Oxygen-deficient zones (ODZs) are major sites of marine fixed nitrogen loss, but the temporal variability of the microbial processes governing nitrogen cycling remains poorly constrained due to the scarcity of sustained observations. Here we present a nearly three-year record from a Biogeochemical Argo float deployed in the Eastern Tropical North Pacific that captured a pronounced decline in nitrite concentrations, indicating a shift in nitrogen redox balance. Using a stoichiometric mass-balance model, we quantified key nitrogen transformations - including dissimilatory nitrate reduction to nitrite (DNRN), denitrification, anammox, and nitrite oxidation - and examined their links to carbonate chemistry. DNRN dominated nitrogen transformations throughout the record, whereas denitrification and anammox exhibited pronounced temporal and vertical variability associated with changes in organic matter supply and biogeochemical conditions. In addition, mesoscale eddies episodically altered carbonate saturation horizons through isopycnal shoaling and deepening, highlighting the physical-biogeochemical coupling within the ODZ. These results demonstrate that nitrogen and carbon cycling in oxygen-deficient waters are highly dynamic rather than at steady state, emphasizing the critical role of sustained autonomous observations for resolving temporal variability and improving our understanding of marine biogeochemical processes.

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