Source-specific categorization of microplastics in nearshore surface waters of the Great Lakes

Actions aimed at reducing microplastic pollution need source-specific information to tailor local and global efforts. We applied a source-specific categorization scheme to quantify and characterize microplastics using robust QA/QC methods in 98 nearshore surface waters collected using manta trawls from three Laurentian Great Lakes. The greatest abundances (max. 2.0 x 10⁷ particles/km²) were found adjacent to the Greater Toronto Area, where fragments consistent with commercial and industrial activities contributed up to 58 % ± 21 % of particles on average (0.335 – 4.75 mm size range; near wastewater discharges in Humber Bay, Lake Ontario). Irregularly-shaped polyethylene microbeads were consistently more abundant than spherical microbeads as is typical in personal care products and contributed significantly (up to 44 % ± 21 % on average; Hamilton Harbour, Lake Ontario) in areas dominated by wastewater inputs. Using source-specific microplastic morphology categories may facilitate 1) the development of specific mitigation initiatives for identified and significant sources, and 2) assessing the effectiveness of implemented reduction measures at local levels in marine coastal environments, estuaries, and freshwaters globally.