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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Corcoran, Patricia L.

Abstract

The awareness of and the data on the prevalence of microplastic (plastic particles <5mm) pollution in freshwater environments is rapidly increasing, as low-degrading polymers are being detected in various environmental matrices of the Laurentian Great Lakes. However, the accumulation, distribution and deposition of microplastics in offshore depositional environments of the Great Lakes, and particularly Lake Huron, is relatively unknown. In this study, benthic sediment from various Lake Huron waterbodies (main basin, Georgian Bay, the North Channel, and Saginaw Bay) was quantified for microplastic particles (fibres, fragments, films, and beads). The North Channel contained the greatest microplastic abundances, averaging 47,398 particles per kg-1 dry weight sediment (p kg-1 dw), followed by Georgian Bay (21,390 p kg-1 dw), the main basin (15,910 p kg-1 dw) and Saginaw Bay (1,592 p kg-1 dw). The results suggest that microplastic abundances in offshore settings are positively correlated with increasing water depth (p=0.004) and are controlled by lake bottom geomorphology. Hydrodynamic processes are a prevailing force driving microplastic dispersion and deposition into the offshore, in contrast to source-based drivers closer to the shoreline. Sediment cores were examined from Lake Huron and Lake Ontario, for which historical microplastic accumulation rates were determined using dated sediment profiles. The 210Pb dating method was used to establish a chronology for the Lake Huron sediment core (LH43), and approximate chronologies were constructed for the Lake Ontario cores (403A and 209C) using previously documented sedimentation rates using the polonium distillation method. The results suggest that microplastics have been accumulating in offshore benthic sediment of Lake Huron and Lake Ontario for ~75 years and ~80 years, respectively. Concentrations at depth (0-15 cm) reveal regional and temporal trends, consistent with increased plastic production since the 1950s. Significant increases in abundance were observed from the early 1950s to the late 1970s, and from the late 1980s to 2014. This implies that surface and subsurface sediment is an effective indicator of past and present microplastic contamination in the Great Lakes. Results from this work provide a benchmark for future microplastic assessments in the Lake Huron basin, and the wider Great Lakes system.

Summary for Lay Audience

Microplastics (plastic particles <5mm) are ubiquitous contaminants of concern commonly found in terrestrial, marine and freshwater environments. These slowly-degrading polymers have been detected in shoreline, tributary, nearshore and offshore surface and subsurface water and sediment of the Laurentian Great Lakes. The accumulation, distribution and deposition of microplastics in offshore depositional environments of Lake Huron have not been reported prior to the present investigation. This study examined microplastics (fibres, fragments, films and beads) in 76 benthic surface sediment samples from various Lake Huron waterbodies including Georgian Bay, the North Channel, Saginaw Bay, and the main basin. Microplastics were detected in every offshore sampling location, with the greatest abundances in the North Channel, averaging 47,398 particles per kg-1 dry weight sediment (p kg-1 dw), followed by Georgian Bay (21,390 p kg-1 dw), the main basin (15,910 p kg-1 dw) and Saginaw Bay (1,592 p kg-1 dw). The results indicate that prevailing surface water currents are significant forces controlling microplastic transport and deposition in the lake. Abundances were positively correlated with increased water depths, suggesting depositional lake environments function as sinks for microplastics accumulation. In addition, microplastic particles were identified down to a maximum of 15 cm in dated sediment cores from Lake Huron and Lake Ontario, suggesting that microplastic particles have been accumulating in sediment of Lake Huron and Lake Ontario for ~75 years and ~80 years, respectively. The sediment cores also reflect temporal depositional changes within the Lake Huron and Lake Ontario basins, as increased microplastic concentrations are consistent with plastic production trends. This study establishes a baseline of past and current microplastic inputs into the Lake Huron offshore depositional environment and is the first to examine microplastic depositional trends between Great Lakes sediment profiles.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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