Quantitative distribution of sublittoral plant and animal communities along the Baltic Sea gradient  <B>Quantitative distribution of sublittoral plant and animal communities along the Baltic Sea gradient</B><P><B>Hans Kautsky</B><I><P> Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden</I><B><P> Abstract</B><BR> In the Baltic Sea, the species composition and biomass of the phytobenthos seems to be largely determined by different environmental vectors, and to a less extent by biotic interactions. In comparison to marine environments, the Baltic Sea is relatively species-poor and the broader niche of the remaining species has been taken as an indication that biotic interactions are less important here. The large-scale spatial differences in species composition and dominance are mainly caused by differences in salinity and climate of the subareas. Within a given area species composition is mainly controlled by type of substratum and wave exposure. Within a given site, besides the type of substratum and wave exposure, depth (i.e. light attenuation, decreased water movement, increased siltation, etc.) is also a major structuring factor. Fairly accurate predictions of the plant communities expected at a locality can be made from looking at sea charts, where depth and type of substratum are indicated, and from which wave exposure and salinity can be estimated. Within a given depth, species interactions may be of structuring importance, but due to the low diversity of the Baltic Sea, there are only a few possible community types. The most important type of species interaction to occur is probably competition for space.<BR> There is a shift in species composition and dominance of functional groups in the Baltic Sea gradient. The biomass composition of plants changes from predominantly red and brown algae in the Baltic proper and in the Bothnian Sea, to green algae and phanerogams in the Bothnian Bay. The plant species adapt to the harsher environments of the north by changing their growth form. Perennial plants dominate in the Baltic proper whereas annual species are common in the Bothnian Sea and totally dominate in the Bothnian Bay. In the Bothnian Bay salinity has decreased to around 0.2 psu, and ice cover lasts 100-150 days per year.<BR> In the Baltic proper, animal biomass is totally dominated by the filter feeder <I>Mytilus edulis</I> comprising over 90% of the dry weight. Excluding <I>Mytilus</I>, animal biomass in the Baltic proper and in the Bothnian Sea is dominated by deposit feeders (<I>Macoma balthica</I> and Hydrobiidae), whereas in the Bothnian Bay grazing snails dominate the biomass. The dominating filter-feeding communities of the Baltic proper are replaced by detritivores and grazers in the Bothnian Sea and Bothnian Bay, respectively.<I><P> Keywords</I>: sublittoral, subtidal, environmental factors, trophic groups, function, Sweden, Bothnian Sea, Bothnian Bay, scuba, phytobenthos.<P>