One-dimensional ecosystem model of the equatorial Pacific upwelling system. Part II: sensitivity analysis and comparison with JGOFS EqPac data

A one-dimensional model of the equatorial Pacific upwelling ecosystem that incorporates two phytoplankton components, two grazers, and three nutrients, Si(OH)4, NO3, and NH4 (Chai et al., Deep-Sea Research II (2002) 2713–2745), was designed to consider the effects of Si(OH)4 limitation on the diatom growth and ecosystem functioning. Model output was obtained for a range of source concentrations of Si(OH)4, 3–15 mmol m−3, coinciding with the range measured at 120 m depth during JGOFS EqPac. NO3 was held at 12 mmol m−3, reflecting the relatively greater concentrations of NO3 compared to Si(OH)4 in the JGOFS data. The model was shown to function as a chemostat-like system with the loss rates, provided largely from zooplankton grazing, controlling growth rates of the phytoplankton. When different source concentrations of Si(OH)4 were applied, surface concentrations of Si(OH)4 varied within a narrow range compared to NO3 as would occur in a chemostat with limiting Si(OH)4 and non-limiting NO3 in the feed water. Vertical profiles of nutrients compared well with field data. Model results are compared with field data for new and total nitrogen production and export of N, Si, and C, and with other models, although none consider Si(OH)4 specifically. The model suggests that the stability of the equatorial system with its narrow range of biological and chemical variables is conferred by the action of diatoms providing food for mesozooplankton whose grazing also depletes the picoplankton. Diatoms increase with source Si(OH)4 concentrations, and picoplankton population and NO3 consumption decrease, resulting in a maximum surface TCO2 and increased CO2 flux to the atmosphere at intermediate source Si(OH)4 concentrations. Diatoms function in the equatorial system as a silica pump to export silica. This means that sedimented biogenic silica under the equatorial upwelling area should be viewed as an amplifier of changes in surface properties, with important consequences to paleoequatorial productivity.