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 po pulations which provide a large food source for bacterivorous and algivorou s protists. In 1996, the protistan community in the freshwater tidal reache s of the Schelde estuary was monitored to evaluate whether these high food levels are reflected in a similarly high heterotrophic protistan biomass. P rotistan distribution patterns were compared to those of metazoan zooplankt on to evaluate the possible role of top-down regulation of protists by meta zoans. Apart from the algivorous sarcodine Asterocaelum, which reached high densities in summer, heterotrophic protistan biomass was dominated by cili ates and, second in importance, heterotrophic nanoflagellates (HNAN). HNAN abundance was low (annual average 2490 cells ml(-)1) and did not display la rge seasonal variation. It is hypothesised that HNAN were top-down controll ed by oligotrich ciliates throughout the year and by rotifers in summer. Ci liate 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 prob ably regulated `internally' by carnivorous ciliates (haptorids and Suctoria ). Our observations suggest that, in this type of productive ecosystems, th e microbial food web is mainly top-down controlled rather than regulated by food availability.