Estuaries are a major boundary in the land-ocean interaction zone where organic carbon (OC) and nutrients are being processed. A large amount of terrigenous dissolved organic matter (tDOM) is mobilized by melting of permafrost and reaches through the estuaries the marine environment. The bioavailability of tDOM is an important factor for the global carbon cycle but our understanding of its decomposition and bacteria-tDOM interactions is rather poor. In order to investigate the role of salinity and bacterial diversity on decomposition of subarctic riverine tDOM, we performed mesocosm experiments in three different Baltic Sea salinity regions at spring conditions. The experiments indicate a significant effect of tDOM addition on the bacterial community in the brackish (mesohaline and marine) mesocosm but not in the mesohaline mesocosm. Ampliconsequencing of 16S rRNA genes revealed that the bacterial communities and their dynamics also differed between marine and brackish mesocosms. A major difference in bacterial community composition between manipulated and control mesocosm was apparent in the first week of the experiment. The relatively small change in native DOM composition by tDOM addition in the oligohaline mesocosm is in concordance with the increased bacterial activity at these conditions and supports the hypothesis that native DOM is preferentially used over introduced DOM. However, first results of similar experiments performed in spring indicate a significant effect of tDOM addition at marine condition suggesting a strong seasonal dependency of tDOM degradation.