Mixing of Top-Down and Bottom-Up Diffusing Reactive Tracers Within the Ocean Mixed Layer and Its Application to Autumn Phytoplankton Blooms

The mixing of reactive tracers within the ocean mixed layer, phytoplankton transported downwards from the sea surface and nutrients transported upwards from the mixed layer depth (MLD), is investigated using large eddy simulation coupled to a Lagrangian plankton model. The study focuses on how vertical and horizontal heterogeneity in tracer distribution is generated and how it influences an autumn phytoplankton bloom. The vertical gradient appears in the profiles of horizontal mean phytoplankton and nutrient concentrations, P and N, and it reduces phytoplankton production by photosynthesis compared to the cases with uniform distributions. The reduction ratio decreases as the mixed-layer mean N increases, but it remains relatively insensitive to other conditions such as MLD, surface forcing, stratification below the mixed layer, and the initial N. Phytoplankton and nutrient concentrations show a negative correlation in the horizontal plane, which becomes stronger with increasing depth. Its contribution to plankton production by photosynthesis is negligible, however, because the correlation is weak near the sea surface and the reaction time scale is much longer than the turbulent mixing time scale. It is also found that the vertical gradients of P and N are smaller, and the negative correlation is stronger in the convective mixed layer than in the shear-driven mixed layer. A simple box plankton model, which takes into account the mixing process of tracers, is proposed and used to investigate how mixing affects the prediction of an autumn phytoplankton bloom.