Mesoscale Eddy Variability Enhances Fixed Nitrogen Loss and Suppresses Nitrous Oxide Production in Oxygen Minimum Zones.

Within oxygen minimum zones, anaerobic processes transform bioavailable nitrogen (N) into the gases dinitrogen (N2) and nitrous oxide (N2O), a potent greenhouse gas. Mesoscale eddies in these regions create heterogeneity in dissolved N tracers and O2 concentrations, influencing nonlinear N cycle reactions that depend on them. Here, we use an eddy‐resolving model of the Eastern Tropical South Pacific to show that eddies enhance N2 production by between 43% and 64% at the expense of reducing N2O production by between 94% and 104% due to both the steep increase of progressive denitrification steps at vanishing oxygen, and the more effective inhibition of N2O consumption relative to production. Our findings reveal the critical role of eddies in shaping the N cycle of oxygen minimum zones, which is not currently represented by coarse models used for climate studies. Plain Language Summary: Nitrogen limits photosynthesis over large swaths of the ocean. Oxygen minimum zones are important oceanic environments where the presence of low O2 concentrations allow anaerobic microbial communities to transform bioavailable forms of nitrogen into the gases dinitrogen (N2) and nitrous oxide (N2O, a powerful greenhouse gas), which together lead to nitrogen loss from the oceans. The large scale ocean circulation acts to maintain these oxygen minimum zones, and the gradients in nitrogen and oxygen concentrations surrounding them. Turbulence also generates eddies and filaments which stir these gradients, promoting chemical heterogeneity. However, it is uncertain how eddies impact the nitrogen transformation rates dependent on nitrogen and oxygen tracer concentrations. Using a high resolution ocean model of the Eastern Tropical South Pacific oxygen minimum zone, we explore the contributions from eddies to total N2 and N2O production. We highlight that roughly half of total N2 production is caused by the presence of eddies, while N2O production is drastically reduced or even consumed. These findings expand our understanding of the ways in which eddies alter nitrogen loss from the oceans, and challenge the ability of non‐eddy resolving models to accurately represent N2 and N2O production in a changing ocean. Key Points: A high resolution model is used to estimate the effect of mesoscale eddies on the N cycle in a major oxygen minimum zoneEddies simultaneously enhance fixed N loss and suppress nitrous oxide productionThese eddy effects result from the progressive inhibition of the denitrification steps by oxygen concentrations [ABSTRACT FROM AUTHOR]