Controls on productivity and carbon export in the Australian Subantarctic Zone of the Southern Ocean

We present data on gross primary production and net community production in the Subantarctic Zone south of Australia, determined as a collaboration with the SAZ-SENSE program (Austral summer of 2007 on Aurora Australis). Gross O2 production in the mixed layer was determined by measuring the triple isotope composition of dissolved O2, which gives an index of the fraction of dissolved O2 derived in situ from photosynthesis. Net community production was calculated from continuous measurements of the mixed layer O2/Ar ratio, which gives an index of biological O2 supersaturation. As carbon is not stored in the summertime mixed layer, NCP mostly sinks out, and NCP approximates carbon export. Ratios of NCP/GPP scattered about a value of 0.2 (corresponding to a carbon f-ratio of 0.4), similar to other regions of the Southern Ocean. NCP was highest just south of the subtropical front, likely due to the mixing of iron-rich subtropical waters with N- and P-replete subpolar waters. Productivities decreased to the south, presumably reflecting increasing iron limitation going away from aerosol and shelf sources. Controls on production were investigated by comparing GPP and NCP with extensive measurements of complementary physical and biogeochemical properties. Evidence for light limitation comes from the fact that productivities were much higher when mixed layer depths were shallower than 50 m than when they were deeper. Evidence for a link between iron and production comes from the fact that NCP increases with increasing variable fluorescence when comparing these properties for samples with MLD shallower than 50 m. Pigments were measured and used to deduce the abundance of phytoplankton groups by CHEMTAX analysis. The results show that there is no relationship between the abundance of diatoms and regions of high export production.