Conspicuous peak of oligotrichous ciliates following winter stratification in a bog lake

We analyzed early spring protozooplankton peaks following winter stratification in two different basins (northeast, NE; southwest, SW) of an artificially-divided bog lake Grobe Fuchskuhle (Brandenburg, Germany). The maximum ciliate biomass and numbers (660 and 990 mug l(-1) of organic carbon, and 290 and 260 cells ml(-1) in NE and SW, respectively) were reached in the surface layer during the ice melting and then continuously decreased. The surface layers were numerically dominated by species of the genus Urotricha, while in the ciliate biomass during the first part of the study, a mixotrophic oligotrich, Pelagostrombidium mirabile prevailed (C-org up to 940 mug l(-1)). We observed a conspicuous ciliate peak that could not be related to the feeding activity of the dominant ciliate species upon picoplankton. Seemingly, the peak was related to the specific conditions resulting from the ice melting where these ciliates could be concentrated. Additionally, the mixotrophy of oligotrichous species allowed them to penetrate below the oxycline, although feeding on nanoplankton such as flagellates was also very likely. Our direct measurements of uptake rates of bacteria showed that the abundant strombidiids were not nutritionally dependent on bacterivory. However, the ciliate uptake rate (up to 670 bacteria ciliate(-1) h(-1)) contributed significantly to the overall bacteria mortality rate. The maximum (community average) cell-specific ciliate feeding rate of over 200 bacteria ciliates(-1) h(-1) was reached, along with an increasing contribution of scuticociliates, paralleled by a drop in large oligotrichs.