Hypoxia in marine systems is expanding worldwide due to anthropogenic activities. Deep waters of the St. Lawrence Estuary and Gulf are rapidly deoxygenating (∼1 mmol O2 m−3 y−1), driven by changes in Atlantic water-mass intrusion and nutrient‐fueled remineralization, and are simultaneously acidifying (∼0.004 pH unit y−1). These changes alter biogeochemical cycles and degrade habitats, causing biotic and economic consequences. Using a coupled physical-biogeochemical model, we tested whether watershed-scale nitrate reductions could mitigate deep‐water deoxygenation and carbonate-system degradation. Three nitrate‐reduction scenarios reduced riverine St. Lawrence River nitrate loads by about 11%, 30%, and 50% (−2.8, −7.5, and −11.7 mmol N m−3, respectively). Relative to the baseline simulation, these scenarios increased dissolved oxygen concentrations (DO) (+1.6, +4.3, and +6.7 mmol O2 m−3), pH (+0.004, +0.010, and +0.016), and aragonite saturation state (Ωarag) (+0.003, +0.009, and +0.015) in the deep Laurentian Channel (⩾175 m). The volume of deep waters with DO saturation <30% decreased by 3%, 8%, and 13% (−27, −75, −122 km3), while volumes with pH <7.65 declined by 1.0%, 2.8%, and 4.5% (−22, −60, −96 km3). Severely corrosive waters (Ωarag <0.6) dropped by 1.4%, 3.7%, and 5.9% (−26, −71, −112 km3). Over 2014–2024, modelled improvements exceeded small interannual variability but remained within seasonal amplitudes of deep DO concentrations (∼11 mmol O2 m−3) and pH (∼0.015 units), nevertheless pointing to a detectable management signal. Although nutrient‐driven improvements remained smaller than climate‐forced trends, modelled nitrate load reductions could offset ∼56% and 80% of recent observed oxygen and pH declines, respectively, while reducing hypoxic and acidified (pH and Ωarag) water volumes. These findings provide quantitative evidence that watershed‐scale nitrogen mitigation, through improved agricultural practices and wastewater nitrogen removal, which necessitates cooperation across jurisdictions, can alleviate hypoxia and acidification in deep coastal ecosystems, making nutrient management an essential complement to climate adaptation strategies.
Levesque D., Lavoie D., Boivin-Rioux A., Caron A., Vachon D., Riche O., Simo Matchim A. G., Doucet J., Patry B. & Patoine M., 2026. Reducing nitrate loads to the St. Lawrence river could mitigate hypoxia and acidification in its deep estuary and gulf. Environmental Research Letters 21: 18. doi: 10.1088/1748-9326/aea62d. Article.
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