THE SEA OF JAPAN 



763 



for the to 100 m layer. Calanus tonsus was also greatly developed (45 per 

 cent of the total plankton biomass). 



As early as June diatomaceous plankton was replaced by peridinians (some 

 species of the genera Peridiniwn and Ceratium); among the zooplankton 

 Paracalanus parvus was intensely developed. The amount of zooplankton 

 increased to 350 mg/m 3 , and with the warming of the surface water Calanus 



for phytoplanKton 

 for zoopLanKton 



3000 



CO 4 





wo 



200 - 



Fig. 378. Vertical distribution of plankton biomass (mg/m 3 ) in Sea of 



Japan from 31 March to 2 June 1939. 1 Phytoplankton (Coscinodiscus); 



2 Zooplankton; 3 Calanus tonsus (Kusmorskaya, 1950). 



tonsus became the main form. In 1937 the zooplankton biomass in the same 

 area, at the same season, was three times greater (1,300 g/m 3 ) (K. Brodsky, 

 1939), and in 1936 it had even reached 1,640 mg/m 3 . The greatest concen- 

 tration of zooplankton is found at a depth of 24 m (Fig. 378). Phytoplankton 

 consists almost exclusively of Coscinodiscus oculis iridis, while half the zoo- 

 plankton consists of Calanus tonsus — the main food of sardines in the Sea of 

 Japan. In the opinion of many investigators the sardine catastrophe of 1939 

 was the result of an exceptional fall of temperature in the sardine spawning 

 area and also of the consequential scarcity of food for the newly-hatched 

 young. 



