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



The Caspian benthos is distributed into definite zones; moreover widely 

 eurybathic forms and groups such as, for example, Hypania invalida (0 to 

 900 m), Oligochaeta (0 to 400 m), Chironomidae (0 to 400 m), Dreissena 

 grimmi (5 to 300 m) and some others may be noted. 



The vertical distribution of benthos agrees best with Brujevitch's ' structural 

 zones'. The best conditions for benthos development are found in the photo- 

 synthetic subzone with its rich plankton and good aeration ; and in fact at a 

 depth of 1 5 to 25 m the biomass is at its maximum (up to 1 ,200 g/m 2 ). Feeding 

 conditions deteriorate in the nitrite zone and the biomass falls to 70 to 150 

 g/m 2 , and even less at the lower limit of this zone. Benthos biomass is very 

 low in the accumulation zone, with an increasing shortage of oxygen and 

 foodstuffs (often only a fraction of 1 g/m 2 ) (Fig. 288). 



Qualitative and quantitative distribution of benthos. The first survey of the 

 distribution of bottom-living biocoenoses in the Northern Caspian was given 

 by N. Tchugunov. He was the first worker in the U.S.S.R. to use a grab 

 for the study of marine fauna (1923). Ya. Birstein altered Tchugunov's data 

 and added some new ones. A more comprehensive picture was given by 

 L. Vinogradov (1955). 



The biocoenosis of Dreissena polymorpha, Unio pichorum, Viviparus vivi- 

 parus, Pandorites platycheir, Metamysis strauchi and the much rarer Volgo- 

 cuma thelmatophora and Limnaea ovata is settled in the mouth of the Volga 

 and partly along the western coast of the Northern Caspian (Fig. 289). This 

 biocoenosis is adapted to low salinity (2 to 3% ), strong currents, a hard sea- 

 floor, small depths and an abundance of nutrient substances. The biomass of 

 this biocoenosis is sometimes as high as 200 g/m 2 owing to the numerous 

 large fresh-water molluscs and Dreissena. 



The biocoenosis Monodacna caspia, Dreissena polymorpha, Adacna plicata, 

 Chironomidae, Oligochaeta, Corophiwn nobile, С chelicorne, C. monodon, 

 Pterocuma sowinskyi, Pt. pectinata, Schizorhynchus bilamellatus, Gmelina 

 pusilla, Stenoganvnarus similis, S. compressus, Cordylophora caspia and some 

 others extends as a wide band from Agrakhansk Bay to the Ural River. 

 Fresh-water forms are not found here. This biocoenosis is settled on a soft 

 sea-floor, in areas with unstable saline (3 to 7% ) and gas conditions, and in 

 shallow depths (2 to 8 m). Although the biomass of this biocoenosis is fairly 

 low (12 g/m 2 ) the area is the feeding ground of a number of commercial fish 

 (vobla, golden shiners and others). 



The remaining part of the Northern Caspian, except for the Ural Trench 

 and the transition zone to the Central Caspian, is occupied by the hard-sea- 

 bed biocoenosis, adapted to depths of 8 to 12 m and a salinity of 5 to 9% , 

 with Didacna trigonoides as a dominant species. Among the other forms 

 Monodacna caspia, Dreissena polymorpha, Dr. caspia, Adacna plicata, Theo- 

 doxus pallasi, Niphargoides caspius, N. corpulentus, Corophium chelicorne and 

 Dikerogammarus haemobaphes are most developed here. The average biomass 

 is 28 g/m 2 . Ninety-five per cent of the total biomass here is composed of mol- 

 luscs, whereas in the previous biocoenosis they formed only 86 per cent. 



The soft soils filling the Ural Trench are inhabited by a small community 



