THE MICROBIAL LOOP IN A HUMIC LAKE - SEASONAL AND VERTICAL VARIATIONSIN THE STRUCTURE OF THE DIFFERENT COMMUNITIES

Seasonal and vertical variations of the main microbial communities (he terotrophic bacteria, autotrophic picoplankton, auto- and heterotrophi c nanoflagellates, ciliated protozoa and microalgae) and auto- and het erotrophic activities were estimated in a brown-colored humic and mode rately acid lake in central France, the lake of Vassiviere. The result s demonstrated the dominant role of light in the vertical distribution of autotrophic and mixotrophic microorganisms which are confined to t he 0-5 m layer during thermal stratification. The bacterial biomass wa s high throughout the water column probably because of the great avail ability of dissolved organic matter. Consequently, the predatory micro zooplankton and particularly the various trophic groups of ciliated pr otozoa, were distributed in the water column according to the vertical distribution of the particular food resources (detritus, bacteria, al gae). However, despite the great abundance of algae and bacteria, biom ass of flagellated and ciliated protozoa was relatively weak. Most of the phytoplanktonic biomass was filamentous (Diatoms) or colonial (Cya nobacteria) and therefore almost probably difficult to ingest for algi vorous microzooplankton. Regarding the low abundance of bacterivorous protozoa, the relation with the special physicochemical properties of this lake is discussed.