NO3- + NO2- concentration and isotopic composition analysis

The NO3- + NO2- concentration of samples was determined using chemiluminescent analysis (Braman & Hendrix, 1989) in a configuration with a detection limit of 0.05 uM, and +/- 0.1 uM for 1 standard deviation (S.D.). The d15N of NO3- + NO2- was determined using the “denitrifier” method (K. L. Casciotti, Sigman, Hastings, Bohlke, & Hilkert, 2002; Sigman et al., 2001) with modifications (McIlvin & Casciotti, 2011) on samples with NO3- + NO2- concentration >0.3 uM (typically <0.2‰ 1 S.D.) (Supp. Table 1) (Knapp, Casciotti, Berelson, Prokopenko, & Capone, 2016).

DON concentration and isotopic analysis

The DON concentration of samples was determined using persulfate oxidation to convert DON to NO3- (Solorzano & Sharp, 1980), adapted according to (Knapp et al., 2005). The resulting NO3- concentration was then measured using chemiluminescence as described above. In cases where NO3- + NO2- (and/or ammonium, NH4+) was above the detection limit, DON was determined by subtracting the concentration of NO3- + NO2-+NH4+ from the concentration of total dissolved N (TDN). The average standard deviation for duplicate DON concentration analyses of individual samples that have undetectable levels of NO3- in the sample was +/- 0.30 uM, and the propagated error for DON concentration in the presence of detectable NO3- was +/- 0.32 uM.

The d15N of DON was determined according to (Knapp et al., 2005), where DON samples were oxidized to NO3- by persulfate oxidation (as described above in section 2.2), acidified to a pH range of 3 to 4, and measured as NO3- by the denitrifier method. In samples with measurable NO3- + NO2-, the d15N of DON is calculated by mass balance by subtracting the NO3- + NO2- concentration and d15N of NO3- + NO2- from the TDN concentration and TDN d15N measurements. In surface samples with undetectable NO3- + NO2- concentration, the standard deviation of duplicate analyses of DON d15N in a sample is +/- 0.3‰. For subsurface samples with NO3- + NO2- concentration approximately equal to the DON concentration, the propagated error for the calculation of DON d15N using a Monte Carlo method (Press, Teukolsky, Vetterling, & Flannery, 1992), and assuming duplicate analysis of a single sample and the standard deviations for TN concentration, NO3- + NO2- concentration and d15N of NO3- + NO2- given above, is +/- 0.6 0/00. The 15N of DON in samples with NO3- + NO2- concentration exceeding DON concentration, and/or with NH4+ concentration > 0.2 uM, was not determined (i.e., Stations 9, 10, 11, and 12 from the 2010 cruise).

Sampling

Samples were collected on the R/V Atlantis in January through February 2010, and the R/V Melville in March through April 2011 between 10 and 20 ºS and 80º W and 100º W (Fig. 1), with station locations and sample depths, salinities, sigma theta values, chlorophyll a concentrations, nitrate+nitrite concentration, NO3-+ NO2- d15N, DON concentrations, and DON d15N reported in Supplementary Information Table 1. Water column samples were collected by Niskin bottles deployed on a rosette equipped with conductivity-temperature-depth (CTD) sensors. All samples were collected into acid-washed, sample-rinsed HDPE bottles, and samples from the upper 400 m passed a 0.2 um filter before collection, and were stored at -20º C until analysis on land.