Abundance, biomass and size structure of the microbial assemblage in the high mountain lake Gossenköllesee (Tyrol, Austria) during the ice-free period

The abundance, biomass and morphology of the microbial components (picocyanobacteria, heterotrophic bacteria, heterotrophic and autotrophic nanoflagellates, ciliates) of the pelagic food web of Gossenköllesee were investigated over two summer periods. The density of bacteria remained relatively stable not only over time but also in vertical profiles (2.5-5.5x10⁵ cells ml^-1). Bacterial biomass ranged between 35 to 63 mgC m^-2 (5.4-15.3 μgC l^-1). Small rod shaped bacteria with mean cell volumes of ~0.05 μm³ dominated numerically but filamentous forms (longer than 10 μm), mainly found in the upper water layers, amounting to more than 65% of the total bacterial biomass, increased the mean cell volume up to 0.27 μm³ (SD=0.88). Bacterial biomass represented between 48 and 86% of the total microbial biomass (40 - 90 mgC m^-2), however at 8 m depth the biomass of heterotrophic flagellates (HNF) reached up to 26 mgC m^-3 (2,852 cells ml^-1). From 0-4 m depth small spherical HNF species with cell volumes <10 μm³ were dominant, whereas in deeper water layers large flagellates with cell volumes >50 μm³ dominated throughout the sampling period. Ciliate abundance was low in the upper part of the water column. Only Askenasia chlorelligera and Urotricha cf pelagica occurred with numbers of up to 1,500 cells l^-1. At 8 m depth, Balanion planctonicum was the dominant species throughout the study period reaching numbers of up to 16,000 cells l^-1. Ciliate abundance was significantly correlated with chlorophyll-a concentrations (rs=0.55, p <0.01) confirming the algivory of the dominant species. No significant correlation was found between the parameters of chlorophylla, bacteria and HNF but the abundance of ciliates was negatively correlated with the length of bacteria (rs= -0.41, p <0.05).