THE CASPIAN SEA 



545 



cause, as we shall see below, considerable changes in the phenomena of bio- 

 logical productivity. 



According to Brujevitch's computations (1938) the level of the Caspian Sea 

 would remain practically constant with an average Volga inflow of 257 km 3 . 

 The level would inevitably drop with a decrease in the inflow, and would rise 

 with an increase (Fig. 258). 



Fluctuations of the level of the Caspian 

 Sea are complicated by seasonal changes ; 

 during the first half of the summer after 

 the floods, the level is at its highest, and 

 it is at its lowest at the beginning of 

 winter. 



Currents 



The movements of the water masses of 

 the Caspian Sea, like those of any other 

 sea, are expressed in a system of vertical 

 and horizontal displacements due to 

 different causes. The Caspian Sea is 

 encircled by a large cyclonic current, 

 forming two powerful halistatic areas in 

 the Southern and Central Caspian (Fig. 

 259). The speed of this current along the 

 western side of the Central Caspian may 

 reach, according to Stokman (1938), 20 

 cm/sec. On the approach to the shallows 

 of the Northern Caspian the main mass of 

 waters,' moving from the south along the eastern shore of the Sea, turns to 

 the west and, farther on, to the south, receiving the main mass of the dis- 

 charge waters of the Northern Caspian. Part of these waters passing the 

 Mangyshlak peninsula is diverted into the Northern Caspian. 



There is a separate cyclonic current in the southern part of the Southern 

 Caspian with its own halistatic area in the centre. Part of the waters moving 

 southwards along the western side of the Central Caspian runs away from the 

 western shores, at the latitude of the Apsheron ridge, and passes to the eastern 

 side. The Volga waters move south partly along the western coast, partly 

 directly east, creating two anticyclonic gyrations : one to the northwest from 

 the northern end of Kulaly Island, the other to the northeast over the Ural 

 trench. The existence of a circular movement of waters over the Ural trench is 

 confirmed by the accumulation of soft silty deposits in the trench, by the 

 presence of hydrogen sulphide, and by the absence of hydrogen sulphide in the 

 sea-bed encircling the trench as a result of the washing-out effect of the cir- 

 cular current. In the Northern Caspian, however, especially in summer, the 

 picture of the permanent currents is changed by strong winds owing to the 

 shallowness of the Sea, and by on- and off-shore winds (A. Milkhalevsky, 1931). 



According to N. Gorsky (1936) the system of the winter under-ice currents 

 of the Northern Caspian differs greatly from what has just been described. 



2м 



Fig. 259. Currents of the Caspian 

 Sea (A. Mikhalevsky, 1931). 



