Microplastics in invasive mussels (Dreissena sp.) of Lake Michigan: Patterns across sites and relationship to chemical pollutants

Invasive zebra and quagga mussels (Dreissena sp.) in the Great Lakes of North America serve as biomonitors for chemical contaminants, but are also exposed to microplastics (< 5mm). Little research has examined microplastic consumption by dreissenid mussels or the relationship between microplastics and other contaminants. We measured microplastics and contaminant concentration in mussels from Milwaukee Harbor (Lake Michigan, USA) spanning a gradient from reference (i.e., clean water) conditions to sites influenced by wastewater and urban river discharge. Mussels were deployed in cages, collected after 30 and 60 days, sorted by size class, and analyzed for microplastics and contaminants. As expected, microplastic concentrations were high in the largest mussels at the wastewater-adjacent site in July. However, there was no distinction among sites for microplastic concentrations for smaller size classes, and no differences among sites in August. Microplastics and chemical contaminants were unrelated in mussels. Microplastics have a diversity of intrinsic and extrinsic factors which influence their ingestion, retention, and egestion by mussels, and are likely distinct for chemicals relative to particulate pollutants. Dreissenid mussels may not serve as plastic pollution ‘indicators’ as they do for chemical contaminants. However, microplastic ingestion by dreissenid mussels is widespread, with unknown effects on physiology, population dynamics, and mussel-mediated ecosystem processes. These data will inform our understanding of the spatial distribution of microplastics in urban freshwaters, the role of dreissenid mussels in plastic budgets, and models for the fate of plastic contaminants in the Great Lakes and elsewhere.

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