Abstract
Abstract. The gas transfer velocity (k) is a major source of uncertainty when assessing the magnitude of lake gas exchange with the atmosphere. For the diversity of existing empirical and process-based k models, the transfer velocity increases with the level of turbulence near the air–water interface. However, predictions for k can vary by a factor of 2 among different models. Near-surface turbulence results from the action of wind shear, surface waves, and buoyancy-driven convection. Wind shear has long been identified as a key driver, but recent lake studies have shifted the focus towards the role of convection, particularly in small lakes. In large lakes, wind fetch can, however, be long enough to generate surface waves and contribute to enhance gas transfer, as widely recognised in oceanographic studies. Here, field values for gas transfer velocity were computed in a large hard-water lake, Lake Geneva, from CO2 fluxes measured with an automated (forced diffusion) flux chamber and CO2 partial pressure measured with high-frequency sensors. k estimates were compared to a set of reference limnological and oceanic k models. Our analysis reveals that accounting for surface waves generated during windy events significantly improves the accuracy of k estimates in this large lake. The improved k model is then used to compute k over a 1-year time period. Results show that episodic extreme events with surface waves (6 % occurrence, significant wave height > 0.4 m) can generate more than 20 % of annual cumulative k and more than 25 % of annual net CO2 fluxes in Lake Geneva. We conclude that for lakes whose fetch can exceed 15 km, k models need to integrate the effect of surface waves.
Highlights
Lakes are universally regarded as significant sources of CO2 to the atmosphere; the accurate quantification of the magnitude of such emissions currently remains challenging (Cole et al, 2007; Tranvik et al, 2009; Raymond et al, 2013)
While the effects of surface waves on k can be neglected in small lakes, we question whether this assumption holds for large lakes such as Lake Geneva, in which surface waves are frequently observed (Figs. 2, C1)
Investigations of the four main processes generating the gas transfer velocity in the large Lake Geneva demonstrated the importance of considering surface waves during episodic windy events responsible for more than 44 % of annual cumulative k600
Summary
Lakes are universally regarded as significant sources of CO2 to the atmosphere; the accurate quantification of the magnitude of such emissions currently remains challenging (Cole et al, 2007; Tranvik et al, 2009; Raymond et al, 2013). CO2 fluxes can be directly measured with floating chamber or eddy covariance systems (Vachon et al, 2010; Vesala et al, 2006). The former suffers from limited time and space integration (from minutes to hours and from centimetres to metres respectively; Klaus and Vachon, 2020), whereas the latter remains technically difficult and can be influenced by non-local processes (entrainment from the shore or advection; Vachon et al, 2010; Esters et al, 2021).
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