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

Oxygen-deficient zones (ODZs) are hotspots of marine fixed nitrogen loss and greenhouse gas production, but the temporal dynamics of their microbial processes remain poorly resolved. We present a nearly three-year high-resolution record from a BGC-Argo float in the Eastern Tropical North Pacific that captures a sharp transition in nitrite concentrations, reflecting a microbial regime shift. 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, regime, and organic matter supply. Mesoscale eddies episodically modulate carbonate saturation depths horizons via isopycnal shoaling and deepening. Float-based observations reveal features missed by traditional ship-based sampling, resolving fine-scale links between microbial activity and physical forcing. These results demonstrate the power of BGC-Argo floats to capture nitrogen-carbon coupling in ODZs and provide a scalable framework for extending such insights across other oxygen-deficient regions of the global ocean.