The Baltic Sea’s ecosystem

The Baltic Sea is a unique body of water with a specially adapted ecosystem.

A mixture of freshwater and saltwater

The Baltic Sea is a large and almost entirely enclosed marine area, located far up in the cold north. It is neither truly saltwater nor truly freshwater, and takes a generation to replace. This is an enormous drainage basin, where the various activities of the local population ultimately affect the marine environment. These are the harsh conditions for life in the Baltic Sea. It is almost completely enclosed by land. The only links with the oceans are via the Danish straits and the North Sea. The Baltic Sea consists of the Gulf of Bothnia, the Gulf of Finland and the Baltic Proper.

The Gulf of Bothnia comprises the Bothnian Bay, the Bothnian Sea, the northern Sea of Åland and the northern Archipelago Sea. The North Kvarken is situated on the boundary between the Bothnian Sea and the Bothnian Bay. Half the North Kvarken therefore belongs to the Bothnian Bay and half to the Bothnian Sea. According to the National Atlas of Sweden, the Baltic Proper consists of the Northern Baltic, the Central Baltic, the Southern Baltic and the Gulf of Gdansk. According to the Swedish Meteorological and Hydrological Institute, it consists of the southern Sea of Åland, the southern Archipelago Sea, the southern Belt Sea, the Arkona Sea, the Bornholm Sea, the eastern Gotland Sea, the northern Gotland Sea, the western Gotland Sea, the Gulf of Riga and the Gulf of Finland.

In many contexts, the Belt Sea, the Öresund Sound and the Kattegat are also included in what is commonly called the Baltic Sea region.

From north to south

From the Baltic Sea, the water flows northwards through the sounds and along the west coast of Sweden into the Baltic surface currents, making the Kattegat and the Skagerrak rich in species. When the saltwater succeeds in crossing the high thresholds of the Danish sounds, the deep holes of the Baltic Sea are filled with oxygen-rich saltwater. It takes around 30 years to replace all the water in the Baltic Proper. This means that pollutants released into the Baltic Sea remain here for a very long time. The Baltic Sea is often described as a species-poor ecosystem, but the number of species varies from north to south. The Baltic Proper stretches from the Sea of Åland to the Öresund Sound. This is the marine area where the natural conditions are toughest, and it also receives the most contaminants from the populated surrounding areas and the busy shipping traffic. The Gulf of Bothnia is the northernmost part of the Baltic Sea, and consists of two sea basins: the Bothnian Bay and the Bothnian Sea. Around 80 percent of the water in the Gulf of Bothnia is freshwater from the large rivers that run into the marine area. The marine area is fairly shallow, and the low salinity means that the stratification of the water is weak. The water mass is often mixed from the bottom to the surface, and it takes just over four years for all the water in the Gulf of Bothnia to be replaced.

Salinity is the key

The Baltic Sea region is characterised by large differences in salinity. This falls from around 25 parts per thousand (ppt) in the Kattegat to 8 ppt in the Southern Baltic and just 2 ppt in the Northern Gulf of Bothnia. This has a significant effect on life in the Baltic Sea. Only a few species can survive in such brackish conditions. The number of large marine species falls from almost 1,000 in the Kattegat to just 50 in the Gulf of Bothnia. Instead, hardy freshwater species are found here which live near the coast and grow in number the further north we look. The Baltic Sea is therefore a species-poor ecosystem, and many species are close to their distribution limit. Both of these factors mean that the ecosystem is highly sensitive to external influences. Quite simply, it has little resistance to disruption.

The effects of the human population

Ninety million people live within the large drainage basin of the Baltic Sea. Most of them live in the southern section, and around half of them live in Poland. This is a highly industrialised area, with intensive forestry being carried out in the north and agriculture in the south. Nutrients, environmental toxins and other pollutants run off the land and end up in the sea sooner or later, as do many airborne pollutants. Significant efforts have been made to rectify the biggest sources of emissions and discharges. As a result, it is increasingly the many small, diffuse sources that together have the worst impact on the Baltic Sea. Ineffectively removed antifouling paint in sensitive archipelago areas is an example of large-scale impact from many individuals. Nine countries border the Baltic Sea region directly. Five more countries are partly located within the drainage basin. Sweden has by far the longest coastline, and should therefore take a particularly large responsibility.

These pages include factual information from - a site run by Stockholm University, Umeå University, the University of Gothenburg and the Swedish University of Agricultural Sciences (SLU) within the collaborative framework of the Swedish Institute for the Marine Environment, and in cooperation with the web agency Azote.

Baltic Sea Science Center