[en] A method was developed to allow direct measurements of predation exerted by metazooplankton on ciliates. The method relied on the use of ciliates labelled with fluorescent microparticles (FMP). Optimal labelling conditions were determined with ciliates from cultures (Tetrahymena pyriformis) and with natural ciliate assemblages sampled in a river. Labelled T. pyriformis were used as tracer food to determine gut passage time (GPT) and ingestion rates of the rotifer Brachionus calyciflorus in the laboratory. Predation of metazooplankton from the lowland river Meuse (Belgium) was determined by labelling natural assemblages of ciliates and using them as tracer food for metazooplankters sampled in the river. Optimal labels of ciliates, i.e. sharp distribution of FMP in cells, were obtained with short incubations (10 min) and low FMP concentrations (1 x 10(5) mL(-1)). GPT varied between 30 and 45 min for B. calyciflorus and from 25 up to >35 min for rotifers from the river. The ingestion rate of B. calyciflorus fed with T. pyriformis was 3.3 +/- 0.6 ciliate rot(-1) h(-1), i.e. 1.4 +/- 0.3 ngC rot(-1) h(-1). Metazooplankton species for which the ingestion of ciliates could be measured were the rotifers Keratella cochlearis, Euchlanis dilatata and Synchaeta spp. Ingestion rates measured ranged from 0.4 to 12.5 ngC rot(-1) h(-1). The method proposed proved to be useful in estimating the predation of microplankton on ciliates in semi- in situ conditions; in further developments, labelled natural assemblages of ciliates could be used for in situ incubations with the Haney chamber.
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