Linking Multi-Scale Stressors to Fish and Benthic Invertebrates in Non-Wadable Rivers: Implications for River Management

Understanding how multiple stressors operating across spatial scales shape riverine biota is essential for advancing sustainable ecosystem management. This study evaluated the relative importance of hydromorphological alterations and land-use changes for benthic invertebrate (n = 77) and fish assemblages (n = 92) in large Slovenian rivers. Multivariate analyses were employed to disentangle unique and shared effects of stressors at reach, segment, and catchment scales while accounting for natural variability. The results revealed a hierarchical structuring of stressors, with regional factors shaping local conditions. Fish assemblages were predominantly associated with local-scale stressors, particularly at the segment scale, whereas benthic invertebrates were more strongly associated with shared stressor effects across multiple spatial scales. Stressors explained substantially more variability in fish (43.5%) than in benthic invertebrates (19.9%). Catchment-scale land use showed higher explanatory power than local riparian land use. Typological variables moderately improved model performance, emphasizing the contribution of natural environmental gradients. Our findings suggest that large river management may benefit from a multi-scale perspective that prioritizes segment-scale habitat conditions for fish and coordinated catchment-to-local strategies for benthic invertebrates, with potential benefits for ecosystem resilience and long-term sustainability.