Micky Tackx

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 ratio’s 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.

The predation impact of juvenile herring Clupea harengus and sprat Sprattus sprattus on estuarine zooplankton

The consumption of estuarine copepods by juvenile herring and sprat during estuarine residency was estimated using fish biomass data and daily rations calculated from two models of feeding in fish: a bioenergetic model and a gastric evacuation model. The bioenergetic model predicted daily rations that were, on average, three times higher than those estimated by a model based on field records of stomach contents. The biomass of herring and sprat in the estuary was negatively correlated with the daily ration suggesting that the clupeid fish populations were resource-limited. Copepod production decreased towards the winter and peaked in spring and summer. The relative importance of predation changed seasonally in function of the migration pattern of herring and sprat. In the spring and the summer, in situ production of␣copepod biomass was higher than the in situ consumption by fish. During the fall and the winter, consumption exceeded production. This suggests that top–down control exerted by marine pelagic fish may be an important force structuring estuarine copepod populations.

Dynamics and trophic roles of heterotrophic protists in the plankton of a freshwater tidal estuary

Freshwater tidal estuaries comprise the most upstream reaches of estuaries and are often characterised by the presence of dense bacterial and algal populations which provide a large food source for bacterivorous and algivorous protists. In 1996, the protistan community in the freshwater tidal reaches of the Schelde estuary was monitored to evaluate whether these high food levels are reflected in a similarly high heterotrophic protistan biomass. Protistan distribution patterns were compared to those of metazoan zooplankton to evaluate the possible role of top-down regulation of protists by metazoans. Apart from the algivorous sarcodine Asterocaelum, which reached high densities in summer, heterotrophic protistan biomass was dominated by ciliates and, second in importance, heterotrophic nanoflagellates (HNAN). HNAN abundance was low (annual average 2490 cells ml−1) and did not display large seasonal variation. It is hypothesised that HNAN were top-down controlled by oligotrich ciliates throughout the year and by rotifers in summer. Ciliate abundance was generally relatively high (annual average 65 cells ml−1) and peaked in winter (maximum 450 cells ml−1). The decline of ciliate populations in summer was ascribed to grazing by rotifers, which developed dense populations in that season. In winter, ciliate populations were probably regulated `internally by carnivorous ciliates (haptorids and Suctoria). Our observations suggest that, in this type of productive ecosystems, the microbial food web is mainly top-down controlled rather than regulated by food availability.