204 



V. Alexander etal. 



temperature and light conditions, Pmai is the light-saturated rate at the 

 same temperature and /o 5 is the light intensity at which P equals 0.5 Pmax. 



The half-saturation light intensity, /0.5, can be interpreted as an 

 integrated, inverse measure of photosynthetic efficiency, since it changes 

 when either P max or the slope of the photosynthesis-light curve at 

 subsaturating illumination changes. The Pmax, which is proportional to the 

 rate of the dark reactions of photosynthesis, varies directly with 

 temperature, as shown above, and inversely with the degree of nutrient 

 limitation of the phytoplankton assemblage (Myers and Graham 1971, 

 Thomas and Dodson 1972). It remains relatively constant, however, when 

 light changes (Steele 1962, Myers 1970). On the other hand the slope of the 

 curve at low light intensities, which is proportional to the ratio of 

 chlorophyll to carbon in the cell and hence to the rate of the light reactions 

 of photosynthesis, varies inversely with the light intensity ("sun-shade" 

 light adaptation, Steemann Nielsen and Hansen 1961) and directly with 



. 0. Epipelic Algae 

 Pond A 



c 1.2 



o 

 o 



0.8 



0.4 - 



0.1 



0.2 



0.3 



0.4 



Light Intensity, ly min" 



FIGURE 5-15. A typical set of photosyn- 

 thesis-light curves for the tundra pond algae, 3 

 July 1973. 



