Oosterschelde estuary (S.W. Netherlands): a self-sustaining ecosystem?

The Oosterschelde estuary has a special position among the Dutch North Sea estuaries. With relatively unpolluted water and high transparency it combines high biotic diversity and high primary and secondary production. Before 1970 a major part of the water of the rivers Rhine and Meuse flowed into the Oosterschelde. The building of a permeable storm-surge barrier (1986) decreased the exchange between Oosterschelde and North Sea. The construction of two additional dams (1987) reduced the very limited fresh water discharge on the estuary even further. The total effect of these changes was a decrease of the nutrient levels in the water column.The estuary is used intensively for culture of mussels and fishery of cockles. Both zoobenthos groups use together up to 60% (in the western part) of the organic food available in the Oosterschelde estuary. The storm-surge barrier resulted in reduced current velocities and increased sedimentation, accompanied by higher transparency, hence accelerating primary production. This phenomenon is counteracted by the lowered nutrient concentrations throughout the year.In the seventies a preliminary carbon budget study indicated that a substantial import of organic carbon was required to sustain the ecosystem. Later studies did not confirm this hypothesis.In the present paper three different methods are presented to answer the question, if internal production suffices to sustain the foodweb. At first a steady state model is applied, secondly, calculations with actually measured process rates are carried out and, finally, a dynamic simulation model is used.The conclusion is drawn that, before the construction of the storm-surge barrier, primary production of organic matter by phytoplankton is much more important than import, although the latter can not be neglected as additional food source for mussels, cockles and zooplankton. The simulation model predicts that this conclusion will not have to be changed in the future. The Oosterschelde will remain largely a self-sustaining ecosystem.     

Changes in basin geomorphology after implementation of the Oosterschelde estuary project

The completion in 1986/87 of an open storm-surge barrier in the inlet and of secondary dams in the landward parts of the Oosterschelde tidal basin (SW Netherlands) has had and will continue to have a significant impact on geomorphological developments. An analysis of historic data, and of recent detailed bathymetric and morphodynamic process data, indicates that former trends have reversed. At present the Oosterschelde is a sedimentation basin with a degrading intertidal area and silting up of channels. The continuing reduction in intertidal area, the decreasing geomorphological gradients, the increasing fine sediment content of channel deposits, combined with a general reduction in hydrodynamics, imply significant ecological effects.     

Hydrodynamic characteristics of the Oosterschelde in recent decades

Hydrodynamic conditions in the Oosterschelde have changed as a result of the Delta Project. Engineering works were carried out between 1960 and 1970 in the northern branch of the Oosterschelde. The Grevelingen dam was built in 1964 and the Volkerak dam was completed in the spring of 1969. With the completion of these dams, the northern branch of the Oosterschelde was cut off from other estuaries, and river flow was then regulated by a sluice complex in the Volkerak dam. Between 1980 and 1987 a storm-surge barrier was constructed across the mouth of the Oosterschelde, and two compartmentalisation dams were built in the eastern part of the basin. The storm surge barrier decreased the effective cross-sectional area at the mouth from 80 000 m² to 17 900 m². The compartmentalisation dams reduce the surface area of the Oosterschelde basin from 452 km² to 351 km². The hydraulic impact of the works is demonstrated with data from field measurements and data from model calculations.     

Microphytobenthos in the Oosterschelde estuary (The Netherlands), 1981–1990; consequences of a changed tidal regime

During the period 1981–1990 the functioning of microphytobenthos in the carbon cycle was studied in the Oosterschelde, a mesotidal, euhaline estuary (SW Netherlands), both before and after completion of a storm-surge barrier in the sea ward entrance of the estuary in 1986, which reduced the tidal range to 88% and current velocities to 70% of their former values on average.The annual biomass cycle has changed from small spring and autumn peaks into a much larger summer peak. The average biomass during summer has increased from 70 to 170 mg Chlorophyll-a m². The average annual biomass has increased from 115 to 195 mg Chlorophyll-a m². As a consequence the (calculated) primary production by microphytobenthos has increased also, from 150 to 242 gC m^–2 y^–1 (14 045 to 22 265 tonnes C y^–1), and its share in the total primary production of the Oosterschelde has increased from 16 to 30%. These increases in biomass and primary production are mainly ascribed to a decrease in the dynamic forces (water current velocities) over the intertidal flats in most parts of the basin. Increased water transparency in parts of the estuary and increased import of inorganic carbon from the water column towards shoals may have contributed as well.The rate of reworking of the top layer of the soil (0–10 cm) has not changed significantly, as the decrease in Chlorophyll-a biomass with depth has hardly changed.     

A fine-grained sediment budget for the Sylt-Rømø tidal basin

A budget for net accumulation of fine-grained sediment (<63 μm) has been set up for the Sylt-Rømø tidal basin. Net accumulation within the basin was computed from²¹⁰Pb core dating and mapping of the intertidal and supratidal surface sediments. It was found that a yearly mean value of 58·10³ tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64% to the net budget; the fluvial input and primary production contribute 14% and 15%, respectively. Local salt marsh erosion accounts for about 5% of the budget and atmospheric deposition for only 2%. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Rømø tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged water volume between the tidal basin and the sea. This index shows that in the Sylt-Rømø tidal basin, fine-grained suspended sediment “filters” out of the exchanged sea water at a rate that is 12 times lower than in the Gr»dyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical and hydrodynamical parameters and to a lesser degree of sediment availability     

Preliminary results on phosphorus reduction in Lough Neagh - Assessing the effect against a background of change

Lough Neagh is a large (387 km²), shallow (mean depth 8.9 m) eutrophic lake. The phosphorus budget is described in order to illustrate that 75% of the particulate phosphorus input sediments to the bottom and that there is little retention of SRP in the lake. 54% of the SRP loading comes from sewage works and this is being reduced by phosphorus reduction at the major sewage works. There has been a downturn in the maximum algal crops, particularly of blue-green algae. However, there is an underlying upward trend in SRP loading and the paper stresses the importance of analysing trends in nutrient budgets to get an accurate evaluation of reduction strategies.     

Mussel culture in a changing environment: the effects of a coastal engineering project on mussel culture (Mytilus edulis L.) in the Oosterschelde estuary (SW Netherlands)

To evaluate the effects of a large scale coastal engineering project on the mussel (Mytilus edulis) bottom culture in the Oosterschelde estuary (S.W. Netherlands), mussel growth and production in the period 1980–1990 are studied in relation to food supply and the hydrodynamic conditions. Due to the construction of a storm-surge barrier and two additional dams, the risk that mussels are swept away by high current velocities decreased, resulting in an increase of the area in the Oosterschelde potentially suitable for mussel culture and in food availability now being more important as a limiting factor. For the Oosterschelde, a clear relation between mussel growth, stock sizes, and phytoplankton dynamics has been demonstrated. The meat yield of mussels landed in autumn — which is an index for growth rate — seems to be determined by the phytoplankton production in the preceding summer. In years with dense bivalve stocks, phytoplankton production and meat yields are relatively low. It is concluded that an increase of the mussel biomass cultured can result in a reduction of the primary production and, consequently, in a deterioration of the growing conditions for suspension-feeders in the estuary. This conclusion is supported by model calculations. An expansion of mussel culture in the new Oosterschelde is therefore dissuaded. Apart from primary production and stock sizes, food supply for mussels on culture lots appeared to be controlled by the horizontal advection of phytoplankton between and within the tidal channels. An observed decline in mussel landings from certain areas is attributed to the reduced mixing energy of the estuary in relation to the present distribution of the lots over the estuary. Production figures from the experimental lots, established in 1988 in the newly available areas, demonstrate that the yield of mussels can be enhanced by relaying culture lots towards the areas where the phytoplankton is produced. It is expected that by redistributing the culture lots, without expanding the biomass cultured, the carrying capacity of the Oosterschelde for mussel culture can be maintained.     

Nutrients and phytoplankton primary production in the marine tidal Oosterschelde estuary (The Netherlands)

In a shallow marine tidal area, the eastern part of Oosterschelde estuary in the S.W. Netherlands, phytoplankton primary production amounted to 176–338 g C.m^?2.y^?1 during the period 1981–1985. The influence of nutrient concentrations on the phytoplankton primary production is discussed. Phosphate and inorganic nitrogen generally were amply available. Import of inorganic nitrogen into the basin was shown and an intense delivery of ammonia by zoobenthos was suggested. Nitrate was considered to be slightly influenced by phytoplankton consumption and mainly by nitrate reduction at the bottom. Silicate may have played a limiting role in phytoplankton primary production. The first phytoplankton bloom in spring (diatom bloom) always terminated when silicate concentration decreased below K_s values. Further on in 1983 and 1984 both primary production and chlorophyll curves showed a dip when silicate was not available. The influence of available light on the primary production was demonstrated during situations with a low extinction coefficient when primary production reached maximum values. Further on during 1985 the spring bloom occurred already in March when winter extinction coefficients were lower than during preceding winters. Long term production studies are necessary to understand the extreme fluctuations of annual production patterns in relation to the prevailing environmental conditions.     

The Oosterschelde estuary: an evaluation of changes at the ecosystem level induced by civil-engineering works

The interest in storm-flood protection has recently gained momentum, owing to the wide international discussion on the impact of sea-level rise on society. The Oosterschelde project is technically and scientifically unique. The storm-surge barrier represents an important breakthrough in marine civil engineering. The project also offered ample opportunities to perform integrated physical, chemical, geological and biological research. Integration of the knowledge gained, raised the entire project to the level of a case study of a changing estuarine ecosystem, and demonstrated the effects of human interference in a non-polluted estuary. Notwithstanding considerable changes in the environment, the Oosterschelde has retained most of its favourable abiotic factors, labeling the estuary as a high quality marine system. The water quality in the post-barrier period more closely resembles that of the North Sea than in the period before. Significant changes in erosion and sedimentation and the consequent redistribution of fine sediments, are continuing. The ecosystem has shown responses to various factors. Effects of severe winters and impact of mussel- and cockle fisheries could be distinguished from other factors. The physical response of the ecosystem to the civil-engineering project could be quantified in terms of changes in habitat availability, maintenance of biological productivity, and restricted maintenance of the carrying capacity as an internationally recognized wetland and fisheries area.     

An experimental test of the effects of inorganic sediment addition on benthic macroinvertebrates of a subtropical stream

Inorganic sediments of terrestrial origin may impact stream macroinvertebrate communities. Although input of terrestrial sediments to streams may occur naturally, human-induced activities in the catchment amplify this input greatly. We used an in-stream experiment to investigate whether short-term additions of terrestrial sediments of two size classes affected stream macroinvertebrates. The experiment was designed in blocks to minimize the influence of flow velocity and other environmental variables. Four treatments were employed: (i) addition of fine sand (0–0.24 mm), (ii) coarse sand (0.25–0.8 mm), (iii) fine+coarse sand, and (iv) control (water only). Macroinvertebrates were sampled immediately after the addition of sediments (or water). The experiment consisted of 20 blocks. We analyzed the response of the macroinvertebrate fauna in terms of abundance and species richness. Since species richness is strongly dependent on number of individuals sampled, we also analyzed rarefied species richness. Community structure was evaluated using a distance-based Manova on presence/absence and abundance data. The addition of coarse and fine+coarse sand reduced the abundance and species richness of macroinvertebrates in relation to the control. The response in terms of rarefied species richness in the treatments did not differ from the control, indicating that reduction in species richness was a sampling artifact resulting from decreased sample abundance. The Manova analyses indicated that coarse-sand addition caused changes in both species composition and community structure. Addition of fine and fine+coarse sand affected only slightly species composition and community structure. We concluded that even short-term input of terrestrial sediments causes impacts on benthic macroinvertebrates, and recommend that land-use management of tropical catchments should employ practices that reduce input of terrestrial sediments to streams. Handling editor: K. Martens     

Overwintering waterfowl on Swiss lakes: how are abundance and species richness influenced by trophic status and lake morphology?

During the period 1980–1990 long-term physical, chemical and ecological studies were carried out, to study the changes induced by the building of a storm-surge barrier in the mouth of the Oosterschelde estuary and two large auxiliary compartment dams in the rear ends of the estuary. The storm-surge barrier was constructed in the mouth of Oosterschelde estuary (SW Netherlands) during the period 1979–1986. The barrier allows the tides to enter the estuary freely, and, on the other hand, the barrier guarantees safety for the human population and their properties when a stormflood threatens the area.Oosterschelde estuary is isolated from the river input, the rear ends of the ecosystem were separated from the estuary by sea-walls and the strongly decreased tidal exchange with the North Sea induced sheltered circumstances. The Oosterschelde changed from a turbid estuary into a tidal bay, and yet primary production responses appear to be robust and resilient, and the biological communities showed only quantitative shifts from the dominance of specific species assemblages to other assemblages. In many cases predicted changes in the structure of the biological communities could not be verified owing to the large natural variability mainly caused by physical factors (e.g. temperature).     

Interpretation of caesium-137 profiles in lacustrine and other sediments: the role of catchment-derived inputs

The caesium-137 profiles obtained in many investigations which have attempted to use caesium-137 measurements as a means of assessing the rate of accumulation of recent lake sediments, have not conformed to the classic shape expected from the record of fallout input. Such deviations have been accounted for in terms of post-depositional mobility of the caesium-137 input (e.g. bioturbation) and delayed inputs from the drainage basin. There have, however, been few attempts to determine the likely character of the drainage basin input and to analyse the role of such inputs in influencing the precise form of the caesium-137 profile. This paper presents the results of an attempt to employ existing knowledge concerning the behaviour of caesium-137 in soils and the processes of sediment mobilization to predict the likely form of the record of caesium-137 input to a lake or river floodplain from its drainage basin. The influence of this input on the profile shape will depend on the relative importance of the atmospheric fallout to the lake or floodplain surface and the drainage basin input to the total caesium-137 inventory in the sediment core, and on the land use and sediment sources in the drainage basin. By incorporating the drainage basin input into a simple model of caesium-137 accumulation in lake and floodplain sediments, it was possible to account for the profile shapes measured in four cores investigated by the authors and therefore to verify their utility for assessing rates of sediment accumulation.     

Impacts of bottom and suspended cultures of mussels <Emphasis Type="Italic">Mytilus</Emphasis> spp. on the surrounding sedimentary environment and macrobenthic biodiversity

The aim of this study was to quantify the effect of bottom and suspended mussel cultures, cultured in different physical environments, on the sedimentary environmental conditions and thereby the biodiversity structure of the associated macrofaunal community. We compared two bottom cultures (Limfjorden: microtidal, wind-driven; Oosterschelde: macrotidal) and one suspended culture (Ria de Vigo in an upwelling coastal region). The sedimentary environmental conditions (mud fraction, POC, PON, phosphorus content, chl a breakdown products) were significantly elevated underneath and surrounding bottom and suspended cultures compared to culture-free sediments that were nearby and hydrodynamically similar. The relative change in environmental conditions was more pronounced in the Oosterschelde compared to Limfjorden, most likely due to differences in hydrodynamic forcing and characteristics of the mussel bed. The effect of the suspended cultures in Ria de Vigo on the surrounding sediments was influenced by local topographic and hydrodynamic conditions. The impact of mussels on the benthic community due to biodeposition was clearly seen in the community structure. The species composition changed from species which are typically present in sandy environments to more small opportunistic species, which are typically present in organically enriched sediments. The impact of bottom cultures on the benthic community due to changes in the habitat under the presence of mussels was positive, especially in the Oosterschelde where an increase in the number of epibenthic species was seen. The influence of bottom cultures on the sedimentary environment and on the macrobenthic community seems to be very local. Within the mussel site in Limfjorden, differences were detected between sites where none or almost no mussels were present with sites where mussels were very abundant.     

Changes in the sublittoral hard substrate communities in the Oosterschelde estuary (SW Netherlands), caused by changes in the environmental parameters

In order to assess the effects of the execution of the Delta Project, the sessile sublittoral communities on hard substrates in the Oosterschelde estuary and the environmental parameters were quantitatively investigated from 1985 till 1990. During the construction period of the barrier, three communities were sampled in the photic zone and four in the aphotic zone. The distribution of the communities in the photic zone seemed to be determined by the exposition to water movement and depth, while the communities in the aphotic zone were restricted to geographic areas, with differences in tidal current velocities: the mouth of the estuary, the Hammen, the central part and the Zijpe. Two years after the completion of the enclosure works, the community structure changed rapidly, caused by decreases of tidal current velocities, increases of the amounts of sedimentation, especially of fine sediments, and an increase of the transparency of the water. Changes within the associated vagile animals showed the same tendency as the sessile communities: under less exposed conditions the number of organisms remained the same or increased, while at some locations this increase was nullified by increasing amounts of sedimentation.     

Changes in the fish fauna of the Oosterschelde estuary — a ten-year time series of fyke catches

Frequency of occurrence of fish species was monitored on a fortnightly basis in four fykes and a weir in the Oosterschelde estuary from 1979 through 1988. This was done in order to record changes in the fish fauna that may have occurred as a response to the construction of a storm-surge barrier in the mouth of the Oosterschelde (1984–1986) and the concomitant building of compartmentalization dams in the landward part. These compartmentalization dams reduced the freshwater inflow into the system. Principal component analysis using the annual averages in frequency of occurrence suggests a slight shift occurred in the fish community separating a cluster of years 1979–1984 from the cluster 1985–1988. Many of the changes in individual species could be attributed to fluctuations in yearclass strength or were part of changes occurring on a wider geographical scale. The only impact of the construction works seems to be the decrease in a number of anadromous fish. Fish traps seem to be useful as a monitoring tool for a number of species. The value of the data collected could be improved if catch size and length-frequency data are recorded.     

Megafaunal community structure of Andaman seamounts including the Back-arc Basin--a quantitative exploration from the Indian Ocean

Species rich benthic communities have been reported from some seamounts, predominantly from the Atlantic and Pacific Oceans, but the fauna and habitats on Indian Ocean seamounts are still poorly known. This study focuses on two seamounts, a submarine volcano (cratered seamount--CSM) and a non-volcano (SM2) in the Andaman Back-arc Basin (ABB), and the basin itself. The main purpose was to explore and generate regional biodiversity data from summit and flank (upper slope) of the Andaman seamounts for comparison with other seamounts worldwide. We also investigated how substratum types affect the megafaunal community structure along the ABB. Underwater video recordings from TeleVision guided Gripper (TVG) lowerings were used to describe the benthic community structure along the ABB and both seamounts. We found 13 varieties of substratum in the study area. The CSM has hard substratum, such as boulders and cobbles, whereas the SM2 was dominated by cobbles and fine sediment. The highest abundance of megabenthic communities was recorded on the flank of the CSM. Species richness and diversity were higher at the flank of the CSM than other are of ABB. Non-metric multi-dimensional scaling (nMDS) analysis of substratum types showed 50% similarity between the flanks of both seamounts, because both sites have a component of cobbles mixed with fine sediments in their substratum. Further, nMDS of faunal abundance revealed two groups, each restricted to one of the seamounts, suggesting faunal distinctness between them. The sessile fauna corals and poriferans showed a significant positive relation with cobbles and fine sediments substratum, while the mobile categories echinoderms and arthropods showed a significant positive relation with fine sediments only.     

The phosphorus budget of the Marsdiep tidal basin (Dutch Wadden Sea) in the period 1950–1985: importance of the exchange with the North Sea

Nutrient and carbon budgets of the western part of the Dutch Wadden Sea are based on the concept formulated by Postma (1954): import of organic compounds from the North Sea, mineralization in the estuary ans subsequently an export of dissolved inorganic nutrients to the North Sea. In this paper the phosphorus budget of the westernmost part of the Wadden Sea (Marsdiep basin) during the period 1950–1985 is considered to evaluate this concept and to investigate whether eutrophication processes have changed these P-budgets during the last decades. Most attention is paid to the importance of the exchange of P-components between the tidal basin and the open sea. The budgets are based on a general mass balance equation in which the main entries are the loadings via the freshwater sources, the net sediment-water exchange, the exchange with the North Sea, the input from the adjacent Vlie basin, and a transformation term representing the interchange between dissolved and particulate phosphorus. Averaged over the entire period 55% of the phosphate input (SRP) to the basin is delivered by the fresh water sources, and 45% by the Vlie basin. For non-SRP components these numbers are 50% and 40% respectively, while only 10% comes from the North Sea. The import from the North Sea, as calculated by our model thus seems less important than previously documented. There is a clear effect of the eutrophication during the last decades. In the first 20 years of the period investigated there was a moderate increase in the TP input to the basin, partly caused by an increasing input of non-SRP (TP minus soluble reactive phosphorus) from the North Sea. In 1970/71 almost 30% of the non-SRP loading originated from the North Sea. From approximately 1975 onwards this import of non-SRP turns into an export, while the TP-loading from the other sources inclined, until a maximum was reached in 1981. Recently a decrease in the TP-inputs has been observed. It is concluded that the recent output of phosphorus from the Marsdiep basin may have a considerable impact on the adjacent area of the North Sea.Publication no. 2 of the project Applied Scientific Research Neth Inst. for Sea Res. (BEWON).     

The relationship of floods, drying, flow and light to primary production and producer biomass in a prairie stream

Factors related to autochthonous production were investigated at several sites along a prairie stream at Konza Prairie Research Natural Area. Primary production, algal biomass, litter input, and ability of floods to move native substrate were measured. Additional experiments were conducted to establish the influence of light and water velocity on primary production rates and recovery of biomass following dry periods. The study period encompassed two extreme (> 50 year calculated return time) floods, thus we were able to analyze the effects of scour on periphyton biomass and productivity. Biomass of sedimentary algae was reduced greatly by flooding and did not reach preflood amounts during the 2 months following the first flood. Rates of primary production associated with sediments recovered to levels above preflood rates within 2 weeks. Biomass of epilithic periphyton was not affected as severely as that of sedimentary algae. Little relationship was observed between water velocity and photosythetic rates. Production reached maximum rates at 25% of full sun light. Epilithic chlorophyll levels recovered within eight days following a dry period, and chl a was an order of magnitude greater on rocks than sediments 51 days after re-wetting. Estimated annual rates of primary production were 2.6 times greater in the prairie than in the forest reaches of the stream. The ratio of annual autochthonous:allochthonous carbon input was 4.81 for prairie and 0.32 for the forest. Periphyton production in prairie streams is resilient with regard to flooding and drought and represents a primary carbon source for the system.     

Development and ecological importance of phytoplankton in a large lowland river (River Meuse,Belgium)

The ecological importance of the River Meuse phytoplankton with regard to carbon and nutrient transport has been examined in two reaches of the Belgian course of the river.Field measurements of total particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate phosphorus (PP) show that the large autochtonous production of organic matter strongly affects the carbon and nutrient budget of the aquatic system. During the growing season, phytoplankton accounts for nearly 60% of the POC and dominates the PON. Calculations of the carbon and oxygen budget in the upper reach of the Belgian Meuse demonstrates that the ecosystem is autotrophic, i.e. that autochtonous FPOM (fine particulate organic matter) production is the major carbon input. This suggests that in large lowland rivers, primary production (P) may exceed community respiration (R), i.e. P:R>1, whereas they are assumed to be heterotrophic (P:R<1) in the River Continuum concept.The question of maintenance of phytoplankton in turbid mixed water columns is also addressed, and the case of the River Meuse is treated on the basis of studies of photosynthesis and respiration (ETS measurements). The results suggest that the potamoplankton may show some low-light acclimation, through an increase of chlorophyll a relative to biomass, when it comes to deep downstream reaches, and that algal respiration rate may be reduced. A simulation of the longitudinal development of the algal biomass shows the different phases of algal growth and decline along the river and brings support to the importation hypothesis for explaining maintenance of potamoplankton in the downstream reaches.     

The mobile epifauna of the soft bottoms in the subtidal Oosterschelde estuary: structure,function and impact of the storm-surge barrier

Data on the mobile epifauna of the Oosterschelde estuary, collected by beam trawl, were compiled from several studies. Multivariate statistical techniques brought out the fact that the Oosterschelde, when compared with neighbouring areas, has a characteristic epibenthic fauna. Diversity as measured by Hill's diversity numbers N through N ₊ , is higher for the Oosterschelde (N₁ = 4.5) than for the Voordelta (N₁ = 3.5) and the Westerschelde (N₁ = 2.2).Four epifaunal communities can be distinguished within the Oosterschelde, the two most seaward communities being the richest. Annual production is estimated at about 6 gADW m^–2 yr^–1, annual consumption is estimated at over 25 gADW m^–2 yr^–1. These results are highly dependent on the assumptions. Over 85% of the epibenthic production and consumption in the Oosterschelde is accounted for by only six species: starfish Asterias rubens, plaice Pleuronectes platessa, bib Trisopterus luscus, brown shrimp Crangon crangon, shore crab Carcinus maenas and dab Limanda limanda. In spite of its abundance, the sand goby Pomatoschistus minutus contributes little to the production.From the available data it is difficult to assess the impact of the construction of the storm-surge barrier and the compartmentalization dams on the epibenthic fauna. The increase in flatfish in the Hammen area is probably linked to the decrease in current velocities in that area. On the other hand the increase in the gadoids bib and whiting Merlangius merlangus is predominantly due to the stronger year classes in the post-barrier time period. Lower nutrient inputs through the Northern branch, in combination with the increase of the gadoids, may have caused the decline of the brown shrimp in the Oosterschelde. A decrease has also been observed in the sandeel Ammodytes tobianus and the hooknose Agonus cataphractus.