THE BLACK SEA 383 



Water balance 



The Black Sea total water balance comprises the following elements: the 

 annual river inflow of fresh water is 400 km 3 , most of this being Danube 

 water (203 km 3 ) ; the Dnieper and Bug inflow is only 54-7 km 3 and that of 

 the Dniester 8-4 km 3 . A surface current of Azov waters of lesser salinity runs 

 into the Black Sea through the Kerch Strait diluting the northeastern corner 

 of the Sea, while the more saline Black Sea waters (17 to 17-5% ) enter as a 

 deep current the area of the Sea of Azov off Kerch. Black Sea waters of about 

 13% salinity enter the Sea of Marmora as a surface current through the 

 Bosporus (348 km 3 annually), while a deep reciprocal current of saline Sea 

 of Marmora water enters the Black Sea (202 km 3 per year), running down the 

 slope of the floor off the Bosporus. 



Currents 



As in every other sea the main current of the Black Sea has a counter- 

 clockwise circular motion (Fig. 184). In the narrowest part of the Sea, be- 

 tween the Crimean Peninsula and a spit running out from the Anatolian coast, 

 part of the waters moving from the west go north and the Sea is thus divided 

 as it were into two parts, each with its own circular motion. In each of these 

 circular currents is formed its own halistatic area. In the course of the current 

 all the isolines go down while in the halistatic areas, in contrast, they rise in a 

 cupola-shaped pattern. 



Important additions to this system were introduced by N. Knipovitch 

 (1932), E. Skvortzov (1929), V. Nikitin (1929), A. Dobrovolsky (1933) 

 and G. Neumann (1942). In the eastern part of the Sea there is not one 

 but two halistatic areas, divided by a current running approximately in the 

 direction Samsun-Tuapse. In the most eastern part of the Sea, in the Batum 

 area, there is another circular current, but here, contrary to the three previous 

 halistatic areas, the circulation of the water has an anticy clonic character, and 

 therefore the iso-surfaces are not cupola-shaped, but form cup-shaped depres- 

 sions. Moreover, the existence of certain more or less important anticyclonic 

 and cyclonic rotations of waters in different parts of the Sea has been estab- 

 lished. As will be shown below, the character of the movement of water masses 

 in the Black Sea is well reflected by the lower limit of plankton distribution. 



The general course of the iso-surfaces is given in Fig. 185, which is a dia- 

 gram of a cross section of the halistatic area of the Black Sea from coast to 

 coast. It is evident from this diagram that the isoline goes down most steeply 

 not off the coast itself, but at some distance from it ; the current too usually 

 does not run near the coast itself. The upper limit of hydrogen sulphide in the 

 centre of the halistatic area rises to 100 m, while in the area of the current 

 itself it goes down to 155 m. As has been suggested by V. Nikitin and E. Skvor- 

 tzov (1926) the descent of the isolines off the coast may also be furthered by the 

 water being driven off and on by winds, which causes considerable vertical 

 mixing. The fact that the hydrological conditions of the Black Sea are under- 

 going substantial secular changes, as a result of the alterations of climate, of the 

 mainland run-off and of the water exchange through the Bosporus and the 



