Macrophyte-based assessment of European rivers: Status, gaps, and a practical checklist for improvement

Summary

Aquatic macrophytes are key indicators of ecological status in European rivers and are recognised as biological quality elements under the EU Water Framework Directive (WFD). Following the adoption of the Water Framework Directive (2000), twenty-nine national macrophyte-based assessment systems have been implemented and intercalibrated across the European Union (EU). These systems have been shown to respond to ecological degradation, with the strongest evidence relating to nutrient enrichment. However, their application has generated contrasting perspectives: while macrophyte indices are widely regarded as effective indicators, they have also been criticised as lacking ecological interpretability or explicit links to human pressures. A balanced evaluation of these perspectives is hindered by the limited accessibility of methods, many of which remain unpublished or are available only in grey literature, and the absence of a comprehensive, integrative overview. Here, we present the first synthesis of 29 macrophyte-based assessment systems across EU Member States and associated countries, explicitly addressing these contrasting perspectives and providing a coherent framework for their interpretation. We examine these systems with respect to their taxonomic scope, metrics, target pressures, pressure–response relationships, and boundary-setting approaches. Most systems rely on indices combining taxa-specific indicator values with abundance data and show strong relationships with nutrient concentrations. Nevertheless, substantial differences persist, including wide variation in taxonomic coverage (40–450 taxa) and methodological design, while the ability to distinguish among multiple pressures remains limited. Invasive alien species (IAS) are inconsistently incorporated into the assessment systems. Emerging tools such as environmental DNA (eDNA) offer promising opportunities to complement traditional assessment approaches but require further development. We identify key gaps and propose a practical checklist to improve accessibility, strengthen ecological interpretation, and guide future development of macrophyte-based assessment systems in the context of climate change and emerging environmental challenges.