How does the salinity of the Baltic Sea compare with that of the open ocean?

The salinity of the Baltic Sea is significantly lower than that of the open ocean. While the open ocean maintains a relatively uniform salinity typically ranging from 33 to 37 parts per thousand (or psu), the Baltic Sea is remarkably fresh due to its nearly landlocked geography and the large influx of river water.312

In the western Baltic, surface salinity reaches approximately 10 parts per thousand, with slightly higher values of about 15 parts per thousand near the bottom. These levels drop even further toward the head of the Gulf of Bothnia, where the water is nearly fresh. In contrast, the open ocean's salinity is largely governed by the balance between evaporation and precipitation, with average values typically falling between 34.5 and 35 psu. The Baltic's isolation from the open ocean prevents this standard salt concentration from being maintained, resulting in its characteristically low salinity.132