October 1954. For collecting the plankton 

 Juday nets of No. 38 gauze with an entrance 

 area of 0. 1 m.^ and of No. 140 gauze, 0. 5 m.^ 

 were used. With these nets stratified hauls 

 were made at the 0-10, 10-25, 25-50, 50-100, 

 100-200, and 200-500 meter levels. Samples 

 were weighed, examined under a binocular 

 microscope, and counts were nnade of the larg- 

 er species such as Euphausiacea, Calanus 

 tonsus, Calanus cristatus, Parathemisto 

 japonica, et cetera. 



Distribution of the Plankton Complexes 



The general distribution of the plankton in 

 the northwestern part of the Pacific Ocean de- 

 pends to a great extent on the character of the 

 water mass and on seasonal phenomena in the 

 area of observations. 



The region of the Pacific Ocean investi- 

 gated by us can be divided into two zones. The 

 first is a zone of boreal waters occupying the 

 whole region from the Komandorsky and 

 Aleutian islands to 40°N. latitude. The south- 

 ern boundary of this region does not run in a 

 straight line but meanders in response to the 

 extensions of the northern front of the warm 

 waters of the Kuroshio. It is characterized by 

 significant changes in the temperature regime 

 of the waters in the course of the year. Thus, 

 the investigations of the Vitiaz ' in Kurile 

 waters in May-July 1953 showed that the tem- 

 perature of the surface layer of the water 

 varied from 1° to 5°C. In the same area in 

 August-September 1954 the temperature rose 

 to 9°-15°C. 



Because of the connparatively great winter 

 cooling of the waters, taking in, thanks to con- 

 vective mixing, the layers of water to a depth 

 of 200 m. , and the summer warming of only a 

 comparatively thin layer of the surface waters, 

 in a considerable part of the aquatorium of the 

 boreal region, and particularly in the areas 

 adjacent to the Kurile Is. and Kamchatka, 

 throughout the year a cold intermediate layer 

 is maintained with temperatures below 1°- 1 . 5°C. 

 The presence of a layer of cold "winter" waters 

 materially influences the vertical distribution 

 of the I lankton (Vinogradov 1954, 1955). 



Waters coming from the Okhotsk and Bering 

 seas have a great sicnificance in the formation 

 of the water masses of this region. Through 

 the interaction of the waters of the ocean with 

 the waters of the Okhotsk and Bering seas in 

 the area of sea lying over the Kurile-Kamchatka 

 Trench there is formed an area of mixing of 

 waters, where, thanks to the stron.j vertical 



circulation, we can observe a phenomenon 

 analogous to the "polar front". 



Compared with the Kurile waters, the 

 warm waters of the Kuroshio are characterized 

 by small seasonal fluctuations in temperature. 

 The cold intermediate layer is absent. Warm 

 waters with temperatures above 10°C. occupy 

 the whole mass to a depth of 400-500 m. The 

 differences in origin, temperature regime, and 

 the character of the stratification of the water 

 masses of the boreal region and the waters of 

 the Kuroshio are reflected above all in the 

 composition of the plankton. 



The moderately cold waters of the northern 

 regions with surface temperatures of 9°-15°C. 

 (August-October) are inhabited by a rather 

 homogeneous plankton complex, common to the 

 waters of the Okhotsk and Bering seas. Most 

 characteristic of this complex are such species 

 as Calanus tonsus, C^. cristatus , Eucalanus 

 bungii, Sagitta elegans (and below 100 m. also 

 Eukrohnia hamata), Parathemisto japonica , 

 and Euphausia pacifica. 



All of these species can form concentra- 

 tions of a very high biomass, as much as 

 2 g. /m. . In the areas adjacent to the Kuriles 

 and Hokkaido they are joined by Metridia 

 okhotensis, and in the region of the continental 

 shelf a notable role may be played by Pseudo - 

 calanus elongatus and larvae of bottom animals. 



Besides these species found in great 

 masses, the plankton complex of the moderate- 

 ly cold waters is characterized by the presence 

 of a series of less numerous but constantly en- 

 countered forms such as, for example: 

 cope pods — Metridia pacifica, Candacia 

 columbiae, Pleuromamma scutulata, 

 Pareuchaeta japonica ; amphipod8-- Cyphocari3 

 challengeri and Hyperia galba ; the medusa 

 Aglantha digitale; and anumber of other species. 



Directly in the zone of contact of the cold 

 waters of the moderate region with the warm 

 waters of the Kuroshio the aspect of the zoo- 

 plankton swiftly and sharply changes. In the 

 surface layers, with a rise in temperature to 

 17°C. the species characteristic of the more 

 northern areas disappear and go down below 

 100 m. or even 200 m. As a consequence of 

 the disappearance from the surface zone of the 

 most abundant species of cold-water plankton, 

 the significance of Calanus pacificus and a num- 

 ber of small copepods in this zone increases; a 

 considerable quantity of species, absent in the 

 more northern areas, appears, Penilia aviros - 

 tris and Pleuromamma sp. , in particular. 



