Biological export flux in the Southern Ocean estimated from a climatological nitrate budget

Diagnostic budgets for heat and nitrate in the Southern Ocean mixed layer are constructed from monthly variation of properties between 80°S and 35°S. The budgets are evaluated using historical CTD and bottle data, including those from WOCE and US JGOFS cruises in the 1990s. A consistent feature in the nitrate budget at all latitudes is the net loss to biological production in the spring/summer. Typical maximum spring nitrate utilization rates are predicted to be 10–20 mmol N m−2 d−1 near Antarctica (south of 70°S) and 4 mmol N m−2 d−1 north of 70°S. South of 55°S much of the nitrate lost to biological production in spring/summer is offset by apparent nitrate production (remineralization) in the mixed layer during the fall/winter. This predicted remineralization, however, may be an artifact of the model and/or limited wintertime data. The annually averaged, area-integrated biological carbon export flux south of 50°S is dominated by the region north of the Polar Front, and is estimated to be 0.9±1.2 Gt C yr−1, assuming a Redfield ratio C/N=6.6. Integrating over the area south of 40°S this flux is increased to 2.0±1.7 Gt C yr−1. However, if we assume that the predicted fall/winter remineralization is in error, and that unresolved physical processes completely replenish the mixed-layer nitrate, then the annual average, area-integrated biological export flux to 50°S increases from 0.9 to 2.2 Gt C yr−1. These results suggest that the Southern Ocean makes a small contribution (<15%) to the global oceanʹs biological pump. The current lack of wintertime hydrographic and nutrient data is the biggest limitation to closing the nitrate budget in the Southern Ocean.