Oral Presentation Society for Freshwater Science 2026 Annual Meeting

How are changes in stream water N2:Ar and CH4:Ar saturation ratios linked to estimates of ecosystem metabolism? (135682)

Robert S. Brown 1 , Jonathan R. Behrens 1 , Scott C. Brooks 1 , Natalie A. Griffiths 1 , Erin R. Hotchkiss 2 , Marie J. Kurz 1
  1. Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
  2. Virginia Tech, Blacksburg, Virginia, United States

Traditional reach-scale O2-derived measures of stream metabolism, gross primary production (GPP) and aerobic ecosystem respiration (ER), overlook anaerobic processes (e.g., denitrification and methanogenesis). The inclusion of anaerobic processes alongside estimates of stream metabolism can improve predictions and understanding of watershed biogeochemical dynamics. Daily estimates of reach-scale GPP and ER are facilitated by high frequency O2 sensors, but simultaneous daily estimates of reach-scale denitrification and methanogenesis remain challenging to conduct across multiple sites in a stream network. To explore linkages between GPP, ER, net denitrification, and net methane flux, we used membrane inlet mass spectrometry to measure nitrogen N2:Ar and CH4:Ar saturation ratios (hereafter N2sat and CH4sat) in water samples along the mainstem and four tributaries of a 4th order stream draining a watershed with heterogeneous landcover. These ratios served as proxies for the direction and magnitude of net denitrification or methane flux where N2sat or CH4sat > 1 indicate net production and likely flux of either N2 or CH4 from the stream to the atmosphere. We collected concurrent water samples for nitrate and dissolved organic carbon analyses to assess potential variation in substrate availability for denitrification and methanogenesis. Additionally, we estimated daily GPP and ER from a time series of dissolved oxygen and temperature data collected at the same locations. N2sat ranged from 0.999 to 1.046 and CH4sat ranged from 1.2 to 8.3. CH4sat was highest in summer and positively related to water temperature. N2sat had apparent peaks in summer and winter and was unrelated to water temperature. Preliminary analyses alongside antecedent GPP and ER (mean values for preceding 7 days) suggest CH4sat was negatively correlated with GPP and positively correlated with ER in colder months. We continue to assess N2sat and CH4sat as functions of antecedent GPP, ER, temperature, precipitation, and water chemistry throughout the year. This research will facilitate generation of hypotheses that shed light on relationships between anaerobic as well as aerobic carbon and nitrogen cycling.