Top-down effects of crustacean zooplankton on pelagic microorganisms in a mesotrophic lake

A series of single-factor in situ experiments was conducted in a mesotrophi c lake in Brandenburg, North Germany, to study the predatory impact of Eudi aptomus graciloides (adults, copepodites, nauplii), cyclopoid copepods (adu lt Diacyclops bicuspidatus, Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina, Daphnia cucullata) on the microbial community (bacteria, a utotrophic picoplankton, flagellates, ciliates). All zooplankton species te sted reduced the ciliate community significantly and ingestion rates were a lways higher for ciliates in the 20-55 mu m size category as compared to sm aller ciliates (10-20 mu m). Adult E. graciloides, which exhibited the high est predatory impact on ciliates, differed from cyclopoids and daphnids by their ability to decimate ciliates to very low abundances. Ingestion rates of ciliates by the crustacean zooplankton followed the sequence E. graciloi des > daphnids = cyclopoids = copepodites. While top-down control was evide nt for ciliates, top-down effects down to the autotrophic picoplankton and flagellates were mostly restricted to Daphnia-dominated treatments. Top-dow n effects were never strong enough to produce negative bacterial growth rat es. For all zooplankton tested, clearance rates for ciliates exceeded those for phytoplankton. Besides the potential of the crustacean zooplankton to influence the structure of ciliate communities, ciliates may contribute to the energy demands of copepods and daphnids, especially when phytoplankton resources are limited.