110 R. T. Prentki et al. 

 Ammonification and Nitrification 



It is obvious from the uptake rates and concentrations that ammonia 

 was being rapidly regenerated. The regeneration rates were measured by 

 the isotope dilution method in which labeled ammonia is added to a 

 sample of water. A part of the water is immediately removed and the 

 isotope ratio of the ammonia fraction is determined. Following 

 incubation, the isotope ratio of the ammonia is again determined. The 

 results give a rate of dilution of the labeled ammonia by unlabeled 

 ammonia from other nitrogen fractions within the water. 



In Pond B in 1971 the ammonia supply rate averaged 1.9 ^g liter'' 

 hr ' (3 samples) while the overall range of eight pond measurements was 

 0.4 to 3.26. In Ikroavik Lake the mean of four measurements was 0.74 and 

 the range was 0.28 to 1 .05 Mg liter ' hr ' . These values are higher than the 

 uptake rates but of the same order of magnitude. Given these rapid rates 

 of resupply of ammonia in the ponds, it is very doubtful that nitrogen 

 would ever be limiting to phytoplankton growth. 



Unfortunately, the evidence for nitrification is indirect for the ponds. 

 One piece of evidence that nitrification is occurring is that high nitrate 

 levels are found in the ponds and in Ikroavik Lake early in the year, 

 suggesting that formation has outstripped uptake. Another bit of evidence 

 is that nitrifying bacteria have been isolated from tundra soils at Barrow 

 (Norrell personal communication). The last evidence is that Kinney et al. 

 (1972) have found significant nitrification in fresh and saline waters some 

 hundreds of kilometers east of Barrow. They were able to follow 

 conversion of added ammonia to nitrite and nitrate, as well as to follow 

 changes in water without added ammonia. The maximum potential 

 nitrification rate found was 3.0 Mg liter' day"' under ice in the winter. 

 While this is a slow rate, it is proof that nitrification occurs in arctic 

 waters. All available evidence suggests that nitrification is an active, 

 although slow, process in tundra ponds and lakes. 



Fertilization Experiments 



Based upon the data on concentrations of nitrogen in the pond water 

 (Figure 4-12, Table 4-8) and in the sediment (Table 4-9), we found that 

 most of the inorganic nitrogen was in the sediment. Yet the photosynthesis 

 rates in the water were so low that photosynthesis could be supported by 

 the DIN in the water with only a little DIN added from ammonification. 

 To gain more insight into the process of nutrient cycling, we asked, "What 

 would happen if phosphorus were added to a pond?" 



In the whole pond fertilization experiment carried out on Pond D in 

 1970, phosphorus addition of 0.3 mg P liter' was followed by an almost 

 immediate increase in particulate nitrogen in the water. This preceded an 



