Tracking phosphorus dynamics in the Changjiang Estuary: Causes and implications

Phosphorus (P) is an essential nutrient for primary production and frequently acts as a limiting factor in estuaries. The Changjiang River Estuary, recognized as one of the largest estuaries globally, has experienced significant changes in nutrient dynamics due to anthropogenic activities. The recent reduction in P loading from the Changjiang River may have significant implications for the dynamics of dissolved inorganic phosphorus (DIP) within this estuarine system. Based on DIP data collected in 2017, 2019, and 2023, combined with historical datasets, we aim to identify the drivers of DIP concentration changes in the Changjiang Estuary under the change in river inputs. The results indicate significant spatiotemporal variations in the distribution of DIP in the Changjiang Estuary, with the highest average concentration in winter. DIP exhibits non-conservative behavior along the salinity gradient, primarily influenced by biological utilization. Long-term DIP variations can be divided into three stages: a low-concentration period (1984-1987), a significant increase (1987-2014), and a decline (since 2015), with a current decreasing trend of 0.024 µmol/(L·yr) (R² = 0.97, P < 0.05). A discernible trend of P depletion in estuarine environments is observed, attributed to diminished riverine load and enhanced phytoplankton fixation. The reduction, and in some cases depletion, of DIP in the Changjiang Estuary has significantly altered the nitrogen-to-phosphorus ratio. The recent changes in total phosphorus (TP) compositions in the Changjiang Estuary are also attributed to a decrease in riverine input. Ongoing terrestrial nutrient management may further lower DIP concentrations, potentially impacting the estuarine ecosystem.