What factors control planktonic ciliates during summer in a highly eutrophic lake?

A mesocosm experiment in 24 enclosures (6 m(3)) started at the end of June 1996 in a highly eutrophic shallow lake, Lake Koylionjarvi (SW Finland). Th e original factorial design with nutrient, fish and macrophyte treatments w as 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 crustacea n biomass among the enclosures and the lake (40.2-417.5 mug C l(-1)), and c hlorophyll a varied from 9.5 to 67.0 mug l(-1). Here, the single-day data o n the 25 planktonic communities is analysed by means of correlation and fac tor 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 fl agellates, abundance and species composition of ciliates, phytoplankton and metazooplankton. The results indicate that although the total ciliate abun dance (ranging from 16.2 to 95.0 ind l(-1)) was controlled by food resource s, the observed differences in ciliate community structure could be attribu ted partly to differential predation by metazooplankton. The effect of Daph nia cucullata, the dominant daphnid cladoceran, was stronger than that of o ther metazoans.