Mapping gas exchanges in headwater streams with membrane inlet mass spectrometry

Abstract

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A new method to quantify air–water gas exchanges in streams based on slug injection and semicontinuous measurement

“…The field application of our method highlighted its repeatability. The determined gas exchange rate coefficients were in the range of gas exchanges found in low-turbulent sections of headwater streams (Knapp et al 2019;Vautier et al 2020). Another valuable test would be a direct comparison with another method, for example, a continuous gas tracer injection performed in the same stream at the same time.…”

“…Helium is nonreactive, nontoxic, and it is not a greenhouse gas. Chloride from NaCl was chosen as a conservative tracer for its low cost and its simplicity of measurement (Genereux and Hemond 1990, 1992; Vautier et al 2020).…”

“…Because of their ubiquity (Bishop et al 2008), their elevated turbulence (Duvert et al 2018), and their connection to biologically active compartments such as the aquifer, the soil, and the hyporheic zone (Öquist et al 2009), headwater streams are responsible for most greenhouse gases emissions emerging from running water ecosystems (Wallin et al 2011; Natchimuthu et al 2017; Duvert et al 2018). Recent studies showed that global CO 2 emission from headwater streams has long been underestimated, partly because of errors in the rates of gas exchanges between streams and atmosphere (Cole et al 2007; Hall and Ulseth 2019; Vautier et al 2020). Thus, broadly applicable methods for quantifying gas exchange rates are needed to improve global greenhouse gases budgets (Duvert et al 2018).…”

“…A gas tracer is often injected together with a conservative tracer, allowing to account for dilution and dispersion. Tracers are released either continuously in the stream until their concentration reaches a stable plateau (Wanninkhof et al 1990; Genereux and Hemond 1992; Tobias et al 2009; Jin et al 2012; Soares et al 2013; Benson et al 2014; Hall and Madinger 2018; Vautier et al 2020), or in the form of a slug injection (Hibbs et al 1998; Jin et al 2012). Continuous injections allow accurate estimates of gas exchange rates, but they are time‐intensive and consume a lot of tracer, which are often costly or even damaging to the environment, as the greenhouse gas SF 6 (Benson et al 2014).…”

“…Today, semicontinuous measurements of gas are more and more common and can be implemented in‐situ using moveable devices such as membrane inlet mass spectrometers (Ketola et al 2002; Janfelt et al 2006; Schluter and Gentz 2008; Machler et al 2012; Visser et al 2013; Brennwald et al 2016; Chatton et al 2017; Knapp et al 2019; Weber et al 2019; Vautier et al 2020). The measured signals contain a large amount of information, not only in the value of the concentrations, but also in their variation through time (Weber et al 2019).…”