Microswimmer trapping in surface waves with shear

Abstract

Many species of phytoplankton migrate vertically near the surface of the ocean, either in search of light or nutrients. These motile organisms are affected by ocean waves at the surface. We derive a set of wave-averaged equations to describe the motion of spheroidal microswimmers. We include several possible effects, such as gyrotaxis, settling, and wind-driven shear. In addition to the well-known Stokes drift, the microswimmer orbits 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.