Deciphering phosphorus dynamics in impacted river systems using phosphate oxygen and lead isotope analysis

Phosphorus (P) is a major driver of eutrophication in river systems, but its sources and cycling remain poorly understood. This study employs a novel combination of phosphate oxygen isotope (δ18O-PO4) and lead isotope (206Pb/207Pb) analyses to trace P sources and biogeochemical processes in river waters, shallow sediments (<5 cm), and sediment cores (up to 60 cm) along a ∼100 km stretch of the River Nene, UK. δ18O-PO4 signatures were used to identify phosphate origins and transformation pathways in river sediments, while 206Pb/207Pb ratios provided insights into sediment provenance and historical pollution. Results reveal significant spatial variability in P sources and sinks, with sewage inputs dominating in some parts, whereas and agricultural and urban contributions evident in others. Shallow (<5 cm) river sediment δ18O-PO4 values are close to those in the overlying river water suggesting microbial cycling or some sorption to iron oxides at the river-sediment interface. Deeper sediments exhibit anthropogenic 206Pb/207Pb ratios throughout the core, with the δ18O-PO4 shifts possibly influenced by organic matter decomposition and remineralisation. Sedimentary deposition conditions and permeability appear critical in determining the extent of mixing and diffusion between sediments and overlying water. These findings highlight the complex and dynamic interactions among microbial cycling, organic matter breakdown, iron reduction and reversible phosphate sorption processes. Integrating δ18O-PO4 and 206Pb/207Pb analyses offers a potential framework for disentangling sediment-associated pollution and improving management of nutrient-enriched river systems.