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



Table 195. Changes in the concentration of plant nutrients, mgjm % , in the Sea of Azov 

 {without the Gulf of Taganrog) in 1957 {by M. Fedosov and E. Vinogradova, 1955) 



* Bottom-samples in parentheses. 



the nitrate content in the water had risen to 30 mg/m 3 . In November it in- 

 creased to 60 and even to 90 mg/m 3 . A specially large amount of nitrates was 

 observed in the Gulf of Taganrog (up to 150 to 200 mg/m 3 ). 



In 1937 the nitrates accumulated during the winter were exhausted in a 

 short time by the spring bloom of phytoplankton. In April-May the nitrates 

 disappeared completely, remaining only in the middle part of the Gulf of 

 Taganrog in amounts of 80 to 300 mg/m 3 . By June-July 1937 there was a 

 small accumulation of nitrates, but in August and September they had again 

 disappeared from the whole area of the Sea. Even in the Gulf of Taganrog the 

 amount of nitrates fell to 8 mg/m 3 and only in the actual estuary of the Don 

 did it reach 100 mg/m 3 . The waters of the Don carry 500 to 700 mg/m 3 nitro- 

 gen in the form of nitrates. 



On the other hand the amount of ammonium nitrogen in the Sea of Azov is 

 exceedingly large, especially during the periods of the mass dying-off of plank- 

 ton; for instance, after spring bloom (up to 900 mg/m 3 ). This is obviously 

 connected with the decomposition of a large bulk of organic matter. The 

 picture of ammonia distribution is the reverse of that of oxygen. The amount 

 of ammonia nitrogen in seas usually appears as a few tens of milligrammes 

 per cubic metre of water, and it rarely exceeds 100 (Baltic and Mediterranean 

 Seas). Only in the deep part of the Black Sea does the amount of ammonia 

 nitrogen reach 1,000 to 1,200 mg/m 3 . 



Unlike the nitrates, the phosphates remain all the year round in the waters 

 of the Sea of Azov, although at times in small amounts ; only in the upper 

 layer may they be completely consumed {Table 195). In April to August 

 phosphorus in the upper layer is either absent or remains at a level of 4 to 12 

 mg/m 3 ; in the bottom layer it may increase from 30 mg/m 3 to 200 or 300 

 mg/m 3 in June, and then fall to 40 or 50 mg/m 3 in August. The usual phos- 

 phorus content in sea-water in winter is 20, 40, 60 and even 90 mg/m 3 . The 

 river Don contains from 50 to 150 mg/m 3 of elementary phosphorus. 



Silicic acid 



Silicic acid content is as much as 2,050 to 3,500 mg/m 3 in October and 



November ; it falls, as spring approaches, to an average of only 250 mg/m 3 in 



