The effect of vorticity on the feeding of a freshwater grazer

The transfer of energy through zooplankton grazing on phytoplankton is one of the most important interactions in aquatic ecosystems, yet the role of hydrodynamic forces is not fully understood. Factors that influence these interactions include algal size and shape and the interactions between grazer and alga dictated by hydrodynamic forces. Hydrodynamic forcing lead to unimodal relationships in grazer clearance rates (CR) because water motion and turbulence increases encounters but interferes with feeding at high levels. We expect higher CR on small algae due to increased encounter rates caused by vorticity generated in a horizontally oriented rotating cylinder to model increasing turbulence, whereas longer cells, tumbling in the flow, may be more difficult to handle. Ultimately, we expect that increased vorticity will interfere with zooplankton swimming. The CR of Daphnia magna followed the unimodal trend, with higher CR on the small, spherical Chlorella vulgaris than on larger, elongated, and colonial Scenedesmus quadricauda. This was likely due to lower handling time and higher concentration of the smaller cells in the feeding radius around D. magna. The normal hop‐sink swimming behavior of the Daphnia was affected by increasing vorticity; hop frequency decreased, displacement increased, and there was a loss of vertical orientation. This pattern in feeding and swimming were predicted by vorticity. These results for Daphnia differ from those obtained for copepods, which do not respond to vorticity. Given the importance of vorticity in nature, it is relevant to examine grazer feeding interactions under ecologically relevant hydrodynamic conditions.