Nutrient loading and stream order shape benthic and pelagic spring algal biomass in a large, temperate river basin...

Eutrophication persists in many freshwater systems despite extensive efforts to control nutrient emissions from point and diffuse sources. While intensely studied at local or regional scales, the joint response of benthic and pelagic algae to nutrient loading across entire river networks remains poorly understood. Here, we assessed spatial patterns of pelagic and benthic algal biomass in response to point source and diffuse phosphorus loading in the Elbe River Basin, a temperate, transboundary river network, based on extensive monitoring data and with the parsimonious hydro-ecological model CⁿANDY (Coupled Complex Algal-Nutrient Dynamics). We referenced our simulations to median river discharge data and phosphorus inputs from point (1,900 wastewater treatment plants) and diffuse sources, determined with the MoRE model and CORINE land cover analysis. We found distinct spatial eutrophication patterns across the river network and complex responses to local and cumulative anthropogenic nutrient emissions. Lower stream orders, particularly those in urban and agricultural areas, showed the highest dissolved phosphorus concentration and benthic algae density. Conversely, pelagic algae dominated higher stream orders, influenced by nutrient transport from lower-order streams to downstream reaches. The validated CⁿANDY model effectively identified eutrophication hotspots, enabling prioritized nutrient and eutrophication management. Although extensive monitoring data were available, systematic gaps in established monitoring schemes limited the model calibration and validation. Therefore, we advocate for a revision and propose model-aided eutrophication monitoring at the river basin scale with representative coverage of all stream orders from up to downstream and the algal biomass in the benthic and pelagic compartments.