DIEL CYCLES IN STABLE-ISOTOPE COMPOSITION OF NITRATE, AMMONIA, DISSOLVED OXYGEN, AND DISSOLVED INORGANIC CARBON IN A HYPEREUTROPHIC STREAM

Silver Bow Creek (SBC), a small stream impacted by historical mining activities in Butte, Montana, also receives excess nutrients (ammonium, phosphate) from a wastewater treatment plant. During summer low-flow periods, large diel cycles exist in dissolved oxygen (DO), temperature, and the concentration and speciation of metals and nitrogenous species (Babcock and Gammons, GSAAP, this meeting). Here we describe concurrent changes in the stable-isotope composition of biologically-active solutes in SBC.

Photosynthesis by aquatic plants and algae produces isotopically-light DO, driving down δ¹⁸O-DO to values as low as +5 ‰ during the early afternoon, far below the expected value (+24.2 ‰) for equilibration with air (see also Parker et al., Chem. Geol., in press, doi:10.1016/j.chemgeo.2009.06.016). Meanwhile, daytime fixation of isotopically-light CO₂ by aquatic plants causes δ¹³C of dissolved inorganic carbon (DIC) to increase by several ‰. During the night, microbial consumption of DO and production of CO₂ reverse these trends. Ammonium decreases in concentration steadily with distance below the wastewater outfall due to bacterial oxidation of NH₄⁺ to nitrite/nitrate. Isotopic fractionation results in an increase in δ¹⁵N-NH₄ with distance downstream from initial values at the sewage outfall near +11 ‰ to final values near +29 ‰. The rate of NH₄⁺ oxidation is faster during the warm, DO-rich daytime hours, and therefore the δ¹⁵N-NH₄ values at any one location in the stream are higher by up to 5 ‰ in the day as compared to the night. The δ¹⁵N values for NO₃^- also cycle diurnally by as much as 7 ‰, and generally follow the same temporal pattern as for NH₄⁺. δ¹⁸O-NO₃ values show an inverse trend, decreasing from +4.4 ‰ at night to -1.2 ‰ during the day. When ammonium is oxidized to nitrate, approximately 1/3 of the O atoms in the NO₃^- molecule are inherited from DO, while the other 2/3 come from water. Although no diel variation was noted in δ¹⁸O of SBC water, δ¹⁸O-DO had a very strong diel signal (as above), enough to impart a robust diel signal in δ¹⁸O-NO₃. This study is the first to show this type of direct link between diel cycles in δ¹⁸O-DO and δ¹⁸O-NO₃ in surface water, and also underscores how isotopic signatures of DIC, N-species, and O₂ can change rapidly (hourly) in a nutrified stream.