Enhanced phytoplankton biomass in the wake of the Gal¿pagos Islands is thought to result from an island-mass effect (IME) fueled by upwelling of the Equatorial Undercurrent (EUC) and by natural iron enrichment from the island platform. Annual means of five variables describing the thermocline, the pycnocline, and the availability of nitrate at the surface were derived from the World Ocean Atlas 1998 (WOA98). The first principal component of these variables explained 55.8% of the variance, corroborating that the Gal¿pagos IME is associated with features of the EUC, mainly a shallow thermocline/pycnocline and its vertical spreading in the vicinity of the Gal¿pagos. Regression analysis of SeaWiFS-derived chlorophyll-a (chl) on the WOA98 variables indicated that the depth of the thermocline and nitrate availability explain 91.9% of the chl variance. A secondary IME of enhanced chl levels associated with the wind-sheltered area north of Isabela Island was evident in the residual variability.