Variations in phytoplankton dynamics and primary production associated with ENSO cycle in the western and central equatorial Pacific during 1994-2003

We examined spatial and temporal variations in phytoplankton distribution and primary production on the equator between 143 degrees E and 160 degrees W during 1994-2003. The study area extended from the nitrate-depleted western Pacific warm pool (WPWP) to the nitrate-replete upwelling region in the central equatorial Pacific. Whereas depth-integrated chlorophyll a concentrations did not differ greatly between the WPWP (30.5 +/- 5.7 mg m(-2)) and upwelling region (34.4 +/- 5.4 mg m(-2)), depth-integrated primary production in the upwelling region was approximately double (990 +/- 61 mg C m(-2) d(-1)) that in the WPWP region (435 +/- 158 mg C m(-2) d(-1)), owing to the presence of deep chlorophyll maximum in the former and higher nutrient availability in the latter. Marked differences in water column structure were observed in the WPWP region between El Nnio and La Nina: eastward advection of the WPWP region with development of El Nino led to thermocline shoaling in the WPWP region, resulting in a significant nitracline uplift (P < 0.01). In this region, the isopleth of 1-mu M nitrate in the nitracline was brought up from an average of 103 m during La Nina to 61 m during El Nino. When the uplift reached the lower part of the euphotic zone, primary production was enhanced. The enhancement occurred at 4% to 10% light depths relative to that at the surface. However, the magnitude of the enhancement was not large, and there was no substantial difference in depth-integrated primary production between El Nino and La Nina.