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Microparticles (MP; particles <5 mm) are ubiquitous in marine environments. Understanding MP concentrations at different spatial scales in the Salish Sea, Washington, USA, can provide insight into how ecologically and economically important species may be affected. We collected mussels across the Salish Sea at regional and localized scales, chemically processed tissue to assess MP contamination, and used visual and chemical analyses for particle identification. Throughout the Salish Sea, mussel MP concentrations averaged 0.75 ± 0.09 MP g -1 wet tissue. At a regional scale, we identified slight differences in concentrations and morphotypes of MP while at a localized scale these metrics were not significant and did not differ from controls. In a subset of particles, 20 % were identified as synthetic materials, which include polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and nylon. Differences in MP sources, heterogeneous transport of MP, and distinct shellfish feeding mechanisms may contribute to plastic contamination patterns in the Salish Sea., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Plastic pollution is a threat to marine life with long term impacts to ecosystems and organisms in the sea. In this study, we quantified the presence of microparticles in wild populations of Pacific oysters (Crassostrea gigas) from the Salish Sea, Washington State. Examination under a dissecting microscope revealed 63% of oysters contained microparticles (~1.75 microparticles per oyster) and microfibers were the dominant type of particles. Using Raman microspectroscopy (RMS) and Fourier transform infrared microspectroscopy (μ-FTIR) we found that only ~2% of these microparticles were synthetic and included polymers such as polystyrene, polyethylene, polypropylene, poly(bisphenol A carbonate), rayon, and polyacrylate. It is important to note that of the 447 microparticles analyzed with RMS, 41% showed fluorescence interference, impeding the determination of their identification. The remaining microparticles were cellulose derivatives, shell fragments, biological or proteinaceous material, salts, minerals, and gypsum. Fourier transform infrared spectroscopy equipped with a diamond attenuated total reflectance accessory (ATR-FTIR) showed the presence of sorbitan derivatives in all samples examined (n = 213). These findings provide the first baseline for microplastic and other particles in oysters from the west coast of the United States integrating results from ATR-FTIR, μ-FTIR, and RMS, in addition to visual sorting. These results suggest there is low retention of plastic particles in Pacific oysters from the Salish Sea, but further research is needed to determine the composition of microparticles with fluorescence interference., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)