212 V. Alexander et al. 



T 3- 



FIGURE 5-22. Primary productivity of phyto- 

 plankton in Pond D before and after the addition 

 of 46.5 gPon25 July and 232 gPon28 July 1970. 



ammonia uptake by the microflora as might have been expected if 

 ammonification at the above-mentioned rates were the only means of 

 supply. Secondly, in phosphorus fertilization experiments in Pond D, 

 1970, during which algal productivity and biomass increased (see below), 

 the rates of ammonia uptake increased an order of magnitude (up to 2.1 to 

 3.5 Mg N liter "^ hr~') relative to control ponds. Given that average NH3- 

 N concentrations in the interstitial water of the center of the ponds range 

 from 2,000 to 3,000 Mg liter ', it seems clear that the additional ammonia 

 taken up in the absence of P-limitation came from the sediments. Finally, 

 it seems unlikely that the epipelic algae, despite their higher biomass in 

 comparison to the phytoplankton, would be N-limited in the presence of 

 such high levels of ammonia in the sediments. 



In contrast to the situation with nitrogen, a great deal of experimental 

 evidence suggests that the pond algae are strongly limited by phosphorus. 

 The most direct evidence comes from Pond D fertilization experiments in 

 1970 to 1972 (Figures 5-22 and 5-23). The first addition of phosphate (46.5 

 g P) to Pond D in 1970 resulted in a slow increase in phytoplankton 

 productivity, and a second, much larger fertilization (232 g P, which 

 resulted in a rise in concentration from 2.5 to 780 Mg P liter ^), completely 

 depressed productivity for the next few days. After this, productivity rose 

 to a peak (Figure 5-22) several times higher than that seen in other nearby 

 ponds in that same summer. After phosphate enrichments in the same 

 pond in July and August of 1971, epipelic algal productivity doubled 

 within a month in contrast to the normal pattern of declining rates in 

 control ponds during that period (Figure 5-23). 



The Pond D fertilization experiments show that phosphorus 

 limitation of algal photosynthesis in tundra ponds occurred throughout the 

 growing season and not just during periods of peak productivity as Kalffs 

 (1971) bioassays had indicated. All the 1970 enrichments to Pond D, as 

 well as the first two in 1972, were made in mid- to late July when 

 phytoplankton productivity in most ponds is normally very low. 



