Fehmarnbelt connects the world’s largest brackish sea, the Baltic Sea, with the Danish straits and the ocean. Each year millions of cubics of brackish water from the Baltic Sea flow through Fehmarnbelt. At the same time saline water from the North Sea enters the Danish straits. When the brackish and the saline water meet, the light brackish water is forced to the surface by the heavier saline oceanic water, thereby creating a very dynamic marine environment with strong currents, and a marked depth gradient in salinity, temperature and nutrient availability.

The shallow coastal areas of Fehmarnbelt are ecosystems in constant change. Plenty of sun light makes life flourish but plants and animals need to be resistant. Light might bring life but the dramatic annual temperature fluctuations are a challenge to all living organisms. The belt is orientated west to east and the strong westerly winds are free to rage and storms can change the living conditions over night. 

The deeper parts of Fehmarnbelt are dark and only a little or no light at all will reach the bottom. Enormous mud flats cover the seafloor as a silent desert, only interrupted by oasis of stone reefs with macro algae reaching out after the sparse light. But because of the saline oceanic bottom water, these mudflats are rich in life. Burrowing worms, mussels and sea urchins inhabits the muddy environment and feed on the remains of plankton and algae produced in the sunny environment above.

Going under the surface in Fehmarnbelt, one will find impressive landscapes of extensive blue mussel beds and eel grass and macro algae meadows. The strong currents are a key stone for these habitats. It ensures a sufficient delivery of free floating micro algae and crustaceans to the filtering mussels, and nutrients to the oxygen producing plants being essential to all life in the sea.

At the surface, this dynamic environment is inhabited by seals, harbour porpoises and water birds, and watchful observers may spot these animals from land or ships sailing through the area

Birds, fish and marine mammals depend on this rich underwater ecosystem.

The mussel beds, west of Fehmarn is a very important feeding area to hundreds of thousands of over wintering sea ducks. Common eider, tufted duck and common scoter forage in the area and each bird can consume up to 1.5 kg of blue mussels per day.

The fish fauna in Fehmarnbelt is dominated by three species, herring, cod and sprat. Herring is the most common fish in Fehmarnbelt. It constitutes up to 40% of the entire fish biomass and feeds on microscopic free floating crustaceans. Two spawning stocks of herring are present in the Fehmarnbelt, the autumn spawning herring in low and vulnerable population size and the more common spring spawning herring. At the bottom, the fish fauna is dominated by cod feeding on mussels, crabs and herring, but also plaice and dab are common at sand flats.

Fehmarnbelt is an interesting area for marine mammals. The Rödsand lagoon harbour the most southern site used by harbour seals which haul out on the sand. In wintertime, large grey seal is found here, in some years even with pups. The male grey seals can reach a maximum weight of nearly 300 kg.  The whole Fehmarnbelt is inhabited by harbour porpoises. Harbour porpoises occur year round in the area, but are assumed to be most numerous during the summer months.

The harbour porpoises, the harbour seals and the grey seals mostly feed on fish. Cod and herring are on the menu, but also squids are a delicious meal in the eyes of a seal or harbour porpoises.