ECOSYSTEM CHANGES IN THE NORTH PACIFIC SUBTROPICAL GYRE ATTRIBUTED TOTHE 1991-92 EL-NINO

SUBTROPICAL ocean gyres are considered to be the marine analogues of t errestrial deserts because of chronic nutrient depletion and low stand ing stocks of organisms(1). Despite their presumed low rates of primar y and export production, oligotrophic habitats contribute significantl y to global productivity because of their large extent(2). Therefore, even small changes in ecosystem production can produce large effects i n the global carbon cycle. The North Pacific subtropical gyre has gene rally been thought to support a homogeneous, stable biological communi ty(3,4), but recent investigations have suggested instead that the eco system of this gyre is temporally and spatially variable(5-7). The cau ses of this variability are not well understood. Here we present evide nce of a major change in the structure and productivity of the pelagic ecosystem in the subtropical North Pacific Ocean, an effect that we a ttribute to the 1991-92 EI Nino-Southern Oscillation (ENSO) event. Dec reased upper-ocean mixing and a change in circulation resulted in an i ncreased abundance and activity of nitrogen-fixing microorganisms and a shift from a primarily nitrogen-limited to a primarily phosphorus-li mited habitat with attendant changes in total and export production an d in nutrient cycling pathways and rates.