Primary productivity in the equatorial Pacific during the 1997-1998 El Nino

Shipboard biological and physical measurements made during 1996, 1997, and 1998 in the equatorial Pacific are used to quantify the effect of the 1997-1998 El Nino on the phytoplankton community. This El Nino was by some measures the strongest ever observed and resulted in extremely low phytoplankton biomass and productivity throughout this normally moderately productive region. At the height of the event in late 1997 to early 1998, in the central Pacific, nitrate was absent throughout the entire euphotic zone (similar to 100 m), resulting in chlorophyll concentrations (0.05 mug L-1) that were among the fewest ever observed in the region and rates of primary production (similar to0.41 g C m(-2) d(-1)) that were approximately half the climatological mean. These conditions persisted until May 1998 when the trade winds resumed and upwelling, with its associated supply of nutrients, was restored along the equatorial Pacific. The phytoplankton community quickly recovered, and by June 1998, nitrate, chlorophyll, and primary productivity levels were comparable to, or in excess of, their respective climatological means.