Gas transfer rate and CO2 flux between an unproductive lake and the atmosphere in northern Sweden

Measurements of the gas transfer rate of CO2 between lake water and the atmosphere present a critical problem for the understanding of lake ecosystem carbon balances and landscape carbon budgets. We present calculations of the gas transfer rate of CO2 from direct measurements of the CO2 flux using an eddy covariance system and concurrent measurements of the concentration of CO2 in the surface water in a lake in boreal zone of northern Sweden. The measured gas transfer rate was different, and in general larger than, rates obtained with the most commonly used models for prediction of the gas transfer rate in lakes. The normalized gas transfer rate (k(600)EC) was well predicted from the wind speed at 10 m height if data were bin classed into wind classes of 1 m/s for winds above 1 m/s. Unbinned data were also correlated to wind speed but also to water temperature, water temperature/air temperature ratio and to incoming photosynthetic active radiation (PAR). These relationships could reflect effects of both physico-chemical reactions and biological activity.