"Autonomous measurement of denitrification in the open ocean" by: Mark Altabet

Wednesday, September 02, 2026 at 12:30pm to 1:30pm

SMAST East 101-103 : 836 S. Rodney French Boulevard, New Bedford MA 02744
Callie Rumbut
c.rumbut@umassd.edu
https://umassd.zoom.us/j/97440069270

Department of Estuarine and Ocean Sciences
"Autonomous measurement of denitrification in the open ocean" Dr. Mark Altabet
Professor and Department Chair, Estuarine and Ocean Sciences, SMAST

Wednesday, September 2, 2026
12:30 - 1:30 pm
SMAST E 101-103 and via Zoom

Abstract:

Denitrification in oceanic oxygen-deficient zones (ODZs) removes a large but poorly constrained fraction of the ocean's fixed nitrogen. The standard observational measure, the nitrogen deficit (Ndef), relies on chemical measurements of nitrate, nitrite, and phosphate.  However, phosphate is not measured autonomously, and the SUNA optical nitrate sensors carried by BGC-Argo floats have not routinely separated nitrate from the nitrite that accumulates in ODZ waters.

We present an approach for estimating Ndef from autonomous platforms alone. Expected nitrate is reconstructed without phosphate, from along-isopycnal mixing between a fresh California Current source and a salty Eastern Equatorial Pacific source, with salinity and expected nitrate endmembers defined as functions of potential density. Nitrite is recovered from the same ultraviolet spectra and returned to the fixed-nitrogen total, correcting a potential overestimate of the deficit in ODZ waters.

The method is validated in the Eastern Tropical North Pacific against three independent shipboard estimates — bottle nutrients, gas-tension N₂, and N₂/Ar mass spectrometry — on two cruises. Agreement is within the propagated uncertainty of each method at essentially every station, and the gas-phase and nutrient-based deficits agree in the mean, supporting the Richards denitrification stoichiometry. A float time series is used to demonstrate variability of the Ndef on mesoscale time/space scales.

Join Meeting

https://umassd.zoom.us/j/97440069270

Meeting ID and passcode required. Please email contact to obtain.

For additional information, please contact Callie Rumbut at c.rumbut@umassd.edu