Sediment Legacy Organic Matter Amplifies Nutrient‐Driven Coastal Hypoxia: A Coupled Benthic‐Pelagic Modeling Study

Abstract Coastal hypoxia is profoundly affected by sediment depletion of oxygen and recycling of nutrients—processes sustained by long‐term accumulation of sediment organic matter. While sediment‐water interactions are widely recognized, quantitative understanding of sediment legacy effects remains limited. To address this gap, we developed a two‐layer sediment model integrated with a physical‐biogeochemical framework for the Pearl River Estuary to elucidate how legacy organic matter perpetuates hypoxia via benthic‐pelagic coupling. Results show that sediment oxygen consumption accounts for 44% of total oxygen depletion in low‐oxygen zones, increasing to 81% in near‐bottom waters. This oxygen demand is significantly supported by legacy organic matter, without which would reduce primary production by 68% and the hypoxic area by 53%. Furthermore, we demonstrate that sediment legacy effects amplify hypoxia sensitivity to nutrient loading and may lead to hysteresis and regime shifts if unaddressed. Therefore, effective hypoxia mitigation requires management strategies tailored to system‐specific sediment‐water coupling dynamics.