188 



V. Alexander et al. 



500 



400 



'E 



?'300 



E 200 

 o 



CD 



100 



FIGURE 5-4. Biomass o/ Rhodomonas and Uroglena in 

 Pond E, 1970. Oil was added to the pond on 16 July. 



the Chrysophyte Uroglena replaced Rhodomonas. In other ponds, 

 Uroglena was usually present but always rare. 



This experimental oil addition was repeated in the summer of 1975 

 with the same results: Rhodomonas was replaced by Uroglena. However, 

 the main effect of the oil was always on the zooplankton which were all 

 killed immediately (see Chapter 9). For this reason, the zooplankton were 

 removed from an experimental pond (Miller et al. 1978a, 1978b); the 

 Rhodomonas again was replaced by Uroglena. There are two possible 

 explanations of this relationship: either the grazing of the zooplankton 

 indirectly controls the Rhodomonas, perhaps by keeping the Uroglena at a 

 low level, or there is a control through nutrient regeneration by the 

 zooplankton. Because the sediments seem to control the nutrient supply 

 rate here, we believe that the grazing of zooplankton controls the 

 Rhodomonas, either directly or indirectly. 



Primary Productivity 



Phytoplankton photosynthesis was measured with ^^C-HCOs. All 

 measurements were made in the morning with a 3- to 6-hour incubation in 

 125 ml bottles placed on their sides on the bottom of the pond. During 

 1970 and 1971, subsamples were filtered through 0.45 /um HA Millipore 

 filters, the filters rinsed with 0.005 N HCl, and the activity taken up 

 determined with a gas-fiow planchette counter. In 1972 and 1973, the 

 filtration step was omitted (Schindler et al. 1972); instead, the incubated 

 sample was acidified with 12 N HCl to a pH of 1 and then flushed with air 

 to remove the inorganic '^C. A portion of this subsample was then mixed 

 with Aquasol and counted on a liquid scintillation counter. This procedure 

 measured not only the '^C fixed and retained within the algal cells but also 

 the '^C lost from the cells as dissolved organic carbon. The uptake data 



