Continuous monitoring of dissolved gases with membrane inlet massspectrometry to fingerprint river biochemical activity

International audienceWater quality in rivers results from biogeochemical processes in contributing hydrological compartments (soils,aquifers, hyporheic and riparian zones) and biochemical activity in the river network itself. Consequently,chemical fluxes fluctuate on multiple spatial and temporal scales, leading eventually to complex concentrationsignals in rivers. We characterized these fluctuations with innovative continuous monitoring of dissolved gases, toquantify transport and reaction processes occurring in different hydrological compartments.We performed stream-scale experiments in two headwater streams in Brittany, France. Factorial injections ofinorganic nitrogen (NH4NO3), inorganic phosphate (P2O5) and multiple sources of labile carbon (acetate,tryptophan) were implemented in the two streams. We used a new field application of membrane inlet massspectrometry to continuously monitor dissolved gases for multiple day-night periods (Chatton et al., 2016).Quantified gases included He, O2, N2, CO2, CH4, N2O, and 15N of dissolved N2 and N2O. We calibrated andassessed the methodology with well-established complementary techniques including gas chromatography andhigh-frequency water quality sensors. Wet chemistry and radon analysis complemented the study.The analyses provided several methodological and ecological insights and demonstrated that high frequencyvariations linked to background noise can be efficiently determined and filtered to derive effective fluxes. Froma more fundamental point of view, the tested stream segments were fully characterized with extensive samplingof riverbeds and laboratory experiments, allowing scaling of point-level microbial and invertebrate diversity andactivity on in-stream processing. This innovative technology allows fully-controlled in-situ experiments providingrich information with a high signal to noise ratio. We present the integrated nutrient demand and uptake anddiscuss limiting processes and elements at the reach and catchment scales.Eliot Chatton, Thierry Labasque, Jérôme de La Bernardie, Nicolas Guihéneuf, Olivier Bour, Luc Aquilina.2016. Field Continuous Measurement of Dissolved Gases with a CF-MIMS: Applications to the Physics andBiogeochemistry of Groundwater Flow. Environ. Sci. Technol