Net community production in terms of C, N, P and Si in the Antarctic Circumpolar Current and its influence on regional water mass characteristics

Abstract

The net impact of biological production and regeneration (Net Community Production) on surface water dissolved inorganic carbon, nitrogen, phosphorus, and silica (NCP DIC, NCP DIN, NCP PO 4 , and NCP Si ( OH ) 4 , respectively) were evaluated from nutrient data collected in the Antarctic Circumpolar Current (ACC) along 170°W between October 1997 and March 1998. Budgets were constructed in zonal bands that subdivided the ACC into a southern, seasonally ice-covered region, and three northern, and increasingly less stratified regions. Frontal positions, air–sea gas exchange, divergent Ekman flow, and meridional nutrient gradients were tracked through time to isolate biological change. The budget resolved the southerly progression of peak productivity with time in conjunction with the retreating ice-edge and the change from a production-dominated to a respiration-dominated system. Seasonal values of NCP DIC, NCP DIN, NCP PO 4 , NCP Si ( OH ) 4 from north of the Antarctic Polar Front (APF) to the South Antarctic Circumpolar Front (SACCF) along 170°W averaged 2.6, 0.27, 0.02, and 1.9 mol m −2, respectively. The influx of atmospheric CO 2 was a significant component (20–38%) of NCP DIC and varied from 6.8 to 12.8 g C m −2 season −1 across the region because of high winds in the north, and large surface pCO 2 deficits in the south. Although seasonal NCP DIC was fairly uniform across the region, NCP DIN was about 1.5 x greater, and NCP Si ( OH ) 4 was 2 x greater south of the APF compared to north of it. The present zonal pattern of NCP Si ( OH ) 4 differs from that suggested previously, in that we estimate greater production north of the APF and near the SACCF and relatively less just south of the APF, mainly because of the inclusion of a seasonally variable horizontal flux in the present analysis. In each sub-region, peak rates of NCP DIN and NCP PO 4 preceded those of NCP DIC and NCP Si ( OH ) 4 . Peak diatom growth periods exhibited NCP DIN / NCP PO 4 ratios (11–14) well below Redfield values. In addition, more rapid remineralization of phosphorus, and to a lesser extend nitrogen, as compared to carbon and silica were apparent late in the season. Increases in the NCP Si ( OH ) 4 / NCP DIN ratio through time were consistent with intensifying iron limitation south of the APF. The inter-nutrient comparisons highlight the functional differences among nutrient fluxes, in that, nutrients with relatively low surface remineralization like silicic acid are dependent on meridional transport to replenish surface pools, while surface pools of rapidly recycled nutrients, such as phosphate, can be replenished mainly by vertical mixing in winter. We suggest that biological processes in the Pacific ACC region play an important role in lowering the N/P ratio of the Antarctic Intermediate Water (AAIW).