Diel cycle of lake‐air CO₂ flux from a shallow lake and the impact of waterside convection on the transfer velocity

Abstract

Abstract Two years of eddy covariance measurements of lake carbon dioxide (CO 2 ) fluxes reveal a diel cycle with higher fluxes during night. Measurements of partial pressure in the air ( p CO 2 a ) and in the water ( p CO 2 w ), during 4 months, show that the high nighttime fluxes are not explained by changes in the difference between p CO 2 a and p CO 2 w . Analyzing the transfer velocity ( k 600,meas ), which is a measure of the efficiency of the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain the diel cycle. During nighttime, when the fluxes are high, the wind is normally low. Thus, a solely wind‐based parameterization of the transfer velocity ( k u, CC ) results in large errors compared to k 600,meas , especially for wind speeds lower than 6 m s −1 . The mean absolute percentage error between k u, CC and k 600,meas is 79%. By subtracting k u, CC from k 600,meas , we investigate how waterside convection influence k 600,meas . Our results show that the difference ( k 600,meas − k u ,CC ) increases with increasing waterside convection. Separating the transfer velocity parameterization in two parts, one depending on the wind speed and one depending on waterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO 2 fluxes can, to a large extent, be explained by waterside convection and that a transfer velocity parameterization based on both wind speed and waterside convection better fits the measurements compared to a parameterization based solely on wind speed.