This study explores the impact of phosphorus on the functional diversity (FD) of freshwater fish globally. The FD metrics functional richness (FRic), functional evenness (FEve) and functional divergence (FDiv) are calculated using a compiled dataset of 36,868 total phosphorus (TP) stations and 4,478,635 fish occurrences, leading to a FD analysis for 2498 buffers.
Using linear mixed models and various potential covariates, the effects of TP and freshwater realm on FD was assessed. The best-performing model for FRic, selected based on ANOVA, marginal R2 and conditional R2, included ecoregions as a random factor and temperature as a covariate. This model explained 57.5% of the variance in FRic, but TP's effect was not significant. The models for FEve and FDiv are singular, therefore no response to TP has been found. Contrary to prior research suggesting a significant effect of phosphorus on aquatic ecosystems, these findings reveal no significant relationship between TP and the FD metrics. This result challenges the expectation of decreasing FRic based on findings of decrease in species richness, highlighting that species richness does not necessarily correlate with functional diversity.
These findings suggest that phosphorus alone may not be a reliable indicator of eutrophication's impact on functional diversity. The results underscore the importance of considering additional factors, such as the nitrogen-to-phosphorus (N/P) ratio, in assessing the ecological effects of eutrophication. Consequently, phosphorus should be reconsidered as the sole metric for evaluating freshwater eutrophication in life cycle assessments, especially when using FD as impact category.