THE CASPIAN SEA 583 



Zoogeographical situation of the Caspian Sea 



V. Sovinsky (1902) examined the typical Caspian fauna, which is fairly 

 markedly repeated in the Aral Sea and which abundantly populates, as we 

 have seen, the fresher parts of the Black and Azov Seas, and he had full reason 

 to distinguish a separate Pontic-Caspian-Aral marine zoogeographical pro- 

 vince consisting of two parts : ' The Black-Azov Seas part, which has retained 

 its Caspian fauna only in the freshened section ; and the Caspian-Aral one, 

 which kept its original fauna completely intact.' According to Sovinsky this 

 province is part of the Celtic-Boreal region. 



However, V. Uljanin (1871) justly pointed out the great preponderance of 

 Mediterranean fauna in the Black and even the Azov Seas ; thus the inclusion 

 of these Seas in one single Pontic-Caspian-Aral province is artificial. Der- 

 zhavin considered this problem in 1925 and came to the correct conclusion of 

 the existence of a Caspian zoogeographical brackish-water and fresh-water 

 province, but not of a Pontic-Caspian-Aral marine one ; he thus brought in 

 an important correction of principle into the appellation given by Sovinsky. 



Caspian fauna with its peculiar history of development and the complexity 

 of its origin from different sources could hardly be included in the Atlantic- 

 Boreal region. It seems more correct to consider it as a separate biogeogra- 

 phical unit, since we cannot relate it to any one marine zoogeographical region. 

 Thus we can assume that the Caspian fauna belongs to a separate brackish- 

 water region of partly marine, partly fresh-water origin. 



The micro-organism population of the Caspian Sea 



Micro-organisms probably play a much greater role in the Caspian Sea than 

 in many other bodies of water. Huge chemical processes take place here with 

 their assistance. Desulphurizing bacteria with a more or less strong reducing 

 effect are found in every bottom sample, as has been shown by A. Maliyants 

 (1933). They are as important here as in the Sea of Azov. Thick bacteria films 

 and whole coverings are formed in the upper layers of mud floors, in the more 

 or less enclosed shallows of the eastern shores and off the deltas of rivers with 

 deposits of organic matter carried there by the rivers. 



The chemical role of Caspian Sea micro-organisms has not yet been pro- 

 perly investigated ; however, some valuable data for the understanding of the 

 main bacterial processes, and, in particular, for their quantitative estimation 

 are given in the works of Voroshilova and Dianova. 



The decomposition of organic matter proceeds, especially in the accumula- 

 tion zones, by means of putrifying bacteria. In the middle part of the Kaidak, 

 for instance, their number rises to 1,000 to 2,500/cm 3 , whereas in the purer 

 waters of the Northern Caspian there are only 1 to 60 specimens/cm 3 . They 

 do not descend into the depths of the sea-bed. Ammonia and hydrogen 

 sulphide are the products of their (life) activity. Further decomposition of the 

 compounds (nitrification) proceeds under the action of the nitrate and nitrite 

 bacteria. Ammonium compounds are oxidized to nitrites in water, and to 

 nitrates in the soil, since the nitrate bacteria are absent from water. The 

 denitrifying bacteria, reducing nitrites and nitrates, are opposite in their 



