152 



NIELSEN 



[CHAP. 7 



photosynthesis as a function of light intensity (Fig. 12) reaches a saturation 

 point. At even higher light intensities the process is depressed. Under ordinary 

 light conditions in nature, the algae found near the surface will be able to 

 utilize the absorbed light in the most efficient way only during a short period 



lOOr 



0. 



5,000 



Fig. 12. The rate of photosynthesis as a function of light intensity. (After Steemann 

 Nielsen.) 



Rate of photosynthesis, relative 



0.20 0.40 0.60 0.80 1.00 1.20 



(a) (b) 



Fig. 13. Rate of photosynthesis in surface water exposed at different depths; (a) South 

 China Sea, (b) South coast of Iceland during May. (After Steemann Nielsen.) 



of the day. During the greater part of the day light saturation is reached, or 

 still worse, light intensities prevail at which photosynthesis is depressed. 



Therefore, in bright weather the highest rate of photosynthesis is found not 

 at the very surface but at a depth where the light intensity is reduced by a 

 factor of about 2 or 3. This is the case both in the tropics and at higher latitudes 

 during the summer (Figs. 13a and b). 



