Microplastics Flowing into Lake Winnipeg: Densities, Sources, Flux, and Fish Exposures

Microplastics (plastic particles < 5.0 mm in diameter) have been detected in freshwater ecosystems worldwide. Recently, surface concentrations of microplastics in Lake Winnipeg, Manitoba were shown to be comparable to those observed in Lake Erie, Ontario, despite large differences between the lakes in terms of population density and industrial activity. To better understand potential sources of microplastics into Lake Winnipeg, two inflowing tributaries (the Red and Assiniboine rivers) and the lake outflow (the Nelson River) were sampled for microplastics. To determine the role of wastewater treatment plants in contributing to microplastic pollution, microplastic densities upstream and downstream of wastewater treatment plants in the city of Winnipeg were compared. Finally, to determine the bioavailability of microplastics to fishes, we evaluated the presence of microplastics in the gastrointestinal tracts of two fish species, common carp (Cyprinus carpio) and sauger (Sander canadensis) collected from the Red River. Microplastics in the Red and Assiniboine rivers were comparable to those from Great Lake tributaries, but were elevated four to six times relative to concentrations observed in the Nelson River, suggesting significant losses to settling in Lake Winnipeg. On average, densities of microplastics downstream of wastewater treatment plants were elevated, and a significant correlation was observed between standardized daily effluent discharge from Winnipeg and river flux of microplastics/m2/s. On average, sauger were found to contain one microplastic particle and carp were found to contain seven microplastics within their gastrointestinal tracts. The number of particles ingested did not appear to affect body condition of fish collected in this study.