Primary Producers 201 

 FACTORS CONTROLLING ALGAE 



Introduction 



Primary production in any ecosystem depends on both the standing 

 crop of producer organisms and their rate of net carbon fixation per unit 

 biomass. The environmental factors controlling biomass and 

 photosynthesis are interactive but their effects can be examined 

 individually. The ultimate purpose of such experiments was to explain the 

 highly characteristic daily and seasonal patterns of productivity and 

 biomass (Figures 5-5, 5-6, 5-10, 5-13) seen for the phytoplankton and 

 epipelic algae of these ponds. The primary biomass controls for plankton 

 and epipelic algae appear to be zooplankton grazing and sediment mixing, 

 respectively, while the important photosynthetic controls are phosphorus 

 limitation, light intensity, and temperature. 



The basic approach for assessing short-term effects was comparison 

 of '"C-productivity estimates of natural samples exposed to different 

 amounts of light, different temperatures, and different nutrient 

 concentrations. These 4-hour experiments were done at weekly intervals 

 throughout the summer and occasionally over 24-hour diurnal cycles 

 (details in Stanley 1976b). A variety of techniques were used for studying 

 nutrient uptake directly including ^^P and '^N tracers, analytical water 

 chemistry, and cell extraction procedures. Longer term effects were 

 evaluated by comparing biomass, productivity, and species composition 

 changes in whole ponds and subpond enclosures before and after treatment 

 with heat, nutrients, oil, etc. The magnitude of grazing effects was 

 determined from experimental measurements of filtering and ingestion 

 rates of the zooplankton and benthic herbivores and from experiments in 

 which the zooplankton were removed from ponds. 



Temperature 



The generally cold climate of the Barrow site, with the resultant brief 

 open-water season and low water temperatures, is undoubtedly a major 

 cause of the low annual production levels of both phytoplankton and 

 epipelic algae. For some reason, the responses of the pelagic and benthic 

 photosynthetic processes to temperature were very different (Figure 5-14). 

 The temperature optimum of the phytoplankton averaged about 15°C over 

 the summer, but the optimum of the epipelic algae exceeded 20°C, the 

 highest temperature tested. This difference can only rarely be of con- 

 sequence since the ponds average only 5° to 9°C during the summer and 

 seldom waim to above 15°C. However, differences in the photosynthetic 



