SECT. 2] 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



153 



The most efficient utilization of the absorbed light takes place near the lower 

 boundary of the photic zone. Light saturation never occurs here. It has been 

 demonstrated (Steemann Nielsen and Hansen, 1959 ; Ryther and Menzel, 1959) 

 that the algae are able to adapt those enzymes active in photosynthesis to the 



10,000 

 Lux 



15,000 



Fig. 14. Light intensity and relative rates of net photosynthesis in arctic summer plankton 

 from three different depths ; 1 = surface, 2 = 25 m, 3 = 50 m. (After Steemann Nielsen 

 and Hansen, 1959.) 



100 - 



tE 



Fig. 15. Photosynthesis-light intensity curves of Sargasso Sea phytoplankton from depths 

 to which 100% (open circles), 10% (half-filled circles) and 1% (filled circles) of the 

 surface light penetrated. October. The whole abscissa = 12 x 10 3 ft-candles. (After 

 Ryther and Menzel, 1959.) 



different light intensities and temperatures found at the habitat (see Fig. 14). 

 At the same time the light-induced decrease in photosynthesis in algae adapted 

 to high light intensities sets in only at very high light intensities, whereas this 

 decrease in "shade" adapted algae is found already at relatively low in- 

 tensities (Ryther and Menzel, 1959; and Fig. 15). 



