Observations of Turbulent Mixing in a Phytoplankton Thin Layer: Implications for Formation, Maintenance, and Breakdown

Coincident measurements of chlorophyll a (Chl a) and temperature microstructure from Monterey Bay, California, U.S.A., are used to investigate the dynamics of a phytoplankton thin layer in the context of vertical turbulent mixing. The thin layer was situated in the thermocline of a 20.5-m water column, between a strongly turbulent surface mixed layer and a weakly turbulent stratified interior. The differential mixing established an asymmetric layer with a stronger Chi a gradient at the base of the layer than at the top. Sinking by Akashiwo sanguinea could have balanced turbulent mixing above the layer, but cannot explain the mid-column convergence needed to maintain the thin layer. Swimming by A. sanguinea directed towards a mid-column nitracline could have balanced turbulent mixing on both sides of the layer and supported the observed Chi a gradients. Results from a Eulerian advection-diffusion model suggest that cell swimming may have varied over the lifetime of the layer, with stronger or more directed swimming required for layer formation and weaker or less directed swimming needed to maintain the layer. In-layer growth is unlikely to have played a primary role in supporting the observed Chi a gradients. From the one-dimensional (vertical) observations, the role of shear-induced straining cannot be evaluated, but shear instabilities appear important to forcing elevated rates of turbulent diffusion within the layer. Finally, a Lagrangian particle-tracking model is used to explore the implications of a balance of vertical velocity and turbulent diffusion on the vertical motions of individual cells that constitute the thin layer.