Seasonal nutrient depletion and carbon export in the Subantarctic and Polar Frontal Zones of the Southern Ocean south of Australia

Seasonal surface water nitrate and phosphate depletions were calculated for the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) of the Southern Ocean along the World Ocean Circulation Experiment SR3 repeat section south of Tasmania, Australia, and three other transects slightly to the west. The depletions were calculated between a single winter (July) section and eight other sections from October to March in the years 1991-1998 and corrected for frontal movements and changes in water masses between the cruises using observed quasi-linear nutrient-salinity relationships. Arranged in a seasonal progression, depletion began in approximately October and continued through March, in good agreement with satellite biomass-based estimates of production. Nitrate depletion in the SAZ (510 mmol N m-2) was approximately twice that of the PFZ (250 mmol N m-2), but phosphate depletions were similar (29 and 24 mmol P m-2 for the SAZ and PFZ, respectively). The SAZ exhibited a Redfield N/P depletion ratio (15.1 ± 2.3), but the PFZ ratio was much lower (8.3 ± 5.4). While the SAZ exhibits greater seasonal nutrient depletion, the relative magnitudes of export in the two zones remain uncertain because supply of nutrients from south of the Polar Front may lower the observed nutrient depletions in the PFZ. However, comparisons with sediment trap and production measurements for the region south of Australia suggest the SAZ and PFZ differences derive from differences in production. Assuming a Redfield C/N ratio (6.6), carbon export for the longest period examined (July-March) averaged 3400 ± 430 mmol C m-2 in the SAZ, with a range between 2700 and 3900 mmol C m-2 depending on latitude. In the PFZ, carbon export averaged 1600 ± 350 mmol C m-2, with a range of 1100 to 2000 mmol C m-2. These averages represent minimum annual export production estimates because neither resupply nor export outside the July-March observational period is included. Copyright 2001 by the American Geophysical Union.