Long-term assessment of nutrient-induced eutrophication in Xiamen Bay: Implications for integrated watershed-coast management.

Coastal eutrophication, driven by increasing anthropogenic nutrient inputs, represents a major environmental threat to marine ecosystems worldwide. This study presents a comprehensive analysis of the spatiotemporal dynamics of eutrophication in Xiamen Bay, a subtropical semi-enclosed coastal system in Southeast China, based on a decadal monitoring dataset (2008-2018) covering spring, summer, and autumn. We applied an adapted version of the HELCOM Eutrophication Assessment Tool (HEAT) to evaluate eutrophication status and identify its key drivers. The results revealed pronounced spatial heterogeneity: dissolved inorganic nitrogen (DIN) concentrations were highest in the Jiulong River Estuary, reflecting dominant riverine inputs, whereas elevated dissolved inorganic phosphorus (DIP) levels were found in Western Xiamen Bay and Tongan Bay, associated with localized sewage discharges. Seasonally, nutrient concentrations and eutrophication severity peaked in spring and autumn but were lower in summer, a pattern attributed to increased dilution and flushing during the wet season. Nutrient levels were identified as the primary determinant of eutrophication, with the Eutrophication Ratio (ER) closely tracking spatial and temporal nutrient distributions and reflected by chlorophyll-a responses. A persistently high molar DIN:DIP ratio (71.7-107) indicated that phosphorus (P) was the limiting nutrient throughout the study period. This HEAT-based assessment underscores the critical need for integrated watershed-coast management in Xiamen Bay. Effective mitigation requires a dual-nutrient (N and P) reduction strategy, implemented through advanced wastewater treatment, optimized agricultural practices, and strengthened cross-regional governance. The findings provide a scientific foundation for sustainable eutrophication management in rapidly developing subtropical coastal regions.