The effects of a large-scale nature-based solution on the macroinvertebrate diversity in a gravel river in The Netherlands

Nature-based solutions (NbS) aim to restore river ecosystems and improve biodiversity, but their effectiveness in enhancing aquatic ecological quality remains uncertain, especially across different waterbody types. To address this gap, we assessed macroinvertebrate communities in the Border Meuse valley following NbS interventions. Sampling was conducted across the main channel, secondary channels, floodplain lakes, and tributaries. We analysed abiotic conditions, taxonomic composition, abundances, diversity indices, and contributions to regional ß-diversity. A total of 218,810 individuals representing 152 taxa were recorded. Lakes and tributaries supported the highest abundances and most unique taxa (~20 each), while the main channel and secondary channels contributed least to regional diversity. Shannon-Wiener diversity was highest in lakes and lowest in the Meuse. Differences in abiotic conditions further separated lakes and tributaries from other waterbody types. Secondary channels shared characteristics with the Meuse and supported similar macroinvertebrate assemblages, suggesting limited added ecological value. Despite restoration efforts, taxa characteristic of fast-flowing gravel rivers remains largely absent. These results indicate that NbS measures in the Border Meuse, particularly the creation of side channels, had limited effectiveness in restoring aquatic biodiversity. Contributing factors include low habitat heterogeneity, pre-existing modifications in tributaries, and the lack of new floodplain lake creation. We propose five lessons for future NbS planning: conduct thorough system analysis, gather pre- and post-project data, set ecologically realistic goals, implement adaptive management, and promote knowledge sharing. A functional, ecosystem-based approach is essential for improving biodiversity outcomes in large river restoration projects.