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A mesocosm experiment in 24 enclosures (6 m3) started at the end of June 1996 in a highly eutrophic shallow lake, Lake Köyliönjärvi (SW Finland). The original factorial design with nutrient, fish and macrophyte treatments was lost due to strong winds causing leakages. However, after the walls were made leak-proof again on July 11, the planktonic communities developed in divergent ways. On July 31 there was a tenfold variation in total crustacean biomass among the enclosures and the lake (40.2?17.5 µg C l-1), and chlorophyll a varied from 9.5 to 67.0 µg l-1. Here, the single-day data on the 25 planktonic communities is analysed by means of correlation and factor analysis in order to identify factors controlling the protozoans, with particular emphasis on ciliates. The data set comprised: total phosphorus, nitrogen, chlorophyll, bacteria, autotrophic picoplankton, heterotrophic flagellates, abundance and species composition of ciliates, phytoplankton and metazooplankton. The results indicate that although the total ciliate abundance (ranging from 16.2 to 95.0 ind l-1) was controlled by food resources, the observed differences in ciliate community structure could be attributed partly to differential predation by metazooplankton. The effect of Daphnia cucullata , the dominant daphnid cladoceran, was stronger than that of other metazoans.
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