Mechanisms controlling vertical variability of subsurface chlorophyll maxima in a mode-water eddy

Abstract

An intense subsurface chlorophyll enhancement was found repeatedly within the core of a modewater eddy during a 2-month period. Two controls on chlorophyll concentrations in this deep chlorophyll maximum (DCM) layer are noted: chlorophyll concentration is controlled by nutrients at low nutrient concentrations and by light when nutrients are saturating. To synthesize these results, a simple one-dimensional nutrient-phytoplankton model is developed by including the effects of phytoplankton self-shading for light attenuation, depth-dependent phytoplankton specific loss, and density-associated nutrient fluctuation in the deep layer. The model is parameterized using eddy data including not only vertical diffusivity, sinking velocity, and chlorophyll-to-carbon ratios, but also rates of phytoplankton growth and nutrient regeneration. Our results suggest that the observed DCM variability is controlled by nutrient-light interaction leading to a change of phytoplankton physiology and hence vertical enrichment of chlorophyll within the core of the stratified eddy. Further theoretical analyses indicate that variation of nutrient and light availability in the DCM layer of the eddy core is largely driven by change of the vertical nutrient fluxes as a result of isopycnal motions in the deep layer, which is also subject to influences by processes including vertical mixing, particle sinking, and nutrient regeneration.