The open Baltic Sea foodweb is characterized by low vertebrate species richness as relatively few fish species are tolerant of its brackish water conditions. The fish fauna is typically characterized by a single predatory fish (cod) and its pelagic prey (herring and sprat), and also by three-spined stickleback in some areas. In contrast, the coastal foodwebs are more complex and species rich. In the late 1980s and early 1990s, the open central Baltic went through an ecosystem regime-shift due to environmental and anthropogenic changes, where cod biomass collapsed and that of sprat increased steeply. Simultaneously, changes were observed in the zooplankton composition. Due to its shallow nature, the benthic-pelagic coupling is an important mechanism in transferring energy within the Baltic Sea foodweb. The intensity of eutrophication in the Gulf of Bothnia is less than in other parts of the Baltic Sea, with a large part of the energy transferring to higher trophic levels coming from the microbial loop.
