Trapping of gyrotactic organisms in an unstable shear layer

To explore new mechanisms for planktonic thin layer formation, particle and continuum models of gyrotactically swimming phytoplankton are embedded in simulations of a dynamically unstable stratified shear layer. Two trapping mechanisms are observed in the developing Kelvin–Helmholtz (K–H) billow train. Within the K–H billows, a particle can remain preferentially in downwelling regions, cancelling its upward swimming motion. In the braids that separate the billows, intense shear defeats the gyrotactic stabilization mechanism and causes cells to tumble. Particle and continuum models are compared statistically to reveal both consistencies and weaknesses in each. A scaling based on Reynolds number and swimming speed is used to predict the maximum concentration generated by an instability event. Although K–H billows are short lived in comparison with planktonic thin layers observed in the coastal oceans, the resulting trapping causes rapid aggregation. We conclude that trapping in a growing K–H instability could contribute to the development of the observed cell concentrations.