Water column samples were collected by Niskin bottle on a CTD rosette (“CTD profile”). NO3-+NO2- concentration was measured using a chemiluminescent method described by Braman and Hendrix, 1989, with a detection limit of 0.1 µM. NO3-+NO2- d15N and d18O analyses were by the “denitrifier method” and followed the methods described by Sigman et al., 2001, Casciotti et al., 2002, McIlvin and Casciotti, 2011, and Weigand et al., 2016. Briefly, NO3-+NO2- was quantitatively reduced to N2O by Pseudomonas aureofaciens and Pseudomonas chlororaphis, which was then cryogenically focused and analyzed on an isotope ratio mass spectrometer. A volume of sample was added to each bacterial vial to achieve a final quantity of 10 or 20 nmols N2O, which was then purged from the vial using a helium carrier gas. The d15N of N2O in samples was calibrated with the international isotopic reference materials.

The average precision of the nitrate+nitrite concentration measurement was <0.2 µM. The average precision of nitrate+nitrite d15N measurements was <0.2 per mil and for d18O was <0.3 per mil, but with the standard deviation for duplicate analyses of each sample reported here. NO3-+NO2- δ15N δ18O analyses were calibrated with IAEA N3 and USGS 34 NO3- d15N isotopic reference materials as described in McIlvin and Casciotti, 2011. NO3-+NO2- d18O were also calibrated with the USGS 35 isotopic reference material as described in McIlvin and Casciotti, 2011.