High rates of daytime respiration in three streams: Use of δ¹⁸O_O2 and O₂ to model diel ecosystem metabolism

Photosynthesis and respiration determine the carbon and oxygen (O₂) balance of ecosystems. Current methods used to estimate ecosystem respiration (ER) do not include diel ER fluctuations, which limit testing predictions about short-term drivers of ecosystem metabolism. Diel changes in δ¹⁸O_O2 can be used to estimate diel ER due to discrimination against ¹⁸O_O2 during respiration. We monitored diel δ¹⁸O_O2, O₂, light, and water temperature in three Wyoming streams and measured respiration fractionation (α_R) against ¹⁸O_O2 in dark benthic flow chambers in two streams. The ranges of measured and literature α_R values were used to estimate uncertainty in metabolism parameters associated with not measuring α_R directly. Daytime ER was 54-340% higher than nighttime ER using δ¹⁸O_O2, but diel ER parameter estimates were highly uncertain relative to traditional estimates of ecosystem metabolism. Diel variations in water temperature only accounted for 4-55% of the range of diel ER calculated using diel δ¹⁸O_O2. Measured benthic flow chamber α_R varied within the range of literature values: from 0.9755 to 0.9954. Metabolism parameter estimates were very sensitive to choice of α_R within the measured and published range of values. The mean and uncertainty of diel ER estimates increased with decreasing α_R, with daily ER more than ten times higher given an α_R of 0.975 vs. 0.999. Diel changes in ER can be modeled using δ¹⁸O_O2 and O₂, but diel ER estimates depend on the choice of α_R, suggesting the need to better understand how α_R may vary within spatial and temporal scales appropriate for δ¹⁸O_O2 metabolism models.