A tiered framework for ecological risk assessment of contaminants: from model species to local pollution-impacted...

Ecological risk assessment (ERA) is widely used to quantify potential adverse impacts of chemical contaminants on freshwater ecosystems, but high-tier assessments incorporating local species composition are usually limited by scarce toxicity data. To bridge this gap, this study constructed a four-tiered framework including risk screening, generic assessment, local assemblage-specific evaluation and probabilistic risk characterization. The proposed framework evolved from conventional approaches based on model organisms and generic thresholds aimed at broad protection to regionally tailored evaluations that incorporated the sensitivity profiles of local natural assemblages impacted by pollution. The tiered evaluation framework was applied to a case study of metal contamination of major rivers and lakes in the lower Yangtze and Huai River basins in eastern China. Chemical sensitivity of untested species was predicted with a taxonomy-related Bayesian approach. Results from the tiered assessment revealed a decreasing trend of proportions of high-risk classifications with increasing tiers of assessment, suggesting that community composition has shifted to more pollution tolerant species. Cadmium, copper, lead and zinc consistently posed high ecological risks through different assessment tiers. While refined probabilistic risk characterization further highlighted copper as requiring priority management, the risks posed by nickel to surviving pollution-tolerant species were negligible. In short, the apparent reduction in high risks was largely attributable to pollution-induced high community tolerance, where external stressors have eliminated most sensitive species. The high-tier risk assessments, based on currently existing pollution-tolerant assemblages, enhance ecological realism and inform short-term management priorities, while further emphasizing the need to validate their applicability in healthy ecosystems and support the recolonization of native sensitive species.