Thin layers of phytoplankton are important hotspots of ecological activity that are found in the
coastal ocean, meters beneath the surface, and contain cell concentrations up to two orders of
magnitude above ambient concentrations. Current interpretations of their formation favor abiotic
processes, yet many phytoplankton species found in these layers are motile. We demonstrated
that layers formed when the vertical migration of phytoplankton was disrupted by hydrodynamic
shear. This mechanism, which we call gyrotactic trapping, can be responsible for the thin layers of
phytoplankton commonly observed in the ocean. These results reveal that the coupling between
active microorganism motility and ambient fluid motion can shape the macroscopic features of the
marine ecological landscape.