Quality Assurance and Quality Control. The objective of this research was to extract MP debris (30 µm to 1 mm) from wrack line samples. Therefore, the quality control used reflects the need to limit contamination within that size range, which is mostly particulate in shape, rather than smaller MP, including fibers, and nanoplastics. Briefly, quality control procedures included washing and rinsing all glassware and utensils with DI water before being muffled at 500˚C for 4 hours and then storing covered to reduce airborne contamination.27 Laboratory contamination of samples was assessed using four blank samples. The materials from the blank samples consisted of an average of 12.8 ± 3.0 microfibers. Similar fibers were not found in the samples and therefore, no corrections were done to the final results. Dyed Cotton laboratory coats and gloves were worn at each step of the analysis. Analysis was done in a laboratory in an air filtration system and all surfaces were cleaned with DI water each time before use.
The percentage recoveries for the isolated MPs in POM were estimated (Table S1) by mixing a proportion of the isolated MPs and POM in 1:5 and repeated the whole extraction process using the binary solvent mixture of ethanol and water (7:3) to recover the MPs. We used two approaches to further evaluate our recovery method. First, we visually observed the POM under the stereo microscope where 6 MP particles was the maximum number found in only 1 of the twelve wrack line samples assessed (PL). Second, 1 g each of the recovered POM was stained with methylene blue to mask the fluorescence of organic matter and reduce interference. After that, the POM was stained with Nile red and viewed under the fluorescence microscope (Olympus CKX53 with Hamamatsu ORCA-Spark 431-3196, Japan). The fluorescent MPs were counted and estimated in particle per gram (particle.g-1) of the POM (Figure S3).