Summary Bacterial community composition was analysed in the slush layers of snow‐covered Lake Redon (2240 m altitude, Limnological Observatory of the Pyrenees, LOOP, NE Spain) in winter and spring and compared with bacteria from the lake water column, using 16S rRNA gene clone libraries and CARD‐FISH counts. The set of biological data was related to changes in bacterial production and to other relevant environmental variables measured in situ. In winter, up to 70% of the 16S rRNA sequences found in the slush were closely related to planktonic bacteria from the water column beneath the ice. Conversely, during spring ablation, 50% of the sequences had > 97% identity with bacteria from the cryosphere (i.e. globally distributed glaciers, snow and ice) and may have originated from remote aerosol deposition. The transition winter to spring was characterized by consistent community changes switching from assemblages dominated by Betaproteobacteria , Verrucomicrobia and Bacteroidetes during snowpack growth to communities essentially dominated by the Bacteroidetes of classes Cytophagia and Sphingobacteria . This strong bacterial composition switch was associated with consistent increases in bacterial abundance and production, and decreasing bacterial diversity.