IN SITU MEASUREMENTS OF METAZOOPLANKTON PREDATION ON PROTOZOOPLANKTON: USE OF FLUORESCENTLY LABELLED PROTISTS.

The importance of trophic relationships between metazooplankton and protozoans in rivers are not yet well known, but as some authors have shown during the last decade, the predation exerted by metazooplankton on these organisms might be of importance in the transfer of energy from the pico- and nanoplankton to the metazooplankton. Fluorescent carboxylate microspheres of 0.5 µm in diameter were used to label natural protozooplankton. Different combinations of labelling times and concentrations of microspheres were tested to achieve an optimal labelling of the protists (i.e. a maximal proportion of individuals labelled with an identical number of microspheres); the most appropriate labelling conditions were with short incubations (10 minutes) and relatively low microsphere concentrations (1.105 FMP/ml). Live labelled protozooplankton was used to determine ingestion rates of the natural metazooplankton of the river Meuse (Belgium). It could be showed for all abundant species (i.e. the rotifers Brachionus calyciflorus, Brachionus angularis and Keratella cochlearis) that the predation exerted on protozooplankton was high (from 34 to 182 protists.rotifer-1.h-1) in in situ conditions