604 



Plankton biomass in the Caspian Sea is inferior to that of many other 

 seas, in particular to the Barents Sea and Sea of Azov ; it is, no doubt, inferior 

 to the latter in productivity also. 



Vertical migration of zooplankton. The phenomena of vertical migration 

 of plankton, some plankton-benthos and even benthic crustaceans (for ex- 

 ample Cumacea and Corophiidae) are extremely pronounced in the Caspian 

 Sea. While it is dark these organisms rise in huge masses to the surface, attracted 

 by its large food resources and oxygen. The water teems with them, and the 

 masses of animals present give it a milky appearance by electric light. This 

 process is most striking owing to its very size. No fewer than 4 to 5 millions 

 of crustaceans move hundreds of metres up and down twice a day. 



N. M. Knipovitch (1921) has already pointed out the daily vertical migra- 

 tion of bathopelagic mysids with Mysis microphthalma, M. amblyops and 

 Austromysis loxolepis as specially characteristic. In daylight the maximum 

 numbers keep within the 250 to 350 m layer ; at night they are in the top layer 

 of the Sea. They may travel as much as 300 m. Their migration is accompanied 

 by a 30 atm pressure change. Twice a day the animals experience, without 

 harm to themselves, these great changes in pressure and correspondingly in 

 temperature. Limnocalanus grimaldi and the larvae of the Caspian sprat also 

 experience this kind of migration. Knipovitch has determined the rate of rise 

 of some mysids as 90 m in 75 minutes. 



The number of the plankton forms in vertical migration given by Jashnov 

 for August 1934 {Table 251) is even more indicative. 



V. Bogorov (1939) has given a comprehensive description of the vertical 



Table 251 



