Estimating lake-atmosphere CO2 exchange

Lake‐atmosphere CO₂ flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake‐water CO₂ storage and with flux predictions from boundary‐layer and surface‐renewal models. Over a 3‐yr period, lake‐atmosphere exchanges of CO₂ were measured over 5 weeks in spring, summer, and fall. Observed springtime CO₂ efflux was large (2.3–2.7 umol m^‐2 s^‐1) immediately after lake‐thaw. That efflux decreased exponentially with time to less than 0.2 umol m^‐2 s⁻¹ within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO₂ concentrations, lake CO₂ storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (−0.17 μmol m^‐2 s¹) to the lake's surface, while late fall flux was trendless and smaller (−0.0021 μmol m^‐2 s⁻¹). Large springtime efflux afforded an opportunity to make direct measurement of lake‐atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air‐water gas‐transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake‐water storage change in CO₂, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary‐layer and surface‐renewal models. Comparison of measured and modeled transfer velocities for CO₂, along with measured and modeled cumulative CO₂ flux, indicates that in most instances the surface‐renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary‐layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.