Primary Producers 223 



level. Another possibility is control through the regeneration of nutrients 

 by zoopiankton; in view of the strong control of nutrients by sediment 

 processes, this should have little effect. 



There is also some control mechanism that keeps diatoms from 

 becoming important in the phytoplankton (they are present in the 

 sediments). This mechanism could be the very low concentrations of silica 

 in the water (almost always less than 0.5 mg liter'). Silica is present at 

 low concentrations in the rainwater and runoff water (e.g., less than 0.1 mg 

 liter ' in the snowmelt in 1971) but does increase when water comes in 

 contact with soil (e.g., 1.5 mg Si liter ' in groundwater, 20 July 1970). 

 Concentrations are low in the water, however, and it is possible that the 

 sediment diatoms keep the silica at this low level. Papers by P. Kilham 

 (1971) and S. Kilham (1975) have pointed out the control of diatom 

 species by the silica concentrations. 



Conclusions 



The algae in the plankton are controlled by both zoopiankton and 

 nutrient supply. When zoopiankton are removed, there is an immediate 

 increase in both production and biomass of the algae. The same increase 

 can be obtained by fertilization of a pond with phosphorus but the 

 response takes many weeks. In nature, algae could increase if the rate of 

 supply of nutrients increased and if the zoopiankton increased very slowly. 

 Usually, the phosphorus is tied up in the sediments and is released slowly. 



The algae at the sediment surface live in a very nutrient-rich 

 environment relative to that of the phytoplankton. However, they can only 

 photosynthesize in a layer a few millimeters thick because of the rapid 

 absorption of light by the sediments. Animals are not important in 

 removing algae by grazing but do remove algae from the surface of the 

 sediments by their activity which mixes algae below the euphotic zone. 

 This is the main control of the epipelic algae and explains why there is no 

 algal mat in these ponds. 



Temperature also keeps algal production at a low level but the effects 

 of raising the temperature are unclear because of the interaction of 

 respiration and other processes. It is possible that the respiration of algae 

 would increase faster than the gross photosynthesis and result in a lowered 

 net photosynthesis at high temperatures. Also, the animals would respond 

 to higher temperatures by increasing their grazing rate or their "sediment 

 mixing" rate. Thus, the rates of individual processes would be speeded up 

 by an increase in temperature but there may well be ecosystem 

 compensation so that there is no overall effect of increased temperature. 



