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



Seasonal changes of plankton 



The Aral Sea plankton is more abundant in variety and numbers in summer 

 time (May to October) ; moreover, a number of forms (for example Brac- 

 hionus mulleri, Evadne camptonyx) have a maximum growth in the warmest 

 time of the year. G. V. Nikolsky has pointed out that a large number of 

 chironomid pupae are observed in the plankton, mainly in the autumn. 



The significance of plankton as food 



The Aral Sea plankton is most important as food for the bottom-fauna and 

 the fish-fry. As has been said above, among the adult fish of the Aral Sea only 

 stickleback can be considered as a typical plankton eater. As the investi- 

 gations of V. Pankratova (1935) and A. Behning (1935) have shown, plankton 

 constitutes 69 per cent of stickleback food, 16 per cent of that of Pelecus, 9 per 

 cent of that of Chalcalburnus and 3 per cent of that of bream. Diaptomus 

 salinus and to a much lesser extent other Copepoda and Cladocera form the 

 main food of these fish. The fry of most fish feeds on plankton. 



The quantitative estimate of plankton 



Quantitatively the plankton of the Aral Sea is somewhat inferior to that of the 

 Caspian and much poorer than that of the Sea of Azov (and other central 

 Asian lakes except Lake Balkhash) (Fig. 312). Its average biomass is about 

 3 cm 3 /m 3 and the number of specimens of plankton organisms is of the order 

 of 8 to 9 millions, mainly flagellates and the diatoms Actinocyclus, Exuviella, 

 Proterocentrum, Glenodinium, Diplosalis and other nannoplankton forms. 

 The quantitative distribution of the plankton of the Aral is illustrated in 

 Fig. 312. The inadequacy of the nutrient salts is regarded by Nikolsky as the 

 cause of the poverty of plankton in the Aral Sea. 



Benthos 



Phytobenthos. According to A. Behning (1935) the phytobenthos of the Aral 

 Sea, except for the flowering plants of the coastal zone, comprises the follow- 

 ing groups : 



Chlorophyceae 4 species 



Diatomaceae 25 species 



Rhodophyceae 1 species 



Characeae 1 species 



Phanerogamae 1 species 



Among these 32 forms the flowering plant Zostera nana, the green algae 

 Vaucheha dichotoma and Cladophora gracilis, the red algae Polysiphonia vio- 

 lacea and Characea alga Tolypella aralica are the specially large mass forms. 



Zostera mainly inhabits silty sand soils in the shallower areas of the bottom 

 of the eastern and northern parts of the Sea. Large accumulations of it are 

 found there. As in the Black and Caspian Seas, Zostera forms floating fields 

 in the Aral Sea and great masses of it are cast up on the shore. Bottom 



