Spatial patterns of microplastics in freshwater bivalves (Bivalvia: Unionidae and Sphaeriidae) relative to municipal wastewater effluent discharges.

This article investigates the presence and characteristics of microplastics in two groups of freshwater bivalves—fingernail clams (Sphaeriidae spp.) and unionid mussels (Lasmigona costata)—and surface waters collected upstream and downstream of three municipal wastewater treatment plants (WWTPs) along the Grand River watershed in Southern Ontario, Canada. The study found microparticles, predominantly fibers sized 80 μm to 2 mm, in all sample types, with about 30% confirmed as plastic polymers. Elevated microparticle counts downstream of WWTPs were observed only in mussel gill tissues, while other bivalve tissues and surface waters showed no significant differences between upstream and downstream sites. Although microparticle abundances in bivalve tissues were higher than previously reported and fingernail clams contained more particles per mass than mussels, no correlation was found between microparticle counts in water and bivalves, suggesting complex exposure pathways. The findings indicate widespread microplastic contamination in the watershed but limited influence of WWTP effluents on microplastic uptake by freshwater bivalves, highlighting the need to consider additional sources and to further study the ecological implications.