VERTICAL AND TEMPORAL HETEROGENEITY OF PLANKTONIC CILIATED PROTOZOA IN A HUMIC LAKE

Seasonal population dynamics and the vertical distribution of ciliates were studied in relation to the particular food resources occurring i n a humic and moderately acidic lake (Lake Vassiviere). The abundance (1.4 x 10(3) - 20.4 x 10(3) cells l-1, mean = 4.8 x 10(3) cells l-1) a nd biomass (0.5-34.6 mug C l-1, mean = 6.0 mug C l-1) of ciliated prot ozoa were low and close to values reported for oligotrophic environmen ts. The species composition of the population varied greatly with dept h. Whereas large-sized species of oligotrichs, some of which were mixo trophic, dominated at the surface, haptorids were best represented in deep waters. The spatial distribution of the various groups of ciliate s was largely determined by light and the vertical distribution of mic robial food resources (detritus, bacteria, algae) within the water col umn of this brown-colored lake.