Internal waves and mixing in the epilimnion of a lake affects spatial patterns of zooplankton in a body‐size dependent manner

Lay Wind‐driven internal gravity waves, oscillations of a fluid density surface, are a common feature within the strongly stratified thermocline region of mid‐latitude lakes. We demonstrate that internal waves can propagate within the surface waters, or epilimnion, of Lake Opeongo, Ontario, during periods of weak stratification. We also note that within the epilimnion, zooplankton are not uniformly distributed spatially but are often distributed in patches as a result of the combination of their own movements and the effect of water currents. Using field data from July and August 2009 and 2010 we report observations of the relationship between enhanced heterogeneity of distribution in zooplankton and the presence of internal waves in the epilimnion. To quantify this relationship we compare measurements of small‐scale spatial distributions of zooplankton with a measure of wave activity. For the smallest size ranges of zooplankton (284–450 μm) we find that spatial variability is statistically greatest when internal waves are most active, whereas no such relationship exists for the two larger zooplankton size classes. The vertical velocities associated with the movement of internal waves are estimated to be faster than the swimming speeds of small zooplankton, essentially rendering them passive. This supports our assertion that the movement of internal waves contributes to the increased spatial variability of zooplankton in lakes. Zooplankton are not uniformly distributed in space but are patchy at multiple scales as a result of interactions between their directed motion and both large‐ and small‐scale water currents. Using field data from July and August 2009 and 2010 we report observations of the relationship between enhanced small‐scale spatial variability in zooplankton and the presence of internal waves in the weakly stratified epilimnion of Lake Opeongo, Ontario. To quantify this physical–biological coupling, we compared the variance of isotherm displacement and gradient Richardson number (Rig) measured using moored sensors, with the small‐scale spatial distributions of zooplankton measured using an optical plankton counter towed along linear transects intersecting the moorings. For the smallest size ranges of zooplankton (284–450 μm) we found that spatial variability was statistically greatest at intermediate Rig (0.25