A SIMULATION ANALYSIS OF VARIOUS BIOLOGICAL AND PHYSICAL FACTORS INFLUENCING THE DEEP-CHLOROPHYLL MAXIMUM STRUCTURE IN OLIGOTROPHIC AREAS

A detailed analysis of the Varela et al. (1992) coupled physical/biolo gical model shows the influence of several physical and biological fac tors on the depth and magnitude of the deep chlorophyll maximum (DCM). We show here the importance of a careful computation of the eddy diff usion coefficient below the thermocline for a correct estimation of th e depth and magnitude of the DCM. We also show that light extinction i s critical in determining the DCM depth. Atmospheric nitrate supply in fluences the vertical phytoplankton distribution only in the upper sur face layer, leaving unaffected the DCM. Among the biological factors c onsidered, zooplankton grazing on large phytoplankton and heterotroph grazing on small phytoplankton affect the DCM magnitude but not its de pth. The large phytoplankton cells (> 20 mu M) seem to have advantages when compared to smaller cells (< 20 mu M) when the nutrient supply i ncreased, but small cells almost always constituted the dominant phyto plankton fraction.