『Abstract
　Diel and synoptic studies were undertaken in Silver Bow Creek, a small, highly eutrophic stream receiving municipal sewage from the city of Butte, Montana, USA. During mid-summer baseflow conditions, oxidation of ammonium from the Butte wastewater treatment plant created a 2-km long reach marked by nightly hypoxia and extreme growth of aquatic plants and algae. Large diel cycles in the concentrations and isotopic compositions of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were inversely related, and are explained by the daily cycle of photosynthesis and respiration, modified by the effects of nitrification. Nutrification rates were higher during the day than at night due to a combination of higher water temperatures and higher DO concentrations. Changes in nitrification rate imparted diel cycles in the concentrations and N-isotopic compositions of NO3^- and NH4⁺, whereas total dissolved inorganic nitrogen (DIN) concentrations and δ¹⁵N-DIN values showed minimal diel variation. δ¹⁵N-NH4 steadily increased with distance downstream over a 5 km reach, whereas δ¹⁵N-NO3 showed a more complex spatial pattern. Plant assimilation caused downstream decreases in DIN and soluble reactive phosphorous: however, it was not possible to determine whether the plants assimilated NO3^-, NH4⁺, or both. An important new finding of this study is the recognition that synoptic and diel changes in δ¹⁸O-DO in the zone of active nitrification imparted a corresponding change in δ¹⁸O-NO3. Future studies examining the O-isotope composition of nitrate need to consider whether DO has a constant or shifting isotopic composition at the source of nitrification. The results of this study underscore the complex and dynamic interactions between biologically-active solutes in nutrient-rich streams, and also emphasize the importance of recognizing these cycles when collecting samples for scientific or monitoring purposes.

Keywords: Nitrification; Dissolved oxygen; Ammonium; Nitrate; Stable isotope; Respiration; Photosynthesis; Diel; Diurnal』

1. Introduction
　1.1. Site description
2. Methods
3. Results
　3.1. Field parameters and nutrient concentrations
　3.2. Isotopes of water and dissolved oxygen
　3.3. Concentration and isotopic composition of DIC
　3.4. Isotopes of nitrate, ammonium, and N2
4. Discussion
　4.1. Factors affecting DO and DIC
　4.2. Factors affecting nitrate and ammonium
　4.3. Practical implications
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References