Conspicuous peak of oligotrichous ciliates following winter stratificationin a bog lake

We analyzed early spring protozooplankton peaks following winter stratifica tion in two different basins (northeast, NE; southwest, SW) of an artificia lly-divided bog lake Grobe Fuchskuhle (Brandenburg, Germany). The maximum c iliate biomass and numbers (660 and 990 mug l(-1) of organic carbon, and 29 0 and 260 cells ml(-1) in NE and SW, respectively) were reached in the surf ace layer during the ice melting and then continuously decreased. The surfa ce layers were numerically dominated by species of the genus Urotricha, whi le 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. Seemin gly, the peak was related to the specific conditions resulting from the ice melting where these ciliates could be concentrated. Additionally, the mixo trophy of oligotrichous species allowed them to penetrate below the oxyclin e, although feeding on nanoplankton such as flagellates was also very likel y. Our direct measurements of uptake rates of bacteria showed that the abun dant 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 (communi ty average) cell-specific ciliate feeding rate of over 200 bacteria ciliate s(-1) h(-1) was reached, along with an increasing contribution of scuticoci liates, paralleled by a drop in large oligotrichs.