Fixed-nitrogen loss in a deoxygenating coastal ocean: Insights from the Estuary and Gulf of St. Lawrence

Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from fixation, while denitrification and ANAMMOX removes bioavailable nitrogen in oxygen-depleted conditions. Traditionally considered limited to highly suboxic (i.e., < 5 µM) waters, recent studies suggest 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 35 µM) along the Estuary and Gulf of St. Lawrence using N cycle tracers (, and ). Notably, we observe significant production when concentrations fall below 57-52 µM, including unexpected water column fixed-nitrogen removal processes above suboxia. Benthic production remains unaffected under intensifying deoxygenation from 50 down to 34 µM, but sedimentary nitrification contribution to denitrification diminishes with intensifying deoxygenation. Combined, water column and benthic fixed-nitrogen removal processes drive anomalies and strong deficiency in bottom waters. Additionally, observed concentration threshold triggers production, unveiling the profound impact of ocean deoxygenation on nitrogen cycling, challenging conventional expectations even at hypoxic concentrations.