The Bothnian Sea begins on the northern side of the Sea of Åland, and continues to Kvarken, the narrowest part of the Bay of Bothnia, which is only 70 to 80 km wide. The post-glacially rebounding coastline of Kvarken is Finland’s first natural site on the Unesco World Heritage List. Post-glacial rebound continues to make Kvarken narrower and shallower, and within a couple of thousand years, the water connection is estimated to disappear completely. The deepest point of the Bothnian Sea is nearly 300 m deep, and located close to the coastline of Sweden.
An underwater threshold separates the Gulf of Bothnia from the actual Baltic Sea basin. Nutrient loads in the Bothnian Sea are lower than in the Gulf of Finland or the actual Baltic Sea, and on a Baltic Sea scale, the water can be said to be quite clean. The waters are also less stratified than in the actual Baltic Sea or the Gulf of Finland, and the status of seabeds is relatively good. Bladderwrack, one of the key species of the Baltic Sea’s underwater environment, is plentiful all through the Bothnian Sea. Bladderwrack is also an indicator species for a healthy aquatic environment, as one of the preconditions for thick bladderwrack forests to build up is that the water is clear enough. Underwater bladderwrack forests are an important environment for many smaller animals and fish fry of the Baltic Sea, and also double as an important spawning area for many fish species.
Species richness is similar to that of the Archipelago Sea, and water salinity does not become too low for marine species before the northern side of Kvarken, i.e. in the Bothnian Bay.