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



the circulation, and in spring the upper column of water will be better 'ferti- 

 lized' than in years when the upper layer becomes fresher and the lower limit 

 of circulation moves upwards. As a result there may be years more or less 

 favourable for the quantitative development of phytoplankton, and conse- 

 quently of zooplankton and all the succeeding links in the food chains. 



During the last few years the level of the Caspian has gone down by 2 m. 

 At the same time these years were characterized by an extremely vigorous 

 plankton development. Brujevitch has pointed out that during a 22 cm fall 

 in the level of the Sea from 1933 to 1934 the salinity of the upper 100 m 

 column of water must have been raised by almost 0-1 % . The quantity of 

 nutrient salts at a depth of 50 to 100 m in 1934 was higher than that in 1933 ; 

 this probably indicates their rise from great depths as a result of more intense 

 vertical circulation {Table 234). 



Table 234 



IV. FLORA AND FAUNA 



General characteristics 



The Caspian Sea fauna (Fig. 270), qualitatively very poor, is very varied in its 

 origin ; its basic forms are descended from the Tertiary marine fauna, which 

 underwent considerable evolution as a result of changes in the orography and 

 in the whole hydrological conditions of the Sea. The remains of the fauna of 

 Tertiary seas of the Sarmatian and Pontic periods are represented by such 

 characteristic groups of the Caspian Sea as : herrings, bullheads, Bentho- 

 philus ; the molluscs by various forms of Cardae (except Cardium edule) ; and 

 by Dreissena, Bryozoa Victorella, the polychaetes Hypania, Hypaniola Parhy- 

 pania and perhaps Manayunkia caspica ; some of the Turbellaria ; all the Deca- 

 poda except prawns and Heteropanope ; Cumacea; most of the mysids; 

 Gammaridae ; Porifera ; the medusa Moerisia and the hydroid Cordylophora. 

 Later immigrants from the northern (Arctic community) and western (Medi- 

 terranean community) seas and from fresh waters are mixed in considerable 

 numbers with this basic part of the fauna. 



This fourfold genesis of the Caspian Sea fauna is a striking peculiarity 

 of its biology. During the periods of its history when its salinity was 

 greatly reduced it became a body of almost fresh water (for example the 

 Glacial transgression) ; at least into some of its component parts, a fresh- 

 water fauna made its way there and partly adapted itself to the subsequent 



