Lake thermal structure influences macroinvertebrate predation on crustacean zooplankton

Changes in lake thermal structure, which are predicted with future climate warming, may alter predator-prey interactions if foraging rates or the spatial overlap of predators and prey depend on thermal conditions. Small Boreal Shield lakes are particularly responsive to weather-induced changes in thermal structure. They are often fishless, with macroinvertebrate predators regulating crustacean zooplankton communities. We performed a mesocosm experiment to examine how thermal structure (stratified and isothermal) influences the predation impact of surface-orienting Buenoa macrotibialis and vertically migrating Chaoborus punctipennis on crustacean zooplankton. We expected predation from surface-orienting predators to be greatest in stratified conditions when food resources are concentrated near the surface in proximity with predators. Surprisingly, surface predators had no effect on zooplankton abundance, and zooplankton avoided surface predators regardless of thermal habitat structure. In contrast, Chaoborus had a strong predation impact and reduced total zooplankton abundance, but only in isothermal conditions. We hypothesize that this predation effect was due to increased predator metabolism, foraging and ingestion rates when migrating through a thermally homogenous warm water column without access to cool bottom waters. These results demonstrate that changes in lake thermal structure may result in strong, unexpected consequences for predator-prey dynamics.