A large eddy simulation study of the formation of deep chlorophyll/biological maxima in un-stratified mixed layers: The roles of turbulent mixing and predation pressure

Recent experimental measurements of fluorescence values and turbulent energy dissipation rates, recorded in weakly stratified boundary layers in the open ocean, have highlighted a significant correlation between the formation of deep chlorophyll maxima (DCM) and turbulent mixing. Specifically, the depth of many DCM are observed to lie below, but within about one standard deviation, of the point at which the energy dissipation rate profile reaches its maximum. This correlation of DCM and turbulent mixing is both exciting and curious, as conventional thinking tends to see the latter as a destructive rather than a constructive agent in regards to the formation of deep biological maxima (DBM), for which DCM data is usually interpreted as a proxy. In order to investigate this phenomenon, a three-dimensional large eddy simulation (LES) of the ocean boundary layer was combined with a generic nutrient-phytoplankton-zooplankton (NPZ) type biological model, in order establish what mechanisms might be driving the experimental observations. Simulations of the LES-NPZ model, based upon various sets of generic biological parameters, demonstrate DCM/DBM formation occurs at normalized depths close to those seen in the experimental observations. The simulations support the hypothesis that the DBM are generated by a combination of zooplankton predation pressure curtailing phytoplankton growth near the surface, and a decline in the strength of the vertical mixing processes advecting nutrient through the boundary layer. In tandem, these produce a region of the water column in which predation pressure is relatively low and nutrient aggregation relatively high, suitable conditions for DBM formation.