Abstract
It has recently been shown that a rich community of microorganisms inhabits the slush layers of the winter cover of high mountain lakes. In this study, temporal changes in species assemblages and environmental conditions in the ice and snow cover of Lake Redó in the Pyrenees (Spain) are presented. The winter cover was a highly dynamic environment, in which major changes occurred in physical structure and chemical characteristics. The biomass and species composition of the microbial groups present (autotrophic and heterotrophic flagellates, ciliates, and bacteria) reflected these physicochemical alterations. After an initial phase, during which the ice sheet formed and the first snows accumulated with little or no development of slush layers, there were two stages of microbial assemblages, which coincided with the two main phases in the physical change of the cover: the growth period (from January to mid‐April) and the ablation period (from mid‐April to ice‐out in June). Initially, microbial biomass originated from inputs of phytoplankton‐rich (5.9 μg chlorophyll a liter −1 ) surface lake water that flooded the cover as a result of the hydrostatic adjustment induced by the progressive accumulation of snow on top of the ice sheet. This was followed by a differential growth of mixotrophic or heterotrophic flagellated species, such as Ochromonas sp., Cryptomonas spp., Monosiga ovata and Oikomonas termo , which peaked at different times. Species depended primarily on bacterivory for their growth, since during this period, light did not reach the slush layers. By April, the transition from cover growth to ablation was marked by a reduction in biomass and in the number of species. During the ablation period, the rate of change of the cover was greater, and microbial assemblages were characterized by the vertical segregation and by the appearance of new species, some of which were typically nonplanktonic. During that time, the cover was highly influenced by the large amount of water from the melting of the snowpack within the catchment. This water likely provided chemicals and organisms, while increased light availability allowed for the growth of algae ( Pteromonas sp. and other volvocales) and of the associated food web in the upper slush. In deeper layers, however, a bacteria‐based food web provided prey for large ciliates ( Urosoma sp., Dileptus sp., and Lacrymaria sp.), probably carried from littoral or watershed soils.