Southern Ocean Phytoplankton Community Structure as a Gatekeeper for Global Nutrient Biogeochemistry

Abstract

Upwelling and the biological pump in the Southern Ocean control the amount and stoichiometry of nutrients available for lateral export to lower latitudes, thereby collectively acting as a gatekeeper for the global thermocline nutrient distribution and global ocean productivity. Yet, the exact role played by phytoplankton and its community composition in this gatekeeping has not been well established. Here, we investigate this role using a high-resolution model of the Southern Ocean (ROMS-BEC) with an explicit parametrization of silicifying diatoms and calcifying coccolithophores. Consistent with expectations, diatoms are very efficient in consuming the upwelled Si (Formula presented.) south of the Antarctic Polar Front, and exporting it to depth at a rate of 91 Tmol Si (Formula presented.). This leads to Si (Formula presented.) being trapped in the Southern Ocean, preventing it from leaking laterally into the Subantarctic. Model experiments reveal that this trapping is driven by both high Si-to-N diatom uptake ratios and the relatively slow dissolution of the exported opal, with the latter being the dominant mechanism. The low (Formula presented.) consumption resulting from this high uptake ratio allows a good fraction of the upwelled (Formula presented.) to reach the Subantarctic where it fuels the growth of a mixed community including coccolithophores. Coccolithophore production and export of calcite at a rate of 0.16 Pg C (Formula presented.) facilitates an efficient transfer of the exported organic matter to depth, thereby further modifying the laterally exported nutrient levels and stoichiometry. Our results thus demonstrate a key role of phytoplankton community structure in controlling the Southern Ocean biogeochemical gate.