sediment would become colorless (small sub- 

 tropical and tropical species of copepods). 



In 1953 on the southern stations of the 

 meridional sections we encountered tropical 

 plankton. Evidence that the vessel actually 

 reached the area of the true tropical plankton 

 fauna was the common occurrence of forms 

 found by us and listed in the station catalogues 

 of plankton forms from the tropics (Wilson 

 1942, Sewell 1932, 1947 and 1948, Vervoort 

 1949, and others). Thus, as common forms 

 there appeared species of the genera 

 Acrocalanus, Clausocalanus , Undinula, Can- 

 dacia, Pleuromamma, Eucalanus, Canthocala- 

 nus, Neocalanus , Nannocalanus, Corycaeus, 

 Cjpilia , Sapphirina , and many others. How- 

 ever, establishing the similarity of the plank- 

 ton fauna studied in the 1953 collections with 

 the tropical plankton from the open sea, it is 

 possible to establish also the well-known spec- 

 ificity of the plankton of the area of the 

 Karoshio under study, i. e. , the presence in 

 this plankton of a significant mixture of neritic 

 forms making up at times in ternns of the 

 number of individuals the predominant part (at 

 some locations the first place in ternns of quan- 

 tity of specimens in the samples was occupied 

 by Penilia avirostris). 



Plankton of such a composition is typical 

 of coastal areas in the tropical latitudes, par- 

 ticularly at the Great Barrier Reef (Farran 

 1936), on the west coast of India (George 1933, 

 Chidambaram and Menon 1945, Jacob and 

 Menon 1947, Chacko 1950 and others), on the 

 Australian coast (at Sydney) (Dakin and Colefax 

 1933), and from other places in the tropics. In 

 the works listed, particularly in the Indian 

 ones, there are indications of the predominance 

 of Penilia avirostris, species of the genera 

 Acartia , Labidocera, Noctiluca, Acantho- 

 metridae among the Radiolaria and salps in the 

 plankton. 



The neritic character of the plankton in 

 our samples varied with the time and place of 

 collection. Thus, in July it showed up mostly 

 in the southern part of the western sections, 

 while in September such neritic species as 

 Labidocera japonica and others extended all the 

 way to the easternmost section. It is possible 

 to postulate that the water of the Kuroshio 

 passing along the coast of Japan is somewhat 

 transformed and produces favorable conditions 

 for occurrence of the Japanese neritic fauna at 

 a great distance from Japan, almost to the area 

 lying south from Cape Lopatka in Kamchatka, 

 i.e., over a distance of almost 1,000 miles. 

 However, the neritic character of the plankton 



did not extend to the southernmost stations of 

 the 1953 sections. Thus, in July on the western 

 section neritic species were not encountered 

 south of 36°56'N. latitude or were represented 

 by single individuals. South of this latitude the 

 plankton took on the character of the oceanic 

 tropical plankton. It is difficult to say how far 

 the region occupied by this plankton extends, but 

 judging by the character of the circulation of the 

 water the fauna found at the southernmost 

 stations of the 1953 sections probably extends 

 over a greater area, perhaps all the way to the 

 North Equatorial Current. In other words, in 

 the northwestern part of the Pacific Ocean only 

 the northern boundary of the tropical plankton is 

 clearly pronounced, and to the south of it over a 

 broad area there occurs a plankton of similar 

 composition (Wilson 1942). We know little about 

 the southern boundary of the tropical plankton in 

 the Southern Hemisphere (Steuer 1933). 



The zonality in the distribution of the 

 plankton of which we have spoken earlier is nnost 

 clearly pronounced on the sections running from 

 the southern half of the Kurile chain and becomes 

 unclear with the fading out of the boundaries of 

 the zones in the easternmost part of the area in- 

 vestigated. This evidently is connected with 

 changes in the position and most importantly the 

 force and character of the Kuroshio as that 

 current moves farther from the Japanese coast 

 to the east (see Marine Atlas, Vol. 2). 



Comparison of the distribution of the plankton 

 with the circulation of water in the surface 

 layer . By comparing our data on the plankton 

 with Soviet and Japanese material on the circu- 

 lation of the water we disclose a great deal of 

 coincidence with our hydrological data. Thus, 

 the general regionalization of the plankton made 

 on the basis of the 1953 material reflects very 

 well the scheme of the circulation of the water. 

 The sharp change in the plankton (between the 

 zone of Calanus tonsus and the tropical zone) 

 clearly indicates the northern frontal zone of the 

 Kuroshio. The distribution of certain species 

 indicates the presence on the one hand of nuclei 

 of cold water in the area near the Kuriles (dis- 

 tribution of Calanus cristatus) and on the other 

 hand the existence of tongues of warm water ex- 

 tending through the straits of the Kurile chain 

 into the Sea of Okhotsk (distribution of Calanus 

 pacificus ). 



Not only can we follow very well through the 

 changes in the connposition and distribution of 

 the plankton the distribution and origin of the 

 water masses, but through the seasonal changes 

 in the plankton we can even define the speed of 

 transport of the water. Thus, a comparison of 



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