588 



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



The diatoms play a very important role in the Caspian Sea phytoplankton. 

 The diatom biomass in the Central Caspian constitutes 12 to 20 per cent of the 

 total phytoplankton ; their quantity is even higher in the Northern Caspian, 

 especially in its northwestern corner, where it has been known to reach 10 

 g/m 3 . Mass development of Rhizosolenia calcar-avis was first recorded in the 

 Caspian plankton in 1934 in its south- westernmost corner (more than 6 

 g/m 3 ) ; by 1935 it had already spread through the whole of the Sea. Rh. calcar- 

 avis produces a biomass of 5 or 6 to 9 g/m 3 in different places in the Sea 



Fig. 276. Distribution of phytoplankton biomass (g/m 3 ) in northern part of Caspian 

 Sea (Usachev). A August 1934; / Exuviella biomass 4-5 to 6-5 g/m 3 ; // The main 

 zone of Exuviella gathering; III Isoplankta of diatoms 0-8; /^Isoplankta of dia- 

 toms 0-3; В September 1934; / Exuviella 01 to 0-2 g/m 3 ; // Diatoms 0-3 to 0-8 

 g/m 3 ; /// Blue-green algae 0-3 to 0-5 g/m 3 . 



(Fig. 277). In some cases it constitutes 99 per cent of the total phytoplankton ; 

 it commonly exceeds 80 per cent. This is a completely unprecedented example 

 of a mass development of one single form and of the displacement by it of 

 20 to 25 per cent of another mass form, Exuviella cordata, which even in 

 1934 constituted 56 to 78 per cent of the whole mass of phytoplankton. Its 

 average number of specimens is 10 8 m 3 ; that of Rhizosolenia is 2x 10 7 . This 

 situation remained unchanged in 1936. In 1937 the Rhizosolenia biomass de- 

 creased* on the average to 0-06 to 2*16 g/m 3 . 



Among the blue-green algae of the Caspian Sea the dominant species are 

 the following : Aphanizomenon flos-aquae, Nodularia spumigera, N. harveyana, 

 Anabaena bergii, A. bergii var. minor and Merismopedia tenuissima, Blue- 



* High indices of the phytoplankton biomass were observed, however, only off the 

 coast; in the Central part of the Sea the amount of phytoplankton is measured in tens of 

 milligrammes per cubic metre (10 to 20 mg/m 3 ). 



