Non-native animals in the Baltic Sea:: alteration of benthic habitats in coastal inlets and lagoons

Semi-enclosed coastal inlets are particularly interesting areas for studies on non-native species since they show steep gradients in physical environment, biological communities, pollution and intensity of human activities. Due to the ecotone effect their biota is constituted of a mixture of marine, brackish and freshwater indigenous and non-indigenous species. In comparison with offshore areas the coastal inlets seem to be better invadible, and the effects of introductions are more evident here. This paper presents results of a comparative study on non-native benthic species in the semi-enclosed water bodies of the Southern (Curonian and Vistula lagoons) and Northern Baltic (inner Archipelago Sea and Northern Quark, Gulf of Bothnia), which differ by their origin and present environment, scope of anthropogenic impact and level of euthrophication. These areas presently host at least 18 non-native benthic invertebrate species. The ecological role of these species is evaluated in terms of: (a) their relative abundance and biomass in bottom communities; (b) their `feeding/mobility' status and their ability to alter the physical/chemical environment of the ecosystems they invaded; (c) vacancy/occupancy of the niches before these species introduced. The comparative analysis shows that the non-native species have significantly altered ecosystems of the SE Baltic coastal lagoons, while their role in the northern coastal waters still is much less important. The invadibility of different types of the Baltic coastal lagoons and inlets is discussed on the basis of the present study.