Effect of Biological Invasions on Eutrophication of the Baltic Sea

The greatest number of discoveries of alien species in the Baltic Sea is confined to the most eutrophic areas - bays and inlets of the southern Baltic, the Neva River estuary. Many colonizing species (for example, the North American crayfish Balanus improvisus, Ponto-Caspian mollusc Dreissena polymorpha) also reach the greatest quantitative developments in eutrophic areas. The destruction of benthic communities in the eastern part of the Finland Gulf, because of the deterioration of the oxygen regime, provoked a massive biological invasion, in a result of which almost a monoculture resistant to the hypoxia of alien species polychaetes Marenzelleria arctia populated vast areas of deep-see zones. There are three groups of alien organisms, which actively influence the processes of eutrophication through alteration of biogeochemical cycles and / or structure of the food web. Thus, the results suggest a complex interaction of processes of biological invasions, and eutrophication. However, introduced materials show that very often the consequences of the new species' activity (biofiltration, aeration of sediments) may facilitate reduction of eutrophication and (or) its negative manifestations, and, in this aspect, it can be assessed as a positive event.