Anthropogenic Grains: Microplastics in Benthic Compartments of the Great Lakes Watershed

The Laurentian Great Lakes of North America are constantly threatened by historic and emerging contaminants. The most globally widespread of these contaminants is plastic, which has unique chemical properties that result in its preservation and accumulation. Microplastics (MPs), particles <5 mm in their longest diameter, can sink if i) clays or biofilms adsorb to their surfaces, ii) they contain mineral fillers, or iii) they are egested by organisms. The sunken MPs pose a threat to small animals that dwell in and/or feed on benthic sediment. Microplastics that become buried beneath the sediment surface have the potential to become part of Earth's future geologic record. Locating the depositional sinks and mapping the concentrations of MPs in the Great Lakes Basin has indicated that MP abundances are related to factors such as population density, proximity to plastic production and user facilities, the movement of surface water currents, and shoreline and riverbank morphology. Abundances of MPs in benthic sediment of the Great Lakes and their tributaries range from 0 to tens of thousands of particles per kilogram of dry weight sediment. There is a positive correlation between the amount of organic matter and number of particles, which in turn reflects the preferential preservation of MPs in silts and clays compared to sands. The predominant type of MPs identified in our studies are microfibers, although abundances vary considerably following chemical determination of plastic versus natural fibers. Common fragments from certain locations include paint chips, flashing, and black rubber. Our projects provide results of the first comprehensive investigation of MPs in benthic sediment of the Great Lakes upon which future long-term monitoring will build.