Projecting future riverine nitrogen exports to the Wadden Sea

Despite decades of efforts to reduce nutrient pollution, Europe is still confronted with elevated nutrient concentrations in ground- and surface waters. This is a result of nutrient inputs from intensive agriculture, wastewater collection and treatment and atmospheric deposition. As a consequence, inland and marine waters suffer from persistent eutrophication problems manifested as algal blooms, changes in species composition, oxygen depletion, and no full recovery of seagrass. Water management must address this problem by finding additional nutrient reduction measures and assess their effectiveness to ensure good water quality also under a changing climate. To plan nutrient reduction measures and assess their effectiveness, predictive modelling tools are essential but challenging to apply at large spatial scales and under changing boundary conditions. Here, we present results of the EU-funded project NAPSEA addressing eutrophication and nutrient management in the Elbe and Rhine basins and the receiving Wadden Sea (the intertidal zone of the south-eastern North Sea). More specifically, we explore future trajectories of reactive nitrogen (N) concentrations and loads exported from the Elbe and Rhine basins under different nutrient input scenarios. We use the transit-time based catchment water quality model mQM calibrated to long-term observations (>10 yrs) in more than 140 sub-catchments. The model takes nitrogen surplus as a diffuse source that is routed through different soil compartments and the subsurface, with flowpaths to the streams represented by dynamic transit time distributions. In the river network, inputs from wastewater point source and instream removal are considered. The modelled scenarios address the impact of climate change on the hydrological cycle (RCP4.5) and the planned measures for the different nitrogen pathways between 2022 and 2050. More specifically, we quantify the effects of the new urban wastewater treatment directive, the revised German fertilizer ordinance, the expected reduction in atmospheric deposition and nature-based solutions such as reactivated floodplains. We found that the projected changes in discharge and the joint nutrient reduction measures will have a similar magnitude of effect on nutrient exports. The effectiveness of nutrient reduction measures is spatially heterogeneous,depending on the land use composition and the natural attenuation potential of the different basins. The reduction of agricultural N surplus and atmospheric deposition has a higher impact on the Rhine basin, unlike the Elbe basin where the benefits of regulations on urban wastewater prevail. Overall, our results reveal a 15-27% reduction in nutrient exports to the Wadden Sea (average 2045-2050) compared to the average export in the years 2010-2020. While these results are encouraging, a significant gap remains to the estimated reduction needs to sufficiently reduce nitrate pollution of inland waters and to reach safe ecological boundaries of the Wadden Sea. More ambitious nutrient reduction measures are needed to ensure future a good status of inland and coastal waters.