The Baltic Sea is a vulnerable inland sea

The water of the Baltic Sea is permanently stratified according to the level of salinity. Salty seawater flowing from the North Sea is heavier and sinks to the bottom and deep basins of the Baltic Sea. The surface layer consists of water diluted by rainwater and numerous rivers flowing into the Baltic Sea. The stratification hinders water turnover between the bottom and the surface layer. Oxygenic surface water cannot reach the deeper water layers, and there are recurrent oxygenless periods in the deep basins of the Baltic Sea. The oxygen situation improves periodically, about once per decade, with the help of pulses of saline water (infrequent currents of ocean water), when salty and oxygenic ocean water flows from the North Sea into the basins of the Baltic Sea, mixing with the Baltic Sea's water layer near the bottom.

The population of the Baltic Sea is an unusual combination of freshwater and ocean flora and fauna. The number of flora and fauna adapted to life in the brackish water is small, but there may be large quantities of individual species. Compared to the oceans, the food chains in the Baltic Sea are simple. The number of species reduces from the southern Baltic Sea to the North. The low salinity of the northern Baltic Sea, the cold winters and the sea freezing over set challenges for the adaptation of organisms. Many species in the Baltic Sea live on the extreme limits of their adaptability. The flora and fauna of the Baltic Sea are very sensitive to changes in the environment.