Comparative study on pollution characteristics and ecological risk in Xiangshan Bay and its adjacent eastern sea area

Semi-enclosed bays and open coastal areas differ in pollutant behavior due to topography and hydrodynamics, yet the underlying mechanisms remain unclear. We conducted four surveys in Xiangshan Bay and its adjacent eastern sea area during 2023-2024. Water temperature, salinity, and transparency were significantly higher in the eastern sea area than in Xiangshan Bay (p < 0.05). Dissolved inorganic nitrogen in Xiangshan Bay (0.536-0.936 mg/L) exceeded Grade II year-round. The Nemerow index indicated severe eutrophication increasing spatially from bay-mouth to bay-head and peaking in autumn-winter. Sediment copper in Xiangshan Bay consistently exceeded Grade I. The potential ecological risk index was low-to-moderate for heavy metals in sediments of both areas, but all stations in the eastern sea area shifted to moderate risk in winter. Igeo further showed that cadmium was lightly polluted year-round, and copper seasonally enriched in Xiangshan Bay. Principal component analysis identified spatial differentiation as the dominant driver. Correlation analysis revealed that pollutant behavior in the eastern sea area was governed by physical mixing, whereas Xiangshan Bay was influenced by terrestrial inputs and internal sediment release. Positive matrix factorization quantified these differences: terrestrial/agricultural-aquaculture sources dominated year-round; reducing environment sources increased in autumn-winter, evidencing internal release; industrial/transport sources exhibited a bimodal pattern, dominating cadmium and other metal inputs. The winter risk surge in the eastern sea area resulted from sediment resuspension and enhanced industrial sources. This study elucidates the regulatory mechanisms of topography-hydrodynamics coupling on pollutant behavior and provides a basis for differentiated management of semi-enclosed bays and open coastal waters.