The annual cycle of heterotrophic freshwater nanoflagellates: role of bottom-up versus top-down control

The population dynamics of heterotrophic nanoflagellates (HNF)were analyzed in pre-alpine Lake Constance over three consecutiveyears. A recurrent seasonal pattern led to the identificationof five seasonal phases: winter, spring, clear-water, summerand autumn. HNF biomass was lowest in winter and highest m latespring several weeks after the phytoplankton spring bloom. Theaverage biomass of HNF was 5–12% of bacterial biomassand 13–34% of ciliate biomass respectively. The largestHNF cells were recorded during the spring phase, whereas theaverage cell size was reduced to one-third during the subsequentclear-water phase. The pronounced differences in the mean cellsize were attributed mainly to varying grazing impact on HNFThroughout most of the year, HNF production was balanced bygrazing of microzooplankton, namely ciliates, within the microbialloop. During the dear-water phase, however, the grazing impactwas mainly due to rotifers and daphnids. Changing grazing impactwas primarily responsible for the observed 2-fold interannualdifference m the mean biomass of HNF Overall, top-down controlby grazing was more important in governing the population dynamicsof HNF than bottom-up control by bacterial food supply.