AME 29:123-134 (2002)  -  doi:10.3354/ame029123

ABSTRACT: Active microbial communities formed by autotrophic and heterotrophic flagellates, ciliates and bacteria, inhabit slush layers of the ice and snow cover of high mountain lakes. Our study of the ice and snow cover of Gossenköllesee (Tyrolean Alps) during 2 complete winter periods, with special emphasis on the relationship between slush layers and the water column, confirms the hypothesis that the drastic changes in the physical and chemical structure of the cover determine biomass and composition of microbial assemblages. The temporal pattern previously described in the Pyrenees applies to the Alps, and we could distinguish 3 periods of winter cover: formation, growth and ablation. During the formation period (November to December), the ice sheet forms and slush layers start to develop. In the growth period (January to May), slush layer assemblages are mainly influenced by organisms deriving from lake plankton, predominantly flagellated chrysophytes, which peaked at different times and depths. During the ablation period (May to June/July), however, the cover assemblages are shaped by organisms and processes in the catchment. Microbial communities are characterized by the appearance of new species, such as Gymnodinium sp., red volvocales and large ciliates. The mutual influence between lake water and winter cover assemblages affects only the upper 1 to 2 m of the water column, while changes in the microbial composition of deeper water layers are slow and poorly related to slush layer assemblages. The appearance of Œ2 systems in 1 lake¹ is restricted to ca. 6 mo yr^-1 and the reciprocal influence seems to be even more limited by space and time. During the ablation phase, i.e. when the influence of the catchment is much stronger than that of lake plankton, the slush microbial communities in the Alps and the Pyrenees are more similar to each other than during the growth phase of the cover. Over the whole winter period, the ice and snow cover appeared to be more dynamic, vertically variable and sporadically richer in biomass than the water column.

KEY WORDS: Phytoplankton · Bacteria · Protozoa · Slush · High-mountain lake