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% , but ceases at a salinity of 10 to 

 15% 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 1 to 11% , while in Jutland 

 it does not enter more than 5 to 7% 

 (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. 1 37. 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 water ; 1С Marine forms weakly 

 connected with marine habitats, living 

 mostly in brackish waters; HA Typical 

 fresh-water forms, which penetrate into 

 brackish water; IIB Fresh- water forms 

 of greater mass development in brackish 

 waters; IIC Fresh-water forms weakly 

 linked with fresh-water habitats, living 

 mostly in brackish water; /// Typical 

 brackish-water forms alien to marine 

 and fresh-water forms. 



