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, a utotrophic picoplankton, flagellates, algae) in Lake Pavin (Massif cen tral of France) were investigated during the period of April to Decemb er, 1992. Ciliate abundance varied from 300 to 24,200 cells/1. Peak de nsities 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 algiv orous prostomatids and large mixotrophic oligotrichs were most abundan t in the euphotic zone during summer. Small filter-feeders were presen t at each depth throughout the study whereas haptorids and large size taxa were restricted to 15 m depth during spring and autumn. The highe st density of ciliates occurred in the deep (40 m) cold aerobic hypoli mnion due to the major spring development of the scuticociliate Uronem a nigricans. This species seems to be adapted to cold and dilute fresh water habitats. Evidence for a major impact of availability and divers ity of appropriate feed in determining ciliate composition at differen t seasons and depths are given.