THE DIRECT AND INDIRECT IMPACT OF DAPHNIA AND CYCLOPS ON A FRESH-WATER MICROBIAL FOOD-WEB

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