Effects of freshwater outflows on microbial populations in the Tvärminne archipelago, southern Finland

The effects of freshwater outflows were evaluated in three ways: (1) microbial populations in the outflow area (mainly Tvärminne Storfjärd) were compared with populations outside the area, (2) direct counts were made at three sites to monitor the abundance of bacteria along the seaward course of the outflows, and (3) a survey was made of the distribution of microorganisms in the archipelago. The indices used for the presence of freshwater outflows in the archipelago were low salinity, increased amounts of humic matter, and increased temperatures in the winter. In the outer archipelago, the effects of the outflows were most evident in the low‐salinity surface layer below the ice and were also distinguishable in spring and late autumn, but were noted only sporadically in the summer. The outflows were traced as far seaward as the border of the sea zone. The outflows transported various types of bacteria, bacterial spores and yeasts, but did not significantly affect the community respiration. Many of the microorganisms flowing with the fresh water in the winter evidently originated from sewage. During the other seasons, the roles of outflows from the firth of Pojoviken and runoff from the land became evident, but their contributions could not be estimated because fresh water and sewage move together. However, most of the bacterial spores seemed to originate from the land. The archipelago east of Tvärminne Storfjärd proved to be appropriate for inshore brackish‐water investigations being undisturbed by freshwater outflows.