Trapping of gyrotactic organisms in an unstable shear layer

► Gyrotactic organisms are modeled in a growing Kelvin-Helmholtz instability. ► Gyrotactic instability in braids can increase phytoplankton concentrations 10-fold. ► Aggregation rate scales with Reynolds number and with swimming velocity. ► Motile phytoplankton are trapped in the billows via the Stommel suspension mechanism.