Large-eddy simulation of subsurface phytoplankton dynamics: an optimum condition for chlorophyll patchiness induced by Langmuir circulations

摘要

Phytoplankton patchiness occurs on a plethora of spatial and temporal scales, and can be extremely patchy in both horizontal and vertical directions. This patchiness directly affects the dynamics of the overall bloom; therefore, understanding the mechanisms behind the occurrence of patchiness on each scale is integral to the understanding of plankton bloom dynamics as a whole. This modelling study introduces a mechanism for patch formations, which has received little attention, but is ubiquitous to the oceanic mixed-layer, Langmuir circulations and their interaction with nutrient upwellings to induce patchiness in the plankton. By combining a large-eddy simulation, which resolves Langmuir circulations, with a nutrient-phytoplankton-zooplankton biological model, one can examine the horizontal and vertical patchiness that results from a flux of nutrients into the bottom of the mixed layer. Here, phytoplankton form significant horizontal patchiness at a depth interval where vertical currents from Langmuir cells are apparent and turbulent mixing is not; this comprises the lower region of the surface mixed layer. Aggregations have frequently been observed in lower regions of the surface mixed layer and have been attributed to the high nutrient flux associated with the pycnocline. This modelling study also shows patches occurring in this region and it is hypothesised that Langmuir cells are a catalyst for patchiness. The results clearly demonstrate that for certain levels of wind forcing, which are strong enough to introduce turbulent mixing only to the upper part of the mixed layer whilst inducing deeper Langmuir circulation, patchiness is greatly enhanced.