THE SEA OF AZOV 



473 



occurs usually in May and continues until August. This state may develop with 

 a catastrophic rapidity when conditions make vertical circulation difficult 

 (calm weather, considerable warming up of the upper lower salinity layer) 

 (Fig. 224b). 



This is assisted also by the saline stratification which is especially apparent 

 in the part of the Sea adjacent to the Kerch Strait : the more saline Black Sea 

 waters entering the Sea of Azov through the Strait lie in the bottom layer 

 where they are covered by the diluted waters of the Sea of Azov. 



In June 1937 A. Zhukov observed a 40 to 80 per cent oxygen saturation in 

 the bottom layers throughout the Sea, and in July phenomena of a very in- 

 tense suffocation developed in the bottom layer, from which oxygen dis- 

 appeared throughout most of the Sea. In August the situation became less 

 acute and the September gales broke down the established stratification and 

 the amount of oxygen near the bottom increased. Similar intense suffocation 

 phenomena were observed in 1946. 



Stormy weather mixes up the whole water-column and disturbs the strati- 

 fication. The table given by Knipovitch is a good illustration of this {Table 

 194). 



Table 194 



After calm weather 



After stormy weather 



Bottom zones exposed to frequent suffocation phenomena are the poorest 

 in benthos. Suffocation leads to a mass extinction of bentopelagic organisms ; 

 among fish some species of bullheads suffer most. 



For some areas of the Sea of Azov the bacterium Microspira aestuarii is 

 very characteristic; it is sometimes found in huge amounts reaching 56 

 million specimens per 1 g of soil. The total amount of bacteria can rise to 

 776 million specimens per 1 g of soil (in the Kazantip area). Sulphates are 

 reduced by this bacterium, while carbonates are formed in the process and 

 hydrogen sulphide is evolved ; this can also contribute to a loss of oxygen 

 content, since it is bound to be used for the oxidation of the hydrogen sul- 

 phide formed. Bacteria decomposing cellular tissues with the formation of 

 marsh gas (methane), which requires for its further oxidation large amounts of 

 oxygen, are important among the bacteria of the bottom of the Sea of Azov. 



Nutrient salts 



As regards nitrates, in July 1936 these were everywhere absent. They began 

 to appear in August and by the beginning of the autumn (September-October) 



