Measurement of gas-exchange rate in streams by the oxygen-carbon method

The gas-exchange rate between streams and the atmosphere is needed to measure in-stream ecological processes and C processing in rivers and streams. Current methods include empirical relationships to hydraulics, direct injection of a tracer gas, and modeling based on O-2 or C diel curves. All existing methods have strengths and drawbacks and most are limited to point measurements or are unable to measure diel variation in exchange rate. Researchers continue to search for better techniques, particularly for steep streams with high rates of gas exchange and low primary productivity. We present the O-2-C (OC) method for calculating gas-exchange rates via simultaneous measurement of O-2 and dissolved inorganic C (DIC). Gas-exchange rates are calculated by solving the combined stream transport equation for O-2 and DIC. The output is a time-series of aeration rates at the same sampling frequency as the input O-2 and C data. Field tests in a 4th-order montane stream in Oregon, USA, indicate that the method is suitable for stream reaches with high downstream gas-concentration gradients and saturation deficits. The mean modeled aeration rate adjusted to 17 degrees C (3.25/h) agreed well with the value of 3.22/h from direct gas injection. Net ecosystem production calculated with the modeled aeration rate (-1.69 g O-2 m(-2) d(-1)) was consistent with the result obtained with direct gas injection (-1.60 g O-2 m(-2) d(-1)). An assumption of the model is a constant respiration quotient, but results indicated that the respiration quotient may be time variable. Sensitivity analysis indicated that application of the OC method is limited to reaches with a suitable change in combined O-2 and CO2 concentration >= similar to 4 mu mol/L and combined O-2 and CO2 saturation deficits approximate to 4 mu mol/L, characteristic of smaller gaining streams. Preliminary application of the OC method indicates it could be useful to practitioners interested in continuous measurement of gas-exchange rates.