Policy-driven water quality trends in Qinhuangdao coastal waters, China (2003–2024): Nutrient controls and climate vulnerabilities via high-accuracy hue angle

Coastal water color, as captured by the hue angle from satellite imagery, provides a comprehensive indicator of ecosystem dynamics influenced by environmental and human factors. This study optimizes a bias-correction model for deriving hue angle from MODIS data using a large in situ hyperspectral dataset, achieving improved accuracy ( R ² = 0.71, MAPE = 5.67%, RMSE = 12.05°). Applied to Qinhuangdao coastal waters over 2003–2024, the model uncovers a nearshore-to-offshore gradient, a V-shaped seasonal pattern, and a triphasic interannual trend: degradation (2003–2011), recovery (2012–2023), and a 2024 reversal. Phytoplankton chlorophyll-a (Chl-a) emerges as the dominant driver of color variability ( R ² = 0.67), with dissolved inorganic nitrogen (DIN) as the key environmental influence ( R ² = 0.54), underscoring nutrient-driven eutrophication. Policy measures, including 2008 Olympic pollution controls and the 2018 Bohai Sea remediation, correlate with post-2012 improvements. The hue angle demonstrates greater sensitivity than the Forel-Ule Index, offering a valuable tool for coastal management.