Supplementary file 1_Nutrient dynamics in the Yellow River Estuary: influencing factors and ecological impacts.docx

The estuaries have experienced significant eutrophication and red tides due to anthropogenic activities and climate change. Seasonal and annual variations of nutrients in the Yellow River Estuary and the potential influencing factors were studied based on historical data collected from 2004 to 2023. The seasonal variations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved inorganic silicon (DSi) concentrations were significant, with DIN and DIP concentrations demonstrating autumn maxima, and DSi concentrations peaking in summer. The seasonal variations of DIN and DSi concentrations were mainly controlled by terrestrial input and water-sediment regulation, while changes in DIP concentrations were primarily attributed to biological activities, suspended particulate matter and sediment resuspension. An extreme rainfall event in the watershed, as an exceptional case, can significantly increase DIN and DSi concentrations in the Yellow River Estuary. The DIN, DIP and DSi concentrations in the Yellow River Estuary have generally decreased over the past two decades, while the N/P and Si/P ratios shifted from decreasing trends before the mid-2010s to increasing trends after. The annual variations in DIN, DIP, and DSi concentrations and N/P and Si/P ratios were dominantly affected by Yellow River input. The long-term nutrient variations in the Yellow River Estuary have potential ecological impacts on the abundance and community structure of phytoplankton.