Light Assimilation Curves of Surface Phytoplankton 255 



neritic forms. Oceanic forms, such as Chaetoceros atlanticus and 

 ThalassiotJirix longissima, were found only in a few samples. 



Carbon uptake by one mg of chlorophyll-rt per hour varied 

 from 0.8 mg to 7.1 mg, and that by one diatom cell varied from 

 0.3 X 10~^ mg to 7.4 X 10"^ mg. The variation of carbon uptake 

 by unit weight of chlorophyll or one cell of diatoms was thus 

 great from station to station. No correlation was shown between 

 the amount of carbon uptake and characteristics of population 

 or of environments. 



Figure 2 illustrates the light assimilation curves (photosyn- 

 tliesis-light intensity curves) obtained by fourteen experiments 

 made at different locations. In this figure the light intensity 

 is put on the abscissa taking the half of values of light intensity 

 emitted by four fluorescent tubes because of the reduction of 

 light by the plastic and the glass plates. The rate of photosyn- 

 thesis is plotted in percentage on the ordinate taking the 

 photosynthesis at the maximum light intensity (8000 lux) as 

 100 per cent. Since the maximum intensity of light provided was 

 pretty low, most of the assimilation curves did not reach the 

 light saturated region in photosynthesis. Photosynthesis became 

 light saturated only at about 5000 lux at Sta's 1 and 2, and around 

 8000 lux at Sta's 8, 11 and 12. So far as these five results are 

 concerned, the optimum intensity of light in photosynthesis was 

 far lower than in phytoplankton population reported in other 

 areas by previous workers. Steemann Nielsen and Jensen (5) 

 reported that phytoplankton photosvnthesis became light satur- 

 ated at about 30000 lux in the tropical water, at 20000 lux in 

 the Tasman Sea (46°S) in the summer, at about 15000 lux in the 

 North Atlantic (62°N). Steemann Nielsen and Hansen (6) again 

 reported that light satin-ation in photosynthesis in the Atlantic 

 was shown at about 13000 lux at 69°46'N, at about 10000 lux 

 at 63°35'N and at 6000-7000 lux at 62°00'N. In Davis Strait in 

 the North Atlantic (64°N) it was 11000 lux (7). Dim light of 

 the northern North Pacific in spite of length of daylight time in 

 the summer might have been responsible for the low intensity 

 of optimum light in photosynthesis. In the Bering Sea and 

 Aleutian region, and also western North Pacific misty weather is 

 often met with in the summer; phytoplankton in the surface water 



