THE ARAL SEA 



659 



the open Sea, according to G. V. Nikolsky (1940), is 3-23 cm 3 per 1 m 3 . 

 Diaptomus salinus, Cladocera and Dreissena larvae make daily vertical migra- 

 tions ; in daytime they keep mainly at 



a depth of 10 to 20 m (Fig. 311). 



'These daily plankton migrations', 

 says A. Behning, ' have real significance 

 in the life of the Sea. They enable the 

 plankton animals to use all layers of 

 water in their search for food ; masses 

 of diatoms are found by them in day- 

 time in the depths — Actinocyclus, 

 Campylodiscus, Pleurosigma — and by 

 night in the upper layer they find 

 flagellates and other species of phyto- 

 plankton.' 



In bays and inlets, which usually 

 have a somehat higher salinity and are 

 subject to greater temperature and 

 salinity fluctuations, the most common 

 forms among the plankton, accord- 

 ing to A. Behning, are the algae 

 Chroococcus turgidus, Oscillatoria 

 tenuis, Lyngbya aestuarii, two species 

 of peridinians Cyclotella and Melosira 

 Borreri, among the animals the Rotifera Brachionus bakeri, B. mulleri, 

 and Colurella adriatica, and among the Crustacea Halicyclops aequoreus, 

 Cyclops viridis and Alona rectangula. The mean plankton biomass is here 

 about 2-75 mg/m 3 . Near the mouth of the rivers Amu- and Sur-Daria, 



SEA AREA 

 LITTORAL AREA 

 ШШ ESTUARINE AREA 



Fig. 310. The region of the north- 

 western part of the Aral Sea according 

 to plankton composition (Behning). 



ens 



Fig. 311. Vertical migrations of plankton in the 



Aral Sea in August 1933 according to the number 



of specimens (Behning). 



within the areas of the lower surface salinity, the plankton composition 

 changes, many forms of the saline Aral waters are not found, and there is a 

 considerable admixture of fresh-water forms. The most characteristic are the 

 following : Microcystis aeruginosa, Dinobryon sertularia, Ceratium hirundinella, 

 Eudorina elegans, Fragilaria crotonensis, Keratella aculeata and Diaphanosoma 

 brachyurum. At the confluence of fresh and saline waters there is an increase 

 of plankton biomass caused by the high content of plant nutrients in the 

 river waters. The average wet volume of plankton in the Syr-Dar'ya estuary 

 is 3 mg/m 3 . 



