Water was collected during the MESO-SCOPE research cruise aboard R/V Kilo Moana (KM1709) during June-July 2017. Upper ocean biogeochemistry was characterized at 11 stations along the transect traversing the cyclonic and anticyclonic eddies, using a rosette mounted with 10-liter (L) Niskin® bottles. Water samples for nutrient and nitrate isotope analyses were collected at ~25-meter (m) intervals from 5 m to 500 m with higher vertical resolution (~5 m intervals) near the deep chlorophyll maximum (DCM). Samples were frozen at -20 degrees Celsius (°C) after collection pending analysis.

The N isotope ratios of nitrate (15N/14N) in water samples from stations 4 to 13 were measured with the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001) for concentrations exceeding 0.5 micromoles per liter (µmol L-1). Nitrate was converted to nitrous oxide (N2O) by cell concentrates of the denitrifying bacterial strain Pseudomonas chlororaphis (ATCC 43928, Manassas, VA, USA), which lacks the terminal N2O reductase. The N2O gas was extracted and purified using a custom-modified Thermo Fisher Scientific Gas Bench II fronted by dual cold traps and a GC Pal autosampler, and analyzed with a Thermo Delta V Advantage continuous flow gas chromatograph isotope ratio mass spectrometer (Casciotti et al., 2002; McIlvin & Casciotti, 2011). Working solutions were diluted from primary stocks into nutrient-free seawater to concentrations bracketing sample concentrations to account for potential matrix effects (Weigand et al., 2016; Zhou et al., 2021). Individual samples were measured 3 to 9 times to achieve an analytical uncertainty to ≤ 0.3 ‰. The oxygen isotope ratios of nitrate (ẟ18ONO3) were not measured concurrently as we did not secure sufficient sample volumes to estimate these reliably (see Zhou et al., 2021).