560 



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



main zones of enrichment have the following sequence : silicic acid, nitrates, 

 phosphates. 

 L. A. Zenkevitch had suggested a little earlier (1932) a different vertical 



200 400 BOO 800 1000 1200 1400 1600 1800 2000 Si mp/m 3 

 < i 1 1 1 ■ 1 1 1 ■ ■ o/ 



0.0 K 5 1,0 1, 5 N(N0 2 )mg/m 3 



10 20 30 40 50 60 70 80 90 100110 12 P, N(N0 3 ) mg/m 3 





 100 

 200 

 300 

 400 

 <^500 



-to 



^600 

 700 

 800 

 900 



20 22 24 26 

 t° S%c,0 2 с cm 



Fig. 269. Distribution of main elements of the medium in Caspian Sea 

 waters. / Subzone of photosynthesis ; // Nitrites subzone ; /// Nitrates 

 subzone; / V Reduction subzone (Zenkevitch, 1947). 



division of the water column into zones for the Barents Sea based chiefly on 

 oxygen conditions (see Barents Sea). The two diagrams, however, can be 

 contrasted. 



Chemical conditions of the Northern Caspian 



Hydrochemical conditions of the Northern Caspian differ greatly from those 

 of the rest of this Sea because of its instability, its strong seasonal fluctuations, 

 and its greater dependence, owing to its shallowness, on winds and the 

 chemical properties of its soil. Slightly less than 300 km 3 of fresh water are 

 brought into the Caspian Sea each year by the rivers; rainfall adds about 

 18 km 3 , and about 100 km 3 is lost by evaporation, so that the annual gain in 

 fresh water is of the order of 200 km 3 , i.e. about one-fourth of the whole 

 volume of water of the Northern Caspian. The volume of winter ice in the 



