. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE BALTIC SEA 293 As in other brackish-water bodies the qualitative variety of the flora and fauna of the Baltic Sea is not large; nor are the indices of biological productivity high. Some individual, mostly euryhaline, members of the fauna of the adjacent fully saline sea basins frequently become very numerous. Biocoenoses with a few (mesomixed) or with very few (oligomixed) species are character- istic of such bodies. Many of the forms of the Baltic Sea sink to great depths, penetrat- ing into t
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE BALTIC SEA 293 As in other brackish-water bodies the qualitative variety of the flora and fauna of the Baltic Sea is not large; nor are the indices of biological productivity high. Some individual, mostly euryhaline, members of the fauna of the adjacent fully saline sea basins frequently become very numerous. Biocoenoses with a few (mesomixed) or with very few (oligomixed) species are character- istic of such bodies. Many of the forms of the Baltic Sea sink to great depths, penetrat- ing into the areas with a salinity lower than that of the North Sea, and the fresh-water forms move into areas of higher salinity. Sagerstrale has pointed out (1957) that Macoma baltica and Scoloplos armiger are encountered in the Baltic Sea down to 100 to 140 m (Hessle, 1924). Fucus vesiculosus, which does not go more than 5 m deep into the Kattegat, descends in the Baltic Sea to 10 to 12 m (Waern, 1952). Idothea baltica in the Gulf of Finland reaches a salinity of 3 to 4%0, but ceases at a salinity of 10 to 15%0 near the coast of Jutland (Jo- hansen, 1918). Fresh-water forms, on the contrary, enter much farther into the saline waters of the Baltic Sea. For example, Lymnaea peregra goes up to 10 to 11%0, while in Jutland it does not enter more than 5 to 7%0 (Jaeckel, 1950; Johansen, 1918). Many marine groups do not penetrate, or only penetrate in small numbers, into the Baltic Sea: Porifera, Actiniaria, Madreporaria, Octocorallia, Solenogastres, Scaphopoda, Pteropoda, Cephalopoda, Echinodermata and others. Plankton Qualitative changes of plankton from west to east. Plankton suffers a marked qualitative change as one leaves the Belt and Oresund and enters the Baltic Sea (Fig. 138).. Fig. 137. Composition of barckish-water population. / Marine, euryhaline fauna; IA Typical marine forms d veloping only slightly in brackish waters; IB Marine forms of greater mass development in brackish wat
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