Assessment of nutrient pollution and eutrophication in Hangzhou Bay and its adjacent waters in summer

Based on the ecological environment survey data of Hangzhou Bay and its adjacent waters in summer 2022, by using the single factor pollution index method, nutrient structure and limitation analysis, and eutrophication status index method, the characteristics of surface nutrient and organic pollution in the bay were systematically studied, and the eutrophication status was evaluated. Through the Pearson correlation statistical analysis, the impact of nutrient pollution on the growth of phytoplankton and the control factors of eutrophication in water bodies were preliminarily explored. Results indicate that the pollution of nitrogen (N) and phosphorus (P) nutrients in the region was relatively severe. 55% and 80% of the stations in which dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) content exceeded China's national Class III seawater quality benchmarks, respectively. The N/P and Si/P ratios of surface seawater were imbalanced, and 20% and 5% of the stations showed potential limitations of P and Si on the growth of phytoplankton, respectively The chlorophyll a (Chl a) content was mainly affected by the transparency of the water body, showing a spatial distribution characteristic: higher in the east than in the west, and higher outside the bay than inside the bay. The Chl a content in the eastern waters outside the bay was relatively high, indicating a high density of algae cells and a risk of red tide outbreaks. The survey results of chemical oxygen demand (COD) indicate that the overall organic pollution in the area is not significant, and 95% of the stations could meet China’s national Class Ⅲ water quality benchmarks. Results of the eutrophication assessment indicate that 90% of the stations in the area were in eutrophication state (40% were severely eutrophicated). The order of contribution to the eutrophication was COD>DIN>DIP. Preliminary studies have shown that N and P nutrient pollution still exists in the Hangzhou Bay and its adjacent waters, and the eutrophication of water bodies deserves attention and further research is needed.