Spatial and temporal variability in new production in the equatorial Pacific during 1980–2003: Physical and biogeochemical controls

Abstract

Abstract The total new production in the equatorial Pacific ocean is still subject to debate. Early estimates range from 0.8 to 1.9 Pg C year−1 for the Wyrtki box (5°N–5°S, 180°W–90°W). More recent studies estimate a rate of less than 0.14 Pg C year−1 for the 1°N–1°S equatorial divergence, which is significantly lower than the previous estimates even when the difference in area is taken into account. To address this issue, we employ a coupled three-dimensional physical-biogeochemical model to simulate ecosystem dynamics and to estimate new production over the period of 1980–2003. The biogeochemical model consists of nine components: two size classes (large and small) each of phytoplankton, zooplankton and detritus, and three nutrients (ammonium, nitrate, and iron). The model is able to reasonably reproduce observed surface chlorophyll, ecosystem structure, primary productivity, and new production. The modeled primary productivity and new production are in the ranges of 20–100 and 5–40 mmol C m−2 d−1, respectively. The modeled primary productivity shows relatively weak spatial and temporal variations whereas the modeled new production shows considerable variability over space and on seasonal-to-interannual time scales. The model simulates two peaks in new production (e.g., in boreal spring and boreal fall) in the high-nitrate low-chlorophyll (HNLC) region. The high rate of new production in spring is associated with the seasonal warming and a shallow mixed layer whereas the high rate of new production during fall is related to the high iron concentrations due to the seasonal upwelling and entrainment in the eastern equatorial Pacific. The new production reveals clear spatial and interannual variability in association with the El Nino/Southern Oscillation (ENSO). The rate of new production is highly related to the mixed-layer concentrations of nitrate and iron, and the 20 °C isotherm depth (Z20) in the HNLC region. However, there is no relationship between the rate of new production and either the concentrations of nitrate or iron, or Z20 in the warm pool. Integrated new production from the model is 0.61±0.14 Pg C year−1 for the Wyrtki box, which is significantly lower than some previous estimates.