The Baltic is the largest brackish water system in the world. Its only connection to more open seas are the shallow sounds between Sweden and Denmark. x01x02 The sea is slowly shrinking because of geological uplifting of land after the last glaciation. x01x02 In many respects the Baltic is similar to an inland lake or an estuary. It is unique in that there are areas where freshwater, brackish water and marine species are all present. Its salinity increases from east to west and from north to south. x01x02 The marked vertical and horizontal salinity gradients are reflected in different species communities and species numbers. The highest biodiversity is found in the south-west of the Baltic Sea. x01x02 Many of the marine species are at the limit of their distributions. x01x02 The main threats to biodiversity in the Baltic Sea are: o Eutrophication: this has caused increased amounts of planktonic algae, increased frequencies of toxic blooms of algae, the reduction of oxygen levels in the deep waters of the Baltic and a decline or disappearance of larger perennial macroalgae. o Fisheries: Fishery of the main target fish species such as cod, herring, salmon and eel is presently unsustainable due to over-exploitation and impairment of conditions for reproduction. Bycatches of marine mammals, seabirds and non-target fish species are too high. o Pollution by contaminants and oil: Organic contaminants have caused health and reproduction problems in marine mammals and birds. Oil has killed seabirds and negatively effected benthic communities. o Introduction of non-indigenous species: Changes in the structure and components of the ecosystem are caused by introduced species. Intentional introduction, fouling and ballast water are three important ways organisms have been introduced into the Baltic Sea. The river connections with the brackish waters in the Black and Caspian Seas increase the risk of introduction from these areas.