Temporal variations in surface water CO2 concentration in a boreal humic lake based on high-frequency measurements

We measured surface water CO 2 concentrations continuously at three different depths to reveal the seasonal, daily and diel CO 2 dynamics of a steeply stratified pristine, spring-meromictic, humic lake during April 2005-October 2006. The lake was supersaturated with CO 2 most of the time and was a net source of CO 2 to the atmosphere. The stability of stratification and the depth of water column mixing strongly controlled the surface water CO 2 concentrations. Surface water concentrations as high as 195 μmol l -1 were measured when mixing extended to deep waters during spring ice breakup and autumn turnover. The concentrations were near the level of atmospheric equilibrium, about 15 μmol l -1 , during periods of high stability. The yearly CO 2 fluxes to the atmosphere were 3.7 and 2.5 mol m -2 in 2005 and 2006, respectively.