Summary
The isotopes of nitrate, which show different dynamics than nitrate concentration during physical transport and nitrate assimilation, provide a complementary constraint in the effort to understand nutrient transport and biogeochemical fluxes in the Southern Ocean. For instance, two potential sources of nitrate to the Subantarctic Zone (SAZ) surface, the Subantarctic thermocline below and the Antarctic Zone to the south, have different isotope/concentration relationships, so that the isotope/concentration characteristics of surface nitrate in the SAZ constrain the relative importance of these two sources. We report seasonal nitrate nitrogen isotope data for seawater samples collected as part of the Australian ANTCRC SAZ Program. The concentration and 15N/ 14N of the wintertime surface nitrate are very close to that of the Subantarctic thermocline. The 15N/ 14N of nitrate in the surface increases sharply into the summer even though there is little seasonal change in nitrate concentration. There are two causes for this observation. First, there is significant lateral nitrate supply from the Antarctic surface during the summer. Second, the isotope effect of algal nitrate assimilation appears to be higher than has been estimated elsewhere. We incorporate the data into a simple geochemical box model which simulates salinity, nitrate concentration, and nitrate 15N/ 14N of the SAZ surface mixed layer as it evolves over the course of the summer. Our results suggest that 30 percent of the nitrate remaining in the SAZ at the end of the summer originates from South of the Polar Frontal Zone. Our approach also identifies the necessity of a greater isotope fractionation during nitrate assimilation in the SAZ (suggesting an isotope effect of ~ 8 per mil) than in other subpolar regimes (where the isotope effect is closer to 5 per mil). Given current information on the controls of nitrate isotope fractionation, we hypothesize that the high isotope effect of nitrate assimilation in the SAZ is due to light limitation during the spring/summer bloom, associated with the pervasively deep mixed layers of the region.