Biological and physical controls on N2, O2, and CO2 distributions in contrasting Southern Ocean surface waters

We present measurements of pCO(2), O-2 concentration, biological oxygen saturation (O-2/Ar), and N-2 saturation (N-2) in Southern Ocean surface waters during austral summer, 2010-2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chl a) concentrations in regions of frontal mixing and sea ice melt. pCO(2) and O-2/Ar exhibited large spatial gradients (range 90 to 450 mu atm and -10 to 60%, respectively) and covaried strongly with Chl a. However, the ratio of biological O-2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time scales of O-2 and CO2 air-sea equilibration. We measured significant oceanic CO2 uptake, with a mean air-sea flux (-10mmolm(-2)d(-1)) that significantly exceeded regional climatological values. N-2 was mostly supersaturated in surface waters (mean N-2 of +2.5%), while physical processes resulted in both supersaturation and undersaturation of mixed layer O-2 (mean O-2phys=2.1%). Box model calculations were able to reproduce much of the spatial variability of N-2 and O-2phys along the cruise track, demonstrating significant effects of air-sea exchange processes (e.g., atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment-corrected surface O-2/Ar data, ranged from -40 to >300mmolO(2)m(-2)d(-1) and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea ice melt, reflecting physical controls on surface water light fields and nutrient availability.