High rates of daytime respiration in three streams: Use of δ18OO2 and O2 to model diel ecosystem metabolism

Photosynthesis and respiration determine the carbon and oxygen (O-2) 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 delta O-18(O2) can be used to estimate diel ER due to discrimination against O-18(O2) during respiration. We monitored diel delta O-18(O2), O-2, light, and water temperature in three Wyoming streams and measured respiration fractionation (alpha(R)) against O-18(O2) in dark benthic flow chambers in two streams. The ranges of measured and literature alpha(R) values were used to estimate uncertainty in metabolism parameters associated with not measuring alpha(R) directly. Daytime ER was 54-340% higher than nighttime ER using delta O-18(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 delta O-18(O2). Measured benthic flow chamber alpha(R) varied within the range of literature values: from 0.9755 to 0.9954. Metabolism parameter estimates were very sensitive to choice of alpha(R) within the measured and published range of values. The mean and uncertainty of diel ER estimates increased with decreasing alpha(R), with daily ER more than ten times higher given an alpha(R) of 0.975 vs. 0.999. Diel changes in ER can be modeled using delta O-18(O2) and O-2, but diel ER estimates depend on the choice of alpha(R), suggesting the need to better understand how alpha(R) may vary within spatial and temporal scales appropriate for delta O-18(O2) metabolism models.