One-dimensional ecosystem model of the equatorial Pacific upwelling system. Part I: model development and silicon and nitrogen cycle

A one-dimensional ecosystem model was developed for the equatorial Pacific upwelling system, and the model was used to study nitrogen and silicon cycle in the equatorial Pacific. The ecosystem model consisted of 10 components (nitrate, silicate, ammonium, small phytoplankton, diatom, micro- and meso-zooplankton, detrital nitrogen and silicon, and total CO 2). The ecosystem model was forced by the area-averaged (5°S–5°N, 90°W–180°, the Wyrtki Box) annual mean upwelling velocity and vertical diffusivity obtained from a three-dimensional circulation model. The model was capable of reproducing the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific. The linkage to carbon cycle was through the consumption of assimilated nitrate and silicate (i.e. new productions). Model simulations demonstrated that low-silicate concentration in the equatorial Pacific limits production of diatoms, and it resulted in low percentage of diatoms, 16%, in the total phytoplankton biomass. In the area of 5°S–5°N and 90°W–180°, the model produced an estimated sea-to-air CO 2 flux of 4.3 mol m −2 yr −1, which is consistent with the observed results ranging of 1.0–4.5 mol m −2 yr −1. The ammonium inhibition played an important role in determining the nitrogen cycle in the model. The modeled surface nitrate concentration could increase by a factor of 10 (from 0.8 to 8.0 mmol m −3) when the strength of the ammonium inhibition increased from ψ=1.0 to 10.0 (mmol m −3) –1. The effects of both micro- and meso-zooplankton grazing were tested by varying the micro- and meso-zooplankton maximum grazing rates, G1 max and G2 max. The modeled results were quite sensitive to the zooplankton grazing parameters. The current model considered the role of iron implicitly through the parameters that determine the growth rate of diatoms. Several iron-enrichment experiments were conducted by changing the parameter α (the initial slope of the photosynthetic rate over irradiance at low irradiance), K Si(OH) 4 (half-saturation concentration of silicate uptake by diatom), and μ2 max (the potential maximum specific diatom growth rate) in the regulation terms of silicate uptake by diatom. Within the first 5 days in the modeled iron-enrichment experiment, the diatom biomass increased from 0.08 to 2.5 mmol m −3, more than a factor of 30 increase. But the diatom populations crashed 2 weeks after the experiment started, due to exhaustion of available silicate and increased meso