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Within the scope of the long term vision of the Scheldt estuary, a new disposal strategy was proposed by an international expert team appointed by the Antwerp Port Authority. After the feasibility study, two in situ disposal tests were carried out near the Walsoorden sandbar in the Western Scheldt. Both tests were thoroughly monitored, both morphological as ecological.A project was defined to investigate the impact of bed forms, hydrodynamics and sediment properties on the ecological value in the shallow water area near the Walsoorden sandbar. The goal of this project is to make a habitat classification for the Western Scheldt based on its ecological value, allowing to define which areas are best suited for disposal of dredged material and which areas should be avoided.In the first phase the multibeam echo sounding data were analysed in respect to the occurrence of bed forms. The analysis was executed in four steps, resulting in a limited number of classes.A large spatial variation was found for the analysed parameters. The average length of bed forms ranged from 5 m to 50 m, the average height from 0,10 m to almost 2,00 m, while, considering the asymmetry due to tidal currents, most of the sections were flood dominated.In a second phase the seasonal variation of the bed forms was investigated, analysing the multibeam echo sounding data of the summer condition (September 2007).In the third phase of the project the relation between the occurrence of bed forms and hydrodynamic parameters (using a validated numerical hydrodynamic model) was investigated. With the addition of sediment properties (grain sizes) a classification based on the physical parameters is made. These “physiotopes” were finally validated with the ecological data (i.e. Macrobenthos and fish), trying to make a classification in “ecotopes”.

Voor de beoordeling van de toestand van macrofauna in de Nederlandse kust- en overgangswateren zijn maatlatten ontwikkeld, gebaseerd op de Benthic Ecosystem Quality Index (BEQI). Deze methode gaat uit van een ecosysteembenadering en maakt niet alleen gebruik van informatie over de macrofauna zelf maar ook over zijn leefgebieden en voedselbronnen. Zo is het mogelijk directe en indirecte effecten van door de mens veroorzaakte druk te signaleren. De methode overstijgt de beoordeling van individuele meetpunten en maakt het mogelijk voor een waterlichaam als geheel op een betrouwbare manier vast te stellen of de toestand van de macrofauna afwijkt van een gekozen referentie

Coastal sediments in sheltered temperate locations are strongly modified by ecosystem engineering species such as marsh plants, seagrass, and algae as well as by epibenthic and endobenthic invertebrates. These ecosystem engineers are shaping the coastal sea and landscape, control particulate and dissolved material fluxes between the land and sea, and between the benthos and the passing water or air. Above all, habitat engineering exerts facilitating and inhibiting effects on biodiversity. Despite a strongly growing interest in the functional role of ecosystem engineering over the recent years, compared to food web analyses, the conceptual understanding of engineering-mediated species interactions is still in its infancy. In the present paper, we provide a concise overview on current insights and propose two hypotheses on the general mechanisms by which ecosystem engineering may affect biodiversity in coastal sediments. We hypothesise that autogenic and allogenic ecosystem engineers have inverse effects on epibenthic and endobenthic biodiversity in coastal sediments. The primarily autogenic structures of the epibenthos achieve high diversity at the expense of endobenthos, whilst allogenic sediment reworking by infauna may facilitate other infauna and inhibits epibenthos. On a larger scale, these antagonistic processes generate patchiness and habitat diversity. Due to such interaction, anthropogenic influences can strongly modify the engineering community by removing autogenic ecosystem engineers through coastal engineering or bottom trawling. Another source of anthropogenic influences comes from introducing invasive engineers, from which the impact is often hard to predict. We hypothesise that the local biodiversity effects of invasive ecosystem engineers will depend on the engineering strength of the invasive species, with engineering strength defined as the number of habitats it can invade and the extent of modification. At a larger scale of an entire shore, biodiversity need not be decreased by invasive engineers and may even increase. On a global scale, invasive engineers may cause shore biota to converge, especially visually due to the presence of epibenthic structures.

The Benthic Ecosystem Quality Index (BEQI), has been developed by the Netherlands Institute of Ecology on behalf of the Rijkswaterstaat/Institute for Coastal and Marine Management. It is based on an ecosystem functioning approach (Ysebaert & Herman, 2004), which aims to give an indication about ecosystem structure and functioning, and about biological relationships. BEQI evaluates at the scale of a whole water body, contrary to methods applied by other member states that evaluate the ecological status per sampling station. The Dutch monitoring system of water bodies (MWTL) and ecological monitoring (since the nineties) also aims to assess the status of a water body as a whole. The initial developed BEQI method (2004) was compared with other Member States WFD Benthos methods under development and valuated as the most appropriate to develop further and to apply to all Dutch coastal and transitional water bodies the BEQI method for WFD benthos classification.

The EU Water Framework Directive (WFD) requires development of biological indicators and ecological classification for assessment of the ecological quality status of rivers, lakes, coastal and transitional waters. The NIOO - approach is developed to evaluate the ecological status of benthic invertebrates in coastal and transitional waters. This approach aims to be representative for the important structural and functional characteristics of coastal and estuarine ecosystems. The method uses a set of indicators that take into account the different scales of variability in coastal and transitional waters and evaluate the water body (ecosystem) as a whole and not by evaluating sampling station by sampling station. This approach evaluates the ecological status on three different levels (Escaravage et al., 2004; Ysebaert and Herman, 2004). On the level of the whole ecosystem (e.g. a water body) one can evaluate if the benthic macrofauna fulfils the functional role one might expect given the current ecological circumstances. At this level also integration with other quality measures is most appropriate, and information on the water body can be summarised. On the subsequent level the distribution of habitats (habitat completeness, complexity and presence of certain ecoelements [e.g. mussel banks]) can be evaluated. Finally the biological quality of each distinguished habitat based on benthic macrofauna will be evaluated (within-habitat level), based on four indicators (biomass, density, species richness and species composition changes). These four indicators aim at providing a signal that is capable of showing significant changes/deviations from a certain defined reference state. The indicator results strongly depend on the sampling effort (sediment surface) that is deployed in a habitat and water body. Therefore, the reference values for the indicators were calculated per habitat from permutations executed over increased sampling surfaces. This allows estimating, for any given sampling surface, the reference value that can be expected. The expected reference values for each indicator, based on this randomisation, were determined for each ecological status boundary of the WFD. The ecological status for each indicator is finally determined by classifying the observed assessment value in the right ecological status class, based on their defined reference values. The NIOO - approach uses easily interpretable indicators that are evaluated separately. This allows a transparent assessment method and for any deviation of the overall ecological quality ratio from the reference condition the underlying responsible indicator can be easily traced back and evaluated individually. This tool is currently used by the Netherlands and Belgium to evaluate the ecological status of the benthos, following the requirements of the WFD.

The ecology of estuarine intertidal flats is characterised by low species diversity and high productivity. The consequences of these two main characteristics have been investigated in detail on an intertidal flat in the Schelde estuary in the Netherlands. The existence of a species minimum in the oligohaline section of the estuary, as postulated by Remane, could not be confirmed for nematodes and macrofauna for the Schelde, nor for several other European estuaries. Rather, a linear decrease in species richness from the sea to the freshwater was observed. The productivity of estuarine tidal flats depends on an in situ source of primary produced material, the microphytobenthos, as well as on the capacity of the macrofauna to transfer material from the water column to the sediments. Avery rapid transfer of 13C labelled carbon from primary production to the higher trophic levels was observed.

On October 23th 2000, the EU Water Framework Directive was finally adopted. The Directive was published in the Official Journal on December 22th 2000 and entered into force the same day. The Water Framework Directive expands the scope of water protection of all waters and sets clear objectives that a 'good status' must be achieved for all European waters by 2015 and that water use must be sustainable throughout Europe. According to the Directive, the individual member states are obligated to describe ecological objectives or goals for different waters. The aim of this project was to define ecological objectives for different biological quality elements for the Flemish transitional waters. In this document quality indices for benthos and phytoplankton have been presented. This document includes a study about the different approaches and methods used in Europe to define these ecological objectives. None of the existing indices was satisfactory and applicable on the Flemish transitional waters, neither for benthos nor for phytoplankton. This has to do with the complexity of transitional water systems. For the Flemish transitional waters no temporal or spatial reference is available. The reference is therefore based on the available knowledge about the ecological functioning of estuaries. Concerning phytoplankton, separate indices have been established for the salt and fresh water zones of an estuary. For brackish water, a natural mortality zone for plankton, indices would be pointless. The scoring system for phytoplankton takes into account the trophic status (including the important element silica), the photic status and the danger of flushing. This scoring system has a simple design and determination of the coefficients requires minimal effort. After evaluation by specialists and comparison with other scoring systems, we can state that the proposed phytoplankton index is promising. Nevertheless the index needs testing, because the available data of the Flemish transitional waters were used for its compilation. For benthos indices have been established for fresh water and for salt and brackish water together. These indices are derived from the marine index of Borja et al. (2000), which takes into account the response of species to organic load. For salt and brackish water the 'Borja'- index was combined with an index based on sensitivity/tolerance for current velocity. The 'Borja'-index and the proposed index were tested on the sediments of the Scheldt estuary. Although the proposed indices showed improvement in comparison with application of 'Borja', they, still are not fully satisfying. The lack of any score for absolute density - or biomass values proved to be a shortcoming. The sensitivity/tolerance of species to organic pollution needs to be further examined, especially for estuarine fresh water species. For macro-algae and angiosperms no indices have been established. At present the role of macro algae in the Flemish transitional waters is nihil, and most probably this will not change in the future, so indices would be pointless. The only angiosperms occurring in the Flemish transitional waters are tidal flat vegetation, the discussion whether tidal flat vegetation must be incorporated in the quality index still exists.

Practical considerations in estuarine management, as well as prediction of the consequences of global change on coastal protection, urgently require a better understanding and better modeling of estuarine ecosystems as influenced by ecological, physical, chemical and morphological processes. Recent Dutch examples of such questions are: the impact of enhanced dredging in the Schelde estuary, the impact of sea level rise on the Wadden Sea and Delta area, concerns about the loss of salt marsh habitats, etc. Benthic communities are good indicators of biotic integrity and reflect the present state of the estuarine ecosystem. The analysis of benthic infauna is a key element of many marine and estuarine monitoring programs. In the Dutch Delta area (SW-Netherlands) there is a relatively long tradition on estuarine macrozoobenthos monitoring, such as implemented e.g. in the BIOMON program. This program was designed to detect long-term trends in the average density, biomass and species composition of large parts of different systems (e.g. Schelde estuary, Oosterschelde, Grevelingen), in order to obtain insight in the natural development of estuarine and coastal areas and the anthropogenic influences on these systems. Running now for over a decade, these programs, together with other field campaigns, provide a unique data set on benthic macrofauna (e.g. for the Schelde estuary over 5000 samples are available at the moment). Until recently these data were hardly processed and used for further analysis. However, such data sets offer the opportunity to analyze and predict patterns in occurrence of benthic macrofauna in a much more profound way. Recently, within a cooperation between decision makers (Rijkswaterstaat, Directie Zeeland), advisers (RIKZ) and scientists (NIOO-CEMO), the possibilities and limitations of using these data sets for the predictions of benthic macrofauna at scales relevant to resource management problems are evaluated. In our approach we use different statistical methodologies to quantify, model and predict patterns at different spatial and temporal scales, going from patterns on a single tidal flat to inter-estuary comparisons and from monthly patterns to decennial trends. Several examples are shown that illustrate the use of these data, going from simple classification techniques to more sophisticated predictive modeling: Changes and shifts in benthos communities are shown for a land reclamation area of Rotterdam harbour in the Haringvliet-delta using classification techniques. Ordination analysis on the saline lake Grevelingen, a former estuary, showed long-term changes in macrobenthic community structure as a consequence of changes in salinity, light penetration, etc. This case study will be dealt with in more detail in a separate contribution. In the Schelde estuary, a detailed study was performed to unravel the use of environmental data in predicting benthic macrofaunal species distributions at different spatial scales (from a single tidal flat to the whole estuary). Statistical techniques such as geostatistics, hierarchical analysis and logistic regression were applied. At these different scales a distinct relation between the environment (e.g. salinity, sediment characteristics) on the one hand and macrofaunal species distributions on the other hand was observed. As a consequence, predictions of macrofaunal distributions can be made quite successful from environmental data within the Schelde estuary. An inter-estuary comparison between the Schelde estuary and Oosterschelde revealed that predictive models should also incorporate system-wide properties of estuarine systems, such as primary production and suspended matter concentrations, in order to perform in a more generic way. The results clearly show their use in making more sensible long-term decisions about matters having direct environmental effects. The results also provide information on how the design of monitoring programs could be improved or optimized, depending on the questions asked. As such, a more synergetic and flexible approach is urgently necessary, in which decision makers, advisers and scientists communicate in a more efficient way.

Estuarine ecosystems are charac­terized by largely varying physicochemi­cal condi­tions, especially in the meso-/oligohaline zones. Knowledge of the environmental variability and related population effects on a range of spatio-temporal scales is fundamental to a better understanding of their functioning, stability, resilience and the way they are influenced by human impacts. This study combines spatial and temporal variations in macrobenthic populations (and their environment) on a brackish mudflat in the Schelde estuary (NW-Europe). From 1990 to 1999, long -term year-to-year variations were monitored on 24 sites, short -term monthly variations on two sites. The variation in macrobenthos and the physicochemical environment due to seasonal dynamics, spatial pattern, and annual as well as long-term trends, was quantified. The macrobenthic community on the mudflat was characterized by a few dominant species (Corophium, Heteromastus, Nereis, Oligochaeta), with spatial distributions related to sediment characteristics. Considerable year-to-year variation was observed and seasonality was evident for all species. It is argued that in the meso-/oligohaline zone, where salinity shows large seasonal fluctuations, benthic communities change frequently during the year, resulting in communities that seldom progress beyond early benthic-community succession. Annual variations are less pronounced. The impact of the construction of a containerterminal (1994), adjacent to the mudflat, on the macrobentic species distributions is discussed.

Water column dynamics of carbon, nitrogen and phosphorus in the main creek of the Bangrong mangrove forest, Phuket Island, Thailand, were examined during the dry season. Water sampled from the upper and lower reaches of the creek throughout entire neap and spring tide periods was incubated under saturated irradiation and in the dark. The activity of microbial primary producers and heterotrophs were evaluated from changes in O2, TCO2, DOC, DIN, DON and PO43- concentrations. Gross primary production was moderate (1.6–2.4 µM C h-1) with no pronounced spatial and temporal variations. A large fraction of the assimilated inorganic carbon and nitrogen was released in the form of DOC (50–90%) and DON (50–60%) indicating that primary producers were under stress or nutrient depleted. PO43- and occasionally DIN appeared to be the limiting nutrients. The pelagic heterotrophic community was supported by DOC (50–100%) and DON (40–90%) excreted by primary producers when exposed to light. However, rapid light attenuation in the turbid creek water rendered the entire water column strongly heterotrophic at all times (1.1–2.8 µM C h-1). The microbial heterotrophs are therefore largely supported by particulate and dissolved substrates derived from tidal resuspension, mangrove root exudates and leachates from fallen leaves on the forest floor. The relatively high concentrations of metabolites (e.g. CO2, NH4+) in creek water were primarily supplied by microbial mineralization. Water seeping from creek banks, which was only enriched in TCO2 (2 times) and PO43- (20–100 times) compared with creek water, is not considered an important source of solutes in the waterways of the Bangrong mangrove forest. Although the results obtained here are only strictly representative for the dry season, water column processes in the wet season are not expected to differ much due to the limited seasonal variations with respect to physical, chemical and biological parameters in the Bangrong area. [KEYWORDS: mangrove, creek water, primary production, respiration, seepage]

There is growing awareness of the need to address the issue of scale more explicitly in (benthic) ecology. If spatial heterogeneity is important for the functioning of ecosystems, ecosystem models should include the spatial organization of populations, species assemblages and their environment. Macrobenthos plays an important role in the system dynamics of estuaries and is a good bioindicator. However, few attempts have been made to model and predict responses of macrobenthic species to environmental variables at different spatial scales. For the Scheldt estuary (NW-Europe) a large macrobenthic database is available, covering different spatial (from a single tidal flat to the complete estuary) and temporal (from month to decade) scales. Apart from the biotic information, this database contains several environmental variables such as salinity, current velocities and sediment characteristics. Besides more classical approaches such as classification and ordination (e.g. direct gradient analysis), we applied geostatistics and regression models (e.g. generalized linear models such as logistic regression) to the data in order to elucidate the role of environmental conditions in explaining the occurrence (presence/absence, abundance, biomass) of the benthic macrofauna at different spatial and/or temporal scales. Our results showed that at meso-scales (>100m-10 km) a considerable fraction of the variation in occurrence, density and biomass of macrobenthic species correlated very well with physical factors (depth, tidal current velocity, sediment composition). At larger scales within the estuary salinity changed these relations significantly. However, a large and statistically significant amount of unexplained variation still occurred at the smaller scales (< 100 m). Possible mechanisms evoking these small-scale patterns are discussed.

This paper describes the results of two field campaigns to measure critical shear stress for erosion of muddy sediments in intertidal mudflats of the Scheldt estuary in relation to both abiotic and biotic parameters.The critical erosion shear stress was measured in situ in three intertidal mudflats (Groot Buitenschoor; Galgenschoor and Paardenschor) situated in the Belgian part of the Scheldt estuary; using the HR Wallingford instrument SedErode. The tidal flats are situated in the mesohaline and the turbidity maximum zone of the estuary. On every intertidal flat three sampling locations were chosen. In every location three measurements of erosion stress were carried out and samples of the top layer were taken for sediment and benthos analysis. The sampling stations differed in topographical elevation: the average time of flooding during a spring tide varied between 2 and 9 hours.The pigments chlorophyll a and fucoxanthin were determined as an indication of the presence of microphytobenthos. Large differences in chlorophyll a content were found between both campaigns, with maxima in April, being ten times higher than in September.The number per m2 of all benthic species was measured. A total of 13 different species was found. The highest diversity occurred during September: The benthos density was substantially higher during September than April.Very low pigment content and macrofauna density occurred on the Galgenschoor intertidal flat. The sample locations on this tidal flat are situated low in the intertidal zone and are flooded during a major part of the tidal cycle.The measured critical shear stress for erosion was up to 65% higher during April than September: The value of the critical shear stress is situated between 0.26 and 0.43 Pa (April) and between 0.24 and 0.29 Pa (September). The critical erosion shear stresses measured on the Galgenschoor, where the biological activity was very low, showed little variation between both campaigns. The data do not permit to establish a clear correlation between 050 or mud content and the critical shear stress or density. Some data, however suggest a relation between shear stress and increasing sand content or density. This can be explained by the presence of sand in a muddy bed, which will compact the sediment and increase the erosion resistance. The difference in erosion resistance between both campaigns is for a major part caused by biological parameters.

Het landwaarts verplaatsen van waterkerende dijken, of ontpolderen, is één van de meest substantiële estuariene herstelmaatregelen. Uitvoering hiervan vereist dat de effecten met een aanvaardbare zekerheid ingeschat kunnen worden. In deze studie worden de ontwikkelingen in het Sieperdaschor, de natuurlijk ontpolderde Selenapolder in de Westerschelde, vergeleken met andere natuurlijke en geplande ontpolderingen. Het doel is enerzijds het afleiden van wetmatigheden over te verwachten evoluties in ontpolderingen en anderzijds de verkregen data over het Sieperdaschor daarin te situeren.

The Schelde estuary, with its tidal flats and marshes along an uninterrupted salinity gradient from a marine to a fresh water system, is almost unique in Europe. On top of its important contributions towards national and international biodiversity it performs multiple socio-economic functions of vital importance. To combine all these functions in a sustainable way an integrated management is needed, based on the comprehension of its functioning and aimed at an optimal tuning of all processes and functions involved. With this report we hope to initiate a productive dialogue, one step ahead towards the integrated management of the Schelde estuary. It forms part of an agreement between the Administration of Waterways and Water infrastructure and some Flemish environmental organisations. The agreement was concluded as a result of protest actions against the deepening of the Westerschelde and it should result in a fruitful cooperation towards a more integrated management for the Flemish waterways. This pilot study explores possibilities to couple ecological rehabilitation and the creation of sustainable river related nature with the security measures of the SIGMA plan (a protection plan against floods) and the navigation requisites for the Zeeschelde. It also integrates the proposals from MEIRE et al. (1992), HOFFMANN (1993) and the AMIS study (general environmental impact assessment of the Sigmaplan). The decreased self-cleaning capacity of the river, the highly fragmented ecologica1 infrastructure, the accelerated sedimentation and increased tidal amplitude inhibit a proper functioning of the estuarine ecosystem. They are combined consequences of land reclamation, channelisation, land use and water management in the catchment area, discharge manipulations and the general sea level rise. They bring about the ecosystems limited carrying capacity and resilience, its increased contribution to the eutrofication of the North Sea, impoverished biotic communities, flood hazzard and reduced navigability. Possible restoration measures can be defined at three different levels: expansion of the intertidal area through dike removal and levelling of raised grounds outside the dikes, enhancement of contacts between the river and its catchment through ecological adjustments of dike structures and restoration of the contact between waters on both sides of the dike, structural and functional restoration of inland territories. Application of these measures can yield a more complete ecological infrastructure, improved migration possibilities for biota, more diverse biotic communities, limited input of energy and matter towards the river, enhanced self-cleaning capacity and retention, reduced contribution towards the eutrofication of the North Sea, increased volume and a slow down of the sedimentation rate. According to the adhered vision for the development of the estuary, the proposed restoration measures can be applied in different combinations to enhance its ecological rehabilitation/restoration. In this report three different scenario's are traced out, each of which set a different direction for the area' s development. The first two scenario's aim at a coherent development direction for the estuary, each from a different viewpoint with respect to hydrodynamics, one of the most important guiding determinants for the estuarine gradiënts along which different biotic communities follow upon each other in space and time. The scenario 'Space for the Estuary' leaves as much space as possible for the estuary to allow the tidal wave to die out laterally and longitudinally. Processes, which are steered by tidal dynamics gradually, merge into processes, which are directed by hydrodynamics of precipitation, ground and surface water. In those areas where estuarine expansion is not possible or advisable the attention is rather focused on inland nature development. The full realisation of this scenario would result in a total of 3.544 ha tidal wetland, 846 ha controlled inundation area with a reduced tidal regime and 3.472 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.207 and maximum 3.907 ha. The scenario' Attention for the Alluvium' judges that the greater part of this alluvium in the Schelde River Basin did not come into existence under tidal influence but through seasonal floods from upstream areas. It rather aims at the restoration of the alluvial plains as nutrient-rich wetlands in a continuous transition towards higher and more dry, nutrient -poor soils. Still, in this scenario attention is also paid to estuarine rehabilitation outside the dikes. The full realisation of this scenario would result in a total of 1.663 ha tidal wetland, 774 ha controlled inundation area with a reduced tidal regime and 5.324 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.787 and maximum 5.687 ha.The scenario ‘Funtional and Structural Basic Quality' does not really adhere to a preconceived vision for the development of the estuary. The proposed measures mainly contain the ecological adaptations to the Sigmaprojects as they were suggested in the environmental impact assessment (AMIS-45) and the ecological restoration of all raised fields and dumping grounds outside the dikes. Implementation of this scenario would suffice for the ecological rehabilitation of the estuary, it would at the very most maintain a structural and functional basic quality with the completion of all sigmaprojects, the deepening of the Westerschelde and the structural adaptations of the Zeeschelde to contemporary navigation demands. The full realisation would result in a total of 1.878 ha tidal wetland, 496 ha controlled inundation area with a reduced tidal regime and 1.075 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.508 and maximum 1.694 ha. Not any of these scenario's constitutes a ready-made blueprint for a ‘restored’ Zeeschelde. They picture a few possible designs for the estuary which, by means of models, can be tested for their probable impacts on the ecosystem, its functioning and potential opportunities for ecotopes, species and biotic communities. As such they should be considered as a contribution towards the social discussion that, hopefully, will yield an integrated ecosystem vision in the end.

Based on data, collected in 1980–1990, the intertidal benthic macrofauna of the Schelde and Ems estuaries was compared. The spatial occurrence of the benthic macrofauna along the salinity gradient, including the freshwater tidal area was emphasized. Both estuaries appeared to have a very similar species composition, especially at genus level. The higher number of species observed in the Schelde estuary was probably due to a greater habitat diversity. In both estuaries species diversity decreased with distance upstream. The total density did not vary along the estuarine gradient, whereas biomass is highest in the polyhaline zone.In both estuaries distinct intertidal benthic communities were observed along the salinity gradient: a marine community in the polyhaline zone, a brackish community in the mesohaline zone, and a third community in the oligohaline and freshwater tidal zones of the estuary. These three communities were very similar between both estuaries. Their main characteristics were discussed together with the occurrence and distribution of the dominant species.For the Schelde estuary and to a lesser extent also for the Ems estuary, there was evidence that anthropogenic stress had a negative effect on the intertidal macrobenthic communities of the oligohaline/freshwater tidal zone. Only Oligochaeta were dominating, whereas the very euryhaline and/or true limnetic species were missing. In the mesohaline zone, the Schelde estuary was dominated by large numbers of short-living, opportunistic species, whereas in the Ems estuary relatively more stable macrobenthic communities were observed. A comparison with some other European estuaries showed in general similar trends as those observed for the Schelde and Ems estuaries.

The intertidal benthic macrofauna of the Schelde estuary (The Netherlands and Belgium) was sampled in late autumn of 1990 at 50 stations along the whole salinity gradient (between Vlissingen and Dendermonde), including the freshwater tidal part. All stations were situated in sheltered areas with a relatively muddy sediment. Species richness, diversity and total biomass of the benthic macrofauna decreased along the salinity gradient from Vlissingen to Dendermonde, while total density showed no clear trend. Especially the oligoha- line and freshwater tidal part of the Schelde estuary was characterized by a very impoverished benthic community, composed only of Oligochaeta. No other species (freshwater, marine or brackish) was observed in this part of the estuary. The marine part had a more diverse macrozoobenthos structure than that of the brackish part. Species found only in the marine zone are Cerastoderma edule, Tharyx marioni, Eteone longa, Nephtys hombergii and Capitella capitata. In the brackish part of the estuary, Corophium volutator was a typical, dominant species. However, a lot of the dominant species were common in both the marine and brackish part of the Schelde estuary (e.g. Heteromastus filiformis, Pygospio elegans, Nereis diversicolor, Macoma balthica). The observed radient in species composition and dominance is compared with some other European estuaries. The marine and brackish part of the Schelde estuary is quite similar to other european estuaries. The freshwater tidal part, however, was more impoverished.

The Wester- and Oosterschelde are the only 2 remaining estuaries in the Delta area of SW Netherlands. In the Westerschelde the normal estuarine gradient from a brackish to a marine tidal system is found, whereas in the Oosterschelde major coastal engineering works have profoundly changed the character of the area. There is now no salinity gradient, and turbidity and pollution are very low. The intertidal macrofauna of both estuaries was studied intensively in 1987 and found to be similar in species composition. Multivariate statistical analyses (TWINSPAN and DECORANA) were used to determine similarities between stations and almost no overlap occurred between stations from the Ooster- and Westerschelde. The data indicate that between the Wester- and the Oosterschelde, a clear gradient from a filter-feeder to a deposit-feeder dominated community was found.