New Deoxygenation Threshold for N₂ and N₂O Production in Coastal Waters and Sediments

Pascal, Ludovic ORCID: https://orcid.org/0000-0002-9669-3336, Cloutier-Artiwat, Félix, Zanon, Arturo et Wallace, Douglas W.R. (2025). New Deoxygenation Threshold for N₂ and N₂O Production in Coastal Waters and Sediments. Global Biogeochemical Cycles .

PDF Licence d'utilisation : CC BY 4.0. Télécharger (842kB) | Prévisualisation

Résumé

Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from N₂ fixation, while denitrification and anammox remove bioavailable nitrogen in oxygen-depleted conditions. Traditionally considered limited to highly suboxic (i.e., <5 μM) waters, recent studies suggest that fixed-nitrogen removal processes may extend beyond, elevating global nitrogen loss estimates. This study directly quantifies fixed-nitrogen loss across oxygen gradients (from 140 to 32 μM) along the Estuary and Gulf of St. Lawrence using N cycle tracers (N₂, Ar, N* and N₂O). Notably, we observe significant N₂ production when ambient O₂ concentrations fall below a threshold value of 58.9 ± 1.1 μM, including potential water column fixed-nitrogen removal processes above suboxia. We hypothesis that ambient deoxygenation eases the formation of suboxic microareas in suspended organic matter. Benthic N₂ production remains unaffected under intensifying water column deoxygenation from 50 down to 32 μM, but the contribution of NO₃- produced through nitrification in the sediment to denitrification diminishes as deoxygenation intensifies. Combined, water column and benthic fixed-nitrogen removal processes drive N* anomalies and strong NO₃- deficiency in bottom waters. Additionally, the observed O₂ threshold also triggers N₂O production. Overall, our study highlights the profound impact of coastal ocean deoxygenation on nitrogen cycling, suggesting unexpected shifts even at ambient oxygen concentrations traditionally considered well above suboxic conditions.

Type de document :	Article
Validation par les pairs :	Oui
Départements et unités départementales :	Institut des sciences de la mer de Rimouski (ISMER)
Date de dépôt :	05 août 2025 19:21
Dernière modification :	05 août 2025 19:21
URI :	https://semaphore.uqar.ca/id/eprint/3313