Stricter reductions of nutrient pollution support riverine community recovery in degraded catchments

Excessive nutrient levels remain a significant barrier to community recovery in large rivers globally. Here we ask if nutrient reduction can lead to restoration of riverine communities through investigating key anthropogenic drivers of total nitrogen (TN) and total phosphorus (TP) concentrations that affect riverine benthic macroinvertebrate and diatom communities. We seek to identify potential benefits of stricter nutrient reductions under the European Water Framework Directive (WFD). Nutrient impacts on riverine communities were analyzed across Germany using Structural Equation Modelling to distinguish cascading nutrient contributions from land and in-stream sources, and Bayesian Integrated Nested Laplace Approximation to account for spatial dependency. Across 781 benthic macroinvertebrate and 200 diatom sampling sites, covering 40 % of river catchments in Germany, in-stream nutrient concentrations explained nutrient impacts on riverine communities better than land use proxies. Agriculture contributes the majority of nutrient input loads, but urban areas, despite their small extent (<10 % of land), generate 41 % of TP inputs. Elevated TP had the strongest impact among nutrients on riverine communities. Reductions in nutrient concentrations by up to 50 % can improve highly degraded catchments, but stricter reductions suggest limited benefits for moderate to good quality ecosystems, where other stressors may constrain recovery. Our findings underline the conservation value of land use transformation to reduce in-stream nutrients and the need for stricter regulation of pollutant mixtures to address the complex challenges facing riverine ecosystems.