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.