Southern Ocean annual net community production using combined biogeochemical mass balances on profiling floats

The Southern Ocean is responsible for over 40% of the annual oceanic uptake of anthropogenic carbon and is the source of ~75% of the nutrients that drive global productivity. Understanding the magnitude and variability of net community production in the Southern Ocean is important for improving our understanding of what drives carbon uptake and what sets the pre-formed nutrient concentration of waters as they flow north. Quantifying biological carbon export over large areas is difficult using traditional in situ observations, and satellite-derived algorithms of carbon export need ground-truthing to be able to relate surface observations with processes in the water column. In this work we use recently available profiling float observations of oxygen, nitrate, and inferred dissolved inorganic carbon to create mass balances from more than 200 float years to estimate biogenic carbon export over the entire Southern Ocean. Float data are analyzed using an upper ocean box model, and biogeochemical data are evaluated along with a heat budget to determine where estimates of net community production are robust. This approach allows calculation of net community production in the surface ocean and integrated down to the ventilation depth for comparison with methods that sample only in the surface ocean or only at depth. We are also able to evaluate the seasonal progression of net community production in order to differentiate productivity during the summer growth period from the annual net productivity that includes respiration at depth and during the winter. Southern Ocean net community production from this method follows prior work that indicates a maximum at ~48°S with less to the north and south. Superimposed on this large-scale trend is significant spatial variability, with higher production in the Argentine Basin and the western Pacific Basin. The wide-spread availability of Biogeochemical-Argo floats in the Southern Ocean presents a new tool to link satellite observations with in situ estimates of biological carbon export and to better understand how Southern Ocean net community production impacts air-sea carbon fluxes and pre-formed nutrient concentrations.