The Baltic Sea is characterized by its distinct horizontal and vertical salinity gradients, with the unique feature that 62% of its surface zone is represented by brackish conditions. Salinity-related distribution patterns and a diversity minimum at brackish conditions are known for higher benthic and pelagic organisms. In a first analysis for microorganisms, based on pyrosequencing of bacterial 16S rRNA genes, we were able to show that also bacterial communities experience distinct shifts at different phylogenetic levels along the salinity gradient. Moreover, there is evidence for an adapted brackish water bacterial community, represented by a phylogenetic member of the Verrucomicrobia. This project aims to test how the shift in bacterioplankton community composition from marine to brackish and limnic conditions affects bacterially mediated functions within the pelagic carbon cycle. This will be investigated by two different approaches: (1) in ship-board experimental investigations at stations of different salinity we will assess, besides prokaryotic diversity, microbial functionality covering broad to highly specific bacterial activities related to organic carbon processing, and (2) the direct impact of salinity on the composition and function of surface bacterioplankton in the central, brackish Baltic Sea will be examined by experimentally manipulating salinity levels. Here, as an additional functional factor besides substrate utilization, we will investigate also prokaryotic response towards predation and trophic interactions between prokaryotes and bacterivores.