THE CASPIAN SEA 587 



I. Makarova (1957) distinguished 59 species, subspecies and forms of dia- 

 tomaceous algae in the phytoplankton of the Central and Southern Caspian. 

 There are 17 species and varieties of Chaetoceros (Ch. wighami, Ch. paulsenii, 

 Ch. subtilis) ; 10 species of Coscinodiscus {C.jonesianus, C.j. var. commutatus) ; 

 and 6 species of Thalassiosira. Thus more than half the Caspian diatoms be- 

 long to these three genera. Among the other genera Sceletonema costatum, 

 Cyclotella caspia, Actinocyclus ehrenbergi, and among the immigrants Rhizo- 

 solenia calcar-avis are the dominant forms. The fact that, contrary to animal 

 groups, endemism among the diatoms is poorly marked, is most characteristic. 

 Makarova points out C. radiatus and C. perforatus as the only two endemic 

 species ; both belong to the widely distributed genus Thalassiosira (77г. cas- 

 pica and 77г. variabile). There is also one species of just as common a genus, 

 Actinocyclus paradoxus. On the other hand, there is a pronounced predomin- 

 ance of marine-brackish- water, brackish- water and cosmopolitan forms 

 among the plankton diatoms (about 62 per cent). The composition of the 

 Caspian Sea diatoms has a very great similarity with that of the north- 

 western part of the Black Sea and the Sea of Azov. 



Thirty-five per cent of Caspian species are common with those of the lower 

 Volga and its delta ; 37 per cent of the species are common with those of the 

 Aral Sea, but the greatest similarity is observed with the Sea of Azov (114 

 common forms, or 63 per cent). The species common with the Black Sea 

 constitute 36 per cent, mainly among the diatoms and peridineans ; there are 

 no common species among the blue-green algae. Hence as regards its phyto- 

 plankton composition the Caspian Sea lies between the Sea of Azov and the 

 Aral Sea. 



Phytoplankton biomass. Among the peridinean algae one species — Exuviella 

 cordata with two variants (typica and aralensis) — plays an exceptional role 

 in the biology of all parts of the Caspian Sea ; it forms the basic food of plank- 

 ton animals and plankton-eating fish, producing at times a biomass of 4-5 to 

 6-5 g/m 3 , mostly on the western side of the Northern and Central Caspian. 

 This is probably due to the presence of a powerful current, carrying plant 

 food and running along the western coast. The intensive development of 

 Rhizosolenia calcar-avis since 1934 has resulted in a pronounced decrease of 

 Exuviella. Among the other peridineans Prorocentrum micans var. scutellum 

 and Gonyaulax polyedra have most significance in the Caspian Sea. 



As in the Sea of Azov, and contrary to the open seas, green algae play an 

 important part in the Caspian Sea phytoplankton, especially in its northern 

 part and still more in its freshest part. The majority are fresh-water forms. 

 Dictyosphaerium ehrenbergianum var. subsalsa, Oocystis socialis and Botryo- 

 coccus braunii are the most widely distributed green algae. Among the diatoms 

 the dominant species in the plankton up to 1934 were Skeletonema costatum, 

 Actinocyclus ehrenbergii, Coscinodiscus biconicus, Chaetoceros subtilis, Ch. 

 wighamii, Thalassionema nitzshioides. A new form, Rhizosolenia calcar-avis, 

 appeared in the Caspian Sea in 1934 and later became the dominant form of 

 the whole phytoplankton. The distribution of the diatoms in the northern 

 part of the Sea is given in Fig. 276. 



