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Mapping gas exchanges in headwater streams with membrane inlet mass spectrometry.

Vautier, Camille, Abhervé, Ronan, Labasque, Thierry, Laverman, Anniet M., Guillou, Aurélie, Chatton, Eliot, Dupont, Pascal, Aquilina, Luc et de Dreuzy, Jean-Raynald (2020). Mapping gas exchanges in headwater streams with membrane inlet mass spectrometry. Journal of Hydrology , vol. 581 . p. 124398. DOI: 10.1016/j.jhydrol.2019.124398.

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Résumé

La transcription des symboles et des caractères spéciaux utilisés dans la version originale de ce résumé n’a pas été possible en raison de limitations techniques. La version correcte de ce résumé peut être lue dans le document original.

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 (k2) 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 O2 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 O2, 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.

Type de document: Article
Mots-clés libres: headwater stream; membrane inlet mass spectrometry (MIMS); reaeration; gas exchange; greenhouse gas emission; CO₂ evasion
Centre: Centre Eau Terre Environnement
Date de dépôt: 08 mars 2021 20:18
Dernière modification: 08 mars 2021 20:18
URI: https://espace.inrs.ca/id/eprint/11397

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