<p>Eutrophication due to anthropogenic nutrient enrichment has produced a significant threat to the diversity of freshwater communities. Loss of biodiversity may lead to a decrease in the number of species with similar functions, resulting in reduced functional redundancy (FRe). The reduction of FRe may leave ecosystems without sufficient coping strategies to deal with environmental stress and disturbance. Numerous studies propose higher biodiversity boosts FRe, affecting terrestrial or marine ecosystem stability. However, research is scant on how biodiversity changes from nutrient enrichment impact FRe in river ecosystems. We investigated this in a boreal river basin in northern China, studying the link between nutrient enrichment and the FRe of fish assemblages. The results showed that as nutrient enrichment levels increased, there was a decrease in both species richness and functional richness of fish assemblages; however, a reverse trend was found in both taxonomic and functional beta-diversity, all of which resulted in the decrease in FRe. Elevations in beta-diversity may instigate a marked amplification in differentiation among communities, which could propel evolution of more specialized functions. This notable divergence, accompanied by diminished species richness, may subsequently mitigate FRe in ecosystem with varied nutrient enrichments.</p>