Technical note: drifting versus anchored flux chambers for measuring greenhouse gas emissions from running waters

Abstract

Abstract. Stream networks have recently been discovered to be major but poorly constrained natural greenhouse gas (GHG) sources. A fundamental problem is that several measurement approaches have been used without crosscomparisons. Flux chambers represent a potentially powerful methodological approach if robust and reliable ways to use chambers on running water can be defined. Here we compare the use of anchored and freely drifting chambers on various streams with different flow velocities. The study clearly shows that (1) anchored chambers enhance turbulence under the chambers and thus elevate fluxes, (2) drifting chambers have a very small impact on the water turbulence under the chamber and thus generate more reliable fluxes, (3) the bias of the anchored chambers greatly depends on chamber design and sampling conditions, and (4) there is a promising method to reduce the bias from anchored chambers by using a flexible plastic foil collar to seal the chambers to the water surface, rather than having rigid chamber walls penetrating into the water. Altogether, these results provide novel guidance on how to apply flux chambers in running water, which will have important consequences for measurements to constrain the global GHG balances.

The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

“…We used the measured k-CO 2 and 245 calculated k-CH 4 to estimate the flux for each gas according to: 246 247 Direct water-air flux measurements using floating chambers were excluded during 255 hydrologic phase transitions as this method does not account for the influence of physical 256 forcing by rainfall on the aquatic boundary layer and because anchored chambers tend to 257 over-estimate fluxes in turbulent waters (Lorke et al 2015). Thus, to account for the 258 influence of rewetting events on k we also modelled our data empirically using two 259 Inc.), with significance herein implied at the 95% confidence interval (p<0.05).…”

Groundwater data improve modelling of headwater stream CO&lt;sub&gt;2&lt;/sub&gt; outgassing with a stable DIC isotope approach

“…Because of its decreased uncertainty future CO 2 modelling would further benefit from direct pCO 2 measurement instead of its calculation. Such direct in situ methods include submerged infrared gas analysis (Johnson et al, 2010), equilibrator systems (Polsenaere et al, 2013), offaxis integrated cavity output spectrometer combined with a gas analyser (Gonzalez-Valencia et al, 2014) and nondispersive infrared sensors inside floating chambers Lorke et al, 2015). In addition, modelling on higher 10 temporal resolution is needed to better reproduce dynamics and quantities of CO 2 outgassing.…”

“…For applications in small streams and during changing flow conditions also direct methods such as floating chamber approaches exhibit major drawbacks such as altered outgassing behaviour because of artificially created currents inside anchored chambers (Lorke et al, 2015;Bastviken et al, 2015). In addition, rapid downstream losses of CO 2 often imply that CO 2 -rich groundwater inputs are 15 lost before actual measurements can take place.…”

Groundwater data improve modelling of headwater stream CO&lt;sub&gt;2&lt;/sub&gt; outgassing with a stable DIC isotope approach

“…Such direct in situ methods include submerged infrared gas analysis (Johnson et al, 2010), equilibrator systems (Polsenaere et al, 2013), an off-axis integrated cavity output spectrometer combined with a gas analyser (Gonzalez-Valencia et al, 2014), and non-dispersive infrared sensors inside floating chambers Lorke et al, 2015).…”

“…However, because of large variabilities of gas transfer velocities on different spatial and temporal scales, this type of determination remains controversial, especially for small-scale applications (Marx et al, 2017a;Regnier et al, 2013;Schelker et al, 2016). For applications in small streams and during changing flow conditions, direct methods such as floating chamber approaches also exhibit major drawbacks such as altered outgassing behaviour because of artificially created currents inside anchored chambers (Lorke et al, 2015;Bastviken et al, 2015). In addition, rapid downstream losses of CO 2 often imply that CO 2 -rich groundwater inputs are lost before actual measurements can take place (Reichert et al, 2009).…”