Schelde Estuary Research Articles

Macrofaunal communities in the habitats of intertidal marshes along the salinity gradient of the Schelde estuary

The macrobenthos is important in benthic remineralization processes; it represents a trophic link and is also often used as a bio-indicator in monitoring programs. Variations of the environmental parameters strongly influence the structure of the macrobenthic communities in the marshes and since macrobenthos is the most important food item for marsh-visiting fish species in the Schelde, the variation in food resources can have a strong effect on the higher trophic level. The present study deals with the variation in macrobenthic communities in different habitats of intertidal marshes along the salinity gradient and the differences between the marsh creeks and the intertidal part of the estuary. The study measured density and species richness together with the biomass, and sampled a large intertidal channel and a smaller creek within five marshes along the salinity gradient of the Schelde estuary every six weeks between May and October in 2000. The small creeks had a smaller grain size and higher organic matter content than those in the large channel although the differences in the environmental parameters did not explain the different communities in the two habitats. Marshes had distinct macrobenthic communities but the abundance of macrofauna fluctuated along the estuary without an identifiable spatial trend. In contrast, the total biomass increased towards the euhaline area due to the domination of Nereis diversicolor. Diversity showed a significant positive correlation with the salinity. Comparison of the macrofaunal communities in the marsh with those on the intertidal flats of the estuary indicated similar trends in density, biomass and diversity along the salinity gradient. The density was similar in both habitats whereas biomass was much higher in the intertidal habitats of the estuary, partly due to the higher biomass of molluscs and annelids. Diversity indices were higher in the marsh, and the freshwater area had more species than in the estuary.

Changes in phytoplankton diversity and community composition along the salinity gradient of the Schelde estuary (Belgium/The Netherlands)

Phytoplankton diversity and community composition were studied along the Schelde river–estuary–coastal zone continuum during the summer of 2003. DCA analysis indicated a gradual compositional turnover of the phytoplankton community within the estuary. GAM modelling of species response curves along the estuarine gradient was used to identify taxa that had their population maximum in the river, the coastal zone or within the estuary. Taxa that had their population maximum within the estuary did not form a homogenous community but comprised species with different salinity optima and rather restricted salinity tolerances. The observed changes in community composition along the estuarine transition zone correspond more closely to an ecocline than to an ecotone model. Despite the fact that few taxa had their population maximum at or near the salinity gradient, alpha diversity did not display a minimum around the salinity gradient. This lack of a diversity minimum within the estuary was ascribed to an important contribution of taxa of riverine or coastal origin to alpha diversity within the estuary contributed. On average 55% of the alpha diversity in the estuarine samples was due to riverine or coastal taxa. Beta diversity displayed a clear maximum around the salinity gradient. For planktonic organisms which are subject to mass effects, beta diversity is probably a better indicator for the impact of the salinity gradient on diversity in estuaries than alpha diversity.

Spatiotemporal aspects of silica buffering in restored tidal marshes

Losses of pelagic diatom production resulting from silica limitation have not only been blamed for toxic algal blooms, but for the reduction in ability of coastal food webs to support higher trophic levels. Recent research has shown the importance of advective seepage water fluxes of dissolved silica (DSi) from freshwater marshes to pelagic waters during moments of riverine Si-limitation. In this study, we investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located in former polder areas and characterized by so-called controlled reduced tidal regimes (CRT). Nine mass-balance studies were conducted in a newly constructed CRT in the freshwater Schelde estuary. During complete tidal cycles both DSi and amorphous silica (ASi) concentrations were monitored at the entrance culverts and in different habitats in the marsh. A swift DSi-delivery capacity was observed despite the shifted spatiotemporal frame of exchange processes compared to reference marshes. As silica-accumulating vegetation is not yet present, and difference with reference marshes’ deliveries is surprisingly small, we indicate diatomaceous debris and phytoliths to be the main silica source. Although further research is necessary on the driving forces of the different processes involved, restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the estuary.

Inter-annual variability in phytoplankton summer blooms in the freshwater tidal reaches of the Schelde estuary (Belgium)

The inter-annual variability in phytoplankton summer blooms in the upper reaches of the Schelde estuary was investigated between 1996 and 2005 by monthly sampling at 10 stations. The large inter-annual variations of the chlorophyll a concentration in the freshwater tidal reaches were independent from variations in chlorophyll a in the tributary river Schelde. Summer mean chlorophyll a concentrations were significantly negatively correlated with flushing rate (Spearman correlation: r = −0.67, p = 0.05, n = 9) but not with temperature, irradiance and suspended particulate matter or dissolved silica (DSi) concentrations. During dry summers, low flushing rates permitted the development of dense phytoplankton populations in the upper part of the estuary, while during wet summers high flushing rates prevented the development of dense phytoplankton blooms. Flushing rate was also found to be important for the phytoplankton community composition. At low flushing rates, the community was dominated by diatoms that developed within the upper estuary. At high flushing rates, chlorophytes imported from the tributary river Schelde became more important in the phytoplankton community. The position of the chlorophyll a maximum shifted from the head of the estuary when flushing rates were low, to more downstream when flushing rates were high. Although DSi concentrations tended to be lower during years of high phytoplankton (mainly diatom) biomass, the relation with flushing rate was not significant.

Potential Implications of Sea-Level Rise for Belgium

The Belgian coastal plain and the Schelde estuary are threatened by sea-level rise. While of great economic importance with a threatened population of some 0.8 million (of a total population of 10 million), assessments of these risks are limited. This article describes the physical characteristics of the coast and undertakes a qualitative interpretation of its vulnerability. Low-lying polders are the most vulnerable to sea-level rise where a major problem is water drainage during rainy periods; their varying vulnerability to sea-level rise and increase in rain intensity is assessed, including the relationship between drainage levels and saltwater seepage. Freshwater lenses developed within the dunes are also vulnerable to sea-level rise, leading to threats to drinking water supplies from saltwater intrusion. Belgian coastal defence structures and their effectiveness are discussed. Historical sea-level rise during the past century, wave and wind data, and the evolution of erosion and accretion along the coast are interpreted. For Antwerpen, a harbour city on the river Schelde, the effects of sea-level rise are far from clear. Included here are historical data on changes in tidal amplitude during the 20th century. Future research needs should focus on the quantitative interpretation of data to understand the effect of sea-level rise on beach erosion, flood risk, and fresh and salt groundwater distribution. Furthermore, a thorough socio-economic study should be undertaken to assess the vulnerability of the Belgian coast and the Schelde estuary.

Decomposition of willow (Salix triandraL.) leaves in the Schelde estuary (Belgium)

(2008). Decomposition of willow (Salix triandra L.) leaves in the Schelde estuary (Belgium) SIL Proceedings, 1922-2010: Vol. 30, No. 4, pp. 603-606.

Tuning the tide: creating ecological conditions for tidal marsh development in a flood control area

The Schelde estuary, characterised as a turbid, polluted and eutrophic system, has nowadays reached a turning point in the restoration of its water quality. During the past century, human activities have reduced the intertidal areas, essential in the estuarine ecosystem for nutrient cycling and the self-cleaning capacity. Today, in combination with a master plan to protect the population from storm surges, an opportunity rises to restore areas with a tidal influence. One specific option of combining safety and ecology is the creation of flood control areas (FCA) under the influence of a controlled reduced tide (CRT). These specific areas will differ in many ways from fully tidal areas. However, these areas can fulfill important ecological functions with effects on aeration, nitrification, denitrification, sedimentation and primary production in the estuary. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT showed to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. Possible measures to design a CRT with a rich habitat variation are discussed.

Evaluation of the performance of HPLC–CHEMTAX analysis for determining phytoplankton biomass and composition in a turbid estuary (Schelde, Belgium)

In the upper Schelde estuary in 2002, phytoplankton biomass and community composition were studied using microscopic and pigment analyses. Chlorophyll a concentration was a good predictor of phytoplankton biomass estimated from cell counts and biovolume measurements. The phytoplankton carbon to chlorophyll a ratio, however, was often unrealistically low (<10). CHEMTAX was used to estimate the contribution of the major algal groups to total chlorophyll a. The dominant algal groups were diatoms and chlorophytes. While diatom equivalents in chlorophyll a predicted diatom biomass relatively well, chlorophyte equivalents in chlorophyll a were only weakly related to chlorophyte biomass. The pigment-based approach to study phytoplankton overestimated phytoplankton biomass in general and chlorophyte biomass in particular in late autumn and winter, when phytoplankton biomass was low. A possible explanation for this overestimation may be the presence of large amounts of vascular plant detritus in the upper Schelde estuary. Residual chlorophyll a, chlorophyll b and lutein in this detritus may result in an overestimation of total phytoplankton and chlorophyte biomass when the contribution of phytoplankton to total particulate organic matter is low.

Sediment characteristics and sedimentation-erosion processes on Ketenisse polder one year after levelling

The Ketenisse polder is a former intertidal brackish marsh of 30 ha situated between the fortress of Liefkenshoek and the Kallo sluice on the left bank of the Schelde estuary, Belgium. In the 19th century its central part was embanked as a polder. During the construction of a traffic tunnel in the late eighties, the excavated soil was dumped between a lower summer dike and the damming dike. The area was restored in 2002 as compensation for the construction of a container terminal on a mudflat in 1994-1995. The rubble of the summer dike and the dumped material were removed and the area was levelled with the intention of creating the optimal starting conditions for the development of new tidal mudflats and marshes. Developments concerning morphology, sediment characteristics and sedimentation-erosion processes are monitored. Six transects with sedimentation-erosion plots were placed perpendicular to the shoreline. This article discusses those monitoring results, mainly for the first year after levelling. Sedimentation as well as erosion between 0 and 30 cm was observed in the first year. Local changes in stream current patterns caused erosion on some parts of the former mudflats; sheltered depressions filled up relatively fast. Median grain size showed large variation, both spatial and temporal. Organic matter content of the sediment varied between 0.5 and 15% and was closely related to sediment median grain size.

Looking for general trends in trophic interactions among estuarine micro- and mesozooplankton

Trophic interactions among micro- and mesozooplankton were investigated in the Schelde estuary. Field populations of plankton were separated by selective filtrations (30 and 200 mm). Predation was measured by comparing ciliate and rotifer abundance in treatments with and without potential predators (cladocerans, cyclopids, the calanoid Eurytemora affinis and rotifers). To deal with variability in the data, a cross-calculation method using all replicates separately is proposed. In order to look for general trends in predation behaviour, the predation rates were ranked and analysed in relation to ranked characteristics of the potential prey: numerical abundance, growth rate, individual and population biomass. Cyclopids fed selectively on the ciliates having the highest population biomass and growth rates among the ciliate population. Cyclopids selected the least abundant rotifers. No trends were detected for cladocerans and E. affinis. As predators, rotifers selected the least abundant ciliates in spring. Although no general selectivity patterns for the predators studied can be drawn yet, the potential of the method employed is illustrated and discussed. Its main advantage is the possibility to extend the obtained data set with data from new experiments as well as with extant data on selectivity of the predators.

A fish-based assessment tool for the ecological quality of the brackish Schelde estuary in Flanders (Belgium)

This paper presents a new approach which was developed to find an optimal combination of candidate metrics for creating a fish-based estuarine biotic index (EBI) for defining the quality status of an estuarine area. The key idea was that a powerful index should minimise two prediction errors simultaneously: falsely declaring the status of a site as disturbed while it is not (Type I error) and the reverse, falsely declaring a disturbed site as undisturbed (Type II error). The balance between both errors is an inherent characteristic of an index and can be displayed as a curve. The area under this curve (AUC) is a measure of the misclassification rate (smaller = better). This criterion was the basis for a stepwise approach whereby in each step a metric resulting in the highest reduction of AUC was added. Five metrics were selected and the distribution of their average was the basis to derive the thresholds for the classes of the EBI. This paper presents the fish-based index (EBI) for the brackish Schelde estuary in Flanders (Belgium). The index was calibrated against fyke net data from five sites during the period from 1995 to 2004. The sites ranged in quality from moderately impacted to very disturbed, classes 3 to 5 respectively. Despite there being neither of the highest classes 1 (high) and 2 (good), the EBI presented can serve as an evaluation tool in the highly impacted situation in Flanders as it discriminates well between moderate and highly impacted sites. In addition, its definition complies with the biological status classes of the European Water Framework Directive (WFD).

Seasonal variation in dimethylsulfoniopropionate related to microphytobenthos composition in intertidal estuarine sediments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 320:55-63 (2006) - doi:10.3354/meps320055 Seasonal variation in dimethylsulfoniopropionate related to microphytobenthos composition in intertidal estuarine sediments S. A. van Bergeijk1,3, U. Wollenzien2, K. Schönefeldt1, L. J. Stal2,* 1Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands 2Department of Marine Microbiology, Netherlands Institute of Ecology - KNAW, PO Box 140, 4400 AC Yerseke, The Netherlands 3Present address: Departamento de Biología, Facultad de Ciencias del Mar, Universidad de Cádiz, Campus Rio San Pedro s/n, 11510 Puerto Real, Spain *Corresponding author. Email: l.stal@nioo.knaw.nl ABSTRACT: Dimethylsulfoniopropionate (DMSP) and algal pigments were measured monthly over 1 yr (May 1997 to June 1998) in intertidal sediments at 2 sampling sites along the Wester- and Oosterschelde (Netherlands). Temperature, salinity and dissolved inorganic nitrogen (DIN) were measured in the pore water and qualitative microscopic observations were made on the microphytobenthos present in the sediment. Both DMSP and pigment content correlated positively with temperature and salinity. However, the chlorophyll a- or fucoxanthin-specific DMSP contents were not affected by temperature, salinity or DIN. Simultaneous peaks of DMSP and chlorophyll a and fucoxanthin were observed in spring and in summer, and at 1 station also in winter. These peaks coincided with increases in epipelic diatoms. The DMSP content correlated with chlorophyll a and fucoxanthin and this relationship improved by excluding samples with a low ratio of fucoxanthin to chlorophyll a (chl a) indicative of a low amount of diatoms relative to other microalgae. The sediment molar ratio of DMSP:chl a was 9.2 ± 5.6 and 5.1 ± 3.6 for the Wester- and Oosterschelde, respectively, values similar to the cellular ratio in cultured diatoms. These results suggest that diatoms were the most important source of DMSP in the sediment. In summer, cyanobacteria and green algae were also present. While cyanobacteria did not contribute to the DMSP content, the data suggest that the amount of DMSP produced by other algae (probably Euglenophytes) produced similar to that produced by diatoms. Hence, microphytobenthos biomass and community composition were the main factors determining the DMSP content in the intertidal sediments of the Schelde estuary. KEY WORDS: Dimethylsulfoniopropionate · DMSP · Diatoms · Microphytobenthos · Intertidal sediments · Pigments Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 320. Online publication date: August 29, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Impact of a Flood Event on the Planktonic Food Web of the Schelde Estuary (Belgium) in Spring 1998

To evaluate the effect of short-term changes in discharge on plankton dynamics, a station in the upper reaches of the Schelde estuary was monitored during 1 month in spring 1998 with a sampling frequency of 1–2 days. During this monitoring period, a flood event occurred during which discharge increased about 5-fold. This flood event had a strong effect on the composition of the planktonic community. Rotifers were strongly negatively affected by the flood event while densities of heterotrophic nanoflagellates and scuticociliates increased. Chlorophyll a concentration and abundance of bacteria, oligotrich ciliates and crustacean zooplankton did not respond clearly to the flood event. Although the flood event lasted only a few days it took more than 2 weeks for the planktonic community to return to its original composition.

Migration dynamics of clupeoids in the Schelde estuary: A stable isotope approach

Large numbers of young of the year herring ( Clupea harengus L.) and sprat ( Sprattus sprattus (L.)) typically enter and remain within North Sea estuaries during the winter months. The main purpose of this study was to examine their migration dynamics between the North Sea and the Schelde estuary using C and N stable isotopes. Prior to this, stomach contents were used to verify the isotopic differences between the food sources at the sampling stations. From May 2000 to April 2001 fish were collected monthly in the upper and lower estuary. Muscle tissue and stomach contents were analyzed for δ 13C and δ 15N using an EA-IRMS. Based on the stomach contents, it was demonstrated that δ 15N could not be used as a tracer for fish migration because the longitudinal estuarine δ 15N gradient had reversed completely during autumn. The δ 13C gradient, however, was found to be reliable for studying fish movement in the Schelde estuary. Seasonal movements of clupeoids in the Schelde estuary were analyzed by separating the temporal abundance patterns into migration groups based on their muscle isotopic composition. Immigration and emigration seem to occur continuously throughout the year. Most exchange occurred in November. During winter, immigration remained high but gradually decreased. Although the herring and sprat abundance further declined in the estuary during February and March, large seaward emigration was not fully demonstrated. As temporal overlap between immigration and emigration is concluded the results support the hypothesis that migration to estuarine nurseries is individually based.

Biogenic silica in tidal freshwater marsh sediments and vegetation (Schelde estuary, Belgium)

To date, estuarine ecosystem research has mostly neglected silica cycling in freshwater intertidal marshes. However, tidal marshes can store large amounts of biogenic silica (BSi) in vegetation and sediment. BSi content of the typical freshwater marsh plants Phragmites australis, Impatiens glandulifera, Urtica dioica, Epilobium hirsutum and Salix sp. was analysed year-round. All herbaceous species accumulated silica in their tissue during their life cycle. Of the live plants, P. aus- tralis contained the most BSi (accumulating from 6 to 55 mg g -1 ). Dead shoots of P. australis had the highest BSi content (up to 72.2 mg g -1 ). U. dioica (<11.1 mg g -1 ), I. glandulifera (<1.1 mg g -1 ), E. hirsutum (<1.2 mg g -1 ) and Salix sp. (<1.9 mg g -1 ) had a much lower BSi content. Except for P. aus- tralis rhizomes (<15 mg g -1 ) underground biomass contained low amounts of BSi (<6 mg g -1 ). Sediment BSi content decreased from the surface (9 to 10 mg g -1 ) to deeper layers (5 to 7 mg g -1 ). There was seasonal variation in sediment BSi. Dissolved Si in porewater was highest in summer (ca. 600 µM) and lowest in winter (ca. 400 µM). P. australis vegetation (aboveground and roots) con- tained up to 126 g m -2 BSi, while the upper 30 cm of sediment accumulated up to 1500 g m -2 , making sediment the largest BSi reservoir in the marsh. We conclude that P. australis wetlands could be an essential, but unrecognised, sink for BSi in the biogeochemical cycling of Si.

Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary (Belgium/The Netherlands)

In contrast with the marine reaches of estuaries, few studies have dealt with zooplankton grazing on phytoplankton in the upper estuarine reaches, where freshwater zooplankton species tend to dominate the zooplankton community. In spring and early summer 2003, grazing by micro- and mesozooplankton on phytoplankton was investigated at three sites in the upper Schelde estuary. Grazing by mesozooplankton was evaluated by monitoring growth of phytoplankton in 200 μm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoïd copepod Eurytemora affinis, the cyclopoid copepods Acanthocyclops robustus and Cyclops vicinus and the cladocera Chydorus sphaericus, Moina affinis and Daphnia magna/ pulex. No significant grazing impact of mesozooplankton in any experiment was found despite the fact that mesozooplankton densities used in the experiments (20 or 40 ind. l −1) were higher than densities in the field (0.1–6.9 ind. l −1). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 μm filtered water. Microzooplankton in the 30–200 μm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. l −1. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41–1.83 day −1 and grazed up to 84% of initial phytoplankton standing stock per day. Rotifer clearance rates estimated from microzooplankton community grazing rates and rotifer abundances varied from 8.3 to 41.7 μl ind. −1 h −1. CHEMTAX analysis of accessory pigment data revealed a similar phytoplankton community composition after incubation with and without microzooplankton, indicating non-selective feeding by rotifers on phytoplankton.

Dissolved organic carbon in the freshwater tidal reaches of the Schelde estuary

To unravel the factors that regulate DOC dynamics in the freshwater tidal reaches of the Schelde estuary, DOC concentration and biodegradability were monitored in the upper Schelde estuary and its major tributaries. Although the Schelde estuary possesses a densely populated and industrialized catchment, our data suggest that the bulk of DOC in the freshwater tidal reaches is not derived from waste water. This was concluded from the low biodegradability of DOC (on average 9%), DOC concentrations that are close to the mean for European rivers (4.61 mg l −1) and the absence of an inverse relationship between DOC and discharge. Most DOC originating from waste water being discharged in tributaries of the estuary appears to be remineralised before these tributaries reach the main estuary. Although dense phytoplankton blooms were observed in the upper estuary during summer (up to 700 μg chl a l −1), these blooms did not appear to produce large quantities of DOC in the freshwater tidal reaches as DOC concentrations were low when phytoplankton biomass was high. The fact that DOC concentrations were high in winter and decreased in summer suggests a predominantly terrestrial source of DOC in the freshwater tidal reaches of the Schelde estuary.

Temperature and salinity effects on post-marsupial growth of Neomysis integer (Crustacea: Mysidacea)

There has been an increasing interest in using the brackish water mysid Neomysis integer as a toxicological test species for Western European estuarine systems. In this respect, more data on growth, moulting and development in this species is needed. The influence of prevailing environmental variables (e.g. temperature, salinity) and age on these processes as well as their optimal range have to be known in order to develop optimal laboratory cultures and to differentiate between chemically induced variability and natural variability in toxicity testing. Individual post-marsupial growth (size, intermoult period, growth factor) was studied from first day neonates until adulthood at eight environmentally relevant temperature–salinity conditions. Three salinities (5, 15 and 30 psu) were tested at 15 and 20 °C, and two more extreme temperatures (8 and 25 °C) were tested at a salinity of 5 psu. Survival and growth of N. integer were detected within the whole range tested, but sexual maturation was only possible in the narrower range of 15–25 °C and 5–15 psu. The size at maturity of N. integer increased with decreasing temperature and increasing salinity. Salinity seems to have a stronger effect than temperature on the duration of maturation. The sigmoid von Bertalanffy growth model was fitted to the individual and pooled data, except for the 8 °C experiment where growth was linear. Estimates from pooled data were comparable with individually based estimates, but generally underestimated the asymptotic length. Temperature was negatively correlated with the asymptotic length and positively correlated with the growth constant K. Higher temperatures caused smaller intermoult periods but had no effect on the growth increment, while salinity effects were less straightforward and dependent on the water temperature. A tool is provided to estimate the age, moult number, intermoult period, growth factor and growth rate from the body standard length of N. integer. Experimentally derived von Bertalanffy parameter estimates resulted in a higher growth performance index compared with field-based estimates for the Schelde estuary and Galgenweel populations of N. integer.

Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium)

Compared to knowledge about N and P processing in the aquatic continuum of lakes, wetlands and estuaries, knowledge concerning transport and cycling of Si is only fragmentary. Furthermore, Si research in estuaries has mainly been focused on subtidal benthic sediments and uptake and recycling by diatom communities. The biogeochemical cycling of Si in tidal wetlands, which can contain large amounts of Si, has thus far been neglected. We have conducted several whole ecosystem Si mass-balances on a freshwater marsh located in the Schelde estuary (6 tidal cycles, 2 with BSi included). Our measurements show that the freshwater marsh acts as an important source of dissolved Si to the main river (1–18% more export than import, on average 0.114 g m−2). This export is compensated by import of amorphous silica into the marsh (19–55% more import than export). The marsh was shown to act as silica recycler, resupplying biologically available dissolved Si to the estuarine ecosystem. Extrapolations show that during summer and spring months, when dissolved silica is depleted due to diatom growth, almost half of the total dissolved silica load in the main river channel could result from marsh recycling.

Zooplankton in the Schelde estuary, Belgium and the Netherlands: long-term trends in spring populations

A compilation of available data in between 1967 and 2002 on spring zooplankton abundance was made for the brackish and the freshwater zone of the Schelde estuary. The general picture is a significant increase of 1–2 orders of magnitude in abundance for Rotifera, Copepoda and Branchiopoda (mainly Cladocera) in the freshwater zone, while zooplankton abundance in the brackishwater zone remained more constant. Possible natural and management related causes for this increase in zooplankton abundance are briefly discussed.