Abstract

Southern Ocean phytoplankton biogeography is important for the biogeochemical cycling of carbon, silicate, and the transport of macronutrients to lower latitudes. With the discovery of the “Great Calcite Belt” (GBC), revealing an unexpectedly high prevalence of calcifying phytoplankton between 40-55°S, the relative importance of Southern Ocean coccolithophores for phytoplankton biomass, net primary productivity and the carbon cycle need to be revisited. Here, I present results from a regional high-resolution model with an embedded ecosystem module (ROMS-BEC) for the Southern Ocean (24-78°S). We assess the environmental drivers of Southern Ocean coccolithophore biogeography over the course of the growing season focusing on biotic interactions and the relative importance of top-down versus bottom-up factors controlling growth and abundance. Furthermore, we perform iron fertilization experiments to assess the effects of changed nutrient availability on coccolithophore abundance in the GCB . We find that top down controls are a major control on the relative abundance of diatoms and coccolithophores in the Southern Ocean and that changes in nutrient stoichiometry significantly alter phytoplankton community composition, the relative contribution of particulate organic and inorganic carbon, as well as opal to export, and the supply of nutrients to lower latitudes. Consequently, when assessing potential future changes in Southern Ocean coccolithophore abundance and its implications for biogeochemical cycles, both physical (temperature, light, nutrient availability) and chemical (ocean acidification) changes, but also biotic interactions need to be considered.