The Baltic Sea is the second largest inland sea in the world, the largest being Hudson Bay in North America. The Baltic Sea is also the largest brackish sea area in the world. Inflow of fresh saline water from the North Sea and Atlantic is extremely limited due to the narrow, shallow channel connecting to the Baltic Sea via the Kattegat between Denmark and Sweden. The Baltic Sea is also fed by rivers from a large catchment area four times the size of the sea itself. Many of the rivers discharging into the sea flow through large industrial areas (Neva – St. Petersburg; Vistula & Motława – Gdansk; Daugava – Riga). For example, the River Vistula drains 60 percent of Poland’s land area. Many rivers in Finland, Sweden, and Denmark run through rich agricultural and forestry regions, and as a result large amounts of organic material and fertilizer run-off are deposited into the sea. All these factors lead to making the Baltic one of the most polluted seas in the world.

The Baltic Sea is heavily used by commercial shipping, particularly by tankers and container ships coming from and heading to ports in Russia, as well as other major harbours such as Gdansk, Klaipeda, Liepaja, Ventspils, Tallinn, Stockholm, and Helsinki. The Baltic also plays host to large numbers of cruise ships, each with the pollution footprint of a small town. In addition, there are many commercial ferries on regular routes connecting cities around the coast of the sea. During the summer months the coastal areas of the Baltic attract huge numbers of tourists and pleasure boaters, particularly in the Stockholm archipelago and the Finnish Archipelago Sea areas, as well as along the northern Polish and German coasts. All these factors lead to increased pollution and pressure on the sea and its wildlife.