Summertime physical and biological controls on O2 and CO2 in the Australian Sector of the Southern Ocean

Abstract

Continuous measurements of CO2 partial pressure, dissolved oxygen, and the oxygen argon ratio (O2/Ar) in surface waters, complemented by discrete observations of inorganic carbon, along a late-summer south-to-north transect in the Australian Sector of the Southern Ocean (115°E, WOCE I9S) are presented. The largest net community production (NCP, based on O2/Ar), and air–sea CO2 disequilibrium (ΔDpCO2) were found in three regions: (1) between the southern boundary of the Antarctic Circumpolar Current (63.6°S) and southern extent of the Polar Front (58.6°S, NCP=30mmolCm−2d−1, ΔpCO2=30 to 50μatm); (2) between the northern extent of the Sub-Antarctic Front (45.5°S) and the Sub-Tropical Front (39.8°S, NCP=50mmolCm−2d−1, ΔpCO2=25μatm); and (3) near 35°S (NCP=50mmolCm−2d−1, ΔpCO2=20μatm). Areas of enhanced NCP were correlated with shallow mixed layer depths, suggesting that production is controlled in part by light limitation. We found good agreement between estimates of NCP based on O2/Ar measurements and those derived from seasonal (mixed layer) inorganic carbon deficits. In ice covered areas, and in areas where surface temperature changes act on shorter timescales than the equilibration of oxygen in the mixed layer, we find that physical controls on surface O2 are of equal magnitude to biological controls, which has important implications for the use of oxygen sensors on autonomous platforms to infer NCP.