Temperature and nutrient supply interact to control nitrogen fixation in oligotrophic streams: An experimental examination

We performed two experiments to examine how temperature and nutrients interact to control dinitrogen (N-2) fixation, chlorophyll a (Chl a) biomass, and community composition of periphyton in subalpine oligotrophic streams in the Sawtooth Mountains of Idaho. We grew periphyton on nutrient-diffusing substrata (NDS) in a cold lake inlet (7 degrees C) and a warm lake outlet (18 degrees C). We then switched substrata between the two stream sites to test the effect of incubation temperature on N-2-fixation rates. Periphyton on substrata grown at both sites exhibited greater N-2-fixation rates when incubated in the warm outlet, which indicates physiologic temperature control. Periphyton on P-enriched NDS grown in the warm outlet had the greatest N-2-fixation rates, largest Chl a biomass, and largest percentage of N-2-fixing taxa of any treatment, which indicates that temperature and P interact to influence the community. In the second experiment, colonized rocks and uncolonized NDS were placed in cold (13 degrees C) and warm (18 degrees C) mesocosms. Within 2 days, warm temperature stimulated N-2 fixation by the rock periphyton community two times above cold temperatures, which indicates physiologic temperature control. After 45 days, warm temperatures and P enrichment led to Anabaena sp. in the periphyton community and the greatest rates of N-2 fixation observed in the experiment, which also indicates temperature and nutrient control at the community level. This study indicates that N-2 fixation and periphyton community composition in oligotrophic streams are controlled by both temperature and P supply, with temperature modulating the response to P.