Cross-ecosystem transfer of microplastics via emergent aquatic insects to riparian spiders in a municipal wastewater-impacted system.

Microplastics (plastic particles ≤5 mm) are a persistent and widespread pollutant in aquatic ecosystems, where they can accumulate in the larvae of aquatic insects inhabiting benthic habitats. Aquatic insects cross aquatic-terrestrial boundaries through metamorphosis, yet few studies have investigated their role in transporting microplastics to terrestrial ecosystems as emergent adults. This study sampled upstream and downstream of eight municipal wastewater treatment plants in the Grand River watershed, Ontario, Canada, in 2022 and 2023 to assess whether these facilities increase microplastics occurrence in aquatic and riparian food webs. Larval and adult caddisflies (Hydropsychidae), mayflies (Heptageniidae), midges (Chironomidae), and riparian spiders (Tetragnathidae) were collected to examine microplastics accumulation, retention through metamorphosis, and cross-ecosystem transfer into riparian spiders. Larval Heptageniidae, adult Chironomidae, and Tetragnathidae exhibited higher microplastic abundances downstream relative to upstream, with concentrations approximately 2.4-, 3.7-, and 2-fold higher, respectively, although the magnitude of differences varied across sites; no significant differences were observed for the other taxa. Organic (cellulose-based) and polyester fibres were the most common, representing ∼25-68% and ∼16-50% of particles across taxa, respectively. Relationships between total microfibre concentration and trophic position (δ15N) were generally weak and site specific, while trophic transfer factors >1 were observed between biofilm and larval taxa at most sites but were generally <1 between adult insects and spiders. Together, these findings demonstrate that emergent aquatic insects retain and transfer microplastics across aquatic-terrestrial boundaries to riparian predators but provide limited evidence for consistent trophic magnification across the food web. This research addresses a critical knowledge gap by advancing our understanding of microplastic uptake and movement through nearshore food webs via emergent aquatic insects.