THE SEA OF AZOV 475 



March. In April and May its content falls to 50 mg/m 3 in the surface layer and 

 to 100 mg/m 3 in the bottom one. These sharp fluctuations in silicic acid 

 content should be attributed to its being consumed by plankton diatoms. 

 During the autumn dying-off of plankton some silicic acid appears in the 

 water again. 



Thus the most typical features of the chemical conditions of the Sea of 

 Azov are due to an abundant discharge of detritus and plant food by the river 

 Don, which ensures an exceptionally intensive development of plankton and 

 benthos life. 



Chemical characteristics 



However, a good supply of oxygen is required for the development of life 

 on this scale and for the oxidation of huge amounts of organic substances. 

 Since in the Sea of Azov the process of aeration is at times, and in the region 

 of the Straits always, impeded by salinity and temperature stratification, 

 catastrophic suffocation of the benthopelagic fauna may occur, accompanied 

 by an accumulation of large amounts of ammonia in the bottom layer. Two 

 maxima of nitrate and phosphate accumulation are observed during the year, 

 with at times a complete consumption of nitrates in April-May and August- 

 September. Owing to the shallowness of the Sea of Azov, large amounts of 

 phosphates and silicic acid can accumulate on the bottom; they may also 

 be dissolved in the water. 



All aspects of the chemical conditions of the Sea of Azov are to a great 

 extent determined by the course of phytoplankton development, both by its 

 increased multiplication and its dying off. The oxidation conditions, the 

 accumulation of ammonia, the phosphorus, nitrogen and silicon cycles all 

 reflect the various phases of plankton development, especially because of the 

 shallowness of the Sea. 



The soils 



A diagram of the distribution of soils appears in Fig. 225a. There are few 

 rocky shores in the Sea of Azov, and these are mainly situated on the southern 

 coast. A wide band of sands with a smaller or larger admixture of shell 

 gravel encircles the central part of the Sea and of the Gulf of Taganrog, 

 occupied by silty mud and shell-gravel mud. In the southern part of the Sea 

 this band of sands is narrower than on the other shores, and the mud bottom 

 approaches the coast more closely. The proportion of fines (less than 0-01 mm) 

 in these muds reaches 30 to 50 per cent ; in the silty muds of the central part 

 of the Sea it is always in excess of 50 per cent. The deepest parts of the Gulf 

 of Taganrog, beginning from a depth of 3-5 m, have a soft mud floor, with a 

 characteristically large number of Ostracoda shells. F. Mordukhai-Boltov- 

 skoy (1937) thinks that these muds might be called Ostracoda muds (up to 

 50,000 or 100,000 and more live Ostracoda per 1 m 2 ). Sand stretches in a more 

 or less narrow band along the coast, entering deep into the Gulf only with 

 shoal heads. Vast areas of the bottom, especially in the part of the Gulf 

 farthest west, are occupied by mixed mud, sand and shell gravel. 



