THE BLACK SEA 



395 



amount of nitrogen in the photosynthetic zone of the ordinary sea is either 

 zero or very small ; it increases, however, with depth. 



In the depths of the oceans the amount of nitrogen in the form of nitrates 

 usually does not exceed 006 to 0-07 mg/lb ; at the surface it may rise to 0-1 1 to 

 0-16 mg/lb. The amount of nitrate nitrogen in the seas is usually expressed in 

 microgrammes per pound. 



The ammonia content of the upper layer of the Black Sea is also practically 

 the same as that of the open seas and oceans ; it increases, however, with depth, 



1000 2000 3000 4000 5000 6000 YEARS 



Fig. 190. Reconstructed course of the alteration of salinity in the Black Sea after 

 the break-through of the Bosporus waters (Wodjanitzky, 1948): A Salinity at sur- 

 face ; В Mean salinity. 



and at 1,500 to 2,000 m the amount of ammonia nitrogen is 1-10 to 1-46 

 mg/lb. 



The content and distribution of nitrates and phosphates in the Black Sea 

 were first investigated by Danilchenko and Chigirin in 1929 and 1930. Twenty 

 years later their work was repeated by V. Datzko, and considerable deviations 

 from the earlier data were found. Lately M. Dobrzanskaja (1958) has investi- 

 gated the distribution and changes of phosphates throughout the Black Sea 

 column of water. This author notes the frequent absence of phosphates from 

 the upper region of the water (50 to 60 m) in spring and summer, although in 

 some years phosphates are present throughout the year in the upper layer of 

 the whole Sea during the periods of marked deficiency of nitrates. In some 

 areas of the Sea there is a pronounced increase in phosphate content as a 

 result of the off- and on-shore winds and the phenomena resulting from them. 

 Within the halistatic areas the phosphate iso-surfaces rise, and off-shore they 

 sink, with fluctuations of 50 to 100 m {Table 156). 



