Biological and physical controls on N sub(2), O sub(2), and CO sub(2) distributions in contrasting Southern Ocean surface waters

• Published in: Global biogeochemical cycles 2015-07, Vol.29 (7), p.994-1013
• Main Authors: Tortell, Philippe D, Bittig, Henry C, Kortzinger, Arne, Jones, Elizabeth M, Hoppema, Mario
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1002/2014GB004975

We present measurements of pCO sub(2), O sub(2) concentration, biological oxygen saturation ( Delta O sub(2)/Ar), and N sub(2) saturation ( Delta N sub(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 sub(2) and Delta O sub(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 sub(2) accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time scales of O sub(2) and CO sub(2) air-sea equilibration. We measured significant oceanic CO sub(2) uptake, with a mean air-sea flux (~-10mmolm super(-2)d super( -1)) that significantly exceeded regional climatological values. N sub(2) was mostly supersaturated in surface waters (mean Delta N sub(2) of +2.5%), while physical processes resulted in both supersaturation and undersaturation of mixed layer O sub(2) (mean Delta O sub(2phys)=2.1%). Box model calculations were able to reproduce much of the spatial variability of Delta N sub(2) and Delta O sub(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 Delta O sub(2)/Ar data, ranged from ~-40 to >300mmolO sub(2)m super(-2)d super(-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. Key Points * Biological and physical controls on Southern Ocean gases are quantified * Sea-air CO sub(2) fluxes significantly exceed regional climatological values * Net community production estimates are corrected for physical processes