Mapping gas exchanges in headwater streams with membrane inlet mass spectrometry

Using continuous injections of helium coupled to in-situ continuous flow membrane inlet mass spectrometry (CF-MIMS), we mapped the gas exchanges along two low-slope headwater streams having discharges of 25 L s(-1) and 90 L s(-1). Mean reaeration rate coefficients (k(2)) were estimated at 130 d(-1) and 60 d(-1), respectively. Our study revealed that gas exchanges along headwater streams are highly heterogeneous. The variable morphology of the streambed causes gas exchanges to be focused into small areas, namely small cascades made up of stones or wood, with reaeration rate coefficients up to 40 times higher than in low-turbulent zones. As such, cascades appear to be hot spots for both oxygenation and greenhouse gases emissions. Additional O-2 and CO2 measurements effectively showed fast exchanges between the stream and the atmosphere in the cascades, following the partial pressure gradients. These cascades allow a fast oxygenation of the eutrophic streams depleted in O-2, which sustains respiration. Simultaneously, cascades release the oversaturated CO2 originating from groundwater inputs to the atmosphere. By comparing measured reaeration rate coefficients to ten predictive equations from literature, we showed that all equations systematically underestimate reaeration rate coefficients, with significantly higher discrepancies in cascades than in low-turbulent zones. The inadequate characterization of the processes occurring in cascades causes empirical equations to have poor predictive capabilities, leading to a global underestimation of CO2 emission from headwater streams.