When phosphorus returns: Emerging upward trends following long-term decline in a temperate basin of Western France

Study region Vilaine River basin, France. Study focus Phosphorus (P) concentrations have declined in Western countries since the 1970s due to reductions in point and diffuse sources, but recent concerns about a potential P return have emerged. We analyzed long-term trends in soluble reactive P (SRP) and total P (TP) across 18 subcatchments of a 10,137 km² temperate basin in western France. A discharge-normalization approach was used to separate the effects of hydrological variability from changes in P sources or forms. We also examined synchronies with chemical proxies and relationships with geographic variables to support attribution of observed trends. New hydrological insights From 2007–2024, 61% of subcatchments showed increasing SRP concentrations (0.3 – 7.7 μg P/L/year), whereas only 28% showed increasing TP (1.1 – 12.1 μg P/L/year). Because increases occurred mainly during baseflow conditions, trends in annual P loads were often not significant. Changes in discharge explained less than 10% of SRP trends. Instead, results point to remobilization of legacy P, associated with rising temperatures and decreasing dissolved oxygen, as the main driver. Remobilization occurs in both the surface water network (streams, lakes, ponds) and artificial systems such as riparian buffer strips and waste stabilization ponds, acting as sources of P to the stream network. Future work should identify where remobilization mainly occurs to improve targeted mitigation measures.