Distribution of CO2 species, estimates of net community production, and air-sea CO2 exchange in the Ross Sea polynya

Measurements of surface total carbon dioxide (TCO2), alkalinity, and calculated pCO2, along with water column nutrients and hydrography, were made on two cruises to the Ross Sea polynya (NBP 94‐6, November‐December 1994 and NBP 95‐8, December 1995 to January 1996). The polynya experiences an intense phytoplankton bloom during a short period of open water conditions from mid‐December to mid‐February each year. Our biogeochemical observations were used to determine the temporal variability of CO2, fluxes of carbon within the ocean, and rates of air‐sea exchange of CO2. Depletions of TCO2, pCO2, and nitrate+nitrite were considerable (∼70–150 μmol kg−1, 80–150 μatm, and 10–20 μmol kg−1, respectively) and associated primarily with biological uptake during Phaeocystis and diatom blooms. Alkalinity was a conservative tracer of salinity and nitrate+nitrite. Surface ρCO2 was undersaturated by ∼50–150 μatm, and air‐sea gas exchange of CO2 during open water conditions was directed from atmosphere to ocean. Observed surface stoichiometric C:N ratios were 6.66: 1 and 6.77:1 for the 2 years, consistent with global “Redfield” ratios, while C:P and N:P ratios were variable (75–141:1, 12–18:1). Estimates of net community production (NCP) rates were made using in situ changes in TCO2 and nitrate+nitrite across repeated transects along 76°30′S. Mean NCP rates across the polynya ranged from 0.86 to 0.98 g C m−2 d−1. These values may be underestimated by 5–25% because of the contribution of atmospheric CO2 to the surface layer through gas exchange. Export of carbon from the surface to depth was at least 55–60% of NCP rates.