Lake-size dependency of wind shear and convection as controls on gas exchange

High-frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u_*) and convection (w_*) to turbulence in the surface mixed layer. Seasonal patterns of u_* and w_* were dissimilar; u_* was often highest in the spring, while w _* increased throughout the summer to a maximum in early fall. Convection was a larger mixed-layer turbulence source than wind shear (u _*/w_*<0.75) for 18 of the 40 lakes, including all 11 lakes<10ha. As a consequence, the relative contribution of convection to the gas transfer velocity (k, estimated by the surface renewal model) was greater for small lakes. The average k was 0.54mday^-1 for lakes<10ha. Because u_* and w_* differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake-air gas exchange, especially for small lakes.