ABSTRACT
The Baltic Sea has a short geological and ecological history. It was formed after the latest glaciation about 15 000 yr BP. The salinity of the Baltic Sea has since then altered between fresh water and saline water and the present condition with brackish water was formed about 2000 BP (Westman et al. 1999). Due to its narrow outlet and shallow sills, and the considerable river input of fresh water, the Baltic Sea may be classified as a large estuary. Because the Baltic Sea is a relatively young brackish water ecosystem, and because the environmental conditions are extreme with steep environmental gradients (low salinity, variation in temperature, water stagnation, etc.) (Leppäkoski & Bonsdorff 1989, Kautsky & Kautsky 2000), relatively few species have adapted to the Baltic conditions. Some infaunal species are of fresh water origin (for example Monoporeia affinis and Saduria entomon) and some are saltwater species adapted to lower salinity (for example Nereis (= Hediste) diversicolor, Pontoporeia femorata and Macoma balthica). Currently, new additions to the fauna (e.g. Marenzelleria viridis) also play an important role in the benthic system (Olenin & Leppäkoski 1999, Leppäkoski & Olenin 2000). Thus, the salinity gradient, which characterises the waters around Scandinavia, is an important factor structuring the faunal communities. Off the Swedish west coast the fauna is diverse both with respect to species richness and function. The Bothnian Bay, on the other hand, is characterised by a low diversity of two to three species (Rumohr et al. 1996, Bonsdorff & Pearson 1999, Laine 1999). Further information about the Baltic ecosystem and the benthic fauna can be found in Elmgren (1984) and Laine et al. (1997), and on successional stages and functional groups of the benthos in Rumohr et al. (1996), Bonsdorff & Pearson (1999), and Rosenberg (2001).
