Trophic links in the lowland river Meuse (Belgium): importance of the predation exerted by rotifers on bacterio- and protozooplankton.

Trophic interactions within the plankton of the lowland river Meuse (Belgium) were measured in Spring and Summer (2001). Consumption of bacteria by protozoa was measured by monitoring the radioactivity disappearance of 3H-thymidine labelled bacteria. The proportion of protozoan bacterivory attributable to flagellates and ciliates was determined with fluorescently labelled bacteria (FLB). Metazooplankton bacterivory was assessed with 0.5 µm fluorescent microparticles (FMPs) and predation of metazooplankton on ciliates was measured by using natural ciliate assemblages labelled with FMPs as tracer food. Grazing of metazooplankton on flagellates was determined through in situ incubations with manipulated metazooplankton densities. flagellates were the main bacterial consumers their grazing being 91  10 % of total bacterial grazing. Metazooplankton, essentially rotifers, grazing on bacteria was negligible compared to grazing by protozoa and to bacterial production. Protozoa grazing balanced bacterial production in early samplings but was lower in July. Predation of rotifers on heterotrophic flagellates (HF) was generally low (1.765 3.432 mg C m-3 d-1 ), the higher contribution of HF in the diet of rotifers being observed when Keratella cochlearis was the dominant metazooplankter. Predation of rotifers on ciliates was low in the first samples (0.557  0.004 mg C m-3 d-1) in contrast to measurements performed in July (8.722  1.363 mg C m-3d-1). The proportion of protozoa in the diet of rotifers was generally low compared to phytoplankton (< 30% of total carbon ingestion) except when phytoplankton biomass decreased below the incipient limiting level (ILL) of the main metazooplantonic species. In such conditions, protozoa (mainly ciliates) constituted ca. 50% of total rotifer food. These results give evidence that microbial organisms play an important role within the plankton of the river Meuse, ciliates providing an alternative food for metazooplankton when phytoplankton becomes scarce.