Measured DO vs predicted DO.

<div><p>Monitoring the seasonal and diurnal variations in headwater stream metabolic regimes can provide critical information for understanding how ecosystems will respond to future environmental changes. In East Fork Creek, a headwater stream in middle Tennessee, week-long field campaigns were set up each month from May 2022 to May 2023 to collect stream metabolism estimators. In a more extensive field campaign from July 2–5 in 2022, diel signals were observed for temperature, pH, turbidity, and concentrations of Ca, Mg, K, Se, Fe, Ba, chloride, nitrate, DIC, DO, DOC, and total algae. Gross Primary Productivity (GPP) and Ecosystem Respiration (ER) were calculated based on a Bayesian model using the dissolved oxygen (DO) time series approach. DO showed diurnal swings between oversaturation in daytime and undersaturation at night, with DO amplitudes being greatest in summer. GPP measurements have a clear seasonal variation, peaking in July and staying low in winter, and strong diel signals that couple with the daily light regime variation. ER does not vary seasonally except for a slight increase in Fall which might be caused by terrestrial organic inputs. The dominant control on GPP is light intensity and on ER is temperature. East Fork Creek shows a heterotrophic metabolic regime for 54 of 57 campaign days and therefore consumes O<sub>2</sub> and emits CO<sub>2</sub> to the atmosphere throughout the year. If carbon inputs are not a limiting factor, the positive temperature dependence of ER may cause increased CO<sub>2</sub> emissions from headwater streams and more frequent hypoxia events in a warming climate.</p></div>