Modeling carbon and silicon cycling in the equatorial Pacific

The equatorial Pacific is a region of significant particulate inorganic carbon (PIC) and biogenic silica sedimentation, the majority of which is carried out by coccolithophorids and diatoms. We developed an ecosystem model that explicitly includes three phytoplankton functional groups (picoplankton, coccolithophorids, and diatoms), two zooplankton functional groups (microzooplankton and mesozooplankton), nutrients (nitrate NO3, ammonium NH4, and silicate Si(OH)4), detritus (particulate organic matter, biogenic silica, and PIC), total alkalinity, total CO2, and partial pressure of CO2 at the surface water (pCO2sea). The model is capable of reproducing many biogeochemical features for the region, such as high-nutrient low-chlorophyll condition, significant exposure of phytoplankton under grazing controls by zooplankton, and large CO2 release to the atmosphere. The export ratio of PIC to particulate organic carbon (rain ratio) to the deep water was 0.16, higher than the global-mean values, implying predominant PIC sedimentation in the equatorial Pacific upwelling region. Comparison between calcification and no-calcification model results indicates that when coccolithophorids are present, the community interactions actually induce more diatom biomass, export fluxes of detritus, and CO2 release to the atmosphere. The model results show remarkable calcification in the subsurface layers, which suggests more field data on calcification processes are needed. Increase of source (120 m depth) Si(OH)4 concentration associated with the tropical instability waves lead to a linear increase in biogenic silica export. Higher Si(OH)4 concentration stimulates diatom growth, which causes a decrease in picoplankton because feeding pressure by mesozooplankton switched from picoplankton's grazer, microzooplankton, to the abundant diatoms. Surface coccolithophorid biomass has its maximum at intermediate source Si(OH)4 concentrations as a result of higher grazing pressure on coccolithophorids and higher NO3 regulation on coccolithophorid growth, with lower and higher source Si(OH)4 concentrations, respectively. Surface total alkalinity has its minimum and TCO2 has its maximum at intermediate source Si(OH)4 concentrations. The two effects on pCO2sea result in maximum CO2 release to the atmosphere and PIC export to the deep water, with nearby standard source Si(OH)4 concentration of 7.5 (mmol Si m−3). The enhanced changes in biogenic silica export flux than in surface diatom biomass, confirmed by the model sensitivity study, suggest sedimented detritus under the equatorial Pacific upwelling region acts as an amplifier of changes in surface properties. The model results suggest that physical forcing, such as tropical instability waves, Kelvin waves, and La Niña, which is capable of changing Si(OH)4 and iron concentrations in the euphotic zone, significantly affect both carbon and silicon fluxes in the region.