Microswimmer trapping in surface waves with shear

Many species of phytoplankton migrate vertically near the surface of the ocean, in search of either light or nutrients. These motile organisms are affected by ocean surface waves. We derive a set of wave-averaged equations to describe the motion of microswimmers with spheroidal body shapes that includes several additional effects, such as gyrotaxis, settling and wind-driven shear. In addition to the well-known Stokes drift, the microswimmer trajectories depend on their orientation in a way that can lead to trapping at a particular depth; this in turn can affect transport of organisms, and may help explain observed phytoplankton layers in the ocean.