Microplastics Associated to Emerged Aquatic Insects Highlight a Potential Pathway of Environmental Transport: a Case Study of the Mayfly Cloeon Dipterum.

Microplastics can move among terrestrial, aquatic, and atmospheric systems. However, the role of aquatic insects in transporting microplastics, and their potential contribution to cross-ecosystem transfer, remains poorly understood. In this study, we examined the accumulation of three microplastic types (polyethylene (PE) fragments, tire wear particles (TWPs), and polyacrylonitrile (PAN) fibres) in the merolimnic mayfly Cloeon dipterum. Larvae were exposed to two microplastics concentrations (500 and 5000 particles/L), and microplastics associated with larvae and newly emerged adults were quantified and characterized. All three microplastic types were detected in larval digestates, indicating their association with the organisms in the aquatic environment; however, ingestion and surface adhesion could not be distinguished. PE fragments were the most abundant (16-49 particles per larva), followed by TWPs (3-9 particles per larva), and PAN fibres (≤2 particles per larva). Microplastics were also found in both the washing water of emerged adults (representing surface-adhered particles) and their digestates (representing ingested particles), demonstrating that particles remained adhered to and inside the organisms after metamorphosis. Particle size analysis revealed preferential retention of smaller PE fragments and TWPs during both ingestion and adhesion, with the median size of recovered particles being 30-57% and 24-42% smaller, respectively, than that of the particles introduced into the system. Exposure to PAN fibres at the higher concentration (5000 particles/L) significantly reduced the time to 50% emergence by one day, whereas none of the treatments affected adult fatty acid profiles. Overall, our findings demonstrated that all three tested microplastic types can be associated with emerged aquatic insects, highlighting a potential pathway for the biologically mediated redistribution of microplastics. Further studies are needed to identify the mechanisms governing microplastic retention and to disentangle the effects of particle characteristics, such as polymer type, size, and shape, on these processes and their ecological significance.