Primary Producers 245 



Total production was around 10 g C m"^ yr ', about 10 times the 

 planktonic production. 



Factors Controlling Algae 



Planktonic algae have a temperature optimum for photosynthesis 

 between 15° and 20°C while the epipelic algae have an optimum above 20° 

 (likely above 25°). The Qio response of photosynthesis is also slightly 

 different as it is 3.0 for the plankton and 2.2 for the epipelic algae (2.5° to 

 12.5°C). In fact, if higher temperatures did occur, the algal productivity 

 might not increase. Thus, we found that a 4°C increase in the temperature 

 of an experimental pond did not change the productivity. The reason for 

 the lack of a temperature effect may be that the rates of algal respiration 

 and animal grazing also increase. 



Light usually limits photosynthesis (Ps) in lakes but the ponds are so 

 shallow that there is always more than enough light for the algae to have 

 maximal rates of Ps. During June and the first half of July, there was even 

 adequate light for net Ps to proceed all night. When there were no clouds 

 or fog, the Ps rate at midnight was only 50% less than the rate at mid-day 

 or mid-afternoon. Experiments relating Ps to light and temperature 

 showed that the Ps increased to a maximum as the light increased (the 

 curve was similar to a Michaelis-Menten saturation relationship). At low 

 temperatures, 2° to 8°C, the Ps of the planktonic algae was inhibited at 

 high light levels (>0.2 ly min"^) while the Ps of epipelic algae was not 

 inhibited. This may be a result of a slowed rate of repair of photo-damage 

 to chlorophyll. The lack of inhibition of the epipelic algae may be a result 

 of shading from sediment particles. It was impossible to separate the 

 sediment from the algae so some algae was always wholly or partially 

 shaded. Increasing the amount of light, therefore, will increase the Ps even 

 if some algae might really be inhibited. 



The half-saturation level of the Ps by light, /o.s, is a useful value for 

 comparison of the phytoplankton responses at different times of the year. 

 In general, they fell throughout the year; this indicates that the algae 

 respond to the lowered light levels by becoming more efficient. A very 

 rapid decrease in /o 5 in late June and early July may indicate a nutrient 

 limitation. The specific growth rate (mg C fixed (mg algal C) ' ' day ') 

 ranges from 0.2 to 3.0 for the phytoplankton and from 0.4 to 1.0 for the 

 epipelic algae. These values, especially those for the plankton, are 

 extremely high (reproduction several times per day). The actual 

 populations do not change as rapidly so there must be an almost equally 

 high removal rate. For the planktonic algae, the removal is due to grazing 

 by the zooplankton; for the epipelic algae, the removal is due to downward 

 mixing of the algae into the sediment. 



