We report nitrate (NO3−) nitrogen isotope ratios for seawater samples collected in the Subantarctic Zone of the Southern Ocean during both winter and summer as part of the Australian Antarctic CRC Subantarctic Zone (SAZ) Project. The concentration and 15N/14N of the wintertime surface nitrate are very close to those 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 possible end-member explanations for this observation. First, there may be significant equatorward nitrate transport during the summer, including a supply from the Antarctic surface. Second, the isotope effect of algal nitrate assimilation may be higher than has been estimated elsewhere, for example, for the seasonal sea ice zone of the Antarctic. We use a simple geochemical box model of the SAZ surface mixed layer as it evolves over the course of the summer to simulate salinity, nitrate concentration, and the 15N/14N of nitrate and sinking N. Our results suggest that a significant portion (∼30%) of the summertime SAZ nitrate is supplied from south of the Subantarctic Front and that N export is ≥3.5 mmol N m−2 d−1. Our approach also identifies the necessity of an isotope effect for nitrate assimilation in the SAZ of ≥7‰ and probably 8–9‰. Comparison to laboratory results suggests that this relatively high isotope effect may result from light limitation of algal growth in the SAZ.