616 



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



(average biomass— 11-28 g/m 2 ) which differs from the preceding one by the 

 absence of some species (Dikerogammarus haemobaphes) and a poor develop- 

 ment of Corophidae and Cumacea. This community lives at a depth below 

 1 1 m at a comparatively high and constant salinity (more than 9%„). Pandorites 

 podoceroids is its dominant form. 



The zone adjacent to the Central Caspian, with a salinity of 10 to 12% , 

 a hard sea bed and depths of more than 1 1 m, is populated by a typical mid- 

 Caspian fauna rich in its composition and biomass (an average of 124 g/m 2 ). 

 Its dominant forms are Didacna barbot-de-marnyi and Dreissena caspia, and 



Fig. 289. Diagram of distribution of benthos biocoenoses in the 

 Northern Caspian according to spring surveys 1947 to 1951 (Vino- 

 gradov). Biocoenoses: 1 River Dreissena ; 2 V. viviparus and other 

 fresh-water forms, low salinity and coastal forms; 3 Ural-Caspian 

 Dreissena ; 4 Adacna minima ; 5 Adacna costata ; 6 Adacna plicata ; 

 7 Oligochaetes, chironomids and crustaceans: brackish water; 8 

 Marine Dreissena; 9 Monodacna; 10 Didacna trigonoides: salt- 

 loving relict; 11 Marine Didacna; 12 Dreissena caspia; Mediter- 

 ranean ; 13 Nereis ; 14 Carditim edule ; 15 Mytilaster. 



some Monodacna sp., Mytilaster lineatus, Cardium edule and Didacna trigo- 

 noides are also found. 



The biomass distribution in the Northern Caspian shows a pronounced 

 drop at 2 to 8% salinity (Fig. 290a) ; this is due to the fact that only a few 

 fresh-water species can endure a salinity above 1 to 2% , while the Caspian 

 autochthonous species ready to Uve at a salinity below 8% are rare. Hence 

 within the zone of a salinity of 2 to 8% life is poor both in number and in 

 variety. The same phenomenon occurs in the zone where fresh- and sea- 

 waters mix in all seas. 



Here, however, apart from salinity the gas conditions of the bottom layer 

 are also of great importance. Salinities of 2 to 8% are found within the zone 

 of the 12 to 18 ft heap of wrack. The soft soils here owe their origin to the 

 deposition of suspended particles under the effect of the coagulative action 



