THE CASPIAN SEA 



593 



the Caspian Sea in 1935 is given in Fig. 278. It is evident that the plankton 

 biomass in the estuarian zone of the Caspian Sea had already reached the 

 huge amount of 100 g/m 3 (in some individual cases 140 g/m 3 ). 



Fig. 281. Alterations of the mean phyto-plankton 

 biomass (g/m 3 ) with depth (/) and of that of Exuviella 

 separately (//) in September 1934 in the coastal part of 

 the Northern Caspian. /// and IV: same for Central 

 Caspian; V and VI: same for Southern Caspian with 

 the appearance of Rhizosolenia; VI: same for Rhizo- 

 solenia separately (Usachev). 



Usachev has compared the phytoplankton biomass of the Caspian and 

 Azov Seas. The mean phytoplankton biomass of the Sea of Azov during the 

 blootii of Rhizosolenia calcar-avis was found to be 2-0 to 5-2 g/m 3 higher than 

 that of the Caspian Sea ; in 1925 it was 100 g/m 3 higher. 



Phytoplankton productivity. Valuable data on the characteristics of plankton 

 distribution in the Sea are obtained from the quantitative estimation of phyto- 

 plankton (productive part of plankton). The determination of productivity 

 on the basis of biomass data is most difficult and at present almost impossible. 

 As we have seen in Vorobieff ' s work on the Sea of Azov, such data can be 

 computed for the benthos since the indices of growth, multiplication, dying 

 off, and consumption by fish can be obtained. The problem, however, 

 generally becomes most difficult for plankton species. It can be solved in 

 part for zooplankton. Let us recall, for example, the Barents Sea plankton, 

 80 to 85 per cent of which is composed of a one-year-old population of 

 Calanus finmarchicus. The estimation of phytoplankton productivity cannot 

 be approached by means of population census. On the other hand, the exist- 

 ence of phytoplankton is closely linked with the chemistry of the water, with 

 the amounts of oxygen, carbon dioxide, phosphorus and nitrogen present, 

 and with its pH value. 



S. P. Brujevitch (1937) has determined phytoplankton productivity in the 



2p 



