Major Findings 13 



that chemical reactions in the surface sediments, especially those reactions 

 involving iron, set the concentration of DRP in the water and in this way 

 control the productivity of the ponds. 



Flux of Nitrogen 



The main inputs of nitrogen to the pond came from the rain water 

 (1 1.5 mg inorganic N m ' yr ') and from nitrogen fixation (28 mg N m ^ 

 yr"'). The ponds appear to accumulate some nitrogen each year but the 

 total, somewhat less than 80 mg N m^^ yr"\ is very small compared to 

 the 38,400 mg N m~^ stored in the top 5 cm of sediment. In the water 

 column, ammonia was more abundant than nitrate, 20 to 40 Mg 

 NH3 N liter ' vs. 2 to 13 Mg NO3-N liter \ Uptake in the plankton was 

 slow (Figure 1-10) so that turnover times ranged from 30 to 100 days for 

 the inorganic nitrogen. Measurements with ''NH3 indicated that the rate 

 of supply of ammonia from within the water column was high enough to 

 replace the NH 3 in 6 to 48 hr. As expected, we found no evidence of 

 nitrogen limitation upon the primary production of the algae in the pond. 

 When the uptake of nitrogen was used to calculate primary productivity, 

 by taking the ratio of C uptake to N uptake as 100 to 12, the results 

 exactly matched the ^"C primary productivity measurements. 



In the sediments, the interstitial water contained high amounts of 

 ammonia except where plant roots were present. For example, there were 

 0.7 to 2.7 mg NH3-N liter"' in the sediments in the plant-free center of the 

 pond but only 0.01 to 0.08 mg NH3-N liter"' inside a Carex bed at the 

 pond edge. Based upon production calculations, the Carex may turn over 

 all the ammonia each day. Despite this relatively high rate of removal, the 

 Carex appears not to be limited by nitrogen concentrations. 



The only evidence for a limitation by nitrogen was that nitrogen 

 fixation by sediment algae began when the ponds were continually 

 fertilized with phosphorus. It is likely that the algae were phosphorus- 

 limited; when excess amounts of P were added, the uptake rate of both 

 nutrients increased and eventually the N became limiting. At this point, 

 the blue-green algae gained a competitive advantage by fixing nitrogen. 



Effects of the Arctic Environment 



The annual primary production of the ponds is low, but this is largely 

 a result of the short ice-free season. When compared to the daily 

 production of other ecosystems the ponds are reasonably productive. 

 Thus, food supply is adequate in spite of the low temperatures. There is, of 



