The Baltic Sea contains a unique mix of marine and freshwater species adapted to the brackish conditions, as well as a few true brackish-water species. Where salinity levels are low in the Baltic’s northern and eastern waters, fewer marine species thrive, and the communities of organisms are dominated by freshwater species, especially in estuaries and coastal waters.
The limited number of species in the Baltic Sea food webs implies that each individual species has a special importance in terms of the structure and dynamics of the whole ecosystem. The disappearance of a single key-species could destroy the functioning of the system. The Baltic Sea ecosystem is considered to be very vulnerable to external disturbances.
The biotopes in the Baltic Sea can cover either large homogenous areas, small patches or be a part of a small-scale mosaic. The size and structure of the biotopes depends on variations in the seafloor substrate types, bathymetry and other environmental gradients. Baltic Sea biotopes exhibit a great diversity in function and structure. Some biotopes are dominated by large perennial vegetation that creates a threedimensionally complex biotope, such as the bladderwrack (Fucus vesiculosus) on rocky bottoms or the common eelgrass (Zostera marina) on sandy bottoms. Biotopes in the aphotic zone are typically dominated by semisessile macrofauna that either attach to the hard surface or burrow into soft substrates, for exampleblue mussels (Mytilus spp.) or the ocean quahog (Arctica islandica), respectively.
Baltic Sea habitats and species are threatened by eutrophication, elevated amounts of hazardous substances entering the sea as a result of long-lasting human activities in the surrounding catchment area and at sea. In addition, biodiversity is affected by socio-economic activities such as fisheries and shipping.