/.. 



2a 



(2) 



(a+b) + (a+c) 



where a is the number of species common forj- and k-stations; 

 b is the number of species to be found only at station]; and c is 

 the number of species to be found only at station k. 



Results 



Species Composition and Species Complexes 



Seventy-four units of hydrobionts were found as a result of 

 analysis of samples of mesozooplankton collected in the 

 100-m upper layer of the Bering and Chukchi Seas (Table 1 ). 

 The most diverse and numerous group of organisms — copepods 

 (Copepoda) — counted 26 species and dominated plankton on 

 the majority of stations both in number and biomass. Data in 

 Table 1 shows that epipelagic zones of all of the investigated 

 water areas of the Bering and Chukchi Seas are marked by an 

 active development of populations of two eurybiontic epipelagic 

 species of copepods ( \.e..Pseudocalamus minutus and Oithona 

 similis). Plastic species also could be found in the samples; the 

 hydromedusa Ag/fl/!r/!(7 digitate, appendicularians Oikopleiira 

 labradoriensis and Fritillaria borealis. Some regions are 



characterized by abundance of individuals of subarctic and 

 boreal species of organisms that form a number of species 

 complexes (Table 2). 



The calculated regions were defined on the basis of common 

 character of taxonomic composition of the plankton 

 (equation 2). Resultsof J,, cluster analysis are shown in Fig. 1. 

 At the index value of 0.75, the stations were combined into 

 three groups; their geographic position provided for the division 

 of the northern part of the Bering Sea into four regions with 

 relatively homogenous taxonomic composition of 

 mesozooplankton community. Species composition of the 

 southern part of the Chukchi Sea had more similarity than the 

 northern part of the Bering Sea, which is why the fonner was 

 regarded as the region inhabited with taxonomically 

 homogeneous plankton fauna. 



Distribution of Mass Species Across the Study Region 



Oithona similis was the most numerous of the investigated 

 populations. It was most developed at the East Polygon, the 

 number of copepodites of this species alone averaged 

 91 1,000 ind/m-. A somewhat lower concentration was found 

 at the South Polygon — 765,000 ind/m'. The number of 

 O. similis in the northern part of the Bering Sea was considerably 



TABLE 1 



Species composition of mesozooplankton and frequency (%) of occurrence in the Bering and Chukchi Seas. 



No. Species 



Areas of the Bering Sea (Station Nos.) 



77 

 4 

 19 



27 

 4 



12 



12 



27 

 5 



2 

 2 



13 

 6 



3 



42 

 162 



14 



5 



27 

 27 



64 



27 

 8 



15 



4 

 35 

 38 



100 



75 



25 



17 



42 



100 



20 

 2 

 2 



42 

 6 



3 



55 



48 

 3 



92 



