Multidimensional riverine biodiversity reveals decadal and sub-decadal effects of hydrological and water-quality...

Freshwater biodiversity is increasingly affected by compounded anthropogenic pressures and climatic variability. However, the current understanding of how multiple environmental stressors reorganize biodiversity across taxonomic, functional, and phylogenetic dimensions at different temporal scales remains poorly understood. It is particularly evident in highly climatic and hydrologically dynamic systems, such as in the Mediterranean climate and monsoonal areas. Using two decades of macroinvertebrate data from three contrasting Mediterranean watersheds, we examine how multidimensional biodiversity patterns vary across decadal and sub-decadal scales in response to hydrological variability and environmental stressors. This multi-scale design provides a robust, regionally representative dataset for testing scale-dependent biodiversity dynamics. We partitioned the 20-year dataset into decadal (10-year) and sub-decadal (5-year) periods to assess scale-dependent variability. We found a scale-dependent hierarchy: at the decadal scale, taxonomic richness showed no net change while functional and phylogenetic diversity declined; at sub-decadal scales, phylogenetic diversity exhibited detectable variability and environmental filtering, whereas functional assembly remained predominantly random. Hydrological variables and organic nutrients were among the key correlates of this scale-dependent variability. Our work reveals a mixed assembly framework where random processes dominated (60-80% of communities), but within the non-random subset, phylogenetic clustering varied across time periods, indicating that scale-dependent environmental filtering operates alongside strong stochasticity. This demonstrates that different biodiversity dimensions respond to environmental change at distinct temporal grains, with even coarse phylogenetic proxies revealing detectable filtering at fine resolutions, necessitating scale-aware, phylogeny- and trait-based conservation strategies to accurately assess resilience under escalating global change.