As a consequence of the salinity gradient, the number of marine benthic species significantly decreases towards the northern Baltic Sea and inner coastal waters, and the dominance of freshwater species increases. As a result, benthic communities in the southwestern part of the Baltic Sea are much more diverse than communities further north. Benthic life is scarce or absent in many deeper basins below the halocline, particularly after longer periods without any saline water inflow, due to seasonal and permanent oxygen depletions. Hypoxia and anoxia in bottom waters of both deep and shallower areas lead to impoverished communities and altered foodwebs. The polychaetes Marenzelleria spp. are some of the most successful invasive species in the Baltic Sea. In many areas, these polychaetes dominate the composition of the benthic communities, and may even increase the functional diversity, as deep bioturbators influencing sediment nutrient fluxes.
