Bacterial freshwater species successfully immigrate to the brackish water environment in the northern Baltic

We studied the distribution and seasonal dynamics of five species from the genus Flavobacterium and one species from the genus Marinomonas over the course of a year along a northern Baltic Sea river–marine transect. All of the species had been previously demonstrated as important consumers of riverine dissolved organic carbon. Quan-titative DNA–DNA hybridization data showed that two of the Flavobacterium spp. and the Marinomonas sp. had highest abundance in the river water (maximum 20,000 cells ml21), with maximum relative abundance of 0.5–2.5% of the bacterial community. These species declined in abundance from the river to the estuary and the offshore site. Abundance and dynamics in the estuarine environment suggested successful immigration of freshwater bacteria, accompanied by growth in the brackish water environment. Two of the three abundant species showed high cell numbers also during late autumn to early spring in the estuary, indicating a selective advantage when riverine dissolved organic carbon was the main carbon source. The remaining three species showed more episodic abundance close to the detection limit of the method, providing weaker evidence of occurrence in the freshwater environment. Some bacterioplankton consuming riverine organic carbon in the brackish water environment in the northern Baltic are therefore freshwater species, with a selective advantage during winter. Riverine carbon can contribute significant carbon and en