300 



BIOLOGY OF THE SEAS OF THE U.S.S.R. 



southern part of the Sea are much lower than those of the corresponding parts 

 of the Atlantic (Fig. 143). 



R. Kolbe (1927) similarly noted the stimulating effect of the slightly brack- 

 ish water on the development of fresh-water diatoms. A high concentration 



OCEAN 



(Г-ЗРЗОЧР 4О'-5(Г50'-б(Г ёРЖ BALTIC 

 LATITUDE SEA 



Fig. 143. Comparison of plankton biomass at 

 different latitudes in the Baltic Sea and the 

 Atlantic (Gessner). 



of nutrient matter must be considered the main factor conditioning the mass 

 development of these forms in low-salinity water. N. Tchougounov has 

 observed a similar phenomenon in the Caspian Sea opposite the Volga delta. 

 Some Arctic species as, for example, Goniaulax catenata, Achnanthes 

 taeniata, Fragilaria cylindrus, Melosira hyperborea and others break out into 

 intensive flowering in the cold springtime waters of the eastern and northern 

 parts of the Baltic Sea. 



Benthos 



Bottom vegetation — qualitative composition. The distribution of flora in the 

 Baltic Sea is wholly similar to the qualitative distribution of its fauna. Among 

 the vegetable organisms marine, true brackish, and fresh-water forms may 

 also be distinguished, and each of these groups includes euryhaline and 

 stenohaline representatives. 



The impoverishment of the flora owing to the lowering of salinity as one 

 moves from the North Sea to the Baltic is shown in Table 124, which is 

 copied from K. Hofmann (1940). A comparison of Tables 123 and 124 reveals 

 a much more intense qualitative impoverishment of the fauna than of the 

 flora. Many representatives of the green algae have an unusually luxuriant 

 group in the Baltic Sea. Among the brown algae some, like Pylaiella rupincola, 

 develop intensively there also. As a rule, however, sea algae do not grow pro- 

 perly in the Baltic Sea ; thus, for example, the large marine algae Laminaria 

 saccharina in the Arctic region grows to a size of only a few centimetres. The 



