Hydrologic Extremes and Legacy Sources Can Override Efforts to Mitigate Nutrient and Sediment Losses at the Catchment Scale

Combating eutrophication requires changes in land and water management in agricultural catchments and implementation of mitigation measures to reduce phosphorus (P), nitrogen (N) and suspended sediment (SS) losses. To date, such mitigation measures have been built in many agricultural catchments, but there is a lack of studies evaluating their effectiveness. Here we evaluated the effectiveness of mitigation measures in a clay soil‐dominated headwater catchment by combining the evaluation of long‐term and high‐frequency data with punctual measurements upstream and downstream of three mitigation measures: lime‐filter drains, a two‐stage ditch, and a sedimentation pond. Long‐term hydrochemical data at the catchment outlet showed a significant decrease in P (−15%) and SS (−28%) and an increase in nitrate nitrogen (NO 3 –N, +13%) concentrations. High‐frequency (hourly) measurements with a wet‐chemistry analyzer (total and reactive P) and optical sensor (NO 3 –N and SS) showed that the catchment is an abundant source of nutrients and sediments and that their transport is exacerbated by prolonged drought and resuspension of stream sediments during storm events. Lime‐filter drains showed a decrease in SS by 76% and total P by 80% and an increase in NO 3 –N by 45% compared with traditional drains, potentially indicating pollution swapping. The effectiveness of two‐stage ditch and sedimentation pond was less evident and depended on the prevalent hydrometeorological conditions that drove the resuspension of bed sediments and associated sediment‐bound P transport. These results suggest that increased frequency of prolonged drought due to changing weather patterns and resuspension of SS and sediment‐bound P during storm events can override the generally positive effect of mitigation measures. Core Ideas We assessed the efficiency of catchment mitigation measures to reduce water pollution. At the catchment outlet, concentrations of P and sediments decreased by 15 and 28%. Lime‐filter drains reduced P and sediments by 80% but increased nitrate by 45%. Prolonged drought and resuspension of sediments exacerbated stream nutrient export. High‐frequency data show that legacy sources can override mitigation efforts.