Modelling Holocene riverine phosphorus concentrations in the Belgian loess belt: Integrating land-cover dynamics and population pressure

Most European water bodies fail to meet good ecological status, largely due to excessive phosphorus (P) concentrations. Despite policy efforts, little improvement has occurred over the past 15 years. To put the situation into historical perspective, this study investigates long-term human impacts on riverine P concentrations by modelling P export throughout the Holocene. Three approaches were compared: a spatially lumped nutrient export coefficient (EC) model, a spatially explicit sediment export model adapted for P export (WaTEM/SEDEM), and a hindcasting model based on population density. Model outputs were validated using present-day total phosphorus (TP) concentrations and a Holocene TP time series derived from dated floodplain sediments. Results indicate that land cover change alone may have triggered eutrophication as early as 2000 years ago, well before the widespread use of mineral fertilizers. Erosion mobilized P naturally present in soils and concentrated by human activity. Both the EC and WaTEM/SEDEM models suggest that historical exceedances of the freshwater TP threshold (140 μg L -1 ) have occurred, with critical land cover thresholds ranging from 17% to 46%. Population-based estimates indicate this threshold was crossed around 300 years ago at ∼56 inhabitants km -2 . Predicted pristine TP concentrations ranged from 6 [95% CI: 4–8] μg L -1 (WaTEM/SEDEM) to 35 [18–124] μg L -1 (EC), while sediment records suggest 48 μg L -1 . The intricate link between water quality, land use change, and human population dynamics is crucial to consider for effective future water quality management.