Modeling controls of phytoplankton production in the southwest Pacific sector of the Southern Ocean

Abstract

Abstract The Southern Ocean is the largest high-nutrient, low-chlorophyll region in the world's ocean and a potentially important site for the sequestration of carbon. We present a one-dimensional physical/biogeochemical model that integrates biogeochemical measurements obtained during the AESOPS (U.S. JGOFS) study in the southwest Pacific sector to elucidate the controls of primary productivity and export. The model is applied to a series of four stations along 170°W spanning the different biogeochemical subsystems in the Polar Frontal Zone, the Polar Front, and the Seasonal Ice Zone south of the Polar Front. Since horizontal fluxes of heat, freshwater and nutrients are found to be important but cannot be resolved explicitly in a one-dimensional model, we employ a restoration of temperature, salinity and nutrients. The surface fluxes of light and momentum are modified during ice-covered periods to account for the effects of sea ice. The biological model component includes the elemental cycles of nitrogen and silica. Diatoms are represented as a separate phytoplankton group, and small phytoplankton and zooplankton are tightly coupled. The effect of the low iron availability in the region is implicitly taken into account by using typical phytoplankton growth rates and a typical, high Si:N stoichiometry of 4 for the diatoms. The model captures the essential features of the distinct subsystems including the low-chlorophyll condition north of the Polar Front, the diatom blooms in the vicinity of the Polar Front and to its south, the differential drawdown of nitrate and silicic acid, and the seasonal patterns of biogenic silica, primary production and vertical particle flux. “Top-down” control of the small phytoplankton by efficient microzooplankton grazing and “bottom-up” control of diatoms by light and silicic acid are the main factors for the simulated behavior. A sensitivity analysis of the biological model component shows that the growth parameters for the two phytoplankton groups are most important in constraining primary productivity and overall model behavior. This implies that changes in growth rates induced by variations in iron supply as assumed over glacial–interglacial transitions can affect primary and export production substantially.