Impacts of Combined Microplastics and Heavy Metal Pollution on Aquatic Ecological Health: Insights from Trophic Levels

Microplastics (MPs) and heavy metals are widespread co-occurring contaminants in aquatic environments. Their interactions can alter metal mobility and bioavailability and may influence contaminant transfer and toxicity across aquatic food webs. This review critically synthesizes current evidence on MP–metal interactions, trophic transfer, and biological effects across trophic levels. Particular attention is given to the distinction among co-occurrence, bioaccumulation, trophic transfer, and biomagnification. MP–metal adsorption and desorption are jointly regulated by particle properties, metal speciation, and environmental conditions, resulting in substantial variations among exposure systems. Current evidence indicates that MPs can modify metal partitioning, bioaccessibility, and dietary transfer under some conditions. However, persistent particle-bound co-transport and biomagnification across complex aquatic food webs remain insufficiently demonstrated. Biological responses to MP–metal co-exposure are also context dependent, with both strengthened and attenuated effects reported across species and exposure conditions. Stronger or weaker responses under co-exposure should not be interpreted as synergistic or antagonistic without comparison with an appropriate non-interaction reference model. Experimental evidence further suggests that MPs can alter metal bioaccessibility, transport, and biological responses, whereas human evidence remains insufficient to establish increased systemic metal exposure or adverse health outcomes. Future studies should emphasize environmentally relevant exposures, well-defined prey–consumer pathways, quantitative transfer assessment, and standardized approaches for evaluating mixture toxicity.