Most plastic pollution originates on land. As such, freshwater bodies serve as conduits for the transport of plastic litter to the ocean. Understanding the concentrations and fluxes of plastic litter in freshwater ecosystems is critical to our understanding of the global plastic litter budget and underpins the success of future management strategies. We conducted a replicated field survey of surface plastic concentrations in four lakes in the North American Great Lakes system, the largest contiguous freshwater system on the planet. We then modeled plastic transport to resolve spatial and temporal variability of plastic distribution in one of the Great Lakes, Lake Erie. Triplicate surface samples were collected at 38 stations in mid-summer of 2014. Plastic particles >106 µm in size were quantified. Concentrations were highest near populated urban areas and their water infrastructure. In the highest concentration trawl, nearly 2 million fragments km −2 were found in the Detroit River-dwarfing previous reports of Great Lakes plastic abundances by over 4-fold. Yet, the accuracy of single trawl counts was challenged: within-station plastic abundances varied 0-to 3-fold between replicate trawls. In the smallest size class (106-1,000 µm), false positive rates of 12-24% were determined analytically for plastic vs. non-plastic, while false negative rates averaged ∼18%. Though predicted to form in summer by the existing Lake Erie circulation model, our transport model did not predict a permanent surface "Lake Erie Garbage Patch" in its central basin-a trend supported by field survey data. Rather, general eastward transport with recirculation in the major basins was predicted. Further, modeled plastic residence times were drastically influenced by plastic buoyancy. Neutrally buoyant plastics-those with the same density as the ambient water-were flushed several times slower than plastics floating at the water's surface and exceeded the hydraulic residence time of the lake. It is likely that the ecosystem impacts of plastic litter persist in the Great Lakes longer than assumed based on lake flushing rates. This study furthers our understanding of plastic pollution in the Great Lakes, a model freshwater system to study the movement of plastic from anthropogenic sources to environmental sinks.

[1] In most thermally stratified lakes, the summer thermocline has the shape of a "dome", with a shallower depth offshore than nearshore. This configuration is accompanied by a lake-wide cyclonic circulation. Lake-wide observations of subsurface temperature in central Lake Erie revealed an atypical "depressed" or "bowl-shaped" thermocline in late summer, with a deeper thermocline in the middle of the lake and a shallower thermocline nearshore. Currents measured in the central basin when the bowl-shaped thermocline was observed were anticyclonic, forming a single basin-wide gyre. It is suggested that the unusual bowl-shaped thermocline is the result of Ekman pumping driven by anticyclonic vorticity in surface winds. The bowl-shaped thermocline can lead to greater hypoxia in bottom waters and negative effects on biota by reducing the hypolimnetic volume.

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