Spatial-temporal distribution of nitrogen and phosphorus nutrients and potential eutrophication assessment in Bohai Bay, China

This study analyzes the spatiotemporal distribution characteristics of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), chemical oxygen demand (COD_Mn), and dissolved oxygen (DO) in Bohai Bay, based on long-term observational data collected from 16 stations between 2020 and 2024. The eutrophication status of the bay was assessed using the eutrophication index (EI), nitrogen-to-phosphorus ratio (N/P), and single-factor pollution index (SFPI). The results indicate a rising trend in DIN concentrations, while DIP levels are declining, leading to an N/P ratio far exceeding the Redfield ratio of 16:1, suggesting a persistent nitrogen surplus in Bohai Bay. Eutrophication levels were found to be higher in spring and summer, with EI values peaking at 3.593 in spring 2024 and 4.705 in summer 2021. Additionally, coastal regions exhibited significantly higher eutrophication levels than the central bay. A strong negative correlation between DO and DIN suggests that nitrogen-driven eutrophication has intensified hypoxia. These findings highlight the combined impacts of human activities and climate change on nutrient dynamics and underscore the need for a balanced approach to nitrogen pollution control and ecosystem response management in the future.