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Abstract

To obtain information about the planktonic protozoans that form a link in the flux of matter from bacterio- to metazooplankton in the eutrophic Belauer See, one of the lakes in the Bornhoveder Seenkette of North Germany, abundance, biomass, production, and grazing loss rates of the planktonic bacteria, algae, ciliates, flagellates, rotifers, and crustaceans were estimated. Different trophic levels were isolated using the size fractionation technique, and each was exposed for 24 and 48 h to ambient conditions. Changes in their population densities were recorded. Toward the end of winter, herbivorous protozoan plankton developed together with small algae. Protozoans that feed on algae were predominant during our experiments in spring. They were preyed upon in turn by rotifers and crustaceans. The protozoan community structure was influenced by the presence of cyclopoid copepods, which were also the main predators on rotifers. Cylopoids were the main top-down controlling organisms in the pelagic food web during the spring