plankton in it is well shown in a section from 

 the island of Iturup (Etorofu) eastward (fig. 6). 

 In that part where the section passes through 

 the waters near the Kuriles, where there is a 

 cold layer, the 100-200 m. level is shown to be 

 somewhat impoverished in plankton. However, 

 in the area 300-600 miles and more distant 

 from the Kuriles the plankton biomass in the 

 100-200 m. layer is higher than in the area of 

 the Kurile-Kamchatka Trench. 



of the zooplankton. Such quantitative poverty of 

 the plankton is also characteristic for other 

 regions of the Pacific Ocean lying between 20°H 

 and 40°N. latitude, where the vertical circula- 

 tion of the water is very weak. Only in the zone 

 of the equatorial divergence does the plankton 

 biomass again increase (Graham 1941). 



Seasonal Changes in the Plankton 



The impoverishment of this layer, in 

 comparison with the layers lying above and be- 

 low it, is very small or completely unnoticeable, 

 and the plankton biomass decreases evenly with 

 depth; in our opinion this is related to the dis- 

 appearance in these regions of the cold 

 intermediate layer. 



The vertical distribution of the plankton has 

 an altogether different character in the warm 

 waters of the Kuroshio, the temperature of 

 which drops slowly with depth and even at 400- 

 500 m. does not go below 8°-10°C. In the 

 uppermost surface waters, strongly illuminated 

 by the sun, the biomass is negligible. Below 

 10-25 m. it increases insignificantly and, after 

 reaching a maxinnunn at 50-100 m. , it again 

 gradually decreases with depth. At night the 

 quantity of organisms in the surface layers in- 

 creases somewhat as a consequence of the 

 ascent from the deeper layers of various 

 bathypelagic crustaceans, squids, and fishes. 



The small seasonal temperature changes in 

 the waters of the Kuroshio do not bring about 

 conditions permitting vertical circulation and 

 mixing of the water. Therefore the surface lay- 

 ers are not enriched with biogenetic elements. 



The absence of nutritive substances limits 

 the development of the phyto-, and consequently. 



We can give an evaluation of the seasonal 

 regime of the plankton only in the boreal 

 waters. 



In the spring months (May- June), in the 

 surface zone, there accumulate for feeding an 

 enormous quantity of zooplankton, consisting in 

 the main of Calanus tonsus, Calanus cristatus, 

 and Eucalanus bungii. 



In sumnner the crustaceans, having trans- 

 formed into the V copepodite stage and having 

 stored up fat, begin to descend deeper. Judging 

 by the data which we have on the plankton of the 

 Bering Sea, this downward movement proceeds 

 most intensively in the autumn, before the onset 

 of the winter homothermy. As a result of the 

 downward movement of the plankton into the 

 deeper layers of the water, the quantity in the 

 surface layers decreases appreciably. In 

 addition, in these areas whales, saury and other 

 plankton-eating fishes, and squid are feeding 

 heavily. This also diminishes the concentration 

 of plankton in the surface layers. Thus, in 

 May-July 1953 Calanus cristatus was encoun- 

 tered over the greater part of the aquatorium in 

 quantities greater than 5 per nn.-^ in the upper 

 100-m. layer, in August there were in all 1.6 

 per m.3, and in September and October almost 

 everywhere the quantity of this large crustacean 

 became less than 1 per m.-^. 



e iffnjjpyn 



Figure 6. --Distribution of the plankton biomass on sections in the 

 northwestern part of the Pacific Ocean, a - in a region with a 

 cold intermediate layer; b - in a region without a cold inter- 

 mediate layer. 1 - more than 500 nng. Irr\.\ 2 - 500-200 

 nng. /m.-*; 3 - less than 200 mg. /m,^. 



13 



