Microplastic contamination in fish from the St. Lawrence River and Estuary: Roles of semisynthetic polymers, passive uptake, and wastewater inputs

Semisynthetic polymers, such as rayon, are inconsistently reported in microplastic monitoring as most studies focus on synthetic polymers. However, evidence is growing for their ecological impacts. We measured microplastics, including semisynthetic polymers, in water and four fish species from the St. Lawrence River and Estuary (SLRE, Canada), a freshwater-marine corridor and good model for large river-estuary systems. Microplastic abundance was 0.44 ± 1.13 (mean ± SD) in fish gastrointestinal tract, 1.34 ± 2.12 (n/sample) in fish gills, and 2.17 ± 3.68 (n/L) in water. Rayon was the dominant microplastic in both water (41 %) and fish (40-100 %), revealing an underreported but significant contribution of semisynthetic polymers to aquatic microplastic burdens. This finding underscores the need to integrate semisynthetic polymers into future monitoring frameworks. In large piscivorous fish, gill uptake contributed more to microplastic accumulation than oral ingestion, unlike most non-piscivorous species reported in the literature, which accumulate more microplastics in the gastrointestinal tract. Comparisons of sites upstream and downstream of wastewater treatment plants (WWTPs) showed no significant difference in total microplastic abundance, but downstream waters contained more particles <100 μm and a broader diversity of polymers and colors. Given the greater environmental risks of smaller microplastics, these patterns highlight gaps in WWTP performance metrics that focus solely on total counts. Our findings provide evidence to expand monitoring frameworks to include semisynthetic polymers, incorporate non-oral exposure pathways into risk assessments, and improve WWTP metrics to inform global policy against microplastic pollution.