Bacterial diversity and community structure in Vistula river estuary (Southern Baltic) studied at the 16S rRNA gene fragments level. Samples were collected four times (in spring, summer, autumn and winter) at three sites differing in salinity. The goals of the study are the following: i) to find which bacterial taxa are dominant in freshwater, mixing zone and marine waters during the four seasons ii) identification of rare species, iii) how the three zones differ in bacterial diversity and if the differences result from changes in rare species subcommunity iv) how the diversity and community structure changes seasonally v) what is the contribution of freshwater taxa to marine assemblages and vi) which freshwater taxa are active in marine waters.