THE SEA OF OKHOTSK 791 



mixed, with a predominance of neritic, re-deposited and fresh-water species. 

 At depths of 5 to 1 1 m below the surface of the soil oceanic diatoms again 

 become markedly predominant; there is, however, a considerable ad- 

 mixture of neritic forms. In the fourth horizon (10 to 16-5 m), as in the second, 

 diatoms become scarce, while the neritic (possible glacial) and fresh-water 

 forms are again predominant. Below about 16 m there is again a greater 

 abundance of oceanic species with some neritic ones and some bottom-living 

 diatoms of the Pliocene Age. Zhuze thinks it possible to synchronize the 

 layers rich in oceanic diatoms and those which are poor in them but have 

 an admixture of neritic and fresh-water forms with two periods of glaciation 

 and two inter-glacial periods. The contemporary period has the most 'oceanic' 

 aspect, and exchange between the Okhotsk and Bering Seas and the open 

 Ocean is on a greater scale now than ever. Changes in the Foraminifera in the 

 bottom deposits of the Sea of Okhotsk have also been comprehensively in- 

 vestigated (Kh. Saidova, 1953, 1955). About fifty such species were recorded, 

 including those in the Bering Sea, and almost all of them exist at present. 

 Examination of the successive layers of soil cores led Zhuze to the conclusion 

 that during the deposition of the layer of the sea-bed examined the Sea of 

 Okhotsk trenches underwent a submersion. The numbers of shallow-water 

 and cold-water Foraminifera increase with the depth. 



Cold-water organisms are predominant in the summer zooplankton in the 

 north of the Sea of Okhotsk. The greatest plankton biomass (1,000 to 3,000 

 mg/m 3 ) was recorded in 1949 in the east of the region at depths of more than 

 25 m (Fig. 394) at some distance (100 to 150 km) from the coast (M. Kuhn, 

 1951). There was a considerable predominance of Metridia sp., Oithona 

 similis, Pseudocalanus elongatus, Microcalanus pygmaeus, Acartia longiremis, 

 Sagitta sp. and Themisto libellula in the upper horizons (less than 25 m). 

 In the lower, most productive layers (below 25 m) and in addition to the 

 Metridia sp. (45 to 50 per cent of the biomass) and Themisto libellula men- 

 tioned above, there was a predominance of Calanus finmarchicus, C. tonsus, 

 С cristatus and Pareuchaeta japonica. The temperature of these lower horizons 

 is, however, below freezing point in summer. Some zooplankton species 

 {Metridia sp., Themisto libellula and Calanus finmarchicus), in spite of the 

 markedly cold intermediate layer, migrate freely through it. 



A considerable admixture of warm-water and partly subtropical members 

 of the Calanoida group appears in two areas of the Sea of Okhotsk. They are 

 brought into the most southwesterly corner of the Sea through La Perouse 

 Strait with branches of the Tsushima current. Warm-water plankton forms 

 are also brought through the Kuril Straits by the warm Pacific waters into 

 the southeastern part of the Sea. Species of the genera Clytemnestra, Claudo- 

 calanus, and Pleuromamma can be mentioned among these warm-water 

 forms. 



In the most northwesterly part of the Sea (Bay of Sakhalin) not only are 

 estuarine and brackish-water species (Eurytemora asymmetrica, E. herdmani, 

 E. americana, Acartia bifilosa, Totanus derjugini, Sinacalanus tenellus) greatly 

 developed under the effect of the fresh water of the river Amur, but also the 

 true fresh-water groups (Rotifera and Crustacea) are plentiful. 



