THE BERING SEA 829 



diatoms, however, become markedly predominant {Thalassiosira norden- 

 skioldi, Th. gravida, Fragillaria oceanica, Amphiprora hyperborea, Porosira 

 glacialis, Coscinosira polychorda and others.) Anadyr phytoplankton, how- 

 ever, is mainly Arctic or Arctic-boreal, the boreal forms being predominant 

 only in the surface layer in summer. 



The same pronounced difference is observed between the phytoplankton 

 compositions in the western and eastern sides of the Bering Strait, Arctic and 

 Arctic-boreal forms being predominant in the first and boreal in the second. 



The phytoplankton of the eastern side of the northern half of the Bering 

 Sea is characterized by the predominance of boreal forms with an admixture 

 of brackish-water and neritic species {Thalassiosira japonica, Coscinodiscus 

 granii, Actynoptychus undulatus, Rhizosolenia alata, Ditylum brightwellii, 

 Actinocyclus ehrenbergii, Bellerochea malleus, Asterionella japonica, Peri- 

 dinium excentricum). Arctic and Arctic-boreal species are just as character- 

 istic of the western side {Thalassiosira nordenskioldi, Th. gravida, Chaetoceros 

 socialis, Ch. radians, Porosira glacialis, Bacterosira fragilis, Eucampia groen- 

 landica proceeding from cold-water to warm-water forms) (I. Kisselev, 

 1937). Apart from these zoogeographical changes seasonal alterations are 

 observed in summer, especially in the surface layer of the western part of the 

 Sea. 



The Bay of Anadyr phytoplankton is characterized by the predominance of 

 Arctic and Arctic-boreal species even in summer (except for its warmed sur- 

 face layer). It is possible, however, that small, terminal branches of warm 

 Pacific waters enter the Bay of Anadyr and currents stimulate a rich develop- 

 ment of boreal forms in summer. 



Four main groupings of zooplankton may be distinguished in the Bering 

 Sea according to their distribution (K. Brodsky, 1954; M. Vinogradov, 1956) 

 (Fig. 420). 



The southern Bering Sea oceanic group in the 200 m surface layer is char- 

 acterized by a selection of forms similar to those of the surface waters of the 

 northwestern part of the Pacific Ocean ; they penetrate into the Bering Sea 

 with the warm Pacific waters. Calanus cristatus, C. tonsus and Eucalanus 

 bungii are the mass forms of this group ; Racovitzanus antarcticus, Scolecith- 

 ricella minor, Parathemisto japonica, Oncaea borealis and others are added 

 to them in smaller numbers. This group penetrates far to the north and into 

 the Chukotsk Sea. The northern Bering Sea group lives on the shelf in the 

 northern part of the Sea, partly overlapping the first group. C. cristatus, С ton- 

 sus, Primno macropa and other warm-water forms are completely absent there, 

 while Calanus finmarchicus and Parathemisto libellula become abundant. 

 Certain cold-water species of this group move southwards with the cold 

 waters along the coast of the Chukotsk Peninsula and Kamchatka almost 

 to the southern end of the latter, forming the western neritic group together 

 with some neritic species ; this third group is very similar in its composition 

 to the eastern neritic group. Podon leuckarti, Centropages mamurrichi, 

 Acartia clausi and A. longiremis play an important role in the plankton of the 

 most shallow regions of low salinity. This group, with the Oceanic and 

 northern groups, penetrates into the Chukotsk Sea. A deep-water Bering Sea 



