Whole-stream metabolism estimates derived from diel oxygen curves facilitate understanding of gross primary production (GPP) and aerobic ecosystem respiration (ER) as basal ecosystem processes. However, traditional estimates of stream metabolism overlook anaerobic metabolic processes. There is a need to include anaerobic processes in estimates of stream metabolism to more accurately predict watershed biogeochemical dynamics. Using membrane inlet mass spectrometry (MIMS), we simultaneously measure nitrogen (N2) and methane (CH4) saturation monthly at 14 sites spanning the headwaters to 4th-order outlet of East Fork Poplar Creek, which drains the city of Oak Ridge, Tennessee, USA. Super saturation of both gases can be a useful indicator of processes not captured by aerobic metabolism models (i.e., denitrification and CH4 production). We collect concurrent grab samples for nitrate (NO3), ammonium (NH4), and dissolved organic carbon (DOC) along with N2 and CH4 measurements. Additionally, we estimate GPP and ER from sensor-collected DO and temperature data. We present N2 and CH4 saturation data, which variably change over time across the watershed. Preliminary results show super saturation in surface water for both N2 (8-13%) and CH4 (25-400%) in October 2024. CH4 was also super saturated at all sites in November, but N2 was 1-2% undersaturated at all sites. This indicates potential for divergent responses among anaerobic metabolic processes as temperatures cool and aerobic metabolism slows down. Ongoing work will assess N2 and CH4 saturation patterns as functions of landcover, water chemistry, and antecedent GPP, ER, temperature, and precipitation throughout the year. This research will facilitate generation of hypotheses that shed light on relationships between aerobic and anaerobic metabolic processes in stream ecosystems.
Oak Ridge National Laboratory (ORNL) is managed by UT Battelle on behalf of the U.S. Department of Energy (DOE). This research represents views of the authors not necessarily those of ORNL, UT Battelle, or DOE.