Quantifying diffuse- vs point-source contributions to phosphorus loading using high-frequency monitoring in a large-scale nested catchment

Despite agri-environmental measures implemented to protect water quality, there are still ongoing water quality issues and eutrophic impacts at different scales. Phosphorus (P) sources are diverse but evidence on the relative contribution of diffuse- and point-sources to P loading in rivers is limited, estimated from models or based on low frequency datasets that don’t capture brief but high magnitude P discharges. Using hourly measurements of P concentrations, the aim of this study was to investigate the relative contribution of diffuse- and point-sources to P loading in a large, complex catchment river noted as major contributor to downstream lake hypertrophication. Stream water total P (TP) concentrations were measured every hour at the outlet of two nested catchments: an ‘upstream’ catchment dominated by diffuse agricultural P sources and a ‘downstream’ catchment which included additional point urban P sources. Hourly TP loads were computed for each catchment to quantify the diffuse vs point P source percentage contributions to TP loading. Results showed that over February-June 2025, median TP concentrations were 0.092 mg L^-1 and 0.137 mg L^-1 at the outlet of the ‘upstream’ and ‘downstream’ catchment, respectively. Total diffuse- and point-sources TP loads were 5.8 tons (22.7 kg km^-2) and 2.4 tons (6.2 kg km^-2) and contributed on average 70% and 30% of the hourly TP load, respectively. When comparing these relative contributions for different flow conditions, the P pressure from diffuse sources increased at high flows (median of 77.7% for the 15% highest flows) while pressure from point sources increased at low flows (median of 42.2% for the 15% lowest flows). The study emphasizes the imperative to reduce both diffuse- and point‑source P losses to safeguard river ecosystems during low‑flow periods, when ecological vulnerability is greatest, and to prevent lakes from receiving excessive P inputs during high‑flow events. This need becomes increasingly critical as climate change amplifies the frequency and severity of hydrological extremes.