Ocean productivity south of Australia during spring and summer

We estimated mixed layer gross and net community production on a total of 20 crossings in the Australian sector of the Southern Ocean during the summer half-years (October–March) of 2007–2010. These estimates were calculated from measurements of O₂/Ar ratios and triple isotope compositions of O₂ in ∼250 seawater samples collected underway. For comparison purposes, we also measured the seasonal drawdown of mixed layer NO-3 and SiO₂ concentrations during 2006–2007 and 2007–2008. Across all samples, average values of gross and net O₂ production (measured by O₂/Ar and O₂ isotopes), were about 86 ± 90 and 18 ± 17 mmol O₂ m⁻² day⁻¹, respectively. Gross production was highest at the Subtropical Front (up to ∼230 mmol O₂ m⁻² day⁻¹), and decreased southward (to ~10 near the southern boundary of the Antarctic Circumpolar Current). In contrast, net community production showed little meridional variation. Net and gross O₂ production increased throughout the spring-to-fall period, although most SiO₂ drawdown occurred in December. Consistent with satellite chlorophyll estimates, we saw no evidence for an intense spring bloom (e.g. as has been observed in the North Atlantic). Volumetric net and gross O₂ production in the mixed layer, normalized to chlorophyll, increased (with considerable scatter) with average irradiance in the mixed layer. These relationships provide a basis for estimating production from Argo float data and properties observed by satellite.