Carbon dioxide and energy fluxes over a small boreal lake in Southern Finland

Dynamics of carbon dioxide and energy exchange over a small boreal lake were investigated. Flux measurements have been carried out by the eddy covariance technique during two open‐water periods (June–October) at Lake Kuivajärvi in Finland. Sensible heat ( H ) flux peaked in the early morning, and upward sensible heat flux at night results in unstable stratification over the lake. Minimum H was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux ( LE ) showed a different pattern, peaking in the afternoon and having a minimum at night. High correlation ( r 2  = 0.75) between H and water‐air temperature difference multiplied by wind speed ( U ) was found, while LE strongly correlated with the water vapor pressure deficit multiplied by U ( r 2  = 0.78). Monthly average values of energy balance closure ranged between 70 and 99%. The lake acted as net source of carbon dioxide, and the measured flux ( F CO2 ) averaged over the two open‐water periods (0.7 µmol m −2  s −1 ) was up to 3 times higher than those reported in other studies. Furthermore, it was found that during period of high wind speed (>3 m s −1 ) shear‐induced water turbulence controls the water‐air gas transfer efficiency. However, under calm nighttime conditions, F CO2 was poorly correlated with the difference between the water and the equilibrium CO 2 concentrations multiplied by U . Nighttime cooling of surface water enhances the gas transfer efficiency through buoyancy‐driven turbulent mixing, and simple wind speed‐based transfer velocity models strongly underestimate F CO2 .