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Abstract

Experiments were conducted in a mesotrophic North German lake to examine the influence of metazoan zooplankton on the microbial food web. The presence and absence of Daphnia and Cyclops were manipulated in two cross-classified in situ experiments conducted in May and June 1994, during and after the clear-water phase. Ciliates had high population growth rates in the absence of predation during the clear-water phase, but had much lower growth rates 1 month later. Cyclops had strong predation effects on both Daphnia and ciliates. During the clear-water phase, manipulating metazooplankton abundance resulted in shifting the algal grazer community to either primarily metazoans or ciliates, but did not alter the final chlorophyll a concentration. After the clear-water phase, Cyclops had a negative effect on picoautotroph abundance, possibly due to reduced nutrient recycling resulting from the suppression of ciliates by Cyclops. Daphnia had both direct predation and indirect enhancement effects on bacteria, dependent on Daphnia biomass. These experiments indicate that while multiple strong links exist between the classic and microbial food webs, the net effect is not necessarily a clear trophic cascade from metazoan zooplankton to bacteria.