Primary Producers 203 



environment. The pond sediments are, in fact, warmer (up to 4°C on sunny 

 days) than the overlying water; the sediment algae, with their higher 

 optima, appear to take advantage of this fact. The planktonic algae, in 

 contrast, appear to consist of species which photosynthesize more 

 efficiently at the lower temperatures of the pond water but which are more 

 strongly inhibited when temperatures are high. Since our ^lo's are 

 calculated from Pma^ values rather than from rate measurements at a 

 single light intensity, the increased efficiency of planktonic photosynthesis 

 at low temperatures is probably a result of increased enzyme activity in 

 the dark reactions of photosynthesis. Low temperature optima, and 

 presumably increased photosynthetic efficiency at low temperatures, have 

 been seen by Goldman et al. (1963) in plankton in the Antarctic and by 

 Bennett and Hobbie (1972) in a planktonic alga from Swedish Lapland. A 

 direct comparison of Pmai per unit carbon for similar arctic, subarctic, and 

 temperate species would show the magnitude of this effect at low 

 temperatures. 



No clear seasonal trends in the ^lo's or optimal temperatures were 

 observed for the phytoplankton despite the species succession that 

 occurred each year (Figure 5-2). This suggests that the temperature 

 responses of the dominant phytoplankters were similar. However, we were 

 not able to make measurements on individual species. 



In general, then, normal diurnal and seasonal temperature 

 fluctuations are capable of changing photosynthetic rates by as much as 50 

 to 75%. The relationship will normally be a direct one, since average pond 

 temperatures are well below those for the algal optima. However, higher 

 temperatures also stimulate other processes, such as respiration and 

 herbivore grazing, which tend to reduce net photosynthesis or biomass. 

 For this reason, the net effect of temperature increases on algal biomass in 

 the ponds depends on whether photosynthesis can outpace the measured 

 losses. For example, when temperature was increased 4°C in a sub-pond 

 enclosure in Pond B in 1972, epipelic algal productivity showed no increase 

 relative to the control sub-ponds and to Pond B itself. 



Light 



The hyperbolic form of photosynthesis-light curves for algae is well 

 known (Vollenweider 1965, Steele 1962). Over a low illumination range 

 the rate of photosynthesis varies almost linearly with light intensity (/), 

 while at higher light intensities the rate rises more slowly and eventually 

 reaches a maximum rate, P max, at light saturation. The relationship can be 

 described by 



P=Pma.{f){I + h.5)'' 



where P is the photosynthesis rate per unit biomass under a given set of 



