THE ARAL SEA 657 



Table 280. Mean content of phosphates {P mg/m z ) 



Northern Central Southern 



Period section section section 



May-June 0-7 0-8 1-8 



August-September 0-8 3-1 2-5 



October 0-8 0-8 — 



it never decreases to zero. Whereas there are hundreds and thousands of mg/m 3 

 of silicic acid in the surface waters of the Caspian Sea, there is only 1 50 to 

 250 mg/m 3 in the Aral Sea. 



The hydrophysical and hydrochemical characteristics of the Aral Sea are 

 given in Figs. 308 and 309. 



IV. FLORA AND FAUNA 

 The general characteristics of the population of the Aral Sea and its' history 

 have been given above. 



Composition of phytoplankton 



The Aral Sea plankton is poor in numbers and in species. 



A. Behning (1935) gives the following basic composition of phytoplankton : 



Cyanophyceae 6 species 



Flagellates 11 species 



Conjugatae 2 species 



Chlorophyceae 2 species 



Diatomaceae 18 species 



Among these 39 forms the most common blue-green algae are Chroococcus 

 turgidus, Merismopedia glauca and Anabaena bergi ; among flagellates — Exu- 

 viella cordata aralensis, Prorocentrum obtusum, Glenodinium trochoideum, 

 Gonyaulax levanderi, Peridinium achromaticum, P. subsalsum, Diplosalis cas- 

 pica and D. pillula ; among the conjugates — Spirogyra spp. and Mougeotia spp. 

 and among the green algae — Oocystis socialis, Botryococcus braunii ; among 

 the diatoms — Actinocyclus ehrenbergi, Chaetoceras wighanii, Ch. subtile, 

 some Campylodiscus spp., Coscinodiscus granii var. aralensis, Sceletonema 

 costatum, Melosira borreri, Thalassiosira dicipens, Bacillaria paradoxa and 

 others. 



The flagellates are of essential significance in the Aral Sea nannoplankton, 

 forming the basic group in the food of Rotifera and Crustacea. 



The diatoms dominate the plankton by their mass ; the principal form among 

 them is Actinocyclus ehrenbergi var. crasa, which sometimes produces over a 

 million specimens per 1 m 3 . Actinocyclus is the usual food of Diaptomus 

 salinus, the highest mass form of Aral Sea zooplankton, which in its turn is 

 the basic food of the young of most fish. 



Botryococcus braunii is also of significance in the phytoplankton and in the 

 food of zooplankton. 



2t 



