556 



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



SOUTHERN CASPIAN 



Fig. 266. Distribution of phosphate phosphorus in the Caspian Sea (mg/m 3 ) in 

 autumn in cross section (Brujevitch, 1934). 



column of water. Deeper down the nitrites are absent. Nitrites are accumulated 

 in the summer at a depth of 50 to 100 m with the development of phyto- 

 plankton and the establishment of temperature stratification. Below 100 m the 

 content of nitrites gradually decreases, and below 400 m it falls to zero. 



Unlike ammonia and nitrite nitrogen, nitrate nitrogen gives an original 

 picture of distribution different from that of other seas. Intensive accumu- 

 lation of nitrates proceeds at 100 to 600 m (mainly at 200 to 400 m). Above 

 100 m, within the zone of intense vertical circulation and the consumption 

 of plant nutrients, the nitrate content is either very low (in winter in the Central 

 Caspian 5-10-15 mg per 1 m 3 ) or absent (in the summer). The lower limit of 



SOUTHERN CASPIAN 



Fig. 267. Distribution of silicon in the Caspian Sea (mg/m 3 ) in autumn in cross 

 section (Brujevitch, 1934). 



