Dataset: Total dissolved nitrogen (TDN) concentrations and δ15N measurements from samples collected on R/V Nathaniel B. Palmer GO-SHIP cruises NBP1706 and NBP1707 in the South Pacific from Jul 3 to Sep 30, 2017

Total dissolved nitrogen (TDN) samples were collected from Niskin bottles deployed on a CTD rosette. All samples were filtered through a PES filter with 0.2 µm nominal pore size and frozen at -20 ˚C until analysis in the lab at Florida State University. We used the persulfate oxidation approach adapted from Knapp et al., 2005 to convert all TDN to nitrate, and the resulting nitrate concentration was determined using a chemiluminescent method, which quantitatively reduces NO₃^-+NO₂^- to NO gas in a heated, acidic vanadium (III) solution (Braman & Hendrix, 1989). DON concentrations were computed by the difference between TDN and NO₃^-+NO₂^- concentrations, with NO₃^-+NO₂^- concentrations measured at sea using colorimetric methods (Armstrong et al., 1967) and reported at CCHDO Hydrographic Data Office (2023).

After TDN concentration analysis, each TDN sample was acidified to a pH = ~4, and the TDN δ¹⁵N was measured by the “denitrifier” method (Sigman et al., 2001; Casciotti et al., 2002; McIlvin & Casciotti, 2011; Weigand et al., 2016). In samples with measurable NO₃^-+NO₂^- concentration, the δ¹⁵N of DON can be calculated by mass balance by subtracting the concentration and δ¹⁵N of NO₃^-+NO₂^- from the TDN concentration and TDN δ¹⁵N measurements (Knapp et al., 2005).