Primary Producers 209 



a.20'C 



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b. I4°C 



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^Periods of 

 Inhibition 



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FIGURE 5-20. Ratios of P 

 /o 'Pfrjaxfor the phytoplank- 



tonic and epipelic algae at 

 four temperatures (a-d), 

 1973. Solid areas represent 

 periods of inhibition; 

 hatched areas represent un- 

 dersaturation. Pond water 

 daily maximum tempera- 

 ture (e) is given in the bot- 

 tom panel. In each pair of 

 panels in a-d, the upper 

 panel represents epipelic 

 and the lower planktonic. 



temperatures. Since pond water temperatures seldom rose over 10°C in 

 1973, it is clear that in that summer the phytoplankton were indeed light- 

 inhibited during the middle portion of most days. The reasons for the 

 differing responses of the benthic and pelagic algae to high illumination 

 are unclear because of the difficulties in measuring true incident light 

 intensities for the benthic group. Thus, the epipelic algal community may 

 never reach inhibitory light intensities because of sediment shading of a 

 part of the population. 



The quantitative effect of photoinhibition on seasonal phytoplankton 

 productivity is probably even greater than would be predicted from 

 calculations of the duration of supraoptimal light intensities over the 

 summer. Since photoinhibition apparently results from chlorophyll photo- 

 oxidation (Yentsch and Ryther 1957), a process which is injurious to the 

 cell, photosynthesis does not recover immediately when light intensity 

 again falls below inhibitory levels. Instead, rates probably remain 

 depressed for a period of time proportional both to temperature and to the 

 absolute intensity and duration of the inhibitory illumination. In 

 consequence, the diurnal photosynthetic maximum may not occur in the 

 tundra ponds until midnight (Kalff 1969). 



