554 



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



Caspian. In 1934 even at the greatest depths there was some 0-13 to 0-64 cm 3 /l. 

 of oxygen. According to N. M. Knipovitch's data (1914-15) for the Central 

 Caspian, at a depth of 700 m oxygen was disappearing and hydrogen sulphide 

 appearing (up to 0-3 to 0-4 cm 3 /l.). A smaller amount of hydrogen sulphide 

 was recorded in the Southern Caspian. In 1934 S. Brujevitch recorded some 

 small amounts of hydrogen sulphide (about 0-2 cm 3 /l.) in the Southern Cas- 

 pian at a depth of 700 m. 

 As in the Black Sea, the hydrogen sulphide is mainly the result of anaerobic 



SOUTHERN CASPIAN 



Fig. 264. Oxygen content (percentage saturation) of the Central Caspian in cross 



section (Brujevitch, 1934). 



reduction of sulphates due to the activities of bacteria of the Microspira type. 

 Moreover A. Pelsh (1936) discovered in the Caspian Sea a new group of 

 bacteria (Hydrogenthiobacteria) capable of synthesizing hydrogen sulphide 

 from solid sulphur and gaseous hydrogen. In contrast to that of the Black 

 Sea, animal life in the Caspian Sea penetrates to the greatest depths. 



Suffocation phenomena of the type found in the Sea of Azov have not been 

 recorded in the Caspian Sea. If they do exist in the Northern Caspian they are 

 probably local and limited ; this is confirmed by the absence from the Caspian 

 of zones of blackened shell gravel, so typical of the Sea of Azov. This is ex- 

 plained by the wide distribution of sand and large-grain soils in the shallows 

 encircling the Caspian depths, which indicates a sufficient aeration of the 

 bottom layer in shallow areas. It is different in the shallows with mud-accu- 

 mulations, where a very marked shortage of oxygen (4 to 20 per cent of satu- 

 ration) has been recorded at times. A mass accumulation of soft mud-beds 

 in protected regions and bottom hollows, however small, is due to abundant 



