The fissure fault in the Kosterfjord is 247 metres deep and reaches all the way out to the Atlantic Ocean. The deep fjord provides oceanic salinity (35‰) in the bottom water, and constant deep-water circulation.
In the depths of the Koster fjord there are animals typical for the oceanic continental slopes and the deep fjords of the Norwegian west coast. For example, different species of sponges and sea-pens, as well as lima mussels and reefs of Lophelia coral.
Other diverse habitats close-by include exposed rocky shores, narrow sounds with strong currents, shallow areas with sand and/or silt bottoms often supporting beds of eelgrass. The Koster archipelago is also home to a large population of harbour seals. About 6 000 marine species are found in the Koster area, of which more than 200 are not existing elsewhere in Swedish waters.
Kosterhavet Marine National Park, the first national park in Sweden with a marine focus, was inaugurated in 2009. One of the objects is to promote research and education regarding conservation and sustainable use of marine ecosystems.