The El Niño-Southern Oscillation (ENSO) is a natural climate phenomenon that alters the biogeochemical and physical dynamics of the Eastern Tropical Pacific Ocean. Its two phases, El Niño and La Niña, are characterized by decreased and increased coastal upwelling, respectively, which have cascading effects on primary productivity, organic matter supply, and ocean-atmosphere interactions. The Eastern Tropical South Pacific (ETSP) oxygen minimum zone (OMZ) is a source of nitrous oxide (N2O), a potent greenhouse gas, to the atmosphere. While nitrogen cycling in the ETSP OMZ has been shown to be sensitive to ENSO, we present the first study to directly compare N2O distributions during both ENSO phases using N2O isotopocule analyses. Our data show that during La Niña, N2O accumulation increased six-fold in the upper 100 m of the water column, and N2O fluxes to the atmosphere increased up to 100-fold. N2O isotopocule data demonstrated substantial increases in δ18O up to 60.5‰ and decreases in δ15Nβ down to -10.3‰, signaling a shift in N2O cycling during La Niña in the oxycline compared to El Niño. N2O production via the hybrid pathway and incomplete denitrification with overprinting of N2O consumption are likely co-occurring to maintain the high site preference (SP) values (17‰ – 26.7‰), corroborating previous hypotheses. Ultimately, our results illustrate a strong connection between upwelling intensity, biogeochemistry, and N2O flux to the atmosphere, and highlight the importance of repeat measurements in the same region to constrain N2O interannual variability and cycling dynamics under different climate scenarios.

Marine oxygen deficient zones (ODZs) are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N2O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N2O production and distribution from the offshore waters of the Eastern Tropical South Pacific (ETSP). On each cruise, high-resolution measurements of ammonium ([NH4+]), nitrite ([NO2-]), and N2O were combined with 15N tracer-based determination of ammonia oxidation, nitrite oxidation, nitrate reduction and N2O production rates. Depth-integrated inventories of NH4+ and NO2- were positively correlated with one another, and with depth-integrated primary production. Depth-integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap-collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaeal genes and ammonia oxidation rates and between -like 16S rRNA genes and nitrite oxidation rates. N2O concentrations in the upper oxycline reached values of greater than 140 nM, even at the western extent of the cruise track, supporting air-sea fluxes of up to 1.71 umol m-2 d-1. Our results suggest that a source of N2O other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore ETSP and that air-sea fluxes of N2O from this region may be higher than previously estimated.