Intensified water column hypoxia drives disproportionate benthic P release and N:P imbalance via enhanced sedimentary sulfate reduction

초록

Ocean deoxygenation, driven by human activities and climate change, increasingly threatens the sustainability of marine ecosystems. This study investigates how water column hypoxia (WCH; dissolved oxygen (DO) < 63 μM) intensity regulates sedimentary sulfate reduction (SR), sulfur–iron–manganese–phosphorus (S–Fe–Mn–P) dynamics, benthic nutrient fluxes (BNF) and nitrogen (N):P ratio in Jinhae Bay, Korea, characterized by seasonally recurring WCH. As bottom-water DO declined from 206 to 17 μM, SR rates in surface sediments increased 6-fold, from 46.0 to 281 nmol cm−3 d−1, accompanied by elevated pore-water sulfide (H₂S) and depletion of Fe(III)/Mn oxides. These redox changes markedly enhanced ammonium (NH4+) and phosphate (PO43−) release into the overlying water, with NH4+ and PO43− fluxes 6.5- and 17-fold, respectively, higher under severe hypoxia (NH4+: 7.70; PO43−: 0.52 mmol m−2 d−1) than under normoxia (NH4+: 1.18; PO43−: 0.03 mmol m−2 d−1). Intensified SR and dissolution of Fe(III)/Mn oxides further enhanced the disproportionate release of P relative to N, leading to a threefold decrease in the N:P ratio of benthic nutrient fluxes (N: P = 14.8 under severe WCH vs. 45.2 under normoxia). Long-term monitoring (1997–2024) revealed persistent bottom-water P enrichment, despite reduced external (terrestrial) inputs following environmental regulations, underscoring the dominance of internal (benthic) sources. Our findings demonstrate that intensified WCH enhances SR-driven internal BNF, reinforcing eutrophication and challenging the effectiveness of external nutrient reduction strategies in hypoxia-prone coastal systems. © 2026 Elsevier Ltd.

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