ANNUAL AND INTERANNUAL VARIATIONS OF PHYTOPLANKTON PIGMENT CONCENTRATION AND UPWELLING ALONG THE PACIFIC EQUATOR

The following variables along the Pacific equator from 145-degrees-E t o 95-degrees-W were employed: surface layer phytoplankton pigment conc entrations derived from Nimbus 7 coastal zone color scanner (CZCS) mea surements of ocean color radiances; vertical velocities simulated at t he 90-m bottom of the euphotic layer from a wind-driven ocean general circulation model; and nitrate concentrations estimated from model-sim ulated temperature. The upward flux of nitrate into the euphotic layer was calculated from the simulated vertical motion and nitrate concent ration. The CZCS-derived phytoplankton pigment concentration was unifo rm from 175-degrees to 95-degrees-W. Longitudinal profiles of upwellin g, phytoplankton biomass, and 90-m nitrate flux were of different shap es. The small annual cycles of the phytoplankton pigment concentration and nitrate flux were in phase: increased phytoplankton biomass was a ssociated with increased upward nitrate flux, but the phase was not co nsistent with the annual cycles of the easterly wind or of the upwelli ng intensity. Variation of phytoplankton pigment concentration was gre ater during El Nino than during the annual cycle. The substantially re duced phytoplankton pigment concentration observed during El Nino was associated with smaller upward nitrate flux. Phytoplankton biomass dur ing non-El Nino conditions was not related to nitrate flux into the eu photic layer.