THE CASPIAN SEA 



557 



the layer rich in nitrates is linked with the horizon of the sharp fall of oxygen 

 content. Within the water column, at 200 to 400 m deep, nitrate nitrogen con- 

 tent fluctuates between 110 and 180 mg/m 3 ; below 600 m nitrates disappear. 



Fig. 268. The carrying out of nutrient substances by river waters in the northern 



Caspian (mg/m 3 ). A Phosphorus content by the end of August (Brujevitch and 



Ivanov) ; В Nitrates in front of the Ural delta in February and March (Brujevitch 



and Fedosov); С Silicon in September (Brujevitch and Ivanov). 



This is similar to the distribution of nitrates in the Black Sea, only the upper 

 zone of impoverishment is thinner there (above 50 m) ; it frequently contains a 

 considerable amount of nitrates. The lower limit of this zone is at 200 m. 

 Both Seas have similar amounts of nitrates. 



The Caspian waters are kept continuously enriched in nutrient salts by 

 river waters (Fig. 268). 



Phosphorus 



As in other seas the phosphates are completely absent from the upper layer 

 of the Caspian Sea in summer. In winter they are found in small amounts in the 

 upper layer, but not everywhere (up to 6 to 9 mg/m 3 ). From 100 m downwards 

 the phosphorus content increases to 60 to 80 mg/m 3 {Table 230 and Fig. 266). 



Table 230. Average amounts of phosphate phosphorus in mg/m 3 in the Caspian Sea 



