112 R. T. Prentki et al. 



2 4 6 8 10 



Phosphorus Infusion, ^g P liter" doy" 



FIGURE 4-14. Rate of nitrogen uptake 

 [yig N (ixg N)-' hr'J in Pond B and C sub- 

 ponds, 1970, at daily phosphorus infusion 

 rates ofO, 2, and 10 yig P per liter of pond 

 volume. 



To try and resolve the question of a possible nitrogen limitation to 

 planktonic photosynthesis, larger (675 liter) subponds were constructed in 

 Pond B in 1972. The experimental treatments included complete darkness, 

 partial shading (50%), heating (+4C°), added lights, and added 

 phosphorus (5 and 25 ng P (liter of pond) ' day *). Nitrogen fixation 

 (Table 4-14) was stimulated by the added light and stopped by the shading. 

 Even more interesting, nitrogen fixation was strongly stimulated by added 

 phosphate. These data can also be expressed as the average seasonal 

 fixation per hour. When results from the two phosphate addition 

 experiments and the controls are plotted against the average seasonal 

 phosphate concentrations (Figure 4-15), it is seen that fixation was directly 

 proportional to P concentration both for dissolved reactive P (r = 0.98) and 

 for total dissolved P (r = 0.95). It should be noted, however, that no 

 response of fixation to added P was seen in the previous whole-pond 

 fertilization experiments or in the 1970 sub-pond fertilizations. 



The stimulation of N fixation by high amounts of P agrees with the 

 data reported in the review by Schindler (1977). He views the occurrence 

 of stimulation in lakes as one stage in the evolution of some lakes as they 

 receive larger and larger quantities of P from their drainage basins. In the 

 Barrow ponds, it is likely that the algae in the pond are usually phosphate- 



