### Abstract

[1] Gas fluxes from lakes and other stratified water bodies, computed using conservative values of the gas transfer coefficient k_{600}, have been shown to be a significant component of the carbon cycle. We present a mechanistic analysis of the dominant physical processes modifying k_{600} in a stratified lake and resulting new models of k_{600} whose use will enable improved computation of carbon fluxes. Using eddy covariance results, we demonstrate that i) higher values of k_{600} occur during low to moderate winds with surface cooling than with surface heating; ii) under overnight low wind conditions k_{600} depends on buoyancy flux *β* rather than wind speed; iii) the meteorological conditions at the time of measurement and the inertia within the lake determine k_{600}; and iv) eddy covariance estimates of k_{600} compare well with predictions of k_{600} using a surface renewal model based on wind speed and *β*.