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

Abstract

<jats:p>High‐frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (<jats:italic>u</jats:italic><jats:sub>*</jats:sub>) and convection (<jats:italic>w</jats:italic><jats:sub>*</jats:sub>) to turbulence in the surface mixed layer. Seasonal patterns of <jats:italic>u</jats:italic><jats:sub>*</jats:sub> and <jats:italic>w</jats:italic><jats:sub>*</jats:sub> were dissimilar; <jats:italic>u</jats:italic><jats:sub>*</jats:sub> was often highest in the spring, while <jats:italic>w</jats:italic><jats:sub>*</jats:sub>increased throughout the summer to a maximum in early fall. Convection was a larger mixed‐layer turbulence source than wind shear (<jats:italic>u</jats:italic><jats:sub>*</jats:sub>/<jats:italic>w</jats:italic><jats:sub>*</jats:sub> &lt; 0.75) for 18 of the 40 lakes, including all 11 lakes &lt;10 ha. As a consequence, the relative contribution of convection to the gas transfer velocity (<jats:italic>k</jats:italic>, estimated by the surface renewal model) was greater for small lakes. The average <jats:italic>k</jats:italic> was 0.54 m day<jats:sup>−1</jats:sup> for lakes &lt;10 ha. Because <jats:italic>u</jats:italic><jats:sub>*</jats:sub> and <jats:italic>w</jats:italic><jats:sub>*</jats:sub>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.</jats:p>