Functional diversity complements taxonomical diversity in productivity-biodiversity relationships in freshwaters

Productivity is widely considered a key driver of biodiversity, yet whether taxonomic and functional diversity respond similarly to productivity gradients remains unclear. Using phosphorus as an indicator for productivity gradient, we compiled 78 freshwater datasets spanning > 30,000 sites globally to test how productivity–biodiversity relationships (PBRs) vary across diversity dimensions, trophic groups, and ecosystems. We show that PBRs are predominantly unimodal across taxonomic and functional dimensions, with slightly weaker curvature for functional diversity. Unimodal PBRs are more pronounced in producers than in consumers and show stronger curvature in lakes than in rivers. Functional reorganization also varies along the phosphorus gradient, with the clearest contraction in resource-related trait space and marked shifts in community functional centroids. Climatic, geographic and human land-use gradients further modify PBR shape. These findings indicate that predicting biodiversity responses to productivity benefits from considering functional diversity, ecological context, and broad-scale environmental gradients.