THE ANNUAL CYCLE OF HETEROTROPHIC FRESH-WATER NANOFLAGELLATES - ROLE OF BOTTOM-UP VERSUS TOP-DOWN CONTROL

The population dynamics of heterotrophic nanoflagellates (HNF) were analyzed in prealpine Lake Constance over three cosecutive years. A recurrent seasonal pattern led to the identification of five seasonal phases: winter, spring, clear-water, summer and autumn. HNF biomass was lowest in winter and highest in late spring several weeks after the phytoplankton spring bloom. The average biomass of HNF was 5-12% of bacterial biomass and 13-34% of ciliate biomass respectively. The largest HNF cells were recorded during the phase, whereas the average cell size was reduced to one-third during the subsequent clear-water phase. The pronounced differences in the mean cell size were attributed mainly to varying grazing impact on HNF. Throughout most of the year, HNF production was balanced by grazing of microzooplankton, namely ciliates, within the microbial loop. During the clear-water phase, however, the grazing impact was mainly due to rotifers and daphnids. Changing grazing impact was primarily responsible for the observed 2-fold interannual difference in the mean biomass of HNF. Overall, top-down control by grazing was more important in governing the population dynamics of HNF than bottom-up control by bacterial food supply.