MS Thesis Defense
Microplastics in the surface water and sediments of western Lake Superior as determined via microscopy, Pyr-GC/MS, and FTIR
While the presence of plastic pollution is well known in the world’s oceans and is beginning to be documented in the world’s freshwater systems, there is not yet an in-depth understanding of the distributions, chemical compositions, fates and ecological impacts of plastic particles in most aquatic systems. Microplastic particles are of particular concern due to their direct biological effects (such as false satiation), their roles as sorbents of other chemical compounds, and as vectors for invasive species. In this study, we evaluate the magnitude, distribution, and common polymers of microplastic pollution in surface waters and sediments in western Lake Superior, the deepest and most pristine of the Laurentian Great Lakes. Microscopy, Pyrolysis-Gas Chromatography/Mass spectrometry (Pyr-GC/MS), and Fourier Transform Infrared spectroscopy (FTIR) was used to quantify and identify microplastic particles. Despite the low human population density in Lake Superior’s watershed, microplastic particles (particularly fibers, fragments, and films) were identified in western-lake surface waters at levels significantly greater than those previously reported in Lake Superior’s eastern basin (p-value < 0.05). Microplastic concentrations in western-lake surface waters were found to range from 11,000-120,000 particles•km-2 (n=15, mean: 43,000 particles•km-2, standard deviation: 28,000 particles•km-2, and 95% confidence interval: ±15,000 particles•km-2). The average abundance is similar to average surface-water values in the North Atlantic and South Pacific gyres, and almost an order of magnitude lower than the average value for the North Pacific gyre. Fibers were the most frequently observed morphology in lake surface waters and sediments. The most common polymer in surface waters and sediments was PVC; for surface waters, PP and PE were the next most frequently observed, and PET was the only other polymer observed in sediments. Our ability to evaluate microplastic abundances in Lake Superior’s waters and sediments was limited by the challenges due to contamination from atmospheric deposition of microplastics during sampling and sample processing. Sediment analyses were also limited by the low sample area of sediment that can be obtained with coring techniques that also maintain the sediment-water interface.