Eutrophication Dynamics and Driving Factors in Xiangshan Bay (2000-2024) in China

This research investigates the spatiotemporal characteristics and influencing factors of eutrophication in Xiangshan Bay from 2000 to 2024. By integrating long-term monitoring data and statistical analyses, the study elucidates the evolutionary trends, spatial distribution patterns, and underlying driving mechanisms of eutrophication in the bay. Evaluations using the Eutrophication Index (EI) and the Trophic Index (TRIX) reveal that the water quality in Xiangshan Bay has remained in a eutrophic state for at least the past 25 years, exhibiting an overall trend of initial increase followed by a decline. The eutrophication level reached its peak in 2016 and has since shown a downward trend, a shift largely attributed to the effective implementation of environmental protection policies by the Chinese government. These policies include measures such as pollution reduction, industrial restructuring, and the optimisation of spatial distribution of pollution sources. Chemical Oxygen Demand (COD), Dissolved Inorganic Nitrogen (DIN), and Dissolved Inorganic Phosphorus (DIP) were identified as the primary pollutants driving eutrophication changes in Xiangshan Bay, with their concentrations heavily influenced by human activities and land-based pollution inputs. Spatiotemporal analysis further highlights significant spatial heterogeneity across the bay, characterised by a distinct gradient from the “port area” to the “central bay area” and the “inner bay area”. Seasonal variations were also observed: COD concentrations were significantly higher in winter compared to spring, summer, and autumn, while DIN concentrations peaked in summer. Conversely, DIP concentrations were elevated in autumn and winter relative to spring and summer. The seasonal growth and decline of phytoplankton were identified as key drivers of eutrophication dynamics. This research provides critical insights into the mechanisms of eutrophication in semi-enclosed bays and offers a scientific foundation for developing targeted management strategies to mitigate eutrophication. Furthermore, it contributes to the broader goal of promoting the sustainable health and resilience of coastal ecosystems.