THE BLACK SEA 



419 



(both animal and plant) in the open part of the Sea. He has determined the 

 total plankton biomass as approximately 7 million tons (6,937,714 t), half of 

 the plankton being contained in the upper 50 m layer, while the lower 150 to 

 175 m layer contains only 1 per cent of its bulk (Fig. 201). The Sea of Azov is a 

 body of water attached to the Black Sea which is remarkable in many respects. 



Fig. 201. Vertical distribution of total plankton bio- 

 mass in open parts of Black Sea, tons (Nikitin). 



It is essentially a broad, very shallow inlet of the Don, with water only 

 slightly saline. Owing to a number of circumstances it is supplied with abun- 

 dant' mineral substances. 



Investigations made in recent years have led to a situation where the Sea 

 of Azov can now perhaps be placed among those seas of the u.s.s.r. which 

 have been most comprehensively studied. 



The average plankton biomass decreases steadily from top to bottom 

 (Table 166). 



Table 166 



Depth, m 

 Biomass, mg/m 3 



0-25 

 210 



25-50 

 147 



50-75 

 121 



75-100 

 84 



Depth, m 100-125 



Biomass, mg/m 3 90 



125-150 



54 



150-175 



(150-225) 



38 



The increase of biomass in the 100-125 m layer as compared to the layer 

 above it, is explained by the accumulation in it of such cold water forms as 

 Calanus helgolandicus and Pseudocalanus elongatus throughout the greater 

 part of the year. 



