Gyrotactic trapping can by hydrodynamically unstable*

Several meters below the coastal ocean surface there are areas of high ecological activity that contain thin layers of concentrated motile phytoplankton.  Gyrotactic trapping has been proposed as a potential mechanism for layer formation of bottom-heavy swimming cells like Chlamydomonas, especially in flows where the shear stress varies linearly with depth.  In this study, we use a continuum model for dilute swimmer suspensions to examine gyrotactic trapping in a pressure-driven channel flow.  While we find that the parabolic base flow does generate a thin layer with high cell concentration, an analysis of its linear stability reveals that the layer is hydrodynamically unstable due to negative swimmer buoyancy over the relevant range of parameter values.  Our results suggest that layers formed by gyrotaxis should be transient and the stability of such layers can instead be associated with perhaps a more complex biological mechanism.