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



be due to a sufficient oxygen supply in the deep waters of the latter Sea 

 (Table 306). 



The plankton of the Sea of Japan is divided into definite biogeographical 

 zones (Fig. 373). Its most northern part, adjacent to the Tartary Strait, is 

 occupied by a cold-water biocoenosis with Calanus finmarchicus. Calanus 

 tonsus is the dominant species in its central part, while the warm-water species 

 Calanus pacificus, Oithona plumifera, Paracalanus parvus and Coryceus sp. 

 inhabit the southeastern part of the Sea (M. Kun and I. Meshcheryakova, 

 1954). A neritic biocoenosis of zooplankton (with Labidocera, Epilabidocera, 

 Centropages, Acartia, Evadne, Podon and the larval forms of bottom- 

 living animals) encircles the Sea. 



In K. Brodsky's opinion (1941) the list of warm- water organisms should 

 include Cladocera sp., Paracalanus parvus, Oithona brevicornis and Calanus 



Table 306. Change in number of species and specimens of Copepoda with depth 



{K. Brodsky, 1952) 



pacificus, and the list of the cold-water organisms — Calanus cristatus, C. fin- 

 marchicus, С tonsus, Pseudocalanus elongatus and in part Oithona similis. 



Plankton is much more developed in the open sea, furnishing a biomass in 

 excess of 1 g/m 3 (Figs. 374 and 375). 



In winter (December to February) the zooplankton biomass in the to 200 m 

 layer remains fairly well developed in the central part of the Sea throughout 

 the whole layer (30 to 500 mg/m 3 , Fig. 376) (I. Meshcheryakova, 1954). 

 Calanus cristatus, Thysanoessa raschii, Themisto abyssorum and Calanus 

 tonsus are the dominant forms in the southern part. There is also an admixture 

 of C. finmarchicus in the coastal areas, and in the southern parts are found also 

 the warm-water Oithona plumifera, Clausocalanus arcuicornis and others. 

 The boundary between the area of winter conditions and that of spring 

 conditions runs at that season approximately along the fortieth parallel. 



Zooplankton distribution in the northern part of the Sea of Japan has some 

 unusual winter features (L. Ponomareva, 1954). The most abundant plankton 

 may be concentrated in the uppermost 25 m layer. Plankton biomass de- 

 creases rapidly within the 100 to 200 m layer, and deeper down it becomes 



