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BGC-Argo floats reveal shifting nitrogen cycling in the Eastern Tropical North Pacific

Posted by mmaheigan 
· Friday, August 14th, 2026 

Oxygen-deficient zones are regions of the ocean where microbes consume nitrate in lieu of oxygen. This drives major losses of biologically available nitrogen. In a recent study, we used a BGC-Argo float in the Eastern Tropical North Pacific to remotely observe these processes over nearly three years. Using a new method to extract nitrite from UV nitrate sensor spectra (Bif and Johnson, 2025), the float captured a progressive decline in the secondary nitrite maximum over time, revealing a shift in microbial nitrogen cycling through time.

By combining float observations with a stoichiometric biogeochemical model, we found that nitrogen transformations reorganized in response to changing organic matter supply and redox conditions. The results show that oxygen-deficient zones are highly dynamic environments rather than stable systems. This also demonstrates how autonomous observing platforms can now monitor hidden nitrogen and carbon cycling processes across the global ocean in near real time.

Figure. BGC-Argo float observations in the Eastern Tropical North Pacific oxygen-deficient zone. (a) Float WMO#5906484 trajectory. (b) Time-series distribution of nitrite concentrations. The white contour indicates the upper boundary of low-oxygen waters of less than 1 µmol/kg. (c) Nitrate concentrations over the same period. (d–e) Relative contributions of anammox and denitrification estimated with a stoichiometric mass-balance model under contrasting biogeochemical conditions.

Authors:
Mariana B. Bif (University of Miami)
Colette Kelly (Woods Hole Oceanographic Institution, now
Mark A. Altabet (University of Massachusetts Dartmouth)
Annie Bourbonnais (University of South Carolina)
Claire Elbon (University of Washington)
Edgart Flores (University of Colorado Boulder
Alanna Mnich (University of Massachusetts Dartmouth)
Josh Plant (Monterey Bay Aquarium Research Institute (MBARI))
Kenneth S. Johnson (Monterey Bay Aquarium Research Institute (MBARI))

 

 

Citations:

Bif, M.B., & Johnson, K.S., 2025. BGC-Argo floats reveal nitrite and thiosulfate dynamics in the oceans with high spatiotemporal resolution. Global Biogeochemical Cycles, 39, e2024GB008473. https://doi.org/10.1029/2024GB008473

Bif, M.B., Kelly, C., Altabet, M.A., Bourbonnais, A., Elbon, C., Flores, E., Mnich, A., Plant, J. and Johnson, K.S., 2026. BGC-Argo float reveals shifts in nitrogen-carbon cycling in an oxygen-deficient zone. Communications Earth & Environment, 7(1), p.294. https://www.nature.com/articles/s43247-026-03410-5

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