Kattegat, 0resund, Arkona Basin, Kvarken and Bothnian Bay: (HELCOM, 2012). Key habitats in the Atlantic region are kelp forest, smaller seaweed species, seagrass meadows, blue mussel beds and soft and sandy sediments. Entering the Baltic Sea, the large kelp species disappear (due to low salinity) leaving selected seaweed species, seagrass meadows, blue mussel beds and soft and sandy sediments as the most important habitats, with decreasing diversity along a decreasing salinity gradient (Fig.