Ecotoxicological Impacts on Riverine Fish Assemblages

Abstract

Riverine fish assemblages serve as integrative bioindicators of catchment-scale pollution, reflecting the cumulative exposure of aquatic communities to agricultural chemicals, pharmaceuticals, heavy metals, and microplastics that enter river systems from diffuse and point source inputs. Ecotoxicological impacts on fish include direct mortality, sublethal physiological disruption (endocrine disruption, immunosuppression, oxidative stress), behavioural modification, and community-level restructuring as sensitive species are replaced by tolerant taxa -- yet the dose-response relationships and species-specific sensitivities across multi-contaminant mixtures representative of real-world riverine exposure are poorly characterised. This study presents a comprehensive multi-catchment assessment of ecotoxicological impacts on riverine fish assemblages at 184 sites across 24 European rivers spanning a full water quality gradient, integrating standardised fish assemblage surveys, multi-contaminant water chemistry profiling (47 analytes), physiological biomarker measurements on sentinel species, and a 12-month mesocosm experiment testing mixture toxicity at environmentally relevant concentrations. Agricultural pesticide mixture exposure was the strongest predictor of fish assemblage degradation (partial R2 = 0.54; p < 0.001), followed by pharmaceutical mixture (0.38; dominated by endocrine-active compounds) and heavy metals (0.28). Sensitive salmonid species showed 84.7% probability of local extinction when summed Toxic Units from pesticides exceeded 1.0 -- the threshold regulatory assessment level. The mesocosm experiment confirmed that sub-regulatory mixture exposures (each compound at < 0.1 x PNEC individually) produced statistically significant endocrine disruption in four indicator species when combined at environmentally realistic ratios.