GRAZING BY ROTIFERS AND CRUSTACEAN ZOOPLANKTON ON NANOPLANKTONIC PROTISTS

Predation on nanoflagellates by metazoan zooplankton was investigated using a radioactively labeled flagellate, Poterioochromonas malhamensi s, as a tracer cell in laboratory incubations of freshly collected pla nkton assemblages. Experiments conducted in the fall, winter and sprin g indicated that rotifers dominated the grazing on nanoflagellates by metazoans in the winter (68%) and spring (92%). Rotifer grazing was no t determined in the autumn. It is likely that the greater impact of ro tifer grazing in the spring was due to the occurrence of abundant fila mentous cyanobacteria and gelatinous colonial phytoplankton which sele ctively depressed feeding rates of crustaceans compared to rotifers. C rustacean predation on nanoflagellates was highest in the autumn when cladocerans (primarily Daphnia spp.) were abundant. Predation by metaz oan zooplankton in this lake appeared capable of removing the total st anding stock of heterotrophic and phototrophic nanoplankton in < 1 d. Impacts of ciliated protozoa on nanoplankton, calculated from abundanc es and literature feeding rates, ranged from approximately one-third t o four times that of metazoan predation depending on season and method of calculation. The relative importance of the different groups of pr edators appears to vary seasonally which is expected to alter the tran sfer of energy, carbon and nutrients from bacteria to higher trophic l evels.