Quantifying the Size Fraction of Microplastic Debris Throughout the Water Column of Western Lake Superior

Abstract

Plastic pollution is a widely acknowledged global problem that has various negative impacts on the environment. As of 2019, 9.2 billion tonnes of plastic have been produced, and roughly 80% of plastic has become waste. Poor waste management, combined with the designed durability of plastics, has allowed plastic pollution to exponentially accumulate in aquatic systems. Once in the environment, plastic debris can weather and break down into smaller particles known as micro- and nano-plastics. This work determines the distribution of microplastics within different size fractions at different depths in the water column at four sites in Western Lake Superior. Samples were collected in May and August of 2021 with both manta net trawls, which collect microplastics >333 µm at the air-water interface, and McLane pumps fitted with three filter sizes to sample the water column. Microplastics were detected with visual microscopy or FTIR microscopy, depending on the size class. Our research shows that the Lake Superior water column contains large amounts of microplastic waste, with an increasing number of microplastic particles per volume as particle size decreases. As well as a decreasing number of microplastics per volume as depth increases. Additionally, the spatial distribution of microplastics in Lake Superior also seems to fluctuate throughout the year as the characteristics of the water column change seasonally. Overall, this is the first study to collect bulk samples from the water column of any Laurentian Great Lakes and the first to report the size fractionation of microplastic debris within the water column; therefore, furthering our understanding of the plastic pollution problem in the Great Lakes.