Primary productivity in the Arabian Sea: A synthesis of JGOFS data

Temporal and spatial variations in phytoplankton biomass exist in the Arabian Sea at all scales from the diurnal to the seasonal, and from fine to large scale. Phytoplankton physiological rate parameters and productivity measurements suggest that phytoplankton are not strongly limited by either irradiance or nutrient supply. Grazing activities, in most cases, match phytoplankton net photosynthetic rates and growth. The variations in biomass are therefore explained from variations in mixing at diurnal to seasonal scales, and spatial variations caused by upwelling near the Omani coast and the presence of mesoscale eddies. Vertical mixing regulates the supply of irradiance and nutrients, but vertical mixing is never deep enough to limit phytoplankton productivity, and nitrogen does not appear to be a factor limiting phytoplankton growth. Vertical mixing, however, also affects grazing by diluting micro-grazers along with phytoplankton. It is argued here that mixed layer deepening acts as a natural ‘dilution experiment’ that allows phytoplankton to escape grazing losses and grow, and thereby create the observed variability in phytoplankton biomass.