during Spring Time in the Eutrophic Belauer See

In spring 1991, we investigated the development of the planktonic community of a eutrophic lake (Belauer See, North Germany), fo-cusing on the feeding behaviour of Vorticella rhabdostyloides and Tintinnidium fluviatile, the two most abundant ciliate species in the pelagic zone of this lake during spring. Experiments on the uptake of fluorescently labelled particles in different size classes and fluores-cently labelled bacteria nd algae permitted the determination f the importance ofthose kinds of food. Vorticella rhabdostyloides ingest-ed particles about 1.0 lam and 3 gm long and autotrophic picoplank-ton (APP). Tintinnidium fluviatile preferred particles 0.5 ~m to 1.0 ~tm and 5.0 gm long, APP and diatoms in the genus Nitzschia. In-gestion rates ranged between 100 and 120 particles h 1 for each TintimTidiumfluviatile and 60 to 230 particles h 1 for each Vorticella rhabdostyloides. APR bacteria, centric diatoms and nanoprotozoans were evaluated as foods for both ciliates. Tintinnidium fluviatile was found to exert a strong short-term control on picoplankton (rs =-0.941, n = 6, p < 0.05) and small centric diatoms (r s = 0.700, n = 6, p < 0.05). The different diets of both ciliates may explain the coexistence ofboth species during spring