Seasonal patterns of bacterial communities in the coastal brackish sediments of the Gulf of Finland, Baltic Sea

Coastal areas are critical in mitigating the impact of nutrients runoffs and downstream eutrophication of aquatic ecosystems. In the Gulf of Finland, the easternmost sub-basin of the Baltic Sea, seasonal and long-term oxygen depletion at the surface of the sediment feeds back the eutrophication loop by promoting the release of nutrients locked in the sediment matrix. In order to understand how the bacterial community responds to the seasonal variations in the local conditions, we sequenced ribosomal gene fragments from the top sediment layer at two coastal sites on the southern coast of Finland in spring, summer and late autumn during two consecutive years. Analysis of the samples collected at a shallow (11 m) and deep (33 m) revealed that the overall composition of the community was rather constant over time and there was an extensive collection of shared OTUs between sites. The dominant taxa were related to sulfate reducers and the variation in community structure was linked to the source and availability of organic matter in the surface sediment. The taxa characteristic of spring samples belonged primarily to Actinobacteria, possibly of fresh water origin. Summer communities were characterized by the increase in number of reads associated with heterotrophic bacteria such as Bacteroidetes which feed on labile organic matter from spring bloom. Taxa typical of autumn samples were linked to Cyanobacteria from the overlying water and to bacteria feeding on organic matter drifting from the phytal zone.