The phytoplankton bloom in the NW Arabian Sea peaked during August-September, extended from the Oman coast to about 65°E, and lagged the development of open-sea upwelling by at least one month. The bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore Ekman pumping. Coastal upwelling was evident from May through September, yielded the most extreme concentrations of phytoplankton biomass, and along the Arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton.