I conducted field surveys and experiments to examine the role of nitrogen fixation in oligotrophic lakes and streams in the Sawtooth Mountains of Idaho. Nitrogen fixation surveys in three inlet-lake-outlet systems showed that nitrogen fixation rates were higher in outlet than inlet streams and high between 0.5 – 5 m in the benthic zone of lakes. Scaling nitrogen fixation rates to whole lakes and streams revealed that nitrogen fixation could contribute nitrogen (N) equal to 12, 18, and 32 percent of the nitrate flux into three lakes and 4, 15, and 136 percent into three outlet streams on a single day in July. These comparisons are conservative, as nitrogen fixation estimates made using our slurry incubation technique are underestimated 7.5 +/- 1.8 (SE) times compared to less disturbed algal communities. Comparing nitrogen fixation rates to stream nitrate and ammonium uptake rates revealed that nitrogen fixation rates in two outlets were statistically similar to nitrate uptake rates and between 10 and 100 percent of ammonium uptake rates, suggesting an important role for nitrogen fixation in these streams.
Experiments showed the temperature and nutrient supply interacted to control nitrogen fixation rates in Sawtooth mountain streams. A field bioassay using nutrient diffusing substrata (NDS) showed that N additions, with or without simultaneous phosphorus (P) addition, suppressed nitrogen fixation by 73 percent at 9 of 19 study sites. P additions alone significantly stimulated nitrogen fixation by an average of 2500 percent at five sites. A reciprocal transplant experiment where periphyton were grown on NDS in a cold lake inlet (7.1 degrees C) and a warm lake outlet (18.0 degrees C) showed that periphyton on P-enriched NDS grown in the warm outlet had the greatest nitrogen fixation rates and largest percentage of nitrogen fixing taxa of any treatment. These results were confirmed by a streamside temperature experiment using cold (13 degrees C) and warm (18 degrees C) mesocosms. After 45 days, warm temperatures and P enrichment stimulated Anabaena sp. in the periphyton community and caused the greatest rates of nitrogen fixation in the experiment, indicating that nitrogen fixation and periphyton community composition were controlled by both temperature and P supply with temperature modulating the response to P.