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, k600, 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 k600 than other approaches. Using results from an EC study of a small, boreal lake, we modelled k600 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 k600 values using these higher CO2 concentrations. This approach led to greater congruence between measured and modelled k values during the stratified period. k600 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 k600, the same pattern is evident at higher winds, but k600 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 k600 provide estimates of gas evasion at least 200% higher than earlier wind-based models. Our improved k600 estimates emphasize the need for integrating within lake physics into models of greenhouse gas evasion.
Highlights
Lakes are disproportionately active sites in carbon cycling relative to their surface area (Cole et al, 2007), and with more accurate estimates of their numbers and surface area (Downing et al, 2006), emissions have been found to be significant relative to the terrestrial sources and sinks
We developed an equation for the gas transfer coefficient based on boundary-layer theory, which took into account both wind speed and buoyancy flux
We used the data from the 0.2 m probe as the surface CO2 concentration, but we computed expected CO2 concentration within the footprint taking into account the extent of thermocline deflection based on calculations of the Wedderburn and Lake numbers and our estimates of the depth of the actively mixing layer, and denote the gas transfer velocities obtained as ksite and kfp, where site stands for measurement site and fp for footprint
Summary
Lakes are disproportionately active sites in carbon cycling relative to their surface area (Cole et al, 2007), and with more accurate estimates of their numbers and surface area (Downing et al, 2006), emissions have been found to be significant relative to the terrestrial sources and sinks. Spatial variability would result if the mixing from nocturnal cooling, which brings dissolved gases to the airÁ water interface (Crill et al, 1988), penetrated to different depths as would be expected if some parts of a lake basin were more exposed to wind and with concomitant higher evaporation rates Processes such as differential cooling would bring gases produced nearshore to offshore sites where they could be mixed vertically by convection. We developed an equation for the gas transfer coefficient based on boundary-layer theory, which took into account both wind speed and buoyancy flux This equation is independent of the empiricism inherent in applications of the surface renewal model which currently rely on similarity scaling from ocean sites for estimates of turbulence (MacIntyre et al, 2001, 2010; Eugster et al, 2003; Read et al, 2012). We assess the physical processes which need to be included in modelling greenhouse gas evasion in small lakes
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