Seasonal dynamics and vertical distribution of planktonic ciliates and their relationship to microbial food resources in the oligomesotrophic Lake Pavin

The planktonic ciliated protozoa and their potential prey (bacteria, autotrophic picoplankton, flagellates, algae) in Lake Pavin (Massif central of France) were investigated during the period of April to December, 1992. Ciliate abundance varied from 300 to 24,200 cells/l. Peak densities were observed during mixis for all sampling depths and in May-June in the cold aerobic hypolimnion. During the rest of the year, the species composition varied greatly with depth and season. Small algivorous prostomatids and large mixotrophic oligotrichs were most abundant in the euphotic zone during summer. Small filter-feeders were present at each depth throughout the study whereas haptorids and large size taxa were restricted to 15 m depth during spring and autumn. The highest density of ciliates occurred in the deep (40 m) cold aerobic hypolimnion due to the major spring development of the scuticociliate Uronema nigricans. This species seems to be adapted to cold and dilute freshwater habitats. Evidence for a major impact of availability and diversity of appropriate food in determining ciliate composition at different seasons and depths are given.