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 mesotrophic lake in Brandenburg, North Germany, to study the predatory impact of Eudiaptomus graciloides (adults, copepodites, nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus, Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina, Daphnia cucullata) on the microbial community (bacteria, autotrophic picoplankton, flagellates, ciliates). All zooplankton species tested reduced the ciliate community significantly and ingestion rates were always higher for ciliates in the 20-55 mu m size category as compared to smaller ciliates (10-20 mu m). Adult E. graciloides, which exhibited the highest 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. graciloides > daphnids = cyclopoids = copepodites. While top-down control was evident for ciliates, top-down effects down to the autotrophic picoplankton and flagellates were mostly restricted to Daphnia-dominated treatments. Top-down effects were never strong enough to produce negative bacterial growth rates. 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.