SUBTROPICAL ocean gyres are considered to be the marine analogues of terrestrial deserts because of chronic nutrient depletion and low standing stocks of organisms(1). Despite their presumed low rates of primary and export production, oligotrophic habitats contribute significantly to global productivity because of their large extent(2). Therefore, even small changes in ecosystem production can produce large effects in the global carbon cycle. The North Pacific subtropical gyre has generally been thought to support a homogeneous, stable biological community(3,4), but recent investigations have suggested instead that the ecosystem of this gyre is temporally and spatially variable(5-7). The causes of this variability are not well understood. Here we present evidence of a major change in the structure and productivity of the pelagic ecosystem in the subtropical North Pacific Ocean, an effect that we attribute to the 1991-92 EI Nino-Southern Oscillation (ENSO) event. Decreased upper-ocean mixing and a change in circulation resulted in an increased abundance and activity of nitrogen-fixing microorganisms and a shift from a primarily nitrogen-limited to a primarily phosphorus-limited habitat with attendant changes in total and export production and in nutrient cycling pathways and rates.