New estimates of Southern Ocean biological production rates from O2/Ar ratios and the triple isotope composition of O2

We report O2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O2 (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1–2° meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O2 production, and gross O2 production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O2 production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous 18O and 14C incubations along 170°W. O2/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O2/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O2-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O2 production scatter about 0.13, corresponding to f ratios of ∼0.25. Net community/gross O2 production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O2 production rates are in satisfactory agreement with comparable published estimates. Net and gross O2 production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O2 production and gross O2 production with other environmental variables, the strongest correlations are between net O2 production and sea surface temperature (SST) (direct correlation), climatological [NO3−] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO2 and iron are the leading influence or if lower SSTs promote greater export in this region.