Functional trait syndromes structure macrophyte diversity and functional redundancy across flowing and impounded freshwater systems

Trait-based approaches provide a powerful framework for understanding community assembly and ecosystem stability in freshwater ecosystems, yet functional responses of aquatic macrophytes to hydrological regime and environmental gradients remain incompletely resolved. We analyzed macrophyte communities across flowing (rivers and artificial canals) and impounded (lakes and reservoirs) freshwater systems in Serbia using an explicitly audited functional trait matrix. Multivariate ordination of species traits revealed a structured functional space organized along two principal gradients: (i) a growth-form and mechanical axis contrasting emergent, rigid species reproducing predominantly by seeds with submerged, flexible species relying mainly on vegetative propagation, and (ii) a persistence axis separating annual from perennial strategies. Projection of site-level communities into this trait space indicated significant but partial functional differentiation between hydrological types, with substantial overlap between flowing and impounded systems, suggesting that hydrological regime acts as a directional rather than deterministic filter. RLQ analysis further revealed coherent multivariate covariation between environmental gradients and coordinated trait syndromes, whereas individual trait-environment associations identified by fourth-corner analysis were not significant after correction for multiple testing. Finally, spatial analyses showed that functional similarity declined more gradually with geographic distance than taxonomic similarity, indicating substantial functional redundancy across the regional macrophyte assemblage. Together, these results show that coordinated functional trait syndromes structure macrophyte diversity across flowing and impounded freshwater systems while maintaining functional redundancy that may enhance ecosystem stability.