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 <10 ha. 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 0.54 m day −1 for lakes <10 ha. 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.