Quantitative apportionment of multiple water sources and their implications for coastal nutrient dynamics: A mult-tracer framework of river-dominated system

Quantifying water source contributions and their environmental impacts is essential for managing water quality in river-dominated coastal systems and sustainable socio-economic development, yet remains methodologically challenging. This study integrated stable isotopes (δD, δ¹⁸O) and rare earth elements within an Optimum Multi-Parameter analysis to resolve source contributions (precipitation, river, shelf, and porewater) in the Changjiang (the world's third-largest river), Jiaojiang, and Oujiang estuaries and the adjacent coastal area. Results delineate distinct source roles. The Changjiang Diluted Water dominates north of 30.5°N with contributions exceeding 60%, restricting shelf water intrusion into Hangzhou Bay. Although geographically confined, the plumes of the smaller Jiaojiang and Oujiang rivers deliver critical nutrient subsidies to proximate fisheries and, crucially, dominate coastal nutrient stoichiometry by elevating inorganic nitrogen to phosphorus (N/P) and depressing silicate to nitrogen (Si/N) ratios—a shift linked quantitatively to river water fraction (r = 0.88 for N/P, r = –0.63 for Si/N) and favoring non-diatom algae. Precipitation contributes 3–8% seasonally, acting as a direct atmospheric material pathway, while sediment porewater exhibits strong anthropogenic gadolinium anomalies (Gd/Gd* up to 1.37), identifying coastal muddy deposits as a major contaminant sink for terrestrially derived contaminants. Furthermore, a robust empirical logarithmic relationship was established between river discharge and the spatial extent of its plume (R₅₀ = 55.1 × ln(Q + 41.6) – 174), providing a predictive link between hydrologic forcing and biogeochemical impact zones. The findings provide critical scientific support for managing coastal environmental issues and demonstrate a novel, transferable methodological framework for quantitative source apportionment applicable to other major estuarine-coastal systems worldwide.