Oceans and seas

Bothnian Sea

May 13, 2013, 7:41 pm
Content Cover Image

Bothnian Sea from Ă…land. Source: Manolo Frias

The Bothnian Sea (Swedish: Bottenhavet, Finnish: Selkämeri) is the designation of the southern part of the Gulf of Bothnia, the northern arm of the Baltic Sea between Sweden and Finland. The sea is shallow with a mean depth of 60 meters,  and the water brackish (with salinity averaging approximately five to six per mg/l).

North of the Bothnian Sea is a region known as The Quark and beyond that Bothnian Bay. To the south lies and the Sea of Åland and Archipelago Sea. These regions combined constitute Gulf of Bothnia. Just beyond the northern limit of the Bothnian Sea and within the Qaurk is the Kvarken Archipelago, part of a a World Heritage Site.

caption Figure 2. Sub-regions of the Baltic Sea. Source: Helsinki Commission


The most common definition of the geographic area of the Bothnian sea are the latidudes of 60.5ºN and 63.5ºN, and the coasts of Finland andSweden. However, HELCOM,  governing body of the Convention is the Helsinki Commission, defines the lmits of the Bothnian Sea more precisely as between the lines Hörnefors (Sweden) and Vaasa (Finland) in the north; and between the towns of Ormön and Understen (Sweden) and Emskär, Eckerö, Saltvik and Uusikaupunk (Finland) in the south.

The Bothnian Sea covers about 220,765 km2 and received a volume about 124 km3 of runoff in the year 2000.

caption Source: Adpated from Google maps

 The largest coastal towns, from south to north, are Rauma and Pori in Finland, and Gävle and Sundsvall in Sweden. Umeå (Sweden) and Vaasa (Finland) lie in the extreme north, near Bothnian Bay.

Helsinki Convention

Snow and ice cover on the Gulf of Bothnia, March 15, 2002. Source: NASA
Pack Ice on the Bothnian Sea. Source: The Baltic Sea - Discovering the sea of life (2006)  by Helena Telkanranta, Helsinki Convention. Photo by Raimo Sundelin

The Bothnian Sea is included within a ten-nation pact to address pollution on a comprehensive basis for the entire Baltic Sea known as the Helsinki Convention. The governing body of the Convention is the Helsinki Commission - Baltic Marine Environment Protection Commission - also known as HELCOM. The goal of the Convention is that: "The Contracting Parties shall individually or jointly take all appropriate legislative, administrative or other relevant measures to prevent and eliminate pollution in order to promote the ecological restoration of the Baltic Sea area and the preservation of its ecological balance." In addition to contracting parties agreed to: apply the precautionary principle, promote the use of Best Environmental Practice and Best Available Technology, apply the polluter-pays principle, ensure that measurements and calculations of emissions from point sources to water and air and of inputs from diffuse sources to water and air are carried out in a scientifically appropriate manner, and ensure that the implementation of the Convention does not cause transboundary pollution in areas outside the Baltic Sea Area.


The Bothnian sea is part of the Baltic Sea large marine ecosystem characterized by its temperate climate, high level of nutrients, and being strongly influenced by human-induced eutrophication, river runoff and a lack of rapid exchange with the adjacent ocean. It is considered healthy compared to much of the rest of the Baltic Sea because it has fewer problems associated with eutrophication, which is a major problem in the Baltic Sea as a whole.

The 2010 HELCOM Initial Holistic Assessment of the Ecosystem Health of the Baltic Sea summarized the state of the Bothnian sea as follows:

The assessment and classification of the ecosystem health of open parts of the Bothnian Sea indicate that the status is impaired. However, one assessed area in the Swedish coastal waters is classifi ed as good. Biodiversity of the Bothnian Sea in general is good, both for the open parts and the majority of coastal waters. The open parts of the Bothnian Sea, a few coastal areas in the southern Swedish parts, and Finnish coastal waters are affected by eutrophication, while the Swedish northern coastal waters are not affected by eutrophication. Regarding hazardous substances, all assessed areas are disturbed by hazardous substances.

HELCOM views the herring fishery in the Bothnian sea as robust and currently managed in a sustainable manner.

Saikku and colleagues at the University of Turku note that the Bothnian Sea represents a unique inland sea environment for the scientific community to study due to its shallowness and low salinity. They note that:

Due to the physical barriers set by the coastline, the groups of islands in the south as well as the shape of the seabed, the nutrient-rich bottom water from the Baltic cannot flow into the Bothnian Sea. This constitutes the main reason why eutrophication has not yet become for the Bothnian Sea the major problem which it is for the Baltic. However, the situation in the Bothnian Sea is now beginning to change as a result of the nutrient-rich water draining into the Bothnian Sea from the many Swedish and Finnish rivers

While the Bothnian Sea is an economically important region with the majority of herring in the Baltic being caught in the Bothnian Sea (Lehtonen, 2005, Parmanne, 1998), it has generally not been the focus of much scientific effort. The reason for this could be its rather good condition. The same is true for monitoring efforts as there are fewer monitoring sites, temporally and spatially, in the Bothnian Sea than in the smaller Archipelago Sea (HELCOM, 2009).

The Bothnian Sea is home to the Baltic Ringed seal a subspecies of the Ringed seal which had declined until recently reaching a nadir of 5000 to 8000 individuals in the late 1990s. Since then, their numbers have increased at Bothnia Bay to the north (the location of three-quarters of this subspecies), declined further in the Gulf of Riga, and stabilized in the Gulf of Finland (although to a very low population of 300 individuals). The Grey seal also occurs in the Sea.

The Bothnian Sea is also home to the Baltic Sea Harbor Porpoise, a subpopulation of the Harbour porpoise

The terrestrial ecoregion that flanks most of the Sea is Scandinavian and Russian taiga with temperate mixed forests. This ecorgions gives way to Sarmatic mixed forests in the south.

There is extensive ice cover in the winter, particularly near shore.

Protected Areas

There are a number of protected areas in the Bothnian Sea. In addition, in early 2011, Finland established a new protected area, the Bothnian Sea National Park, covering 900 square kilometres of mostly open marine waters west of Uusikaupunki and Rauma.

Futher Reading:

  • Seas of the world
  • Finland Adds Two New National Parks. thisisFINLAND. Finland Promotion Board, Ministry of Foreign Affairs. June 2011. Retrieved 18 July 2011.
  • Natural Science of the Bothnian Sea During 1975-2008: A Review, Reetta Saikku, Minna Alhosalo, Sari Repka & Anne Erkkilä,Publications from the Centre for Maritime Studies, University of Turku
    A 50, 2009
  • HELCOM (2009) Eutrophication in the Baltic Sea – An integrated thematic assessment
    of the effects of nutrient enrichment and eutrophication in the Baltic Sea region. Balt.
    Sea Environ. Proc. No. 115B.
  • HELCOM, 2010, Ecosystem Health of the Baltic Sea 2003–2007: HELCOM Initial Holistic Assessment. Balt. Sea Environ. Proc. No. 122.
  • Lehtonen, H. (2005). Selkämeren kalat. Teoksessa: Sarvala, M.& Sarvala J.
    (toim.) Miten voit Selkämeri? Ympäristön tila Lounais-Suomessa 4. Lounais-Suomen
    ympäristökeskus. Turku, S. 102-107.
  • Parmanne, R.( 1998). Herring fishery in the Bothnian Sea (southern Gulf of Bothnia)
    and the North Sea: similarities and differences. Boreal Environment Research 3:321-




Saundry, P. (2013). Bothnian Sea. Retrieved from http://www.eoearth.org/view/article/150739


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