Sediments Provide Amplifying Feedback to Coastal Eutrophication: Disproportionate Nutrient Recycling under Increasing Organic Matter Deposition

Coastal eutrophication is driven by anthropogenic nutrient loading, with efficient sediment nutrient recycling delaying ecosystem recovery after input reduction. Using Pinqing Lagoon (South China Sea) as a model system, we investigate how this feedback changes under eutrophication, whether sediments become enhanced nutrient sinks (buffering eutrophication) or disproportionate nutrient recyclers (exacerbating eutrophication) under increasing organic matter loading. We analyze sediments across a water-column productivity gradient and quantify sediment-water fluxes. Results show that the eutrophication-impacted nearshore exhibits a 30% sedimentation increase over two decades, with chlorophyll-<i>a</i> levels and sediment organic carbon flux 3 times higher than the less-disturbed offshore area. This causes a >50% shallower oxygen penetration (1.1 mm nearshore vs 2.4 mm offshore), impeding ammonium oxidation and likely promoting dissimilatory nitrate reduction to ammonium. The thin oxic layer enhances fresh organic matter burial into anoxic zones, stimulating iron oxide reactivity and iron/sulfate reduction. These mechanisms amplify ammonium, phosphate, and dissolved Fe fluxes by 20-, 30-, and 100-fold, respectively, disproportionate to the 2-3 times increase in organic matter remineralization. This superlinear feedback suggests that eutrophic coastal systems may reach critical thresholds, making them progressively harder to manage. The magnified nutrient release may self-sustain eutrophication, with important implications for restoration timelines and management strategies.