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



16-5° to 17-9° near Batumi. The range of temperature changes in the open sea 

 is considerably less than off the shores : according to the data available the 

 winter minimum is 6-6°, while the summer maximum is 27° with an amplitude 

 of more than 20°, while the annual range off the shores is 31°, i.e. 11° more. 



Transparency 



In the open parts of the Black Sea, with depths over 200 m, the water trans- 

 parency (the depth for the disappearance of a white disc) varies usually 

 from 18 to 21 m. Transparency decreases near the coast. The highest trans- 

 parency observed in the Black Sea was 30 m. 



Vertical stratification 



The Black Sea stands sharply apart from all other seas in its physical and 

 chemical characteristics. Moreover, the main factor determining all the others 

 is the great difference between the water density of its topmost layer, of 100 to 

 150 m deep, and that of the deeper mass of water. This difference is so great 

 that the mixing of the two layers proceeds only to a very small extent, and is 

 completely overlapped by the processes of sharply pronounced stratification 

 and stagnation. The layers differ greatly in their temperature, salinity (density), 

 their gas and nutrient salt contents and in the distribution of life in them. 

 Because of this peculiarity, M. Egunov (1900) called the Black Sea the bio- 

 anisotropic sea and N. Knipovitch (1933) called it the most typically anoma- 

 lous body of water. The sharp difference in water density between the 

 two layers is permanently maintained by the fall in salinity of the surface 

 layer which is due to the coastal run-off and the discharge of the Azov cur- 

 rent, and by the rise of the deep-layer salinity as a result of the lower Bosporus 

 current. This difference is so considerable that however much the temperature 

 of the surface water goes down, its density remains higher than that of the 

 deeper layers. The absence of sufficient vertical circulation for the mixing of 

 water is the result of this. 



A picture of the distribution of the surface salinity is given in Fig. 186, 

 and that of the vertical changes of salinity and temperature during the 

 warmest and coldest seasons of the year in the middle part of the Sea is given 

 in Table 151, taken from Nikitin's work. 



Table 151 



Depth, Temperature, °C Salinity % 



m Summer Winter Amplitude Summer Winter 



