A simulation analysis of various biological and physical factors influencing the deep-chlorophyll maximum structure in oligotrophic areas

Abstract

A detailed analysis of the Varela et al. (1992) coupled physical/biological model shows the influence of several physical and biological factors on the depth and magnitude of the deep chlorophyll maximum (DCM). We show here the importance of a careful computation of the eddy diffusion coefficient below the thermocline for a correct estimation of the depth and magnitude of the DCM. We also show that light extinction is critical in determining the DCM depth. Atmospheric nitrate supply influences the vertical phytoplankton distribution only in the upper surface layer, leaving unaffected the DCM. Among the biological factors considered, zooplankton grazing on large phytoplankton and heterotroph grazing on small phytoplankton affect the DCM magnitude but not its depth. The large phytoplankton cells (> 20 μM) seem to have advantages when compared to smaller cells (> 20 μM) when the nutrient supply increased, but small cells almost always constituted the dominant phytoplankton fraction.