830 



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



group lives in the southern part of the Sea below 200 m ; it is an impoverished 

 deep-water plankton of the Pacific Ocean. 



There are considerable seasonal changes in the vertical distribution of 

 plankton of the southern Bering oceanic group (Fig. 421). The cold intermedi- 

 ate layer of the Bering Sea is not so pronounced as that of the Sea of Okhotsk 

 (its temperature is usually above freezing point) ; it has less influence on the 

 vertical distribution of plankton and does not separate to the same extent 



Fig. 420. Faunal grouping of zooplankton in Bering Sea in summer. 1 South 

 Bering Sea grouping ; 2 North Bering Sea oceanic grouping ; 3 West neritic grouping ; 

 4 East neritic grouping (Vinogradov, 1951). 



the surface and subsurface plankton. The main mass of plankton is retained 

 below the 200 m surface layer by a considerable fall of temperature in winter. 

 In spring and summer, when the surface layer is warmed, zooplankton moves 

 upwards, concentrating mostly in the uppermost 100 m for feeding and multi- 

 plication (Eucalanus bungii, Calanus tonsus and C. cristatus), frequently form- 

 ing a biomass of from 1,500 to 2,500 mg/m 3 in the 10 to 100 m layer ; the three 

 species of Copepoda mentioned constitute up to 90 per cent of the total bio- 

 mass in summer. 



A very intensive development of phytoplankton (up to 1 5 or 20 g/m 3 ) has 

 been recorded in the northern shallows in spring, while zooplankton is only 



