THE BLACK SEA 389 



As is shown in Table 151, containing data at great depths taken at one of 

 the stations, the annual fluctuations of temperature and salinity affect only 

 the 150 m upper layer, while deeper down they remain practically constant 

 throughout the year, the temperature being between 8° and 9° and the salinity 

 a Kttle above 22% . The difference in salinity between the surface and deep 

 waters reaches 4 or 5% . 



Oxygen and hydrogen sulphide 



In the Black Sea the amount of oxygen decreases sharply with the depth, 

 while that of hydrogen sulphide increases starting at 150 m; this is shown in 

 Table 152. 



Table 152 



As in other seas, the maximum oxygen content is at a depth of 25 m (up 

 to 124-133 per cent). Moreover, its supersaturation is regularly observed ; this 

 is the result of phytoplankton activity. 



One of the most striking peculiarities of the Black Sea is the very great 

 amount of hydrogen sulphide which contaminates its depths. As early as 1892 

 the chemist A. Lebedintzev, a member of Andrussov's expeditions, the first 

 to investigate the phenomenon of hydrogen sulphide fermentation in the 

 depths of the Black Sea, expressed an opinion on the existence of two sources 

 of hydrogen sulphide, in both cases formed as a result of intensive bacterial 

 activity. 



B. Issatchenko, during his microbiological investigations of the Black Sea 

 (1924), discovered bacteria responsible for the formation of hydrogen sul- 

 phide in both ways. The bottom dwelling bacteria of the genus Microspira 

 (mainly M. aestuarii) are the main source of hydrogen sulphide ; as a result of 

 their vital activity sulphates are reduced, carbonates are formed, and hydro- 

 gen sulphide is liberated. According to P. Danilchenko and N. Chigirin 

 (1926) 99-4 to 99-6 per cent of the whole of the hydrogen sulphide in the 



