Primary Producers 215 



T. 20 



0) 



Q. 



cr 

 o 



4. 



10 



52.6- 



10 ■ 20 

 Jun 



J^J. 



10 20 

 Jul 



L_ 



Aug 



2.0 



FIGURE 5-24. a) Concentrations of dissolved re- 

 active phosphorus in Pond D, 1971. b) Ratio of 

 ^max ^^ biomass of phytoplankton (8°C) in Pond 

 D, 1971. 



temperature assuming a photosynthesis Q\o of 3.0) parallel productivity 

 (Figure 5-19). Since the ratios of P to 5 often decline with increasing 

 nutrient limitation, these patterns suggest that increasing PO4 causes the 

 spring increase in productivity. Soon, the increased algal production 

 depresses PO4 concentrations; because uptake rates for PO4 are so much 

 faster than transfer of PO4 upwards from the sediment this nutrient 

 limitation causes the decline in the bloom. Further evidence of limitation is 

 seen in the fertilization experiments in Pond D, 1971 (Figure 5-24). 

 Following addition of phosphate on 14 July, the P to B ratio increased 

 quickly from 0.49 to 1.54 and then declined as the PO4 concentration 

 declined. The same response also occurred following fertilization on 1 

 August. 



Despite these clear-cut demonstrations of general phosphorus 

 limitation in the ponds, short-term (2- to 4-hour) '''C bioassay tests for the 

 effect of added phosphorus during 1972 and 1973 always gave negative 

 results with both phytoplankton and epipelic algae samples (Figure 5-25). 

 This was true regardless of the combination of temperature and light 

 intensity used. Thus, the effects of phosphorus limitation, although 

 important over the long term, are quantitatively insignificant over a short 

 period in comparison to the effects of light and temperature. 



