Sample Processing:
NO3-+NO2- concentration was measured using a chemiluminescent method described by Braman and Hendrix, 1989, with a detection limit of 0.1 µM. NH4+ concentration was measured using the fluorescent method of Holmes et al., 1999 with a lower detection limit of 0.025 µM. Phosphate (soluble reactive phosphorus) concentration was measured using the colorimetric method of Koroleff (1983) with a lower detection limit of 50 nM. Dissolved organic nitrogen concentration was determined by subtracting the concentrations of nitrate, nitrite, and ammonium from the concentration of total dissolved nitrogen, determined using persulfate oxidation according to Knapp et al. (2005). 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 δ15N of DON was determined using persulfate oxidation of DON to nitrate and then using the denitrified method as described above.

Precision:
The average precision of the nitrate+nitrite concentration measurement was <0.2 µM.
The average precision of the soluble reactive phosphorus concentration measurement was <0.03 µM.
The average precision of the ammonium concentration measurement was <0.02 µM.
The average precision for DON concentration was 0.3 µM.
The average precision for DON δ15N was 0.3 per mil.
The average precision of nitrate+nitrite δ15N measurements was <0.2 per mil and for d18O was <0.3 per mil. 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.

BCO-DMO processing:
- Date formats changed to yyyy-mm-dd
- Parameter names adjusted to comply with database requirements
- Longitudes West were converted to negative numbers
- Latitude and Longitude rounded to 4 decimal places
- Added a conventional header with dataset name, PI name, version date
- Units added to parameter description metadata section
- Missing data identifier of 'nd' (no data) used

nutrients_isotopes.csv