The Baltic Sea is characterized by salinity gradients, providing an ideal system for examining how salinity governs microbial biodiversity and microbially-mediated processes. Little is known about how locally adapted bacteria cope with changes in salinity and which ecological strategy they employ in response to such change. We have performed a full-factorial transplant experiment with bacterial communities originating from three salinity regimes of the Baltic Sea (freshwater, brackish, marine), which were reciprocally incubated under each other’s environmental conditions.