THE BLACK SEA 



385 



Kerch Strait, is noted in recent literature (S. P. Brujevitch, 1953; A. Bog- 

 danova, 1959). A certain rise in salinity in the upper layer and a fall in salinity 

 throughout the water-column have been observed in the Black Sea for a period 

 of 25 years (1924-51). The decrease of salt content through that period for the 

 whole Sea was determined as 2 milliards of tons (A. Bogdanova, 1959). This 

 is caused primarily by the loss of salts through the Bosporus being greater 

 than the supply, a fact which is linked in its turn with a 19% rise in salinity 

 in the upper layer of the sea as a result of a decrease of the mainland run-off. 



In Bogdanova's opinion the decrease in 

 salinity of the deep layers is connected with 

 the slackening of the deep Bosporus current. 

 The change in salinity of the upper layer and 

 the main column of water should have im- 

 proved vertical circulation. In addition, cool- 

 ing of the intermediate layer (75 to 300 m) 

 and some warming up of the deeper layers 

 were recorded. 



In the off-shore zone animals were found at 

 depths of a little more than 200 m; on the 

 other hand in the halistatic area patches were 

 found where plankton animals disappeared at 

 a depth of no more than 87-5 m. Hence the 

 length of the Black Sea water column populated 

 by animals varies greatly in different parts of 

 the Sea. 



2 J o-o-oo-o^ 



Fluctuations of water level 



The ' fluctuations in the amount of water 



coming from the mainland, or from rainfall, 



evaporated from the sea surface, entering the 



Sea as a result of water exchange with the 



neighbouring seas through the straits may 



affect the volume of sea-water. During 



recent decades changes in the Black Sea level, with an amplitude of about 



32-5 cm, have been observed. Seasonal changes in the sea-level have been 



observed with ranges of 15 to 27-5 cm. Finally, the changes of sea-level may 



be due to the wind and tides. The latter during the spring tide reach an 



amplitude of about 8 cm. 



Fig. 185. Hydrological cross 

 section from southern coast 

 of Crimea southwards to 

 Anatolian shore in February 

 1925 (Nikitin, 1930). /Isoxy- 

 gen,% saturation ; 2Isohaline, 

 % ; 3 Lines of equal content of 

 hydrogen sulphide, cm 3 /l. (its 

 upper limit) ; 4 Lower limit of 

 plankton. 



Salinity 



As in any other inland sea having impeded water exchange with a fully 

 saline sea, the salinity of the upper layer may undergo considerable fluctua- 

 tions depending on climatic changes, which, as we shall see below, is of some 

 significance to the development of life. The upper layers of the Black Sea, 

 except for areas adjacent to the river mouths and some parts of the coast 

 subject to salinity fluctuations, have a salinity of 17 or 18% (Fig. 186). The 



2B 



