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



Thus the main supply of food for the zooplankton consists of flagellates, 

 diatoms and to a lesser degree of green algae. 



Composition of zooplankton 



There are 24 main zooplankton forms in the Aral Sea, including : 



Tintinnoidea 2 species 



Rotatoria 8 species 



Cladocera 7 species 



Copepoda 7 species 



In addition, the plankton usually contains a great many Dreissena poly- 

 mer pha larvae (up to 10,000 specimens per 1 m 3 ) and small-sized (and, in the 

 hours of darkness, also fully grown) Pontogammarus aralensis. 



The most usual zooplankton forms are : Infusoria Codonella relicta (up to 

 40,000 specimens per 1 m 3 ); the Rotifera Floscularia mutabilis, Synchaeta 

 vorax, S. neapolitana and Rattulus marinus; among the Cladocera Cerio- 

 daphnia reticulata, Moina microphthalma (up to 3,000 specimens per 1 m 3 ), 

 Cercopagis pengoi, Evadne camptonyx (up to 12,000 specimens per 1 m 3 ), 

 and E. anonyx. Among the Copepoda Diaptomus salinus (producing up to 

 8,500 specimens per 1 m 3 ) is the most important in the Aral Sea. This form is 

 greatly predominant over all the other zooplankton forms ; it is the main food 

 offish-fry, and sometimes of adult fish (stickleback, Pelecus, Chalcalburnus). 

 According to V. Pankratova (1935) D. salinus forms 58 per cent of the food of 

 carp-fry, 32 per cent of that of Chalcalburnus, 20 per cent of that of bream, 

 and 10 per cent of that of vobla. In the open parts of the Sea Mesocy clops 

 leuckarti and M. hyalinus (up to 600 specimens per 1 m 3 ) are the most common 

 of the Copepoda. 



In August the total zooplankton biomass is on the average 0-5 g/m 3 . 

 Copepoda biomass sometimes yields up to 230 mg/m 3 , Cladocera up to 650 

 mg/m 3 and the larvae of molluscs up to 160 to 170 g/m 3 . 



In A. Behning's opinion (1935) no less than 95 per cent of the total biomass 

 of Aral zooplankton is composed of Dreissena larvae and of all stages of 

 Diaptomus salinus. Thus the Aral zooplankton is an example of the pronounced 

 predominance of a few forms (olygomixed). 



Horizontal and vertical distribution of plankton 



A. Behning (1935) has distinguished in the Aral Sea three areas differing from 

 each other in their qualitative and quantitative plankton composition: the 

 open Sea, the coastal areas and the estuarine reaches (Fig. 310). 



The area of the open Sea is exposed to smaller fluctuations of temperature 

 and salinity (10-3 to 10-5% o ), while in the depths the temperature remains low 

 (between 4-5° and 9-5°) all through the year. This area includes the central 

 part of the Bol'shoy More and the open parts of the northern inlets of the 

 Maloe More. There is 3-2 times more plankton (up to 1,200,000 specimens 

 per 1 m 3 ) in the depths than there is in the surface layer (up to 370,000 speci- 

 mens per 1 m 3 ) mainly owing to Actinocyclus ehrenbergi. The mean biomass in 



