Estimating the flux of carbon dioxide (CO2) from lakes is important for understanding the role of these ecosystems in regional and global carbon budgets. The efflux of partially soluble gasses such as CO2 is controlled by near-surface turbulence. Lakes receive turbulent inputs across the air-water interface via two primary mechanisms: wind shear and negative buoyancy flux (convective cooling). We examined the relative importance of wind and convection in 42 temperate lakes from multiple countries that covered gradients in latitude, size, and water clarity. Wisconsin lakes in the analysis included seven North Temperate Lakes Long-Term Ecological Research lakes (Mendota: ME, Trout: TR, Wingra: WG, Sparkling: SP, Crystal: CR, Trout Bog: TB, Crystal Bog: CB) and 4 other bog lakes in northern Wisconsin (Mouser Bog: MB, North Sparkling Bog: NSB, Jekl Bog: JB, Timber Bog: TB).
We calculated the gas transfer velocity (k600) and the turbulent velocity scales for wind (u*) and convection (w*) using high-resolution measurements of wind speed, water temperature, and meteorological drivers, as well as lake-specific properties like morphometry and the diffuse attenuation coefficient. The final dataset exceeded 20 million measurements in total. We found k600 estimates for small and medium sized lakes to be confined between 2 and 3 cm hr-1, the magnitude of which was not strongly related to wind speed. This result counters conventional wisdom that wind is the major driver of gas exchange in lakes. We then compared the daytime u* with the nighttime w* and found convection to be of increasing importance as lakes decreased in size (Fig. 1) potentially explaining why efflux during low wind conditions is often unrelated to wind speed. In lake-rich regions such as Wisconsin’s Northern Highlands, aquatic environments play a major role in the regional C budget (Buffam et al. 2011), and small lakes are numerically dominant and have highest aerial rates of gas flux (Hanson et al. 2004). Recognizing the role of convection in driving gas exchange in these smaller lakes is thus critical for generating accurate estimates of aquatic C fluxes and improving the accuracy of the regional carbon balance.
Buffam, I., M.G. Turner, A.R. Desai, P.C. Hanson, J.A. Rusak, N.R. Lottig, E.H. Stanley, and S.R. Carpenter. 2011. Integrating aquatic and terrestrial components to construct a complete carbon budget of a north temperate lake district. Global Change Biology doi:10.1111/j.1365-2486.2010.02313.x
Hanson, P.C., A. I. Pollard, D.L. Bade, K. Predick, S.R. Carpenter, and J.A. Foley. 2004. A model of carbon evasion and sedimentation in temperate lakes. Global Change Biology 10:1285-1298, doi: 10.1111/j/1365-2486.2004.008005.x
See also Jordan Read's webpage for more information on this project.