GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 375 



during the Ice Age, as suggested by Sadovsky, or did they go through the 

 process of ' Atlantization ' in the Black Sea for a second time under the effect 

 of more stringent conditions of life? It is quite evident that both possibilities 

 must be considered. If the appearance of some characteristics can be easily 

 explained by the effect of Black Sea climatic conditions (size of the body, 

 thickness of the shell), others are easier to understand from the standpoint 

 of their relict origin (shape of the shell, sculpturing). 



Among the group variations through which different Black Sea species may 

 have gone besides the change of size, one may note, for example, the solidity 

 of the mollusc shells. G. Afanas'ev has shown (1938) that the Black Sea 

 molluscs have a lighter shell than those of the fully saline seas. The ratio of 



Fig. 180. Changes in benthos biomass (g/m 2 ) from west to east along the system of 

 southern bodies of water (Zenkevitch, 1947). 



the weight of the shells to that of the body for the Black Sea bivalves varies 

 from 0-95 to 4-5 per cent (average 1-8 per cent), while for molluscs of fully 

 saline seas it varies from 1-25 to 10-8 per cent (average 3-5 per cent). 



Changes of biomass from west to east 



The regular change in the intensity of the processes of biological productivity 

 from west to east is closely linked with the hydrological and hydrochemical 

 conditions of the southern European seas. A marked decrease of benthos 

 biomass, from a few hundred g/m 2 to some tens, is observed as we pass from 

 the Atlantic Ocean to the Mediterranean Sea through the Straits of Gibraltar ; 

 it reaches its minimum in the eastern part of the Mediterranean Sea (a few 

 g/m 2 ). Only in some places off the coast and at the mouths of rivers does the 

 biomass increase. In the Sea of Marmora the biomass is already greater ; it 

 reaches fairly high indices in the Black Sea (100 to 200 g/m 2 and more). In 

 the Sea of Azov the processes of biological production reach their maximum. 

 Farther east a decrease of productive capacities is again observed, less signifi- 

 cant in the Caspian Sea and more marked in the Aral Sea (Fig. 180). 



The Mediterranean Sea can be cited as an example of the least biologically 

 productive sea in the world ; the Sea of Azov, on the contrary, is the most 

 productive. 



The decrease of biomass in the Mediterranean Sea and its subsequent 



