420 



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



The decrease in plankton numbers with depth is accompanied by its quali- 

 tative impoverishment (Fig. 202). Below 50 m there is a considerable decrease 

 in the amount of oxygen and in the pH value, indicating increasing amounts of 

 free carbon dioxide. The average plankton biomass throughout the inhabited 

 zone is 1 18 mg/m 3 (according to Maljatzky, 100 to 130 mg/m 3 ). 



Qualitative 

 P H — Oxygen composition 



Plankton 

 biomass 



of zooplankton 



Fig. 202. Vertical distribution of oxygen, of the course of the 

 active reaction, and of the qualitative and quantitative distri- 

 bution of plankton in Black Sea (Nikitin). Biomass and number 

 of plankton species of the upper horizon are taken as 100 

 per cent. 



A. Kusmorskaya (1950, 1954, 1955) has carefully studied the zooplankton 

 of the Black Sea, chiefly as food for fish, and the life cycles of mass forms. She 

 notes among the fish-food organisms the preponderance of the following 

 forms: Calanus helgolandicus, Pseudocalanus elongatus, Acartia clausi, 

 Penilla avirostris, predatory and voracious forms of Medusa, Pilemo pulmo 

 and Amelia aurita, Pleurobrachia pileus and Sagitta setosa are usually larger 

 in mass than food plankton, which they devour in huge amounts. Among 

 them S. setosa only is eaten by some fish (sprat and hardtail). 



