An improved composite eutrophication index for coastal assessment: Laizhou Bay, China

Coastal eutrophication, driven by diverse environmental factors, poses a global threat to marine ecosystems. This study develops a Composite Eutrophication Index (CEI) constructed from indicators selected according to ecological relevance, with indicator weights objectively determined using an entropy-based method. This assessment framework ensures high flexibility in indicator selection, strong adaptability across regions, and enables the identification of key eutrophication drivers. Using Laizhou Bay (LZB), China, as a case study, nine state and response indicators were analyzed from 1980 to 2023. The CEI revealed that trophic conditions degraded from Good in the 1980s to Moderate in the 2000s and remained persistently Poor after 2004, with eutrophication hotspots concentrated near the Xiaoqing River estuary. The CEI demonstrated strong consistency with chlorophyll-a (Chl a) concentrations and the Assessment of Estuarine Trophic Status (ASSETS), while showing greater sensitivity than the Eutrophication Index (EI). Contribution analysis showed that dissolved inorganic nitrogen (DIN, 21.4 %) and the non-diatom abundance ratio (R_NDia, 21.1 %) were the dominant drivers of CEI, together accounting for over 40 % of its interannual variation. Seasonal differences were pronounced: in May, DIN (31.8 %), the DIN equivalent proportion (R_DIN, 14.5 %), and R_NDia (12.8 %) were most influential, whereas in August, DIN (28.3 %) remained primary but sea surface temperature (20.9 %) and Chl a (15.0 %) gained importance. The CEI framework developed here, with flexible indicator selection and strong adaptability, is applicable to coastal regions worldwide. Its application in LZB supports China's coastal protection and offers reference for global sustainable management.