Comparison of Three Techniques Used To Measure Diffusive Gas Exchange from Sheltered Aquatic Surfaces

Abstract

Three approaches commonly used to quantify diffusive gas exchange across aquatic surfaces were compared in a densely treed, low-wind environment Diffusive surface fluxes of carbon dioxide (CO2) and methane (CH4) from a small boreal reservoir were estimated using (i) surface water concentrations, the thin boundary layer (TBL) equation, and gas transfer velocities (k) calculated using sulfur hexafluoride (SF6); (ii) surface water concentrations, the TBL equation, and k estimated from wind speed; and (iii) static floating chambers (FCs). Comparisons were made during three different approximately 10-day intervals (August 2000, June and September 2001). CO2 and CH4 fluxes estimated from SF6-derived k were on average 1-3 times greater than those determined from wind-estimated k Overall agreement between FC CO2 and CH4 flux estimates and those based on SF6 and wind speed derived kvalues was much weaker, with FC CO2 and CH4 flux estimates ranging from -9 to 23 times those based on SF6 and wind-estimated k values. Chamber deployment likely enhanced gas transfer through disturbance of the surface boundary layer, and results of this study suggest that caution must be exercised concerning the use of FCs on very still water surfaces. Furthermore, findings of this study contradict the common belief that use of wind speed to approximate k is inappropriate for small bodies of water characterized by low winds and surface obstructions.

The relationship between near‐surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange

“…The overall trend toward FC overestimations in our study is still consistent with the conclusions of Matthews et al (2003). Their FC overestimations were much more important in calm-water conditions than they were when waves were present at higher wind speeds.…”

“…Bias and corrections for FC measurements-Our FC evaluation agrees with those of many other studies that found that fluxes are overestimated when measured by FC methods by perturbing the air-water interface (Kremer et al 2003;Matthews et al 2003;Lambert and Fréchette 2005). Tokoro et al (2008) also measured turbulence inside and outside their FC, but they did not find major disagreements between the two types of measurements.…”

“…The chamber moves slightly but constantly on the surface water, even under very weak winds, and the chamber's walls induce artificial turbulence. Matthews et al (2003) suggest that FCs with edges (or a skirt) at the same level as the water surface tend to overestimate fluxes by up to a three-to fivefold measure as compared to chambers with longer edges that enter the water.…”

“…However, FC-derived fluxes are problematic as well. Some authors (Eugster et al 2003;Kremer et al 2003;Matthews et al 2003) have shown that fluxes measured by FCs are higher than those obtained from SF 6 additions, from the oftenused wind-k parameterization, and from the 'eddy covariance' method. Other authors (Guérin et al 2007;Repo et al 2007;Soumis et al 2008) have shown that FC measurements yield values consistent with those of other methods.…”

Regulation of spatial and temporal variability of carbon flux in six hard‐water lakes of the northern Great Plains

“…Caveats to C flux estimates-A variety of methods are available to estimate CO 2 flux from lakes, each with its relative merits (Anderson et al 1999;Matthews et al 2003;Soumis et al 2008). In this study, gas flux estimates were derived from well-established boundary-layer equations (Cole and Caraco 1998) and observed wind speed but did not account for the effects of precipitation or convective mixing (Anderson et al 1999;Eugster et al 2003).…”