Comparison of ciliate communities in the anoxic hypolimnia of three lakes: general features and the influence of lake characteristics

The ciliate communities and their food sources were investigated in the anoxic hypolimnia of three lakes with differing water chemistry. Bacterial biomass and, as a result, the biomass of bactenvor- ous ciliates were correlated with lake trophy. Additionally, high sulfate and sulfide concentrations led to high bacterial biomass of sulfate reducers and anaerobic phototrophic and heterotrophic bacteria, which in turn sustained large ciliate populations. The anaerobic ciliate communities of the lakes shared many characteristics. They were comprised of the same or closely related species; this was attributed to a low diversity of food sources. Ciliate to prey biomass ratios were 1.2-3.8% which is consistent with a low theoretical growth efficiency of anaerobic metabolism. Grazing pressure on anaerobic ciliates by metazoa was insignificant. In all three lakes, ciliate populations showed distinct vertical non-random distribution patterns which were often correlated with the distribution of the corresponding food sources. It is suggested that the microbial communities in anoxic water bodies are largely influenced by few common environmental conditions and are therefore often inhabited by similarly structured ciliate communities.