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



the bottom, are concentrated in the western, deeper part of the Sea; the 

 formation of hydrogen sulphide is facilitated by their presence. 



River mouths are characterized by brown clay mud. 



Sand floors, passing over into mud beds at depths below 10 m, form a wide 

 band along the northern, eastern and southern coasts of the Aral Sea. 



According to K. Gilzen's data (1908) the Aral Sea bed contains very little 

 organic matter. The carbon content recorded at 15 stations fluctuates from 

 0-07 to 0-43 per cent. Its nitrogen content was determined at two stations as 

 00 187 and 0-0068 per cent; the ratio C/N was 6-7 and 10-6 at these places. 



The western and southern shores of the Aral Sea are flat and low (V. Zenko- 

 vitch, 1962). For the most part they consist of the deltas of great rivers over- 

 grown with bullrushes : the Amu-Dar'ya (to the south) and the Syr-Dar'ya. 

 In a wide area between them shores of a specific kind have developed, called 

 'Aral' type by L. Berg (1908). Owing to a slight rise in its level the Sea in 

 this area has entered some troughs between banks of the adjacent wind- 

 borne desert sand, thus creating a very broken coastline. This latter is being 

 slowly levelled out by the effect of the waves. 



The western shore is almost straight and consists of the steep clunka 

 escarpment (up to 200 m high) of a faulted structure origin which has by now 

 been greatly broken up by landslides. Throughout its length this coast is 

 steep (V. Lymarev, 1957). 



The northern coast has characteristic laminated contours, with a few large 

 islands and peninsulas. These shores are not high; they consist of loose 

 Quaternary deposits. They are intensely abraded, forming small local currents 

 of alluvium. 



Transparency 



The waters of the Aral Sea are for a lake exceptionally transparent : in the 

 western part of the Sea a white disc ceased to be visible at a depth of 24 m. 



Currents 



L. Berg was the first to note (1908) that the currents of the Aral Sea, in con- 

 trast to those of other inland bodies of water, move clockwise (Fig. 303) ; 

 thus the Amu-Dar'ya waters spread northwards throughout the western part 

 of the Sea, and the Syr-Dar'ya waters southwards throughout the eastern 

 part. The surface layer, however, is controlled in its movements by the pre- 

 vailing winds. Owing to the shallowness of the Sea and to its low coastline 

 the phenomena of strong on- and off-shore winds sometimes occur to a 

 marked degree. 



In August, the warmest time of the year, the average temperature of the 

 surface waters of the open part of the Sea is 24° to 25°, while in the depths of 

 the western depression it is 2-3°. In June, however, the average temperature 

 near the bottom falls to 0-3° (Table 278). The layer of sudden change usually 

 occurs at a depth of 16 to 28 m, while in the shallow eastern part the water is 

 warmed down to the bottom and the layer of sudden change is not found 

 {Table 278). 



