222 V. Alexander etal. 



10-3 



o 



E 



Q. 



'S 10-'' 



o 



E 



IQ-S - 



8 14 



Temperature , °C 



20 



FIGURE 5-28. The production to bio- 

 mass ratios in three groups of plankton 

 at four temperatures, 1972. 



expect to find is shown in Figure 5-28 where production to biomass ratio is 

 plotted against temperature for three distinct organism types. With more 

 data, such information could yield answers to questions of adaptations of 

 phytoplankton populations and species dominance. 



Another pathway of control of species composition leads from the 

 zooplankton. As described earlier, we noticed that there was a change in 

 species composition of the phytoplankton when oil was added to a pond 

 (Figure 5-4). The same result, a replacement of Rhodomonas by 

 Uroglena, was also found when the zooplankton were removed from 

 subponds (Miller et al. 1978a, 1978b). Also, the Rhodomonas reappeared 

 in Pond E, the 1970 oil treatment pond, at the same time that the 

 zooplankton reappeared (1976). The mechanism for this apparent control 

 is likely the grazing of the zooplankton which keeps the Uroglena at a low 



