Plankton community structure and export of C, N, P and Si in the Antarctic Circumpolar Current

We analyzed the relationship between the estimated export (ei) of carbon, nitrogen, silica and phosphorus from the surface ∼90 m in the Antarctic Circumpolar Current (ACC) region along 170 °W and the seasonal development of the epipelagic community. Export was based on nutrient budgets from repeat sections during the 1997–1998 austral summer to estimate nutrient specific, net community production (NCPi; Green and Sambrotto, 2006). We adjusted NCPi for changes in surface-water particulate material and dissolved organic matter to isolate ei on an approximately monthly basis in four meridional regimes. Estimates of eC, eN and eSi from October to March ranged from 1.9–2.7, 0.11–0.34 and 1.0–2.7 moles m−2 season−1. Although the meridional trends in our export estimates were similar to previously published trends that suggest an increase in carbon export south of the front, our estimates north of the front were about 30% higher than prior estimates in the region. Typically, the peak of eN (and eP) was 4–6 weeks before that for eSi and eC. The lag between eN and eC was similar to the lag between new production and 234Th-based eC in the region as well as similar lags found in upwelling water of the equatorial Pacific and the Arabian Sea. We suggest that this lag is characteristic of physically perturbed regions and reflects the temporal development of the grazing community, a time scale that appears to vary little, despite a 20 °C difference in temperature. Based on pigment markers and direct microscopic analyses, the growth of heavily silicified Fragilariopsis spp. south of the Antarctic Polar Front (APF) was associated with a rapid increase in eSi in December as the ice retreated. However, elevated eC continued at high levels as the diatom community disappeared (particularly in the north). Later successional forms such as Chaetoceros spp. and Corethron spp. appeared to enhance eC by facilitating aggregation. Thus, the seasonal pattern of eC in the ACC was impacted by the nature of the diatom community and was not a simple function of total opal concentration. This aggregator community promoted elevated eC even at relatively low concentrations of diatoms north of the APF. The pigment markers also reflected other important environmental associations including dinoflagellates with the APF and Phaeocystis with the melt water from the retreating ice. Export ratios deviate from Redfield and suggest a combination of environmental and taxonomic influences. Diatom growth early in the season was associated with low eN/eP ratios and the depletion of iron in January was associated with elevated eSi/eN ratios.