666 



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



21. 8ф 



SANDY SILT ZONE 



(Fig. 315, 5) which forms a biomass of 20 g/m 2 in the Bol'shoe More and of 

 58-8 g/m 2 in the Maloe More. In the western depression on black mud with 

 Vaucheris at depths of 28 to 60 m, Dreissena is predominant with a biomass 

 of 12 to 18 g/m 2 (Fig. 315, 6), but at depths of 40 to 60 m the biomass drops 

 to a few tens of milligrammes per 1 m 2 . In the Syr- and Amu-Daria estuarine 

 zones Adacna biocoenosis with an average biomass of 5-290 g/m 2 develops 

 on brown soils at depths of 4 to 10 m. Dreissena polymorpha in combination 

 with chironomid larvae and to a lesser extent with Adacna is the main form of 



the bottom-fauna of the Aral Sea. 

 27.6 z/m* _. . _ Pontogammarus aralensis lives in 



large numbers on the sands of the 

 shore. 



In reality the bottom of the Aral 

 Sea is populated by one biocoenosis, 

 the Dreissena (Fig. 316), with differ- 

 ent variations according to the type 

 of soil. 



This biocoenosis is most clearly 

 marked on grey muds. At times 80 

 per cent of the whole population is 

 composed of Dreissena. Sometimes 

 chironomid larvae are predominant, 

 constituting 60 per cent of the total 

 benthos. On sands Dreissena and 

 Adacna (on the average more than 

 97 per cent) are predominant; the 

 Chironomus and Gammaridae larvae 

 are found only as single specimens. 

 Nearer to the coast and off it in the 

 vegetation beds of the bays and inlets 

 there is a particularly large number 

 of Pontogammarus aralensis. Behning points out that 'after a gale one can 

 frequently observe along the coast whole strips of wrack consisting entirely 

 of these small-sized crustaceans. They are always numerous too among the 

 sea grass cast up on the shore.' 



Bottom-life is scarce in the deeper part with black muds and hydrogen 

 sulphide, and in some areas it may not be found at all. 



In the shallower areas covered by black mud (40 to 50 m) benthos is poorly 

 developed and is represented mainly by Nematoda, Oligochaeta and Ostra- 

 coda. 



Benthos is richest in shell-gravel areas where, according to Behning, it 

 reaches 40 g/m 2 . Bottom-life is scarce or even completely absent opposite 

 the mouths of both great rivers and, like that of the black muds, it is repre- 

 sented here by Nematoda, Oligochaeta, Ostracoda and chironomid larvae. 



A survey of the distribution of bottom fauna in the Adzhibai inlet (south- 

 western corner of the Sea) was undertaken by P. Dengina in 1957. The 

 salinity of the Sea (measured by chlorine) varies from 4% at the end of the 



GREY OOZE ZONE 

 OF BOL'SHOE SEA 



Dreissena 



fJdacna 



Caidlum 



VA/A/A Chnonomidae 



III! Jam ma 1 1 dae 



WW Phiyganldae 



Fig. 316. Composition of bottom-living 



population of Aral Sea (Nikitinsky) : 



Left: On grey ooze zone; Right: On 



sandy silt zone. 



