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



20 g/m 3 with 7 milliards of cells per 1 m 3 ; this conforms with the abundance 

 of (amorphous) silica in the surface layers of the soil (up to 35 per cent and 

 in some parts of the Sea more than 50). Arctic and Arctic-boreal species 

 are predominant during the spring maximum (Thalassiosira nordenskioldii, 

 Th. gravida, Fragilaria oceanica, Chaetoceros furcellatus, Bacterosira fragilis). 

 More thermophilic species {Chaetoceros constrictus, Leptoclyindricus danicus) 

 are predominant during the autumn maximum. Only one maximum — the 

 spring-summer bloom — is recorded in the north of the Sea of Okhotsk 

 (P. Ushakov, 1953). This is a sign of very severe climatic conditions. 



Table 315 



Investigators have established the very interesting fact that the range of 

 diatoms in the water column corresponds exactly with those in the bottom 

 deposits. Only the thin-shelled diatoms may dissolve at considerable depth 

 when sinking to the bottom. It has been found experimentally on the basis of 

 the research mentioned, that the distance from the coast can be estimated by 

 the composition of the diatoms in the deposits. In coastal areas neritic dia- 

 toms form 78 per cent of the deposits ; in oceanic regions the same percentage 

 is composed of oceanic species. 



The composition of the diatoms in cores of the soil (16 to 27 m) from great 

 depths of the Bering and Okhotsk Seas was examined by A. Zhuze (1954). 

 The whole thickness of the deposit, taken in the core, does not go beyond the 

 limits of the Quaternary Period ; moreover, the composition of the diatoms 

 according to horizons is exactly the same in both Seas. In the uppermost 

 layer (1-5 to 1-8 m) are found all the diatoms now living in the plankton, 

 mainly Coscinodiscus oculus iridis, С marginatus, Thalassiothrix longissima 

 and Rhizosolenia hebetata. 



The composition of the diatoms in the 1-5 to 5 m layer of the soil is very 



