Primary Producers 205 



the nutrient status of the algae (Fuhs et al. 1972, Caperon and Meyer 

 1972). Hence, a decline in /o 5 when P^ax is constant is evidence for light 

 adaptation, while an increase in /o s with a decline in Pmax is suggestive of 

 increasing nutrient limitation. 



Photosynthesis-light curves for the phytoplankton and epipelic algae 

 (Figure 5-15) follow the typical hyperbolic form seen both in algal cultures 

 (e.g., Rabinowitch 1951) and in some marine and freshwater 

 phytoplankton assemblages (e.g., Vollenweider 1965, Ryther and Menzel 

 1959); the curves we mentioned do not show the high variability often 

 encountered in temperate lakes. This tendency for algal assemblages from 

 extreme environments to react like unialgal cultures, also noted by 

 Goldman et al. (1963) for shallow Antarctic lakes, simplifies 

 interpretation of field data considerably. 



The /0.5 is also a temperature dependent variable. It increases as 

 much as 3-fold in the epipelic experiments (from 0.04 to 0. 1 2 ly min ~ ' on 3 

 July) over a 10°C range (Figure 5-16). A comparable change was seen by 

 Tailing (1957) in the similar parameter, /* (the intensity at which /* equals 

 0.71 Pmax), which he used as a measure of the onset of light saturation. As 

 Tailing pointed out, parameters such as /* or /o 5 should be expected to 

 vary with temperature because of the nature of the light-photosynthesis 

 curve. Theoretically, at low illumination the instantaneous photosynthesis 

 rate of a given population rises linearly with light intensity and is 

 independent of temperature. At higher illumination, photosynthesis 

 increases more slowly and eventually reaches a light-saturated optimum, 

 Pmax, which is temperature dependent (Rabinowitch 1951). So as Pmax 

 changes with changing temperature, /o 5 must also vary in order to 

 maintain a constant slope in the lower portion of the light curve. 



5 10 



Temperature , "C 



FIGURE 5-16. Effect of temper- 

 ature on the light half-saturation 

 20 for photosynthesis (Iq^ on three 

 sampling dates in 1973. 



