The Baltic Sea is a young ecosystem formed after the latest glaciation, continuously undergoing postglacial successional changes and diversification. It is a semi-enclosed, non-tidal ecosystem and has distinct latitudinal and vertical salinity gradients. There is strong permanent vertical stratification for much of the Baltic Sea. Substrate distribution (Figure 13) is affected by water movement. Muddy sediments and occasionally sand are most common in the deeper parts, whereas rocky and mixed sediments can occur in near-shore and wave-exposed areas. The southern parts, including the Belt Sea, are connected to the Kattegat and show salinity levels around 25–30. Surface salinity levels in the central Baltic Sea are around 7–8, dropping to around 5 at the entrances to the northern Gulfs. In the most northern and eastern parts of the Baltic Sea, conditions are close to those of freshwater.
Oxygen concentrations are low in many areas, notably in deeper basins. In shallower coastal parts of the Baltic Sea, hypoxia may occur during the summer months in connection with high water temperatures. Nutrient input to the Baltic is the major cause of both anoxia and hypoxia. The extent of the affected areas (Figure 14) varies in relation to the intensity and frequency of the major inflows of water from the North Sea. Starting from the beginning of the 1990s, the frequency of the major inflows from the North Sea dropped from one event every second or third year to one event per decade.
In 2017, the areal extent of the oxygen depletion in the Baltic Sea (Figure 15) remained widespread, with hypoxic waters (< 2 ml l−1 O2) representing about 28% of the area and 22% of the volume of the central Baltic and the Gulf of Finland.
