558 



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



Seasonal fluctuations in phosphorus content in the upper layer of the Cas- 

 pian Sea are very small — in the Southern Caspian about 1 mg/m 3 , in the Cen- 

 tral Caspian up to 4-5 mg/m 3 ; these data are commonly much higher in other 

 seas : in the Barents Sea phosphorus is up to 22 mg/m 3 , in the Channel up to 

 18 mg/m 3 . 



Silicon 



As in other seas the quantity of silicon remains very high in the upper layers 

 of the Caspian all through the year {Table 231). 



Table 231. Average amounts of silicon in mg\m z in the Caspian Sea 



There is more silicon in the upper layer in winter, while in the summer the 

 largest quantity is found at a depth of 50 to 100 m; this is connected with 

 the development of plankton and its regeneration in a deeper layer from the 

 sinking dead plankton. Seasonal fluctuations in silicon content in the upper 

 layer are similar to those observed in the Barents Sea and the Channel (about 

 100 mg/m 3 ). 



Vertical zonation 



S. P. Brujevitch has established a definite vertical zonation of the Caspian 

 Sea waters on the basis of his comprehensive study of the hydrochemistry 

 of the Sea; it is related mainly to the distribution of plant nutrients. 

 Brujevitch (1938) calls it structural zonation {Table 232). 



Table 232 



Zones 



Depth, m 



Subzones 



Depth, m 



I. Zone (impoverishment) 



of consumption of plant 



nutrients 0-100 



II. Zone (aggregation) of 



accumulation of plant 



nutrients Below 100 



IA Photosynthesis 



IB Nitrites 

 IIA Nitrates 

 IIB Reduction 



0-25 (50) 

 50-100 

 100^00 (600) 

 Below 400 (600) 



