Effects of cooling and internal wave motions on gas transfer coefficients in a boreal lake

Abstract

Lakes and other inland waters contribute significantly to regional and global carbon budgets. Emissions from lakes are often computed as the product of a gas transfer coefficient, k ₆₀₀ , and the difference in concentration across the diffusive boundary layer at the air–water interface. Eddy covariance (EC) techniques are increasingly being used in lacustrine gas flux studies and tend to report higher values for derived k ₆₀₀ than other approaches. Using results from an EC study of a small, boreal lake, we modelled k ₆₀₀ using a boundary-layer approach that included wind shear and cooling. During stratification, fluxes estimated by EC occasionally were higher than those obtained by our models. The high fluxes co-occurred with winds strong enough to induce deflections of the thermocline. We attribute the higher measured fluxes to upwelling-induced spatial variability in surface concentrations of CO₂ within the EC footprint. We modelled the increased gas concentrations due to the upwelling and corrected our k ₆₀₀ values using these higher CO₂ concentrations. This approach led to greater congruence between measured and modelled k values during the stratified period. k ₆₀₀ has a well-resolved and ~cubic relationship with wind speed when the water column is unstratified and the dissolved gases well mixed. During stratification and using the corrected k ₆₀₀ , the same pattern is evident at higher winds, but k ₆₀₀ has a median value of ~7 cm h⁻¹ when winds are less than 6 m s⁻¹, similar to observations in recent oceanographic studies. Our models for k ₆₀₀ provide estimates of gas evasion at least 200% higher than earlier wind-based models. Our improved k ₆₀₀ estimates emphasize the need for integrating within lake physics into models of greenhouse gas evasion.

• carbon dioxide
• gas transfer
• upwelling
• eddy covariance
• heat flux
• thermocline tilting

5. Acknowledgements

This study was funded through the Academy of Finland Center of Excellence program (project 1118615), Academy of Finland ICOS project (263149), EU ICOS project (211574), Research Foundation of the University of Helsinki, EU GHG-Europe project (244122), EU-project GHG-LAKE, DEFROST (Nordforsk) project, Academy of Finland (project 218094), and University of Helsinki research funds through the project Vesihiisi. Funding was provided by U.S. National Science Foundation Grants DEB 0919603 and ARC 1204267 to SM. We thank H. Miettinen for practical help during the field activities and the staff of the Hyytiälä Forestry Field Station for invaluable assistance and friendly working atmosphere.