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.     

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). Handling Editor: K. Martens     

Benthic variability in intertidal soft-sediments in the mesohaline part of the Schelde estuary

The benthic (zoobenthos and microphytobenthos) and physical characteristics of intertidal sediments were studied in April and September 1997 on 10 locations, differing in elevation and exposure to tidal currents, situated on three mudflats in the mesohaline part of the Schelde estuary. Sediment characteristics were spatially and temporally relatively homogeneous among the sampling locations, all characterized by a high proportion of mud. It emerged that the nature of the sediment (in terms of granulometric characterization, organic matter) did not fully explain the substantial spatial and temporal (seasonal) variability in the benthos (pigment contents and benthic fauna), but that other environmental conditions explained the observed variability. Chlorophyll a and fucoxanthin contents in the top layer (0–0.5 cm) of the sediment were about 7 times higher in April than in September, but a large spatial variation was observed, especially in April, which was mainly attributed to a difference in elevation of the mudflats. The high pigment contents in April were accompanied by a relative low abundance of the benthic fauna. In April sub-surface deposit feeders, mainly Oligochaeta, were numerically most important in the 1000 and 500 m fraction of the zoobenthos, and Nematodes in the 250 m fraction. The surface deposit feeders Manayunkia aestuarina and Copepoda were locally very abundant, showing a strong positive correlation with elevation and pigment contents. The critical shear stress for erosion, measured in situ with the SedErode device, varied between 0.26 and 0.43 N m^–2 in April. The critical shear stress for erosion was lower in September when pigment contents were significantly lower and benthic fauna abundance was higher. In September Corophium volutator, Heteromastus filiformis, Nereis diversicolor and Macoma balthica dominated the 1000 m fraction, Oligochaeta and C. volutator the 500 m fraction, and Nematodes, M. aestuarina, Oligochaeta and Copepoda the 250 m fraction. It is argued that in estuarine systems with high seasonal variability in river flow rate and therefore salinity, the upper-middle estuarine fauna may switch each year between an oligohaline and a mesohaline fauna, which result in communities that seldom progress beyond early benthic-community succession. The majority of the benthic infauna was found in the upper 3 cm in both seasons, with surface deposit feeders (e.g. C. volutator, M. aestuarina) mainly occurring in the top 1 cm of the sediment, showing a strong correlation with both elevation and pigment contents and sub-surface deposit feeders (e.g. H. filiformis, Oligochaeta) occurring deeper in the sediment, showing a positive correlation with the mud content of the sediment.     

Waterbird communities along the estuarine salinity gradient of the Schelde estuary, NW-Europe

The zonation of non-breeding waterbirds along the Schelde estuary (The Netherlands–Belgium), one of the longest estuaries in NW-Europe with still a complete salinity gradient, including a large freshwater tidal area, was described. Numbers of birds were counted monthly over the period October 1991 to June 1997. Highest numbers of waterbirds were observed in late autumn and winter, with annual peak numbers ranging between 150,000 and 235,000 individuals for the whole estuary. Based on a multivariate analysis different waterbird communities were observed along the salinity gradient. The polyhaline areas of the estuary were numerically dominated by the waders Oystercatcher and Dunlin. Due to the presence of a large brackish marsh in the mesohaline zone, the waterbird community in this area was dominated by the herbivores Wigeon and Greylag Goose. In the oligohaline and freshwater tidal areas, the waterbird community was dominated by duck species, with Teal and Mallard being the most important. The international importance of the Schelde estuary for waterbirds was evidenced by the fact that for 21 waterbird species the 1% level criterion, according to the Ramsar convention, was exceeded. The relation of the observed diversity and community patterns with the functional and habitat diversity of the Schelde estuary as well as the effect of recent conservation measures to preserve this habitat were discussed.     

Influence of changes in salinity and light intensity on growth of phytoplankton communities from the Schelde river and estuary (Belgium/The Netherlands)

In the Schelde continuum, a succession in the phytoplankton community is observed along the transition from the river to the freshwater tidal reaches of the estuary and from the freshwater to brackish reaches of the estuary. The goal of this study was to experimentally evaluate the contribution of changes in salinity and light climate to this succession. In summer 2000 and in spring 2001, phytoplankton communities from the river, the freshwater tidal reaches and the brackish reaches of the estuary were incubated under high or low light intensities and exposed to a change in salinity. HPLC analysis was used to evaluate the response of different algal groups to changes in light intensity and salinity. When incubated at a light intensity corresponding to the mean underwater light intensity of the freshwater tidal reaches, growth of phytoplankton from the river as well as from freshwater tidal reaches was significantly lower than when incubated at a light intensity corresponding to the mean underwater light intensity of the river. The phytoplankton community from the freshwater tidal reaches did not appear to be better adapted to low light intensities than the phytoplankton community from the river. Although diatoms were expected to be less sensitive to a reduction in light intensity than green algae, the opposite response was observed. Freshwater and brackish water phytoplankton were negatively affected by respectively an increase or decrease in salinity. However, the effect of salinity was not strong enough to explain the disappearance of freshwater and brackish water phytoplankton between a salinity of 0.5 and 10 psu, suggesting that other factors also play a role. In the freshwater phytoplankton communities from the river and the freshwater tidal reaches, green algae and diatoms responded in a similar way to an increase in salinity. In the brackish water phytoplankton community, fucoxanthin displayed a different response to salinity than lutein and chlorophyll a.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.     

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.     

Clay turbidity and the relative production of bacterioplankton and phytoplankton

A comparison of phytoplankton with bacterioplanktonproduction as each ismodified by high concentrations of suspended clays ispresented. High clayturbidity caused light-limition of water columnphytoplankton production.However, the clay combined with DOC to form aggregateswhich supportedbacterioplankton production. Consequently,bacterioplankton production wasrelatively high at 42% of total small particleproduction in this lake.Bacterioplankton abundance and biomass was stronglycorrelated withphytoplankton chlorophyll a for most of the lake. Because of the association ofbacterioplankton with the clay-organic aggregates, DOCwas not a good predictorof bacterioplankton abundance or production. POC(primarily OC associatedwith clay) was correlated with bacterioplanktonabundance over most of thelake. Bacteria production was substrate limited asshown by much greaterbiomass-specific production at smaller bacteriapopulation sizes. Multipleregression analysis showed that specificbacterioplankton production wasprimarily governed by POC and secondarily by rates ofphytoplanktonproduction. Thus clay, because of light limitedphytoplankton production,negatively impacts bacterioplankton production yet atthe same time facilitatiesbacterial production by concentrating OC with theformation of the clay-organicaggregate.     

Spatial and temporal variation in community composition and photosynthetic characteristics of phytoplankton in the upper Westerschelde estuary (Belgium,SW Netherlands)

Community composition, biomass and primary production of phytoplankton were studied in the east- ernmost section of the Westerschelde estuary in 1984. Photosynthetic characteristics were compared with distribution of some dominant phytoplankton species along a salinity gradient from 5 to 22 Spring phytoplankton, with Cyclotella meneghiniana (freshwater) and Skeletonema costatum (marine) as the dominant species grew faster than summer phytoplankton. In summer, biomass achieved its maximum, due to the riverine Scenedesmus species and the marine diatoms Thalassiosira levanderi and Ditylum brightwellii, as dominants. Growth conditions were more favourable to phytoplankton communities above 15%, than below this salinity. The data were compared with previous studies (1972) of species composition in the area.     

Zonation of intertidal macrobenthos in the estuaries of Schelde and Ems

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.     

Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north-western Ontario, Canada.
2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet-A (UVA) + ultraviolet-B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.
3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the 'base phytoplankton' (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near-surface phytoplankton. On the other hand, production of near-surface bacterioplankton was reduced following a 4-h UVR exposure but had increased after a 48-h exposure to both UVA and UVB compared with the PAR only treatment.
4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.     

Spatial and temporal variation of physico-chemical conditions and phytoplankton during a dry year in the Tagus estuary (Portugal)

During monthly synoptic surveys in a dry year, physicochemical and biological data were collected at 15 stations throughout the Tagus estuary. A Principal Component Analysis (PCA) was used to identify the major sources of variation in the data. The most important variations in the Tagus estuary were the spatial variations, mainly the longitudinal ones showing clearly the riverine and the sea water influenced zones of the estuary. The results also showed that a lateral variation was due to the local pollution as indicated by intense organic matter degradation and high rates of oxygen consumption. The seasonal variation related to temperature and the variation associated with the semidiurnal tidal cycle were also apparent. These, however, contributed less than the spatial variation to the total variability in the measured parameters of the Tagus estuary.     

Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolved organic carbon (DOC) between 15 and 20 mg L^–1 were fertilized with inorganic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake was unfertilized and served as a control. In addition to this lake fertilization experiment, data from different regional surveys were used to assess the role of different limiting factors.
2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inorganic nutrient limitation of bacterioplankton was a function of DOC concentration in water of the investigated region and nutrient-limited bacteria were found only in lakes with DOC concentrations less than around 15 mg L^–1
3. The fertilization experiments demonstrated that the DOC-rich experimental lakes contained a bioavailable pool of P that was not utilized to its full potential under natural conditions. The overall mobilization of energy (bacterioplankton plus phytoplankton) in the experimental lakes was restricted by lack of inorganic N.     

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.     

Integrative ecosystem analyses of phytoplankton dynamics in the York River estuary (USA)

The relative importance of biotic (top-down) vs. abiotic (bottom-up) controls on phytoplankton dynamics was investigated in the York River estuary, Virginia (USA) by a combination of extensive analyses of long-term data sets collected by the U.S. Environmental Protection Agency (EPA) monitoring program over 17 years (1984–2001), field studies (1996–1997) and ecosystem modeling analyses. Results from the analysis of long-term data records collected at three stations along the salinity gradient suggested that phytoplankton are more likely controlled by abiotic mechanisms such as resource limitation than biotic mechanisms such as grazing since annual cycles of primary production and phytoplankton biomass were similar and no grazing effects were evident based on the observed relationship between phytoplankton and zooplankton biomass (R ² < 0.2, p > 0.1). This scenario was supported by short-term field observations made over an annual cycle at three stations in the mid-channel of the estuary where both chlorophyll a and primary production demonstrated similar patterns of seasonal variation. Ratios of fluorescence before and after acidification at all study sites were relatively high suggesting low grazing pressure in the estuary. A tidally-averaged, size-structured plankton ecosystem model was previously developed and verified for the lower York River estuary. The validated ecosystem model was also used to examine this issue and simulation results supported the importance of bottom-up control in the York River estuary.     

Spatial variability in the abundance and composition of the free-living bacterioplankton community in the pelagic zone of Lake Bourget (France)

Spatial variations in the abundance and diversity of the free-living bacterioplankton community of a large Alpine lake, Lake Bourget (France), were investigated in the pelagic zone by means of two two-dimensional samplings taken in 2003. Lake-water samples were collected in winter during water mixing, and in early summer during stratification. The population abundance in each sample was determined by flow cytometry. Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments from organisms measuring less than 2 mum was used to assess eubacterioplankton community composition. In winter, no obvious differences were observed in either the abundance or the diversity of the bacterial community, on either the horizontal or the vertical scales. The only influence detected was that of river water input, but this was at a very minor scale relative to the surface area of the lake. In early summer, changes were found in the community composition on the vertical scale related to the thermal stratification of the water column. There were also marked differences on the horizontal scale at 15 m depth due to internal waves. The implications of these findings for sampling strategies are very important from the perspective of comparative studies of free-living bacterial community diversity and functioning in large and deep lakes.     

Primary production and phytoplankton composition in relation to DOC input and bacterioplankton production in humic Lake Örträsket

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers.
2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket.
3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.     

Nutrients,light and primary production by phytoplankton and microphytobenthos in the eutrophic,turbid Westerschelde estuary (The Netherlands)

Abiotic factors and primary production by phytoplankton and microphytobenthos was studied in the turbid Westeschelde estuary. Because of the high turbidity and high nutrient concentrations primary production by phytoplankton is light-limited. In the inner and central parts of the estuary maximum rates of primary production were therefore measured during the summer, whereas in the more marine part spring and autumn bloom were observed. Organic loading is high, causing near anaerobic conditions upstream in the river Schelde. Because of this there were no important phytoplankton grazers in this part of the estuary and hence the grazing pressure on phytoplankton was minimal. As this reduced losses, biomass is maximal in the river Schelde, despite the very low growth rates.On a number of occasions, primary production by benthic micro-algae on intertidal flats was studied. Comparison of their rates of primary production to phytoplankton production in the same period led to the conclusion that the contribution to total primary production by benthic algae was small. The main reason for this is that the photosynthetic activity declines rapidly after the flats emerged from the water. It is argued that CO₂-limitation could only be partially responsible for the noticed decrease in activity.     

Changes in phytoplankton biomass and primary production between 1991 and 2001 in the Westerschelde estuary (Belgium/The Netherlands)

The Westerschelde estuary is a very polluted and turbid estuary, but the last decade the waterquality improved. Dredging activity also increased in 1997 to allow bigger ships to enter the port of Antwerpen. This could potentially decrease the light conditions for the phytoplankton. Because of all these recent changes in the estuary we studied primary productivity in 2001 and compared it to values in 1991. The results show that due to a decrease in discharge in particulate and dissolved organic carbon the oxygen concentrations in general have increased in the upstream region, although in spring and summer low oxygen concentrations (10–30% saturation) can still be found. Phosphate and ammonia concentrations have decreased and the zone of nitrification which was very large in 1991 has become very small and is now located in the uppermost upstream region of the estuary. Si-concentrations have remained the same. All nutrient concentrations are still high enough not to limit phytoplankton growth. Turbidity remained unaltered as a result of the dredging works, and as a result phytoplankton biomass in most of the estuary did not show a decrease, although there were signs that in the upstream region phytoplankton biomass decreased, possible caused by increased grazing pressure. The relationship between phytoplankton biomass and primary productivity did not change, and from the data it can be concluded that the dredging activity will not influence the gross and net primary productivity of the phytoplankton.     

Modelling Pb and Cd dynamics in the Seine estuary

The Seine estuary (France) is currently one of the world’s most contaminated estuaries, due in particular to its high cadmium and lead content. Proper understanding of contaminant transfer, transformation and retention mechanisms throughout the estuary and up to the adjacent marine zone require a range of studies involving data collections, experiments, and modeling tool. A multivariable transport model (SiAM-3D) was used to simulate dissolved and particulate transport and it is applied to several calculation grids; a speciation model (MOCO) was used to select key species and obtain a schematic system representation. The coupled model for Cd and Pb was compared with field measurements. The complementary character of various tools (model applied to annual time scale, in situ measurements and experiments) allowed to explore and quantify various hypotheses on the high dissolved cadmium concentrations observed during low river flow. The target is to achieve a compromise between acceptable computing times and adequate result accuracy. Although particle and contaminant behaviour is globally well-reproduced by the coarse grid, calculation errors relating to bayward fluxes and stocks deposited inside the estuary were highlighted after comparison with the fine grid.