426 



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



Table 172. Comparison of mean zooplankton biomass (g/m 3 ) of the Black 

 and Caspian Seas according to seasons in the to 100 m layer 



Food-plankton in 

 Season Central Southern western part of 



Caspian Caspian Black Sea 



zooplankton biomass in the Sea of Azov decreased by almost twelve times 

 and the ratio between the peridineans and the diatoms changed greatly and 

 became unfavourable for fish. Nikitin also tried to trace a seasonal change in 

 the plankton biomass. Table 173 indicates his method. 



Table 173 



* V. Nikitin thinks that the magnitudes of plankton biomass obtained by him are con- 

 siderably understated, since Nansen's net was used for the collection and it lets through 

 almost the whole of nannoplankton and also part of the micro-plankton. 



In the upper layers of the Sea (0 to 25 m) the seasonal changes of plankton 

 biomass are marked, but they are already attenuated at depths of 25 to 50 m, 

 while below 50 m they are practically absent. This is in complete conformity 

 with the course of the annual fluctuations of various factors of the environ- 

 ment, primarily with temperature. Moreover, it is to be borne in mind that the 

 main mass of phytoplankton is concentrated in the upper 25 m layer 



V. Wodjanitzky estimates the total Black Sea plankton biomass as 12 to 

 18 million tons (1941), and its annual productivity as no less than 225 million 

 tons.* 



* In V. Nikitin's book The Feeding of Anchovy (Engraulis encrasicholus L.) in the 

 Black Sea off the Shores of Georgia the total plankton biomass is given as 7 million tons, 

 and the annual production as 105 million tons. All these data should at present be con- 

 sidered as provisional. 



