Spatiotemporal distribution characteristics and eutrophication status of nutrients in Qinzhou Bay, South China Sea over the past 22 years

IntroductionElevated nitrogen and phosphorus inputs from human activities have driven coastal eutrophication. Although Qinzhou Bay has long been influenced by anthropogenic disturbances, long-term systematic studies on its nutrient dynamics remain scarce.MethodsBased on long-term monitoring data spanning 22 years (2003-2024), this study systematically elucidates the interannual trends, spatiotemporal distribution patterns, and eutrophication status of nutrients in Qinzhou Bay.ResultsThe concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) showed an overall upward trend amid interannual fluctuations, yet were characterized by markedly distinct, phase-specific dynamics. The average DIN concentration was at its lowest during 2003-2005, increased sharply over the subsequent seven years, then exhibited a clear declining trend from 2014 to 2022. In the most recent two years, however, the concentration rose again, reaching the peak level for the entire period. The average DIP concentration remained at the lowest level from 2003 to 2014, followed by a marked increase during 2015-2020. Notably, a substantial rise in DIP concentration occurred again in the recent period from 2021 to 2024. Spatially, the distributions of nitrogen and phosphorus were highly overlapping, both showing a decreasing gradient from the inner to the outer bay, with a more pronounced gradient for nitrogen. Principal component analysis confirmed that terrestrial input is the primary driver controlling nutrient distribution. Eutrophication assessment indicated that the bay-head area remained in a eutrophic state for 18 of the past twenty-two years.DiscussionThese regular changes in nutrients are closely related to the historical development of China's economy and environmental governance. This study provides a long-term case for understanding the historical evolution of nutrients in key bays, and complements the comparative understanding with existing long-term studies.