Primary productivity and its regulation in the equatorial Pacific during and following the 1991–1992 El Niño

The cycling of carbon in the equatorial Pacific Ocean was investigated by the Equatorial Pacific (EqPac) Study in 1992. As part of that study in situ primary productivity was measured on survey and time-series cruises along 140°W from 12°N to 12°S with methods determined to be trace-metal clean. Primary productivity, chlorophyll and chlorophyll-specific productivity rates varied coherently in relation to two large-scale features: temporally, primary productivity was reduced during the El Niño dominated period (February–April 1992) and increased during the cool period (August–October 1992); and spatially enhanced primary productivity persisted close to the equator relative to the oligotrophic regions poleward of 10°N and 10°S. On the equator in October 1992 during the period of relatively cool water, primary productivity was about twice (125 mmol C m−2 day−1) the value during the peak warm period (60 mmol C m−2 day−1). The climatological mean equatorial productivity in the cold tongue has been recalculated to be about 75 mmol C m−2 day−1 (Chavez et al., 1996). The mean 1992 productivity on the equator (1°S–1°N) was about 25% higher than climatology (95 vs 75 mmol Cm−2 day−1) and about 3 times the value in oligotrophic waters poleward of 10°N and 10°S (95 vs 30 mmol C m−2 day−1). Higher chlorophyll-specific productivity during the cool period relative to the warm period (3.9 vs 2.4 mmol C mg chl−1 day−1) indicates that the increase in absolute productivity did not result solely from a biomass increase, but from a change in the nutrient-regulated specific productivity rate. The regulating nutrient was not a macronutrient, such as nitrate or silicic acid, because macronutrients (and light) were present in uptake-saturating concentrations during both the warm and cool periods of the 1992 EqPac study. Physiological constraint by a micronutrient, such as iron, is implicated as the factor regulating these productivity variations. The change in iron supply resulted from a change in equatorial circulation processes. During the warm period, El Niño-Southern Oscillation (ENSO)-driven changes in pycnocline topography depressed the Equatorial Undercurrent (EUC), thereby decreasing the amount of iron-rich EUC water entrained into equatorial upwelling and vice versa during the cool period. During the August–October cool period of generally increased productivity, two further episodic increases in specific productivity, biomass and diatom abundance occurred during intense and remotely forced upwelling events associated with a front or the passage of a frontal wave. In both mesoscale episodes, temperature and salinity show that the intensified upwelling reached more deeply into the already relatively shallow EUC. Productivity and biomass increases during both of these events were quantitatively similar to those in an in situ iron addition experiment (IronEx) carried out in equatorial Pacific waters in 1993. Variations in the supply of upwelled iron provided by the iron-rich EUC best account for the warm-cool period difference in phytoplankton productivity as well as the episodic increases in specific productivity, biomass and diatom abundance during intense mesoscale upwelling events seen in the dynamic equatorial region in the EqPac study.