Light Assimilation Curves of Surface Phytoplankton 257 



may not usually be exposed to bright sunlight. 



This may suggest tlie fact of adaptation of phytoplankton in 

 their photosyntliesis to the dim daylight in their habitat as stated 

 by Ryther and Menzel (4) and by Steemann Nielsen and Hansen 

 (7). Ichimura (3) experimented on adaptation to light by lake 

 plankton and found that the effect of past history of light con- 

 ditions did not affect the photosynthetic character of algae for 

 long after the algae were transferred to a habitat with different 

 light conditions. Steemann Nielsen and Hansen (7) stated that 

 the rate of the enzymatic process in photosynthesis increases with 

 increase in temperature, and that adaptation is brought about 

 simply by establishing a higher or lower concentration of the 

 enzymes active in photosynthesis. 



Populations in which a single leading species occupied more 

 than 60 per cent of the total cells were found at Sta's 1, 2, 5, 8, 

 9, 11, 12, 13 and 14 (Table 1). The population at Sta's 1 and 2 

 was mainly composed of Chaetoceros clebilis which is known as 

 a boreal neritic species. The photosynthetic activity of this pop- 

 ulation reached the maximum at the light intensity of about 

 5000 lux, while the optimum light intensity for pliotosyntliesis 

 of the population at Sta's S, 11 and 12 which was mainly com- 

 posed of a boreal neritic form, Fragilaria striatula, was at about 

 8000 lux or more; a similar population (Fragihria) at Sta. 9 

 did not reach light-saturated region at 8000 lux. The photosyn- 

 thesis of the population mainly composed of a cold water form 

 Denticiila marina at Sta. 5 seemed to be almost light-saturated 

 at 8000 lux. The mixed population with a boreal oceanic species, 

 Chaetoceros atJanticus, and an oceanic temperate species. 



Fig. 2. Light assimilation curves obtained at 14 stations in the North 

 Pacific 42°N - 61°N. 1) 43°13'N 151°05'E (morning sample), 2) 50° 

 09'N 170°00'E (morning sample), 3) 54°56'N 170°10'E (morning sample), 

 4) 57°00'N 173°19'E (morning sample), 5) 57°00'N 174°50'E (midday 

 sample), 6) 48°24'N 173°11'W (morning sample), 7) 48°22'N 172°00' 

 W (midday sample), 8) 53°09'N 172°00'W (morning sample), 9) 53°3r 

 N 171°55'W (midday sample), 10) 60°14'N 173°24'W (morning sam- 

 ple), 11) 61°01'N 173°24'W (midday sample), 12) 44°06'N 168°40'\V 

 (morning sample), 13) 43°20'N ]70°03'W (midday sample), 14) 49° 

 19'N 174°08'E (morning sample), and 1.5) 42°19'N 172°52'E (midday 

 sample). 



