower, succession of facies belts (Flemming & Davis 1994; Flemming & Nyandwi 1994; Flemming & Ziegler 1995). Instead, the shoreward-fining succession of sedimentary facies was simply truncated by the dikes, although a slight compression of the remaining facies has been observed locally (Mai 1998). As a result, the sediments along the foot of the dikes are today much coarser than they used to be along the former natural shoreline which is now located several kilometres inland (Fig. 1). This clearly demonstrates that energy levels along the diked shoreline are too high to allow the deposition of larger amounts of fine-grained sediments. As a consequence, the settling-lag/scourlag mechanism of van Straaten & Kuenen (1957; cf. also Postma 1961, 1967), which explains the shoreward-fining trend, has to be modified to include an export loop for those sediments which are too fine for deposition along the modern coast (Flemming & Nyandwi 1994; Flemming & Bartholom~i 1997). By implication, the muds in the mixed flats lining the modern shoreline must comprise flocs and aggregates which are hydraulically equivalent to the local sand particles. The shoreward-fining trend thus reflects an energy gradient which is proportional to the mean grain size, and hence the mean settling velocity, of the sediment particles deposited along the gradient (Kr6gel 1997; Kr6gel & Flemming 1998; Mai 1998). Transgressive tide-dominated depositional systems are normally displaced landwards in the course of sea-level rise. Due to land reclamation, the Wadden Sea system of today is instead being squeezed between a migrating barrier island chain and a rigid dike protecting the mainland coast. The magnitude of this coastal squeeze has in the past been partly reconstructed on the basis of borehole data and old land reclamation maps (e.g., Dijkema 1987; Zagwijn 1987; Streif 1989; Vos 1992; Vos & van Kesteren 2000). Such reconstructions have invariably confirmed the former existence of wide mud flats and salt marshes throughout the Holocene from about 8000 years BP to the onset of land reclamation some 1000 years ago (cf. Fig. 1). Thus far, however, no attempts have been made to link the reconstructed sediment successions
Missing Wadden Sea mud flats
259
with hydrodynamic processes other than invoking the settling lag/scour lag mechanism and the asymmetry of tidal currents (Groen 1967).
Figure 1. Schematic shore-normal cross-section through the East Frisian Wadden Sea illustrating the vertical and horizontal sediment succession (modified after Streif 1989). Note the position of the dike relative to the former mainland shore. To tackle this issue, the accommodation space and the composition of fine-grained sediments lost in the course of land reclamation were reconstructed for two East Frisian tidal basins by numerically projecting the observed shore-normal settling velocity gradients landwards into the reclaimed areas. The results of the exercise were then assessed with respect to possible implications for other tidal catchments along the coast, as well as for the future evolution of the Wadden Sea in the course of continued and possibly accelerating sea-level rise.
2. PHYSICAL SETTING The study focuses on the two tidal basins straddling the back-barrier tidal flats between the islands of Baltrum, Langeoog and Spiekeroog in the East Frisian Wadden Sea of Germany (Fig. 2). The two catchments have a combined area of about 164 km 2 (Accumer Ee tidal basin: 90 km2; Otzumer Balje tidal basin: 74 km 2) which makes up 1.76% of the Wadden Sea as a whole (9300 km2; L6zan et al. 1994). The tides are semidiurnal and the mean tidal range varies from 2.6 m in the Accumer Ee inlet to 2.7 m in the Otzumer Balje inlet. Some 1000 years BP the size of the catchments were considerably larger, their shapes and positions having changed in the course of land reclamation (Homeier & Luck 1969). Since the end of the "Little Ice Age" some 300 years ago, the local mean sea level has been rising continuously, the local rate currently being 18 cm/century. Indeed, the mean high-water level, which has a greater impact on shoreline processes than the mean sea level, is currently rising at a rate of 25 cm/century in the wider
260
Mai and Bartholomd
study area. While some barrier islands have responded by shoreward migration, others have remained stationary (Flemming & Davis 1994). Since neither the West Frisian nor the East Frisian Wadden Sea receives a substantial sediment input from remote sources, it is predicted that the entire barrier island chain will eventually respond by landward migration to compensate the back-barrier sediment deficit created by sea-level rise (Flemming & Bartholom/i 1997). This response will be greatly enhanced if sea-level rise should accelerate in coming decades as predicted.
Figure 2. Locality map of the East Frisian barrier island coast, showing the tidal basins investigated in this study. The progressive shoreward fining in grain sizes is illustrated by the spatial pattern produced by individual 0.5 phi grain-size fractions which are arranged in discrete shore-parallel belts (Fig. 3). The coarsest sand fraction (2.0-2.5 phi or 250-177 lJm) is evidently centred over the islands. The next finer one (2.5-3.0 phi or 177-125 1Jm) occupies most of the modern back-barrier tidal flats. The 3.0-3.5 phi (125-88 1Jm) fraction lines the mainland shore, its landward extension having been truncated by the dike. The finest sand fraction (3.5-4.0 phi or 88-63 lJm) has been almost completely eliminated by the land reclamation process, its depocentre occupying the newly created farmlands behind the dike. In the present situation measurable mud deposition starts where the sediments begin to be dominated by very fine sand, i.e. where the 3.0-4.0 phi (125-63 gm) sand fraction reaches a content of about 40% (cf. Flemming & Davis 1994; Flemming & Ziegler 1995). From there onward the sediments are characterised by slightly muddy (5-25% mud content) and muddy sands (25-50% mud content; classification after Flemming 2000), contents >40% being exceptional (Flemming & Davis 1994; Kr6gel 1997).
Missing Wadden Sea mud fiats
261
Figure 3. Spatial arrangement of individual 0.5 phi size fractions in the back-barrier tidal flats of the study area. Note the sequence of overlapping, shoreward-fining belts, the two finest sand fractions being truncated by the dike.
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These muds evidently represent the coarse tail of the floc and aggregate size spectrum making up the suspended sediment fraction. The concurrent deposition of these flocs and aggregates with the very fine sand fraction suggests hydraulic equivalence of the two sediment components, the proportion of mud progressively increasing with decreasing sand grain size (or settling velocity) in correspondence with the shoreward decreasing energy gradient.
3. METHODS 3.1. Settling velocity analysis In this study an existing data set, generated by the analysis of over 3000 sediment samples collected on a grid measuring about 0.15' Lat./0.25' Lon. (ca. 275 x 275 m, cf. Flemming & Ziegler 1995; Kr6gel 1997), was analysed. The raw samples had previously been processed by standard laboratory procedures (e.g. Carver 1971) before settling velocity distributions of the sand fractions were measured in a highresolution settling tube (Flemming & Thum 1978; Brezina 1979; Flemming & Ziegler 1995). The analyses were standardised to a hydraulic shape factor of SF' - 1.18 (Corey 1949), a particle density of 6~ - 2.65 g cm -3, a water temperature of T - 5~ a salinity of S = 30%o, and a local gravitational acceleration of g - 981.37 cm s-2, using the software package SedVar6.2p 9 (Brezina 1997). Textural parameters were calculated on the basis of moment measures and percentile statistics following Inman (1952). A water temperature of 5~ and a salinity of 30%0 were chosen because the highest energy fluxes in the Wadden Sea occur during the winter season, and because the sediment is more mobile in winter due to the higher kinematic viscosities of the sea water at low temperatures. As a result of this, it has been postulated that the sediment distribution in the Wadden Sea is adjusted to average winter temperatures (Kr6gel & Flemming 1998). The muds (grain size fractions <63 gm) were not analysed any further because the floc and aggregate structure of these sediments are commonly destroyed in the course of sampling and laboratory processing. Since the composite particles have a completely different hydraulic behaviour than their constituents, it serves no purpose to perform grain size or settling velocity analyses on disaggregated sediments if hydrodynamic interpretations are to be made (Terwindt 1977, van Leussen 1994; Bartholom~i & Flemming 1997).
3.2. Procedure for the extrapolation of settling velocity gradients Following the reasoning of Flemming & Nyandwi (1994), Flemming & Bartholom~i (1997), and Kr6gel & Flemming (1998), land reclamation has evidently removed a large portion of the Wadden Sea formerly lining the entire West and East Frisian coastlines. In order to estimate the size of the accommodation space and to assess the composition of the sediment lost in the course of land reclamation, a realistic reconstruction of the physical nature and landward extent of the former tidal basins is required. This was achieved in two steps. First, the landward limits of the former
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tidal basins were determined by projecting the elevation of the present mean highwater line (1.4 m above the German chart datum abbreviated as NN) onto highresolution topographic charts of the coastal region bordering the study area. In a second step, the existing mean settling velocity gradients were extrapolated landwards along each of the shore-normal profiles defined by the north-south lines of the sample grid (spaced at 275 m). However, while the conversion of mean settling velocities into equivalent mean settling diameters is a simple computational exercise, it does not automatically provide information on the local sediment composition. In the present context it was of foremost interest to reconstruct the landward progression of sedimentary facies based on mud contents (cf. Flemming 2000). This was achieved by plotting the mean settling velocity of each sediment sample against the settling velocity of its coarsest percentile. The well correlated trend defined by the scatter plot (cf. Fig. 4) was then extrapolated to the point where the settling velocity of the coarsest percentile (1%) corresponded to the equivalent settling diameter of the s a n d / m u d transition. From this point onward the sediment would on average consist of pure mud, i.e. of grain sizes <63 ~tm. At a water temperature of T = 5~ and a salinity of S - 30%0 this transition is defined by a settling velocity of ~0.21 cm s -1 (+2.3 psi). The corresponding mean settling velocity is ~0.107 cm s 1, and the equivalent mean settling diameter ---45 ~tm. The so-called 100% mud line is then reproduced by linking the intersections defined by the 0.107 cm s -1 mean settling velocity along each extrapolated profile.
Figure 4. Plot of mean settling velocity against the settling velocity of the coarsest percentile. By extrapolating the regression line, the mean settling velocity value is determined at which the coarsest percentile reaches the settling velocity of a 63 ~tm particle (valid for T = 5~ and S - 30%0).
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4. RESULTS
The distribution pattern of 1mean settling velocities in the back-barrier tidal basins of the study area is illustrated in Fig. 5. Clearly visible is the shore-parallel trend defined by individual mean settling velocity contours. Furthermore, the distance between some settling velocity contours varies with the width of the back-barrier basin, indicating selective compression of some facies belts. However, even in the case of the tidal flat behind Baltrum, where the apparent compression is most pronounced, the lowest mean settling velocities adjacent to the dike are still 10-20% higher than in the corresponding areas to the east. More significantly, however, the near-dike sands behind Baltrum also have the lowest mud contents of any corresponding sediment in the entire study area (cf. Flemming & Nyandwi 1994).
Figure 5. Map showing the distribution of mean settling velocities in the study area. The spatial settling velocity pattern illustrated in Fig. 5 clearly documents the postulated landward decreasing energy gradient. By extrapolating the computed mean settling velocity gradients along the hundred odd cross-shore sample profiles, the landward limit of the 100% mud line was determined. As mentioned before, the mud line is defined by a mean settling velocity of 0.107 cm s-1 (45 lJm). The results of this procedure are illustrated by two typical gradients, one representing the Baltrum tidal flats, the other the Langeoog tidal flats (Fig. 6). In each case the last point along the x-axis (distance from the back-barrier island shore) marks the sample position closest to the dike. The regression line is extrapolated until it meets the horizontal line marking the mean settling velocity along the y-axis which corresponds to a sediment consisting of pure mud (0.107 cm s-l). Intermediate mud contents were selected in similar manner.
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Figure 6. Illustration of two typical cross-shore settling velocity gradients, one from the Baltrum tidal flats, the other from the Langeoog tidal flats. The landward limits of the tidal basins selected for this study were reconstructed by extracting the elevation contour corresponding to the modern mean high-water level (+1.4 m NN) from a high-resolution topographic map of the region and transferring it onto a base chart which included the islands and tidal basins of the study area (Fig. 7). The chart highlights the theoretical position which the modern shoreline would occupy if the modern dike is removed. The data generated by the procedure illustrated in Fig. 6 was then transferred onto this base chart by adding the mean settling velocity contours corresponding to 10%, 25%, 50%, and 75% mud contents. In this way a sediment facies map was constructed which includes sand flats (<10% mud content), slightly muddy sand flats (10-25% mud content), muddy sand flats (25-50% mud content), sandy mud flats (50-75% mud content), slightly sandy mud flats (75-100% mud content), and mud flats (100% mud content). The area of the mud flats reconstructed in this way amounts to 53.9 km a in the case of the western tidal basin (Accumer Ee inlet), and 105.5 km 2 in the case of the eastern tidal basin (Otzumer Balje inlet). The combined areas of the reconstructed mud flats amount to 159.4: km 2. In a second step an attempt was made to reconstruct the sediment facies succession if no land reclamation had taken place (Fig. 8). In this more realistic scenario the entire depositional system would have shifted landwards by an amount corresponding to the sediment deficit created by the sea-level rise experienced since the "Little Ice Age" (locally ca. 0.5 m). An corresponding reconstruction is based on the assumption that the landward displacement would have amounted to about 2 km.
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Figure 7. Reconstruction of the tidal basin with the associated sediment facies succession by simply removing the modern dike. Note the large areas and huge m u d flats lost in the course of land reclamation. This is the distance resulting from computations based on the assumption that no sands were imported from remote sources. The effect of such a displacement on the sediment facies succession can be judged by comparing the positions of the various m u d contours on Fig. 8 with those on Fig. 7. Since the mean high-water level, and hence the shoreline, remains at a height of +1.4 m NN, the m u d flat and some of the other sediment belts, located near sections of the shoreline, would have been progressively squeezed out in the course of sea-level rise. The areas occupied by pure m u d flats now amount to 47.2 km a in the case of the western tidal basin (Accumer Ee inlet), and 96.3 km a in the case of the eastern tidal basin (Otzumer Balje inlet). Relative to the scenario illustrated in Fig. 7, the reconstructed m u d flats would be 12.4% and 8.7% smaller, respectively.
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Figure 8. Reconstruction of the natural tidal basin with the associated sediment facies succession if land reclamation had not taken place. Note the landward displacement of the whole system relative to Fig. 7, and the resulting loss of some mud flats along the mainland shore.
5. D I S C U S S I O N A N D I M P L I C A T I O N S
The reconstruction of the accommodation space and sediment lost in the wake of land reclamation along a 25 km section of the East Frisian coast in northern Germany highlights several important points. First of all, it demonstrates that the method of landward extrapolation of mean settling velocity gradients observed on modern tidal flats produces realistic and consistent results, and that such gradients are evidently directly proportional to shore-normal energy gradients. It is furthermore shown that large former intertidal areas have been converted into arable land along the East Frisian coast by the active intervention of man in the course of the last Millennium. In fact, the total area of the reclaimed tidal basins
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reconstructed in this study (ca. 222 km 2) is substantially larger than that of the remaining modern tidal basins (164 km2). Land reclamation has thus removed 58% of the potentially available tidal flats in the present case. However, the areas lost in this way vary strongly from place to place along the Wadden Sea coast. Because of the wide coastal plain along the Dutch North Sea coast, the lost areas must be very much larger there, a fact which is clearly implicated in the reconstructions of Zagwijn (1986) and Vos & van Kesteren (2000). Along the Danish coast, on the other hand, the areas lost will approach zero in some places because elevated Pleistocene deposits border directly on the Wadden Sea (cf. Jacobsen 1986; Bartholdy & Pejrup 1994). The reconstructions demonstrate that most of the reclaimed land consisted of intertidal mud flats. Clearly, some landward parts of these mud flats would have been colonized by salt marshes comprising the pioneer plants Salicornia and Spartina. However, at this stage it is not possible to estimate the space such marshes would have occupied. To do this, it would be necessary to generate a theoretical elevation model of the tidal basins in question. According to Erchinger (1987), the seaward limit of the pioneer zone would have to coincide with the position of the <3.5 h tidal submergence level (<3 h quoted in Dijkema et al. 1990). In most places this elevation seems to correspond to the mean neap high-tide level. Such reconstructions are an important challenge for future modelling exercises. Finally, the question arises as to whether the extrapolation method developed in the course of this study can also be applied to other parts of the Wadden Sea coast. Clearly, a prerequisite for this would be the existence of similarly well defined shorenormal settling velocity gradients as those observed along the East Frisian coast. That this is indeed the case is illustrated by a number of analogous cross-shore profiles from other Wadden Sea regions (Fig. 9), the samples having been collected and processed in similar manner as those used in this study. A comparison of the settling velocity gradients from Mando (Danish Wadden Sea), Pellworm (North Frisian Wadden Sea, Germany), and Rottumeroog (Dutch Wadden Sea) with that of Langeoog (this study) not only reveals similarities but also small, though important differences. Thus, all the profiles show highly correlated shorenormal settling velocity gradients. This suggests that the extrapolation method can be successfully applied to the entire Wadden Sea. The gradients are not identical however, a feature that was also evident along the East Frisian coast (cf. Fig. 6). Thus, every area has a slightly different gradient, the one from Pellworm having the lowest slope, the one from Baltrum (cf. Fig. 6) the steepest. Significantly, the Pellworm profile is the longest of the set, the tidal flats being particularly wide here, whereas the Baltrum profile is the shortest, the tidal flats being very narrow. Corresponding to their length, the other profiles more or less occupy intermediate positions. This trend would suggest that settling velocity gradients, and hence energy gradients, get steeper with decreasing tidal flat width. Whether this merely reflects the different orientations of the coastlines relative to each other, thereby enhancing or subduing hydrodynamic energy fluxes, or whether it documents a more complex and fundamental principle, remains to be seen.
Missing Wadden Sea mud yqats
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Figure 9. A comparison of shore-normal settling velocity gradients from the Dutch, German, and Danish Wadden Seas. Note the consistent and highly correlated trends. The results have confirmed that land reclamation is the main reason for the missing mud flats in the Wadden Sea of today. The current sea-level rise will continue to eliminate fine-grained sediments from the landward margin of the Wadden Sea because it is associated with proportional increases in energy fluxes. Any acceleration in sea-level rise will hence aggravate this situation.
ACKNOWLEDGEMENTS
This study was financially supported by the Senckenbergische Naturforschende Gesellschaft and the Senate of the City of Bremen. The support of both are gratefully acknowledged. Astrid Raschke is thanked for her assistance in the laboratory.
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REFERENCES
Bartholdy, J. & Pejrup, M. (1994) Holocene evolution of the Danish Wadden Sea. Senckenbergiana marit. 24: 187-209. Bartholom~i, A. & Flemming, B.W. (1997) Zur Sedimentdynamik in den ostfriesischen R~ckseitenwatten und den Ver~inderungen durch nat~irliche und anthropogene Einfli~sse. In: Schutzgemeinschaft Deutsche Nordseek~iste e.V. (ed.), Klima und K~iste. SDN Schriftenreihe 9: 70-89. Behre, K.-E. (1979) Zur Rekonstruktion ehemaliger Pflanzengesellschaften an der deutschen Nordseek~iste. In: Williams, O. & T~ixen, R. (eds), Werden und Vergehen von Pflanzengesellschaften. Cramer, Vaduz, pp. 181-214. Brezina, J. (1979) Particle size and settling rate distributions of sand-sized materials. 2nd Europ. Symp. Partikelmet~technik, 24.06.-29.06.1979, N~irnberg, pp. 44 Brezina, J. (1997) SedVar 6.2p PC-software for calculating settling velocities and grain sizes. Heidelberg, Granometry | Carver, R.E. (1971) Procedures in Sedimentary Petrology. Wiley-Interscience, New York, 653 p. Corey, A.T. (1949) Influence of shape on fall velocity of sand grains. M. Sc. thesis, Colorado A & M College, 102 p. Dijkema, K.S. (1987) Changes in salt-marsh area in the Netherlands Wadden Sea after 1600. In: Huiskes, A.H.L., Blom, C.W.P.M. & Rozema J. (eds), Vegetation between land and sea. W. Junk Publisher, Dordrecht, pp. 42-49. Dijkema, K.S. (1989) Habitats of The Netherlands, German and Danish Wadden Sea. RIN and Veth. Foundation, Texel/Leiden, 30 p. Dijkema, K.S., Bossinade, J.H., Bouwsema, P. & de Glopper, R.J. (1990) Salt Marshes in the Netherlands Wadden Sea: rising high-tide levels and accretion enhancement. In: Beukema, J.J., Wolff, W.J. & Brouns, J.J.W.M. (eds.), Expected Effects of Climatic Change on Marine Coastal Ecosystems. Kluwer, Dordrecht, pp. 173-188. Erchinger, H.F. (1987) Funktion und Bedeutung der Salzwiesen. In: Nieders~ichsisches Umweltministerium (ed.), Umwelt-Vorsorge Nordsee, pp. 303-316. Figge, K., K6ster, R., Thiel, H., Weiland, P. (1980) Schlickuntersuchungen im Wattenmeer der Deutschen Bucht. Zwischenber. Forschungsproj. KFKI. Die K~iste 35: 187-204. Flemming, B.W. (2000) A revised classification of gravel-free muddy sediments on the basis of ternary diagrams. Continental Shelf Research (in press). Flemming, B.W. & Thum, A.B. (1978) The settling tube - a hydraulic method for grain size analysis of sands. Kiel. Meeresforsch. Sonderh. 4: 82-95. Flemming, B.W. & Davis, R.A. Jr. (1994) Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). Senckenbergiana marit. 24(1/6): 117-155. Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats, southern North Sea. Neth. J. Aquat. Ecol. 28(3-4): 299-307.
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Flemming, B.W. & Ziegler, K. (1995) High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog island, southern North Sea. Senckenbergiana marit. 26(1/2): 1-24. Flemming, B.W. & Bartholom~i, A. (1997) Response of the Wadden Sea to a rising sea level: a predictive empirical model. German J. Hydrogr. 49(2/3): 343-353. Flemming, B.W., Bartholom~i, A. & Mai, S. (1998) Sedimentzonierung im Ostfriesischen Wattenmeer. In: T~irkay, M. (ed.), Wattenmeer. Kleine Senckenberg Reihe 29, pp. 21-24. Griede, J.W. & Roeleveld, W. (1982) De geologische en paleogeografische ontwikkeling van het Noordelijk Zeekleigebied. KNAG Geograf. Tidsskrift 16: 439-543. Groen, P. (1967) On the residual transport of suspended matter by an alternating tidal current. Neth. J. Sea Res. 3: 564-574. Hansen, K. (1951) Preliminary report on the sediments of the Danish Wadden Sea. Meddelelsar fra Dansk Geologisk Forening 12: 1-26. Homeier, H. & Luck, G. (1969) Das historische Kartenwerk 1:50000 der Nieders~ichsischen Wasserwirtschaftsverwaltung als Ergebnis historisch-topographischer Untersuchungen und Grundlagen zur kausalen Deutung hydrologisch-morphologischer Gestaltungsvorg~inge im K~istengebiet. Ver6ffent. Nieders~ichs. Inst. Landeskunde u. Landesentwicklung Univ. G6ttingen, Reihe A. Forschungen zur Landes- und Volkskunde, G6ttingen Wurm in Komm, 28 p. Inman D.L. (1952) Measures for describing the size distribution of sediments. J. Sediment. Petrol. 22: 125-145. Jacobsen, N.K. (1986) The intertidal sediments. Geogr. Tidsskr. 86: 46-62. Kr6gel, F. (1997) Einflut~ von Viskosit~it und Dichte des Seewassers auf Transport und Ablagerung von Wattsedimenten, Langeooger R~ickseitenwatt, s~dliche Nordsee. Ber. Fachber. Geowissens. Univ. Bremen 102, 168 p. Kr6gel, F. & Flemming, B.W. (1998) Evidence for temperature-adjusted sediment distributions in the back-barrier tidal flats of the East Frisian Wadden Sea (southern North Sea). SEPM Spec. Publ. 61: 31-41. Leussen, W. van (1994) Estuarine macroflocs and their role in fine-grained sediment transport. National Institute of Coastal and Marine Management, Den Haag, 484 p. L6zan, J.L., Rachor, E., Reise, K., Westernhagen, H. von & Lenz, W. (eds) (1994) Warnsignale aus dem Wattenmeer. Blackwell, Berlin, 387 p. Mai, S. (1999) Die Sedimentverteilung im Wattenmeer: ein Simulationsmodell. Berichte, Fachbereich Geowissenschaften, Universit~it Bremen, No. 142, 114 p. Postma, H. (1961) Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res. 1(1/2): 148-190. Postma, H. (1967) Sediment transport and sedimentation in the estuarine environment. In: Lauff, G.A. (ed.), Estuaries. Am. Assoc. Adv. Sci., Washington, pp. 158-179. Reise, K. (1998) Coastal change in a tidal backbarrier basin of the northern Wadden Sea: are tidal flats fading away?. Senckenbergiana marit. 29(1/6): 121-127. Straaten, L.M.J.U. van & Kuenen, Ph.H. (1957) Accumulation of fine grained sediments in the Dutch Wadden Sea. Geol. Mijnb. 19: 329-354.
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Streif, H. (1989) Barrier islands, tidal flats and coastal marshes resulting from a relative rise of sea level in East Frisia on the German North Sea coast. In: Linden, W.J.M. van der, Cloetingh, S.A.P.L., Kaasschieter, J.P.K., Graaff, W.J.E. van de, Vandenberghe, J. & Gun, J.A.M. van der (eds), Proc. KNGMG Symp. Coastal lowlands, geology and geotechnology, 1987. Kluwer, Dordrecht, pp. 213-223. Terwindt, J.H.S. (1977) Deposition, transportation and erosion of mud. In: Golterman, H.L. (ed.), Interaction between sediment and fresh water. Proc. Int. Symp., 6-10 September 1976, Amsterdam. W. Junk Publishers, The Hague, pp. 1924. Vos, P.C. & Kesteren, W. van (2000) The evolution of intertidal mudflats in the northern Netherlands during the Holocene: natural and anthropogenic processes Continental Shelf Research (in press). Vos, P.C. (1992) Paleogeografische reconstructie van het Lauwersmeergebied. Intern. Rep. RGD Proj. 40009, Rijks Geologische Dienst Distrikt Noord, 22 p. Zagwijn, W.H. (1986) Geologie van Nederland, Deel 1: Nederland in het Holoceen. Rijks Geol. Dienst, Haarlem, 46 p. Ziegler, K., Flemming, B.W. & Schubert, H. (1990) Sedimentparameter als Indiz ftir Energiegradienten im Riickseitenwatt der Insel Spiekeroog. Nachrich. Deut. Geol. Gesell. 43, pp. 156.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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The Wadden Sea squeeze as a cause of decreasing sedimentary organic loading M. T. Delafontaine,* B. W. Flemming and S. Mai
Senckenberg Institute, Schleusenstr. 39A, D-26382 Wilhelmshaven, Germany
ABSTRACT
In this study the inventories of mud and organic matter in the surficial sediments of two East Frisian tidal basins are compared with those known to have existed in adjacent intertidal flats removed by land reclamation. Based on a dense sample grid (>3000 samples), the average contents of particulate organic carbon (POC) were found to be 0.106% in the sand fractions, 2% in the m u d fractions of the m u d d y sands on the dike flats (5-40% m u d content), and 4.5% in the mud fractions of the very slightly m u d d y sands on the island flats (<5% m u d content). Using an empirically derived dry bulk density relationship, the masses of m u d and POC were determined for the surficial sediments (upper 5 cm) of the modern tidal flats in the two basins. Thus, at the time of the survey, the total mud mass in the upper 5 cm of the Langeoog tidal basin (90 km 2) amounted to 0.313 * 106 tonnes and the POC mass to >6000 tonnes, whereas in the Spiekeroog tidal basin (74 km 2) the m u d mass amounted to 0.4 * 106 tonnes and the POC mass to >8000 tonnes. The reconstructed tidal flats were found to consist of some mixed flats (ca. 60 km a) and larger expanses of pure mud flats (ca. 160 km2). Using the same empirical dry bulk density relationship, the highest concentrations (masses per unit volume) of mud and POC were found to occur in a belt of sandy m u d with a mean mud content of about 60%. At higher mud contents (60-100% mud) the respective masses decreased again due to increasing water contents and associated lower dry bulk densities. Based on these constraints, a total mass of ca. 1.38 9 1 0 6 tonnes of mud and 26,400 tonnes of POC was estimated to have existed in the reconstructed tidal flats of the Langeoog tidal basin, and 2.08 9 106 tonnes of m u d and 36,800 tonnes of POC in the reconstructed tidal flats of the Spiekeroog tidal basin. The mud and POC lost by land reclamation thus exceed those of the modern tidal basins by a factor of about 4.4 and 5.2 (for mud), and 4.4 and 4.6 (for POC), respectively. We conclude that the man-induced Wadden Sea squeeze has resulted in a substantial decrease in the total loads of fine-grained material and organic matter (and pollutants, for that matter) in the modern back-barrier tidal flats. A further loss of mud and organic matter is predicted in the course of continued sea-level rise. * Corresponding author: M.T. Delafontaine e-mail:
[email protected]
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1. INTRODUCTION The Wadden Sea has until recently been thought to act as a long-term sink for fine particles enriched in organic matter, heavy metals, etc. (e.g., Eisma 1981; Eisma & Irion 1988; Van Alphen 1990; Laane et al. 1999). Consequently, episodic events involving large-scale anoxia and faunal mortality such as were recorded in the German sector in June 1996 were interpreted by many to result from progressive loading with contaminants during industrial times (cf. H6pner 1997). This claim is rooted in the settling-lag/scour-lag model proposed more than 40 years ago in the benchmark studies of Van Straaten and Kuenen (1958), and Postma (1961). The model predicts a net import of suspended matter into the tidal flats. However, to explain the paucity of mud and salt marshes along the diked mainland shore of the Wadden Sea, the model has recently been modified to incorporate an export loop for organic-rich fine sediments (Flemming & Nyandwi 1994; Flemming & Bartholom/i 1997). The modified model accounts for the substantial increase in relative energy levels along the mainland dike, as compared to the former non-diked shoreline, resulting from the reduction in tidal prisms and basin perimeters in the course of land reclamation and sea-level rise. It provides a mechanism by which fine-grained sediments unable to settle out are eliminated in a process called the Wadden Sea squeeze. As a consequence, the Wadden Sea can be regarded neither as a net import area of fine-grained material (Flemming & Nyandwi 1994; Flemming & Bartholom/i 1997), nor as a net sink for organic matter (cf. Delafontaine & Flemming 1997). The geological and ecological impacts of this 1000-year old human interference have hitherto been greatly underrated if not simply ignored (cf. Flemming & Delafontaine 1994; Delafontaine & Flemming 1997). For one, how can there have been a historical increase in sedimentary organic loading, as often claimed, if the man-induced reduction in accommodation space for fine-grained material has been so dramatic in the region (Mai & Bartholom/i 2000)? In addition, there is to date no evidence that organic enrichment has occurred in existing mud stocks since the turn of the century (Delafontaine et al. 1996; Delafontaine et al. 2000b). Our main aim is to quantify the historical decrease in accommodation space and loading capacity of Wadden Sea sediments by comparing the inventories of mud and organic matter observed in modern back-barrier tidal flat sediments with estimates of losses resulting from the elimination of mud flats in the course of land reclamation.
2. MATERIALS AND METHODS 2.1. Sampling locations and field work The field work was carried out in the two tidal basins of the upper mesotidal East Frisian Wadden Sea of Germany bordering the barrier islands of Baltrum, Langeoog, and Spiekeroog (Fig. 1). In the Langeoog/Spiekeroog tidal basin (drained by the Otzumer Balje inlet), which covers an area of about 74 km 2, the sampling campaign
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took place in May-July 1986 (cf. Flemming & Ziegler 1995), whereas the Baltrum/ Langeoog tidal basin (drained by the Accumer Ee inlet), which covers an area of about 90 km 2, was sampled in May 1993. Furthermore, in 1995 comparative samples were collected at monthly intervals along a N-S transect on the Steinplate tidal flat located in the latter tidal basin (position I on Fig. 1) to assess the seasonal variability in mud and organic matter contents (cf. Delafontaine et al. 2000b; Kr6gel et al. 2000). Another comparative campaign was conducted in July 1996 on the Janssand tidal flat south of the Otzumer Balje inlet (position 2 on Fig. 1). For the basin-wide campaigns, surficial sediments (upper 2-5 cm) were collected with a scoop at grid intervals of 0.15' Latitude and 0.25' Longitude (ca. 275 x 275 m), resulting in over 1600 samples in each case (Fig. 1). Geographic positions were initially determined by means of a calibrated portable Decca navigation system, and later by a portable satellite navigator (differential GPS). At each locality 5 random subsamples were pooled into a master sample, thereby reducing the probable sampling error by >50% (e.g., Krumbein & Pettijohn 1938). The samples were deepfrozen and stored until they were processed in the laboratory.
Figure 1. Locality map of the two tidal basins investigated in the present study and the sampling grid on which surficial sediments were collected.
2.2. Terminology For clarity, some key terms used in the present study are defined in the following. Accommodation space: geological term denoting the space available for sediment accumulation (cf. Muto & Steel 2000). Decreasing loading: a concentration which decreases with time. Concentration of a substance: dry (desalinated) mass of the substance per unit volume of water-saturated sediment (cf. Flemming & Delafontaine 2000). Content of a substance: dry (desalinated) mass of the substance per unit mass
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of dry sediment, here expressed as a dry weight-percentage (cf. Flemming & Delafontaine 2000). Dry bulk density: mass of dry desalinated sediment per unit volume of water-saturated sediment. Mud: grain-size fraction <0.063 mm. Sand: grain-size fraction from 0.063-2 mm. Sediment: here mud + sand. POC: particulate organic carbon (here used as a proxy for particulate organic matter). Tonne or t: metric ton (= 1000 kg). 2.3. Missing tidal flats The area of lost (or missing) tidal flats is defined as the land surface between the dike and the topographic elevation corresponding to the local mean high-tide level (in this case 1.4 m above NN, the German topographic chart datum). The sediment composition (mud content) of the missing tidal flats was reconstructed by extending computed shore-normal settling velocity gradients (cf. Fig. 2) inland up to the point where the sediment consisted entirely of mud, i.e. where the coarsest percentile of a settling velocity distribution corresponded to a grain size of 63 ~m (cf. Mai & Bartholom/i 2000 for more detail). The line connecting these points is called the 100% mud line. A 60% mud line was determined by the same procedure. The mean settling velocity gradients were computed from running group averages along each shorenormal sample profile spaced 275 m apart (cf. Fig. 1).
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I
2
'
I
''
~
I
mud
'
I
4 6 8 cross-shore distance (km)
'
I
10
'
12
Figure 2. Diagram illustrating the progressive decrease in mean settling velocities of the sediments between the barrier-island beach and the mainland dike. Also shown is the landward extension of the gradient up to the mean settling velocity at which the total sample consists of 100% mud (see text for further explanations).
Wadden Sea squeeze
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2.4. Mud and POC concentrations Concentrations of mud, sand and POC were computed by converting content values into volume-specific masses on the basis of an empirical relationship between dry bulk density and mud content derived from a highly correlated data set (Fig. 3; cf. Flemming & Delafontaine 2000). I n the East Frisian Wadden Sea this relationship is defined by the equation y - -0.7955892+2.3863045e ~ - ~ ~ ~ where y - dry bulk density (grams desalinated dry sediment per crn3 of water-saturated sediment), and x = mud dry weight-% content of the sediment (n - 337, r - - 0 . 9 8 4 7 ) .
East Frisian Wadden Sea 0.6 co o
E o v t.mO .i-, el
t(D O tO O (/) (/)
0.4100 g cm 3
9
9
"." i.
0.2919 g cm: e,,B~
...........
-~ r_.. . . . . . . . . . . . .
!
E -o
6 9 9 o
-
0.4
0.2
!
"O
E
i
9
4
9
i i
i
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I 0.0
oo.~ '
0
I
20
'
I
40
'
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60
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80
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100
mud content (dry weight-%)
Figure 3. Relationship between mud content and mud concentration for surficial sediments on the intertidal flats of the wider study area (after Flemming & Delafontaine 2000).
2.5. Laboratory procedures The frozen samples were processed according to standard laboratory procedures (e.g., Carver 1971), being first thawed and then dialysed overnight to remove salts before being washed through a 63 gm sieve to separate sand and mud. The two sediment fractions were then dried overnight at 70~ weighed, and their respective weight percentages of the total sample calculated. POC contents were determined in aliquots of all the mud samples. The mean POC content of local sands was extracted from Delafontaine et al. (2000a). POC measurements were carried out by means of an Heraeus CHN analyser following standard
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procedures (reproducibility: 4.8%; cf. Delafontaine et al. 1996). Cross-checks against coulometric analyses resulted in a correlation coefficient r = 0.995 (n = 10). 2.6. Data processing
The data were processed by using the kriging method with the exponential variogram model (e.g., Cressie 1991) in the software package Surfer from Windows Golden Software Inc., and Coreldraw software from Corel Corp. Ltd.
3. RESULTS 3.1. Mud and POC contents in m o d e m tidal flat sediments
The results of the basin-wide sampling campaign of 1986 (n = >1600) show that the modern-day depositional system behind Spiekeroog Island is dominated by sandy tidal fiats with mud contents generally lower than about 5% (Fig. 4). Mixed fiats occur adjacent to the mainland dike, but values rarely exceed 30%. Higher mud contents (30-50%) are restricted to the vicinity of intertidal mussel banks (Mytilus edulis) which occupied roughly 10% of the study area at the time of the sampling campaign (May-July 1986).
Figure 4. Distribution of mud contents (dry weight-%) in the surficial sediments on the Spiekeroog tidal flats (modified after Flemming & Ziegler 1995). M = Mytilus edulis banks with biogenic mud accumulations.
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Figure 5. Distribution of POC contents (dry weight-%) of the m u d fractions in the surficial sediments on the Spiekeroog tidal flats.
On the Spiekeroog tidal flats, the spatial distribution of POC contents of the m u d fractions showed a pronounced inverse trend to that observed for the m u d contents (Fig. 5). Thus, with contents of 2.5-4.5% POC, the m u d fractions of the sandy tidal flats (<5% m u d contents) were generally more enriched in POC than their counterparts on the mixed flats (5-40% m u d contents) where <2.5% POC contents were the norm. The same trend was observed in the Langeoog tidal basin (cf. Delafontaine et al. 2000b). The basin-wide trends in m u d and m u d POC enrichment observed in 1986 on the Spiekeroog tidal flats were corroborated by a smaller (n = 20) and more recent (July 1996) data set for the Janssand tidal flat (cf. Fig. 1). In these sandy sediments (mud contents <5%), the POC contents of the m u d fractions showed values of 3.5-4.8%. Thus, the same general trend emerged when comparing the POC contents in the m u d s from the same area but sampled after a time interval of 10 years. These inverse trends in m u d and POC enrichment in the m u d fractions were also recorded in the Langeoog tidal basin. Figure 6 shows a selected data set for the Steinplate tidal flat behind the neighbouring island of Baltrum collected in 1995 (cf. Fig. 1). Close to the island, on the so-called island flats, generally low m u d contents (<5%) were associated with high POC contents (3.5-4.5%) in the m u d fractions. By contrast, in the
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vicinity of the mainland dike (dike flats), m u d contents were higher (ca. 5-10%) but the m u d fractions were by and large less enriched in POC (<2.5%).
island flats on the Steinplate behind Baltrum Island, 1995 P
O 5 co ~ nt
mud
tn !y(dr wt
rn ud
+
tn
N
P 0 c ~,
.m.mm.
tn iy(dr
20 c? ~ Y.(dr
o
1:
.
o ~ m=mmmmmmmmm
J M M J S N month
5
total
sediment
wt. . . . . . .
J M M J S N month
9 ....
J M M J S N month
dike flats on the Steinplate behind Baltrum Island, 1995 PO C co s nte nt 4 (dr ~.
mud
3
.%) ~ 0
m ud 201 co I it 15 (dr
J M M J S N month
P
m
10
o s total coo, ~ s e d i m e n t 7 y(dr !
0
WL
J
M M J S N month
--mm--m.m
mmnni
J M M J S N month
Figure 6. Monthly variations in POC contents (dry weight-%) of the m u d fractions, m u d contents (dry weight-%) of the sediments, and POC contents (dry weight-%) of the sediments on the Steinplate tidal flat in the rear of Baltrum island. On the basis of the results presented above, a value of 2% POC was chosen to reconstruct the POC concentrations of the m u d fractions for the missing m u d flats. For the claculation of POC concentrations in the sand fractions, a value of 0.106% POC content was used (Delafontaine et al. 2000a). 3.2. M u d and POC concentrations in tidal fiat sediments To facilitate basin-wide budget calculations, m u d and POC concentrations (in tonnes dry mass) were first determined for unit volumes of sediment comprising the upper 5 cm of each grid cell centred around a sample point (275 x 275 x 0.05 m3). To calculate larger scale m u d or POC budgets, the values for the unit volumes defining a specific area are added together. The results show that, besides some mixed flats adjacent to the modern dike line, considerable expanses of m u d flats would have occupied the reclaimed lands extending inland from the 100% m u d line to the mean high-water level of each tidal basin (Fig. 7). In the case of the Langeoog tidal basin, the total tidal flat area lost by land reclamation amounts to 78 k m 2, comprising 24 km 2 of mixed flats and 54 k m 2 of m u d flats. Although the area of the modern tidal basin is in this case about 15% larger than the area lost to land reclamation, the m u d mass of 0.313 * 106 tonnes in the present-day tidal basin amounts to only about 23% of that in the area lost to land
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reclamation (1.38 9 106 tonnes). In the case of Spiekeroog, the total reclaimed area is 142 km 2 of which 36 km 2 comprise mixed flats and 106 km 2 m u d flats. In this case the area of the present-day tidal basin is only about half the size (52%) of the lost area. As a result, the mass of m u d in the present-day tidal basin (0.4 9 106 tonnes) makes up only 19% of the m u d lost to land reclamation (2.08 9 1 0 6 tonnes).
Figure 7. Distribution of mud concentrations (volume-specific masses) in the surficial sediments of modern-day and reconstructed tidal flats behind the islands of Langeoog and Spiekeroog. Besides the impressively large mud flats and m u d masses lost in the course of land reclamation, the reconstructed mud concentration pattern in the reclaimed area reveals a peculiar and quite counter-intuitive trend. Instead of simply increasing with increasing m u d content, the concentrations at first increase progressively until they peak at a m u d content of about 60% (ca. 1600 tonnes of m u d per unit area). Thereafter the concentrations progressively decrease once more until they reach a value of about 1100 tonnes of mud per unit area at the 100% m u d line. This trend is a direct consequence of the relationship between m u d contents and mud
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concentrations illustrated in Fig. 3, being produced by the evolution of dry bulk density and water content as a function of mud contents in Wadden Sea sediments (cf. Flemming & Delafontaine 2000). A similar trend is evident in the reconstructed POC inventories of the missing mud flats behind the two islands (Fig. 8). Thus, POC concentrations progressively increase from <10 tonnes per unit area in the present-day tidal flats to a peak value of 35 tonnes per unit area at a mud content of 60% in the reclaimed area behind the dike, only to progressively decrease to about 22 tonnes of POC at the 100% mud line. In terms of mass, the 6000 tonnes of POC in the present-day tidal flats of Langeoog barely reach 23% of the POC lost in the reclaimed tidal basin (26,400 tonnes). In the case of Spiekeroog the ratio between the present-day POC mass and the lost POC mass is almost identical, the 8000 tonnes of POC in the present-day tidal basin comprising ca. 22% of that in the reclaimed area (36,800 tonnes).
Figure 8. Distribution of POC concentrations (volume-specific masses) in the surficial sediments of modern-day and reconstructed tidal flats behind the islands of Langeoog and Spiekeroog.
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Wadden Sea squeeze 4. DISCUSSION AND CONCLUSIONS
In the reconstructed m u d flats of the Spiekeroog and Langeoog tidal basins, areas of highest m u d contents clearly do not coincide with areas of highest mud concentrations. Seeing that the same trend was observed for the reconstructed POC inventories, these findings would evidently influence the standing stocks of other substances associated with fine-grained sediments, notably pollutants. Although POC concentrations in the surficial sediments of the study area at first increase over a short distance before decreasing substantially landwards as sediments became muddier in the reconstructed tidal flats, the POC inventories of the reclaimed tidal flat sediments (sand + mud) exceed those of the modern tidal flats by factors of 2.5 and 3 for the Langeoog and Spiekeroog tidal basins, respectively (Table 1). These ratios are even higher if the POC inventories of the m u d fractions are considered. In this case, respective ratios of 4.4 and 4.6 are observed for the same tidal basins. Table 1. Load ratios in m u d and POC masses for the modern sand-dominated tidal flats and the tidal flats lost in the wake of land reclamation. Sand POC masses/ m u d POC masses
Missing tidal flats/ modern tidal flats Mud mass
Mud POC mass
Sand POC mass
Sediment POC mass
Modern tidal flats
Missing tidal flats
Langeoog Island
4.4
4.4
0.2
2.5
0.8
0.03
Spiekeroog Island
5.2
4.6
0.1
3.0
0.6
0.02
These order-of-magnitude budgets can certainly be refined by considering several aspects which have not been accounted for in the calculations presented in this study. For one, the reconstructed tidal flats would have been crossed by small and large tidal channels. As a result, the inventories have probably been slightly overestimated in this case. On the other hand, the use of a mean POC content of 2% for the m u d fractions of the reconstructed tidal flats has probably resulted in POC inventories being somewhat underestimated in the reconstructed areas, particularly near the mean high water line where, adjacent to salt marshes, the POC contents would certainly have been higher. For example, values of 3-5% POC have been recorded in fine-grained sediments bordering natural Spartina salt marshes in the Danish Wadden Sea (Pejrup 1981). Indeed, no attempt was made in this study to estimate the area potentially occupied by Spartina and Salicornia salt marshes in the reclaimed tidal basins. Finally, in this study the present-day tidal basins were contrasted with reclaimed ones. In reality, if land reclamation had not taken place at all, the total area
284
Delafontaine et al.
and the sediment distribution of the natural tidal basins would have been somewhat different. Thus, the barrier island chain would in all likelihood have been located up to 1 km closer to the present shoreline, i.e. the tidal basins would have been smaller by up to 10% (cf. Mai & Bartholom/i, this volume). The widespread practice of relating biological responses to gradients in content rather than concentration of organic matter, heavy metals, etc. (e.g., see reviews by Bryan & Langston 1992, and Snelgrove & Butman 1994; Rom6o et al. 1994; Wallace et al. 1998) runs the serious danger of producing spurious relationships. Thus, any lack of correlation between POC contents and animal responses is commonly explained by invoking composition ('quality'), bioavailability, saturation thresholds, etc. In this context, the potentially spurious nature of the well-known relationship between bacteria and organic matter had already been identified by Bird and Duarte (1989), Flemming and Delafontaine (2000) providing more evidence of such misapplication in studies dealing with benthic diatoms, carbohydrates, etc., in sediments. The occurrence of highly localised and/or discrete zones with high mud and POC concentrations in back-barrier tidal flats indicates a new approach in unravelling organism-sediment relationships. We conclude that the Wadden Sea squeeze, i.e. the reduction in accommodation space for fine-grained material resulting from land reclamation, has led to substantial decreases in the total loads of mud, organic matter and, for that matter, any pollutants in the backbarrier tidal flat sediments. This depletion will continue as long as the local sea level keeps rising (currently 18 cm/century for the mean sea level, and 25 cm/century for the mean high-tide level). Because of land reclamation the present-day tidal flats are, if anything, underloaded in organic matter rather than overloaded, as has been claimed (e.g. H6pner 1997). Not surprisingly, massive episodic inputs of nearshore planktonic material (the latest event having occurred in June 1996) have had severe but nonetheless short-lived impacts. Finally, we postulate that, by promoting mixing and remineralisation, strong resuspension in the sanddominated tidal flats of today (e.g., Bartholom/i et al. 1999; Delafontaine et al. 1999) can largely explain the now well-documented stability in organic enrichment of the existing mud stocks during industrial times.
ACKNOWLEDGEMENTS
Our hearty thanks go to the captain, motorboat driver, and crew of the research vessel Senckenbergfor their expertise and unfailing good spirits during the fieldwork, oft carried out under difficult weather conditions. We also acknowledge the field work of F. Kr6gel and W. Xu, as well as the laboratory assistance of A. Rascke, R. Frerichs and numerous students. The work is based on data collected in the course of programs sponsored by the Senckenbergische Naturforschende Gesellschaft, Frankfurt, and the Norwegian State Oil Company (Europipe Development Project Contract No. C-170 198).
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REFERENCES
Bartholom~i, A., Flemming, B.W. & Delafontaine, M.T. (2000) Mass balancing the seasonal turnover of mud and sand in the vicinity of an intertidal mussel bank in the Wadden Sea (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Bird, D.F. & Duarte, C.M. (1989) Bacteria-organic matter relationship in sediments: a case of spurious correlation. Can. J. Fish. Aquat. Sci. 46: 904-908. Bryan, G.W. & Langston, W.J. (1992) Bioavailability, accumulation and effects of heavy metals in sediments with special reference to United Kingdom estuaries: a review. Environ. Pollut. 76: 89-131. Carver, R.E. (1971) Procedures in Sedimentary Petrology. Wiley-Interscience, New York, 653 p. Cressie, N.A.C. (1991) Statistics for Spatial Data. John Wiley & Sons, N.Y., 900 p. Delafontaine, M.T., Bartholom~i, A., Flemming, B.W. & Kurmis, R. (1996) Volumespecific dry POC mass in surficial intertidal sediments: a comparison between biogenic muds and adjacent sand flats. Senckenbergiana marit. 26: 167-178. Delafontaine, M.T. & Flemming, B.W. (1997) Large-scale sedimentary anoxia and faunal mortality in the German Wadden Sea (southern North Sea) in June 1996: a man-made catastrophe or a natural black tide? Ger. J. Hydrogr. Suppl. 7: 21-27. Delafontaine, M.T., Flemming, B.W. & Bartholom~i, A. (2000a) Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea). In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Delafontaine, M.T., Flemming, B.W. & Kr6gel, F. (2000b) Organic enrichment in backbarrier sediments of the Wadden Sea: a five-year environmental impact study spanning the Europipe landfall. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Eisma, D. (1981) Supply and deposition of suspended matter in the North Sea. In: Nio, S.D., Sch~ittenhelm, R.T.E. & Van Weering, T.C.E. (eds), Holocene Marine Sedimentation in the North Sea Basin. IAS Spec. Publ. 5, Blackwell, Oxford, pp. 415-428. Eisma, D. & Irion, G. (1988) Suspended matter and sediment transport. In: Salomons, W., Bayne, B.L., Duursma, E.K. & F6rstner, U. (eds), Pollution of the North Sea: an Assessment. Springer-Verlag, Berlin, pp. 20-35. Flemming, B.W. & Bartholom~i, A. (1997) Response of the Wadden Sea to a rising sea level: a predictive empirical model. Ger. J. Hydrogr. 49: 343-353. Flemming, B.W. & Delafontaine, M.T. (1994) Biodeposition in a juvenile mussel bed of the East Frisian Wadden Sea (south. North Sea). Neth. J. Aquat. Ecol. 28: 289-297. Flemming, B.W. & Delafontaine, M.T. (2000) Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications. Cont. Shelf Res. (in press).
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Flemming, B.W. & Nyandwi, N. (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Neth. J. Aquat. Ecol. 28: 299-307. Flemming, B.W. & Ziegler, K. (1995) High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog Island (southern North Sea). Seckenbergiana marit. 26: 1-24. H6pner, T. (1997) Black spots as the result of a cascade of causes? In: Henke, S. (ed.), Black Spots in the Wadden Sea. UBA, Berlin, Texte 3/97: 21-22. Kr6gel, F., Flemming, B.W. & Delafontaine, M.T. (2000) High-resolution sediment distribution patterns and dynamics in the Accumer Ee tidal basin: subtle effects of Europipe. In: Delafontaine, M.T., Flemming, B.W. & Vollmer, M. (eds), Environmental Impacts of Europipe. J. Coast. Res. Spec. Issue 27 (in press). Krumbein, W.C. & Pettijohn, F.J. (1938) Manual of Sedimentary Petrography. D. Appleton Century, New York, 549 p. Laane, R.W.P.M., Sonneveldt, H.L.A., Van der Weyden, A.J., Loch, J.P.G. & Groeneveld, G. (2000) Trends in the spatial and temporal distribution of metals (Cd, Cu, Zn and Pb) and organic compounds (PCBs and PAHs) in Dutch coastal zone sediments from 1981 to 1996: a model case study for Cd and PCBs. J. Sea Res. 41: 1-17. Mai, S. & Bartholom~i, A. (2000) The missing mud flats of the Wadden Sea: a reconstruction of the intertidal accommodation space and sediments lost in the wake of land reclamation. In: Flemming, B.W., Delafontaine, M.T. & Liebezeit, G. (eds), Muddy Coast Dynamics and Resource Management. Elsevier, Amsterdam (this volume). Muto, T. & Steel, R.J. (2000) The accommodation concept in sequence stratigraphy: some dimensional problems and possible redefinition. Sediment. Geol. 130: 1-10. Pejrup, M. (1981) Bottom sediments in Ho B u g t - a Wadden Sea environment. Geogr. Tidsskr. (Copenhagen) 81: 11-16. Postma, H. (1961) Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res. 1: 148-190. Rom6o, M., Mathieu, A., Gnassia-Barelli, M., Romana, A. & Lafaurie, M. (1994) Heavy metal content and biotransformation enzymes in two fish species from the NW Mediterranean. Mar. Ecol. Prog. Ser. 107: 15-22. Snelgrove, P.V.R. & Butman, C.A. (1994) Animal-sediment relationships revisited: cause versus effect. In: Ansell, A.D., Gibson, R.N. & Barnes, M. (eds), Oceanogr. Mar. Biol. Ann. Rev. 32: 111-177. Van Alphen, J.S.L.J. (1990) A mud balance for Belgian-Dutch coastal waters between 1969 and 1986. Neth. J. Sea Res. 25: 19-30. Van Straaten, L.M.J.U. & Kuenen, P.H. (1958) Tidal action as a cause of clay accumulation. J. Sediment. Petrol. 28: 406-413. Wallace, W.G., Lopez, G.R. & Levinton, J.S. (1998) Cadmium resistance in an oligochaete and its effect on cadmium trophic transfer to an omnivorous shrimp. Mar. Ecol. Prog. Ser. 172: 225-237.
Muddy Coast Dynamics and Resource Management, B. W. Flemming, M. T. Delafontaine and G. Liebezeit (eds.) 9 2000 Elsevier Science B.V. All rights reserved.
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T h e u t i l i z a t i o n of c o a s t a l t i d a l flats: a case s t u d y o n i n t e g r a t e d c o a s t a l area management from China Y. Wang,* X. Zou and D. Zhu
State Laboratory of Coast and Island Exploitation, Nanjing University, Nanjing 210093, China ABSTRACT
Coastal tidal flats are valuable resources formed by the interaction of terrestrial and marine processes. New land is continually being created by the large riverine sediment supply to the coastal zone of China. This natural endowment has already brought substantial economic benefits, although the full potential for the heavily populated coastal area in China has yet to be exploited. The tidal flats, for example, are currently still being used on a rather small scale and in an isolated manner. An uncontrolled expansion, however, causes conflicts between users, and distorts the natural ecology of the environment. Such conflicts can be minimised by integrated coastal area management (ICAM) which is to be understood as a continuous dynamic process. By the establishment of an institutional mechanism, interagency coordination and collective management of the marine environment and its resources can be accomplished. In this case study on natural processes and economic development of the tidal-flat resources in China, the authors present a systematic scheme of ICAM as a basis for further discussion.
1. INTRODUCTION Integrated coastal area management (ICAM) is a continuous and dynamic process by which decisions are taken for sustainable use, development and protection of coastal environments and resources. It consists of a legal and institutional framework which ensures that development and management plans for coastal areas are integrated with environmental (including social) goals. Furthermore, such developments should not be made without the participation of all those affected. The purpose of ICAM is to maximize the benefits provided by the coastal area, and to minimize conflicts and harmful effects of such activities upon each other. It starts with an analytical process in the course of which objectives for the development and management of the coastal area are defined. What ICAM should ensure is that the * Corresponding author: Y. Wang e-mail:
[email protected]
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Wang et al.
process of setting the objectives, planning their implementation, and executing them involves as broad a spectrum of interest groups as possible, that the best possible compromises between the different interests are found, and that a balance is achieved in the overall use of the coastal zone (Anonymous 1993a). In the context of this paper, the coastal zone is the interface where the land meets the sea. It is a transitional zone dominated by interactive terrestrial and marine processes. It should cover an area which includes the lower river basins up to 10 km or more inland, the beach and surf zone, the continental shell the continental slope and the continental rise.
2. THE COASTAL SETTING OF CHINA In China the coastal zone is located along the eastern Euro-Asian continental margin adjacent to the Pacific Ocean. It includes the Yellow Sea, the East China Sea and the South China Sea (Fig. 1). The total length of the coastline is 32,000 km. Of this, 18,400 km encompass the mainland coast between the Yalu River in the north, which forms the border between China and Korea, and the Beilun River in the south which forms the border between China and Vietnam. The remaining 13,600 km of coastline are made up of 6500 islands (Wang & Aubrey 1987). Four coastal types can be recognised: 1) bedrock-embayed coast, 2) coastal plain coasts, 3) river mouth coasts, and 3) biogenic coasts comprising coral reefs and mangroves. Tidal flats are widely distributed along the coastal plain and river mouth coasts, including large parts of the Bohai Sea, Yellow Sea, and East China Sea, as well as the mangrove coasts in the South China Sea (Wang 1983; Fig. 1). Muddy tidal flats, which line some 4000 km of the coastal plains and large river deltas, form a valuable land resource, especially in the heavily populated coastal areas of China. Fluvial sediments are discharged by several large rivers in China, supplying nutrients and fertilized soils to the tidal flats. Because of the huge volume of sediment (ca. 1.0x109 tons per year; Wang et al. 1998; Zhu et al. 1998) supplied by the rivers, tidal flats have accreted rapidly and are still prograding gradually towards the sea. Muddy tidal flats are thus a rich endowment contributed by landsea interactions. The tidal-flat area above the theoretical base level (for safe navigation, China has adopted a practical base level, called zero depth, which is equal to the lowest low-tide level) amounts to about 2.07 million hectares, the total area between the base level and the -10 m bathymetric contour contributing another million hectares (Anonymous 1995). About 20,000 to 30,000 hectares of new land are created every year. This new land is of enormous significance in the eastern, heavily populated coastal plain region of China where it forms the basis of advanced economic development. Taking the Changjiang (Yangtze) River delta as an example, more than 73 million inhabitants or 6.09% of China's total population (Anonymous 1993b) live on an area of 99,500 km 2 (1.04% of the territorial land area), contributing 15.4% to the total GDP in 1995 (Anonymous 1995). In recent years, the population is increasing at the rate of 1% per
Integrated coastal area management in China
289
year, while the arable land is decreasing at the rate of 0.05% each year. Agriculture output value in the area is only about 10% of the local GDP. In the light of this, there is an urgent need to harmonize the relationship between the population and the land resources in this region.
Figure 1. Map showing the coastal types of China (extracted from Wang & Aubrey 1987).
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Wang et al.
3. TIDAL-FLAT RESOURCES
As a kind of spatial resource, tidal flats have a huge potential value. In China, they have a multifunctional character, being utilised as a marine and freshwater resource, for the production of tidal energy, salt, aquatic products and terrestrial plants, but also for tourism and harbour development. Integrated development models for farming, animal husbandry, and fisheries are needed in order to sustain an ecologically sound and efficient agriculture. Along the Jiangsu coast of the Yellow Sea, for example, the tidal flats are about 10-13 km wide and, with an area of 600,000 hectares, they represent roughly 14% of the total farmland in Jiangsu province (Wang & Zhu 1994). The tidal flats are currently prograding at a rate of 1400 hectares per year, being a valuable land resource in this densely populated province which is characterized by a highly developed economy with very limited farmland. Although farmland makes up only 0.08 hectare per capita, it is nevertheless decreasing at a rate of 26,700 hectares per year due to industrial expansion and urbanization. The increasing exploitation of the tidal-flat resources will inevitably intensify social and economic conflicts. Problems created by multiple jurisdictions and the competition between users of resources without the benefit of a conflict resolution mechanism, inadequate regulations for protecting resources, and the lack of nationally or locally adapted coastal policies for informed decision making will all translate into a loss of capability for future sustainable development. As a resource is depleted, conflicts may develop to the point of threatening human life and public order. Tidal flats currently offer physical and biological opportunities for human use, and ICAM tries to find an optimum balance between such uses based on a given set of objectives. In particular, there is growing concern about the destruction of the ecosystem by the demands of population growth and economic expansion. The considerable value of the natural ecosystem for the supply of sustainable extractive and non-extractive products is often underated in comparison with other often nonsustainable uses.
4. INTEGRATED COASTAL AREA M A N A G E M E N T (ICAM)
Based on the ecological character, there should be different exploitation models for the different types of tidal flats: 1) Deep-water sections could be used for harbour construction, whereas aquaculture (in particular seaweed, lavers, sea horse, and pearl farming) could be developed in shallow-water sections. 2) In accordance with the different ecological types found in the intertidal zone (sandy flats, silt flats, mud flats and marshes), different types of aquaculture models should be developed (Fig. 2).
Integrated coastal area management in China
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Figure 2. The utilization of tidal flats in Rudong county along the northern Jiangsu coast (modified after Zhu et al. 1998). Zone I: salt meadow; Zone II: mud flat; Zone III: silty mud flat; Zone IV: silt flat. 3) Three-dimensional models are particularly suitable for the development of supratidal zones. Thus, aquaculture should focus on topographic depressions, whereas animal husbandry and protected reserves for rare wild life (e.g., for the Davis deer (Milu) and the red-crown crane) should utilise salt marshes and grassland, etc. (Fig. 3). Different cultivation models should also be considered for the various maturation stages in tidal-flat evolution, for example, cotton-mulberry-wheat or corn-rice fields (Fig. 4).
Figure 3. The utilization of tidal fiats in Dafeng county along the northern Jiangsu coast (modified after Zhu et al. 1998). Zone I: grass flat; Zone II: mud flat; Zone III: silty mud flat; Zone IV: silt flat.
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4) Integrated development models should be designed for farming, animal husbandry, and fisheries in order to make production, transportation and marketing a coordinated process. Industrial activities, such as goods processing, oil and gas extraction, and transportation should proceed in such a way as to achieve a harmonic relationship with the nature of the tidal environment. 5) Model farms are especially useful in demonstrating the successful application of comprehensive development schemes.
Figure 4. The utilization of tidal flats in Qidong county along the northern Jiangsu coast (modified after Zhu et al. 1998). Zone III: silty mud flat; Zone IV: silt flat. With large tracts of tidal flats being still unused, tidal-flat development currently takes place on a rather small scale, following the old pattern of isolated economic development which is only concerned with immediate benefits. This results in sharp contradiction between short-term resource consumption or utilization and long-term supply. Since it is not a type of integrated and three-dimensional development, it is unsuitable for a sustainable development with higher benefit. As a result, it will produce conflicts between activities such as farming, forestry, animal husbandry, fisheries, industry, harbour development, tourism, salt production, reed planting, and chemical extractions. For example, salt ponds constructed behind the crane habitat will destroy the brackish-water environment and thereby threaten the protection of this rare bird species. Due to global warming and sea-level rise, river sediment discharge is decreasing while storm surges, floods and other marine calamities are increasing. Because of the shallow slope and numerous tidal channels, the tidal flats are often threatened by erosional and depositional processes, droughts, and flood disasters which can attain a frequency up to 70% in the region.
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Taking typhoon no. 11 of 18-20 August 1997 as an example, this cyclone hit the south coast of Zhejiang province with wind speeds of 12 Beaufort, i.e. more than 32.7 m/sec. Since it coincided with the highest annual astronomical high tide, the tide reached a level of 6 m, causing a major disaster on the coastal lowlands (2-4 m above sea level). Even after it landed, the water level of the lower reaches of the Changjiang River was still 5 m high, this being the highest water level on record. Land-based pollutants, especially liquid wastes discharged by rivers, often cause red tides, killing fish and shrimps in the process. In China, more than 60% of anthropogenic contaminants are concentrated in the coastal zone. Thus, the Changjiang River delta, which makes up only about 1% of the national territorial area, has taken up 9% of national waste waters, 5% of waste gas, and 4% of other industrial residues (Fu & Li 1996).
5. CONCLUSIONS Integrated coastal area management should be emphasized not only by decision makers but also by stockholders and by the interested public. The combination of research and exploitation, utilization and protection, calamity prevention and benefits will certainly smoothen the way for sustainable development in coastal agriculture. The scheme shown in Fig. 5 is suggested as a systematic approach to integrated coastal area management. D Dikes and dams D Regulation
D Engineeringworks
D Legislation
D Land reclamation
Enforcement
I
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D Placermining
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\ RESOURCE CONSERVATION
SUSTAINABLE ENVIRONMENTAL DEVELOPMENT IMPACTS
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~> Mangroves
D Research
D
coral reefs
~ Planning
~> Erosion
D Wetlands D Shore protection
D Utilization
I Siltation
D Pollution ~> Sea-level rise
~> Stormsurge effects
Figure 5. Proposed scheme for systematic integrated coastal area management (modified after Wang et al. 1997).
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It clearly shows the links between the natural environment and human impacts in the form of coastal law (regulation), education and ICAM to protect and conserve the natural environment and its resources (cf. Wang et al. 1997; Healy & Wang 2000).
ACKNOWLEDGEMENTS
This study was supported by the State Laboratory of Coast and Island Exploitation, Nanjing University (Contribution No. SCIEL21198118).
REFERENCES
Anonymous (1993a) The Noordwijk guidelines for integrated coastal zone management. The World Bank Environment Department, Land, Water and Natural Habitats Division. World Coast Conference 1993, November 1993, Noordwijk, pp. 1-6. Anonymous (1993b) China Population Statistics Yearbook 1993. State Statistical Bureau P.R. China, China Statistical Publishing House. Anonymous (1995) Statistical Yearbook of China 1995. State Statistical Bureau P.R. China, China Statistical Publishing House. Fu, W.X. & Li, G.T. (1996) Ocean pollution and ocean environment protection. In: Marine Geography of China (Chap. 23). Ocean Science Press, Beijing, pp. 474-517 (in Chinese). Healy, T. & Wang, Y. (eds) (2000) Muddy Coasts of the World: Processes, Deposits and Function. Elsevier, Amsterdam (in press). Wang, Y. (1983) The mudflat system of China. Can. J. Fish. Aquat. Sci. 40 (Suppl. 1): 160-171. Wang, Y. & Aubrey, D.G. (1987) The characteristics of the China coastline. Cont. Shelf Res. 7(4): 329-349. Wang, Y. & Zhu, D.K. (1994) Tidal flats in China. In: Oceanology of China Seas (Vol. 2). Kluwer, Dordrecht, pp. 445-456. Wang, Y., Luo, Z. & Zhu, D. (1997) Economic development and integrated management issues in coastal China. In: Haq, B.U., Haq, S.M., Kullenberg, G. & Stel, J.H. (eds) Coastal Zone Management Imperative for Maritime Developing Nations. Kluwer, Dordrecht, pp. 371-384. Wang, Y., Ren, M.-E. & Syvitski, J. (1998) Sediment transport and terrigenous fluxes. In: The Sea (Vol. 10). John Wiley & Sons, New York, pp. 253-292. Zhu, D.K., Martini, I.P. & Brookfield, M.E. (1998) Morphology and land-use of the coastal zone of the North Jiangsu Plain Jiangsu Province, Eastern China. J. Coast. Res. 14: 591-599.
