246 V. Alexander et al. 



The effect of nutrients on photosynthesis of the algae could not 

 usually be demonstrated with the short-term '""C experiments (less than 4 

 hours). However, there were dramatic changes in the Ps rates when 

 nutrients were added to entire ponds. Nitrogen had no effect; as described 

 in Chapter 4, the rate of ammonification in the plankton is likely high 

 enough to supply all the needs of the algae and the sediments contain large 

 quantities as well. In contrast, P concentration is controlled by the 

 sediment. As described in the previous chapter, large amounts of 

 phosphorus are adsorbed onto iron hydroxides in the sediment; the 

 equilibrium allows only a few ng P liter"' to exist in the water column. A 

 relatively high rate of input of P in the spring allows the algae to grow 

 rapidly and bloom. The input soon ceases as runoff stops and the algae 

 rapidly take up the P faster than it can be resupplied by the sediment. As a 

 result, concentrations fall and the algal bloom ends. 



Additional evidence for the importance of phosphorus in controlling 

 the primary productivity comes from the fertilization of whole ponds. 

 When phosphorus was added there was actually a decrease in productivity 

 (inhibition) followed several days later by a rise to peaks several times 

 higher than those of productivity in control ponds. Epipelic algae also 

 responded to fertilization with P; their productivity doubled within a 

 month. 



Ponds where the iron-P trapping mechanism was not well established, 

 such- as newly formed thaw ponds with no accumulated sediments, had 20 

 times the P concentration and 200 times the primary productivity of 

 control ponds at the same date. 



There are still many remaining problems of the interactions between 

 algae and nutrients. For example, what is responsible for the rapid uptake 

 of phosphorus and its release as a low molecular weight organic compound 

 (XP)? Do the algae do this? The bacteria? Is it in a form usable by algae? 



Grazing by zooplankton could be another mechanism controlling the 

 algae in the pond. The dominant zooplankton was a large Daphnia; each 

 animal is capable of filtering 2-6 ml of water per hour at ll°C. The whole 

 population probably filters all of the pond water every 2 days (range, 0.5 to 

 12 days). Two different calculations, one a back-calculation from the 

 zooplankton production and the other based on measurements of 

 zooplankton filtering rates under different conditions of temperature and 

 food density, indicate that about 20% of the total algal productivity is 

 grazed by zooplankton. At certain times of the year the grazing pressure is 

 intense, and so zooplankton could also be very important as a control. 

 When the zooplankton were removed from a pond, the biomass and 

 primary productivity of the algae increased but this increase was much less 

 than expected. Evidently other factors immediately became limiting. In 

 addition to these small changes, the zooplankton removal caused one of 

 the dominant species, Rhodomonas minuta, to be replaced by Uroglena, 

 but it is not known whether the replacement was a result of the release of 



