THE SEA OF AZOV 



509 



others an increase in the number of specimens with a decrease of biomass is 

 observed during the summer ; this is either caused by replacement of the older 

 age groups by the young, or is the result of a reduction of density and bio- 

 mass. 



The Corbulomya maeotica biocoenosis — This biocoenosis is adapted mainly 

 to depths of 1 to 6 m off the coasts. It reaches its maximum at depths of 

 2 to 4 m on pure sand with shell gravel or on slightly silty sand. In the spring 

 it occupies an area of 1,270 km 2 , but in the autumn only 819 km 2 , being re- 

 placed by Cardium edule which comes up from the deeper sector. The most 

 usual components of this biocoenosis are Nereis succinea, Ampelisca diadema, 



BOUNDARIES OF BIOCOENOSES / 



//933 f /334-35 fl936 



Fig. 239. Displacement from west to east of the boundaries of bottom biocoenoses 

 of the .Gulf of Taganrog under the effect of the rise of salinity in 1933-36 (Morduk- 

 hai-Boltovskoy, 1939). / Monodacna-Dreissena-Unionidae; II Monodacna; /// 

 Ostracoda-Hypaniola-Corophium-Tubificidae ; IV Nereis-Ostracoda ; V Cardium. 



Cardium edule and Syndesmya ovata, which form different quantitative com- 

 binations with Corbulomya. 



Many of the biocoenoses of the open parts of the Sea of Azov which have 

 been considered Hve also in the Utlyuksk inlet, where they undergo great 

 changes in their composition owing to a considerable admixture of Black 

 Seas relicts, which have survived there as a result of somewhat higher salinity, 

 and especially of such forms as Cerithiolium reticulatum, Pectinarianeapolitana, 

 Cardium exiguum, Rissoa euxinica, R. venusta and others. 



Seasonal and annual migrations of biocoenoses. A noticeable migration of 

 marine benthic biocoenoses eastwards into the Gulf, brought about by the 

 fluctuation of the outflow from the Don, was observed by F. Mordukhai- 

 Boltovskoy (1939) when he compared quantitative-biocoenotic data on the 

 Gulf of Taganrog benthos in 1933 with those for 1934-36 (Fig. 239). A con- 

 siderable loss of salinity in the Gulf in 1932 was caused by the abundance of 

 the Don spring outflow, which in previous years had been much lower and 



