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

Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from $N_2$ 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 ($N_2/\text{Ar}$, $N^{\ast }$, and $N_{2}O$). Notably, we observe significant $N_2$ production when ambient $O_2$ 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 $N_2$ production remains unaffected under intensifying water column deoxygenation from 50 down to 32 μM, but the contribution of $N{O}_3^{-}$ produced through nitrification in the sediment to denitrification diminishes as deoxygenation intensifies. Combined, water column and benthic fixed-nitrogen removal processes drive $N^{\ast }$ anomalies and strong $N{O}_3^{-}$ deficiency in bottom waters. Additionally, the observed $O_2$ threshold also triggers $N_{2}O$ 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.