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



a luxuriant population of poriferae, hydroids and bryozoans (about 160 species, 

 according to Derjugin). On the chief areas of soft mud bottom a clear pre- 

 dominance pertains to infauna consisting of comparatively few forms of bi- 

 valves (Astarte boreal/ s, A. elliptica, Yoldia hyperborea, Leda pernula, Macoma 

 calcarea, Dacrydium vitreum, Cardium ciliatum), polychaetes (Pectinaria hyper- 

 borea, Maldane sarsi) and echinoderms (Asterias lincki, Ophiocantha bi~ 

 dent at a). 



The census of the bottom fauna, carried out with the help of the Petersen 

 bottom-dredge, makes it possible to note the exact limits of this soft-mud- 

 bottom community, which embraces the central deep depression (Fig. 89). 

 The biomass of this community increases from east to west ; in the eastern 

 part of the sea the average biomass is 16-78 g/m 2 , in the central part 26-86, 

 and in the western part 61-23 g/m 2 ; that is, it increases 3-5 times. A consider- 

 able increase in the biomass of benthos is observed in the Gulf of Onega 

 (according to S. Ivanova, 1957), in the greater part of which the biomass of 

 benthos ranges from 100 to 500 g/m 2 , and in certain parts exceeds this range, 

 with a clear predominance of bivalves. The comparative significance of the 

 separate components of the community changes also, as is shown in Table 92, 

 from east to west. 



Table 92. Change in the composition of predominant forms in the mud-bottom community of 



the lower division of the sublittoral 



The basic role in this community belongs to the bivalves (34 to 64 per cent 

 of the biomass) ; the polychaetes are considerably less numerous (11 to 39 per 

 cent) and the echinoderms rank third (up to 20 per cent). 



The population of the pseudo-abyssal. The pseudo-abyssal zone, which occupies 

 the bathymetric part of the Sea (150 m), is characterized by its small amount 

 of light, absence of vegetation and feeble fluctuations of temperature (about 

 — 1 -4°) and salinity (about 30 per cent) and finally by its brown soils formed 

 of soft silty clay. The average biomass of this community is 20-6 g/m 2 , and 



