Carbon and nutrient cycling in the upper water column across the Polar Frontal Zone and Antarctic Circumpolar Current along 170°W

Abstract

Seasonal changes of upper water column chemical properties integrate the effects of the physical, chemical, and biological processes that control the export of carbon to the deep ocean. Between October 1997 and March 1998, several hydrographic cruises were undertaken in the southwestern Pacific sector of the Southern Ocean, as part of the U.S. JGOFS program. On these cruises, the partial pressure of carbon dioxide (pCO2) and concentration of total carbon dioxide (TCO2) dissolved in seawater were determined in surface waters, along with the nutrient concentrations [nitrate (NO3−), nitrite (NO2−), ammonium (NH4+), phosphate (PO4≡), and silicate (Si(OH)4)]. This interval saw the commencement, culmination, and dénouement of phytoplankton blooms, both north and south of the Polar Front (PF). Nutrient utilization, regeneration, and export ratios, and primary production and export, were estimated from seasonal changes in these properties observed in surface waters across the Polar Front. While the biological drawdowns of carbon dioxide and nutrient concentrations in the euphotic zone were greater south of the front, the estimated primary productivity (1.6–2.7 mol C/m2/yr) and export (1.2–1.5 mol C/m2/yr) are comparable. The observed C/N/P ratios vary temporally, as a function of the dominant process, and spatially, as a function of the phytoplankton assemblage. South of the PF, the maximum biological utilization (or new production) C/N/P ratios are 69 ± 2/10.4 ± 0.5/1 while post‐regeneration biological utilization (or export) C/N/P ratios are 87 ± 3/12.7 ± 0.5/1. North of the PF the C/N/P ratios are similar to the classic Redfield ratios, 100 ± 5/(14–16.5) ± 0.5/1. Silica/(C, N, and P) ratios are also highly variable temporally, reflecting differences between silica and organic nutrient cycling, and the bio‐availability iron. Departures from the “Redfield ratio” arise from excess phosphate uptake by diatoms and the preferential regeneration of phosphate. The disparity in nutrient ratios north and south of the PF reflects difference in phytoplankton assemblages. South of the PF there was a greater percentage of diatoms than to the north.