Employing extreme value theory to establish nutrient criteria in bay waters: A case study of Xiangshan Bay

The Yangtze River estuary and its surrounding seas have had the highest degree of eutrophication in China. Xiangshan Bay, located at the southern bank of the Yangtze River, is not immune from eutrophication, leading to the water quality below the control requirements for marine functional zoning. Clarifying the nutrient criteria is essential to alleviate eutmphication in Xiangshan Bay. We used a generalized Pareto distribution-based peaks over threshold model to analyze 2006-2018 survey data. Through calculation, inorganic nitrogen and reactive phosphate criteria in Xiangshan Bay at the five percentile are 0.443 mg/L and 0.025 mg/L, respectively. Additionally, we also obtained the corresponding confidence intervals. Rationality and sensitivity analyses of the calculation results proved the methods used in this study to be rational and reliable, providing a basis for subsequent monitoring and assessment of nutrient salts in bays.