Sensitivity of δ¹⁵N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean

We report measurements of the δ¹⁵N of nitrate, suspended particulate nitrogen (PN), and sinking PN from cruises and moored sediment traps in the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia. As expected, surface water nitrate δ¹⁵N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal cycles of surface water suspended and sinking PN δ¹⁵N did not fit expectations from nitrate assimilation alone. Rather than increasing, the δ¹⁵N of surface suspended PN was relatively constant in the SAZ (at ∼1‰), and decreased during the summer in the PFZ (from ∼0 to ∼-4‰), most likely due to the production of low ¹⁵N PN by summertime ammonium recycling. Deep sediment trap PN δ¹⁵N also displayed seasonal decreases (from ∼4 to ∼1‰ in the SAZ, and from ∼3.5 to ∼0.5‰ in the PFZ), which correlated with PON flux magnitude. During high-flux periods, exported PN δ¹⁵N values were close to expectations from nitrate-based export, but low-flux periods exhibited higher δ¹⁵N, consistent with either a reduction in the isotope effect of nitrate assimilation or more extensive isotopic alteration of the sinking material during low-flux periods. The mass balance between net nitrate supply and exported PN that links sinking flux δ¹⁵N to nitrate utilization requires only that the annually integrated (rather than the seasonally varying) sinking flux of PN δ¹⁵N correlates with nitrate depletion. While a correlation between annually integrated sinking PN δ¹⁵N to nitrate depletion was observed in both the SAZ and PFZ, the sensitivity of sinking PN δ¹⁵N to nitrate depletion was lower than expected. Moreover, the seasonal observations raise the possibility that loss of the summertime high-flux period represents an alternative explanation to increased nitrate utilization for the high sedimentary PN δ¹⁵N observed during glacial periods.