BGC-Argo float reveals shifts in nitrogen-carbon cycling in an oxygen-deficient zone

Oxygen-deficient zones are hotspots of marine fixed nitrogen loss, but the temporal dynamics of their microbial processes remain poorly resolved. Here we present a nearly three-year high-resolution record from a Biogeochemical Argo float in the Eastern Tropical North Pacific that captures a decline in nitrite concentrations, reflecting a shift in nitrogen redox balance. Using a stoichiometric mass-balance model, we quantify nitrogen transformations, including dissimilatory nitrate reduction to nitrite, denitrification, anammox, and nitrite oxidation, and their coupling to carbonate chemistry. The model shows that dissimilatory nitrate reduction to nitrite dominates nitrogen transformations, while denitrification and anammox vary over depth, biogeochemical transition, and organic matter supply. Mesoscale eddies episodically modulate carbonate saturation depths horizons via isopycnal shoaling and deepening. We show that oxygen-deficient zone biogeochemistry is inherently dynamic rather than steady-state, with temporal variability in nitrogen and carbon cycling that can only be resolved through sustained autonomous observations. Dissimilatory nitrate reduction to nitrite dominates nitrogen transformations, while denitrification and anammox vary over depth, biogeochemical transition, and organic matter supply, based on a three-year high-resolution record from a Biochemical Argo float in the Eastern Tropical North Pacific