THE CASPIAN SEA 



617 



S%„/ 2 3 4 5 В 7 8 9 10 11 12 13 14 2 4 6 8 10 12 14 16 18 20 22 24 26 28 m 



SALINITY DEPTH 



Fig. 290a. Distribution of benthos Fig. 290b. Distribution of benthos biomass 



biomass of Northern Caspian accord- of Northern Caspian according to depth 

 ing to salinity (Birstein, 1939). (Birstein, 1939). 



of sea-water. In calm weather a definite vertical stratification is observed in 

 this zone, since the fresh waters of the Volga and Ural flow over the saline 

 sea- water. When the processes of decomposition of the organic substances of 

 the sea-bed become intensive owing to conditions of vertical stratification, 

 oxygen is used for the oxidation of the soil and the bottom layer loses much 

 of its oxygen. This oppresses many benthic animals and the heap of wrack 

 becomes inhabited by euryoxybiotic forms which can live in water deficient 

 in oxygen. This problem was discussed above when dealing with brackish 

 water. 



In the Northern Caspian the main benthos biomass is found at depths of 

 12 to 16 m, whereas its average depth is only 6 m (Fig. 290b). The relationship 

 between the benthos biomass and the nature of the sea-bed is just as indica- 

 tive. It has been shown for many seas that on mobile hard floor (gravel, 

 shell gravel, large-grain sands) the fauna becomes scarce and sometimes dis- 

 appears. 



This, however, is not so in the Caspian Sea. Low biomass indices are found 

 on soft beds, situated chiefly along the 12 to 18 ft of wrack {Table 261). This is 



Table 261 



