Why some catchments recover faster: Controls on nitrate decline in eutrophication-sensitive agricultural catchments in western France

Brittany, France After decades of rising agricultural nitrate pollution, several regions of Western Europe are entering a recovery phase driven by stringent nutrient-reduction policies. Brittany offers a natural experiment in large-scale mitigation, particularly in “green-tide” catchments affected by recurrent coastal algal blooms. Using 23-year time series (Oct 2000 – Sept 2023) of nitrate concentrations from 286 agricultural catchments, we assessed: i) the magnitude of nitrate recovery and ii) whether recovery has continued or slowed in recent years. We hypothesised that, due to more extensive measures, green-tide catchments would exhibit faster and more sustained recovery than control catchments. We further hypothesised that accounting for initial conditions and intrinsic catchment properties is essential for evaluating policy effectiveness. Nitrate concentrations declined by 34% in control catchments and by 49% in green-tide catchments. Initial nitrate concentration, a proxy for pre-study N management effectiveness, explained 68% of the variance in recovery magnitude, indicating greater improvement potential where initial nitrate losses were highest. Green-tide catchments exhibited significantly stronger and more sustained declines than controls (ANCOVA, p < 0.05), although differences were modest after accounting for initial concentrations and intrinsic catchment properties. The latest programme of measures will be critical in determining whether nitrate in green-tide catchments will continue to decline or begin to plateau, as observed in control catchments. • Nitrate concentrations decreased by 34–49% in Brittany catchments (2000–2023). • Catchments with higher initial nitrate levels exhibited greater recovery. • Green-tide catchments recovered more consistently than controls.