The Baltic Sea is a shallow, semi-enclosed sea that is connected to the North Sea via the intermediate Skagerrak and Kattegat seas (division 3.a on the map in Figure 1) and the Sound and the Belt Sea (subdivisions 22 and 23). The sea’s limited exchange with the open ocean gives it a particular water flow and salinity pattern. As salty water is denser than fresh water, the saline and oxygenated North Sea waters enter the Baltic and propagate into deeper areas. A large amount of brackish water, caused by inflows of fresh water from surrounding land areas, exit the Baltic Sea in the opposite direction as a surface layer. This creates a vertical stratification, with higher salinity at the bottom. The salinity also decreases further north and east with virtually fresh water in the Gulf of Finland and Bothnia.