During the transition between El Ni¿o and La Ni¿a conditions in 1998, a dramatic bloom occurred near the equator around 170¿E, with chlorophyll concentrations reaching more than 0.8 mg m-3 in May. Previous studies attributed this bloom to a wind-driven upwelling and to the shoaling of the Equatorial Undercurrent (EUC), but they did not explain its particular location near the Kiribati Islands. By combining simulations with observations, we determined that these islands were able to locally disrupt the physical dynamics and the nutrient fields to the extent that they were directly responsible for the location and the amazing strength of the bloom. Our results suggest that an island mass effect was responsible for generating the bloom, while the barrier formed by the islands increased EUC shoaling to the west of them, which explained the bloom peak.