A persistent phytoplankton bloom was observed during August-December 1998 around the Marquesas Islands in the South Pacific using a spaceborne ocean color sensor. The enhancement of the phytoplankton production is attributed to the island mass effect. The effect consists of a combination of turbulent mixing and advection from the south equatorial current flowing through and around the islands, and iron-enriched waters originating from land drainage and hydrothermal fluxes through old volcanic formations. The enhanced phytoplankton production effects are noticeable a large distance downstream from the island (500 to 1000 km). This island mass effect is an important contributor to the productivity of the region and therefore potentially important for fisheries in the area. ¿ 1999 American Geophysical Union