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 degreesS and 35 degreesS. 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 degreesS) and 4 mmol N m(-2) d(-1) north of 70 degreesS. South of 55 degreesS 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 degreesS 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 degreesS 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 degreesS 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 ocean's biological pump. The current lack of wintertime hydrographic and nutrient data is the biggest limitation to closing the nitrate budget in the Southern Ocean. (C) 2001 Elsevier Science Ltd. All rights reserved.