THE CASPIAN SEA 



621 



The Nereis biocoenosis (L. Vinogradov, 1953) has not replaced, and could 

 not have replaced, the biocoenosis of small Adacna, higher crustacean and 

 chironomid larvae. Only in one place (Tyuleni Island), forming 1 -8 per cent of 

 the whole area, has the Nereis biocoenosis taken the place of an oligochaete 

 biocoenosis, but it formed a biomass there two to seven times (in different years) 

 greater than the oligochaete biocoenosis. The examination of benthos 

 throughout the whole Caspian Sea carried out in 1956, 18 years after the 1938 



Fig. 291 . Quantitative distribution of benthos in Central 

 and part of the Southern Caspian in 1956 (g/m 2 ) 

 (Romanova, 1960). 



survey (N. Romanova, 1960), has revealed considerable changes in the nature 

 of the distribution of the bottom-living fauna (Fig. 291). The eastern shores 

 of the Northern and Central Caspian are richer in benthos than the western 

 ones. The main mass of benthos is formed by Mytilaster, with a pronounced 

 decrease in the amount of Dreissena, Didacna, Monodacna and Adacna (63 

 per cent of total biomass in the Central Caspian, and 94 per cent in the 

 Southern). The part played by Cardium is increased, and Nereis is strongly 

 developed (Fig. 292) on soils rich in organic matter (Fig. 293). In its central 

 parts the Sea is deeper than 200 m and the benthos biomass there falls below 

 1 g/m 2 . Total benthos biomass reaches its maximum at depths of 10 to 25 m. 

 In the Southern Caspian at depths of 200 to 300 m the biomass decreases to 



