Microplastic pollution in Lake Superior: A comprehensive study of distribution, abundance and variability

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 Simon Courtenay, School of Environment, Resources and Sustainability

 Kara Cox, School of Environment, Resources and Sustainability

Introduction

Microplastic particles in the aquatic environment have been a growing concern since the early 1990s. Most research on microplastic pollution has occurred in marine ecosystems, but microplastics are also widespread throughout fresh waters. Average microplastic concentrations reported in the Laurentian Great Lakes have been as high as or greater than that reported from some oceanic gyres.

Most Great Lakes microplastics research has focused on the more populated and industrialized lakes, Lakes Erie, Michigan and Ontario. Lake Superior, the largest of the five lakes, is often considered to be relatively pristine due to its low population density and largely forested watersheds. This research is the first comprehensive, full lake assessment of microplastics in Lake Superior assessing their abundance, particle size and source. In addition, the study used side-by-side, paired nets to determine if single net sampling produced a representative estimate of microplastic particle abundance and distribution within a large body of water.

Methodology

Surface water sampling for floating plastic particles was conducted from May to July 2014 at nearshore and offshore sites on Lake Superior using paired three-meter-long nets. A total of 94 paired samples were collected yielding a total of 187 individual samples. Plastic particles were found in all 187 samples, yielding a total of 3887 particles (Figure 1).

Figure 1: Average surface water plastic abundance (particles/km²) between two paired samples collected at 94 locations across Lake Superior during summer 2014. For analytical purposes, sampling regions were divided: east or west of the Keweenaw Peninsula (black dotted line), north–south of the USA/Canada border (black solid line), and nearshore-offshore, based on the approximate location of the 100 m bathymetric depth zone around the lake (grey dotted line).

Samples were first emptied into a set of two stacked stainless-steel sieves (0.355 mm and 4.75 mm), with solids within the largest size fraction being manually sorted to remove visible plastic debris. Solids in the smaller fraction were further sieved to separate particles by size fraction. Using a dissection microscope, all microplastic particles were removed, enumerated, categorized by morphology as fragment, pellet, line/fiber, film, or foam. Fourier Transform Infrared spectroscopic analysis was conducted on about 10 per cent of the total particles counted to identify polymers.

To determine if the spatial heterogeneity between the side-by-side nets was statistically significant a paired T-test was performed. Plastic abundance per square kilometer of lake surface was calculated based on the surface area sampled and the number of plastic particles in each sample. Relative comparisons in plastic particle abundances across the lake were made in relationship to predominant currents, cities, north–south, east–west and nearshore-offshore zones (Figure 1).

Outcomes

There was no difference in mean plastic abundances between the two individual samples simultaneously collected at each site, indicating single net sampling methods can produce representative samples.

The abundance of microplastic particles was relatively homogeneous across the surface of Lake Superior with 54 per cent and 36 per cent of locations sampled having an abundance between 20,001–50,000 and 10,001–20,000 count/km² respectively accounting for 90 per cent of all sample locations. Across the lake surface, microplastic abundance averaged ~30,000 particles/km². Based on this average and the surface area of Lake Superior, a total count of more than 2.5 billion plastic particles across the surface area of the lake was calculated (Figure 1).

The majority of microplastic particles collected were contained in the smallest size fraction (62%) with fibers/lines being the most common morphology (67%). Fragments (23%) were the next most common morphology, followed by films (9%). Of the pellets, only a few matched the size, shape and colour of microbeads commonly associated with personal care products (Table 1).

Table 1. Average plastic abundances (count/km²) for Lake Superior distinguished by size and particle type

Average particle abundance was higher in the north, west, and nearshore regions. Areas of high calculated abundances may be attributed to currents, water circulation patterns and proximity to large population centers (Table 2).

Table 2. Average plastic abundances (count/km²) by size classifications for various location designations on Lake Superior. Designations are visualized in Figure 1.

The most common polymers identified were polyethylene and  polypropylene which accounted for 50 per cent and 20 per cent of all particles identified respectively. (Table 3).

Table 3. Summary of spectroscopic results on retrieved pelagic plastic particles. PE = polyethylene; PP = polypropylene; PEST = polyester; PA = polyamide; PS = polystyrene.

Conclusions

This research represents the first comprehensive study of surface water microplastic pollution in Lake Superior. Lake Superior had higher average abundances than Lake Michigan despite its larger size and overall lower industrial and population densities, which may be attributed to Lake Superior’s larger surface area, longer residence time, and perhaps sampling and analytical methodology differences among studies. Conversely, average abundances for Lake Superior were less than those previously estimated for Lakes Huron, Erie and Ontario, which are likely due to an additive effect of these lakes being downstream of Lake Superior and their higher population densities and industry. While Lake Michigan is technically downstream of Lake Superior, the dominant water flow from Lake Superior is through Lake Huron into Lake Erie. Similar to other studies, the highest concentration of particles was found in the smallest size fraction, with the most common polymer types being polyethylene and polypropylene. As these are the most produced polymers worldwide, it is not surprising that they would be the most prominent within these field samples.

Given Lake Superior’s large size, volume of water and long residence time, continued monitoring of microplastic pollution will be required to understand the overall health of the lake and the status of microplastic debris. Climatic conditions may affect microplastic abundance, distribution, and transport, including warming caused by climate change, seasonal changes in currents, as well as changes in population and industry along the shoreline. Monitoring how these conditions impact not only the lake but pollution within the lake will increase the overall understanding of how microplastics interact with the surrounding ecosystem and may provide insight into management practices which can help protect the overall health of Lake Superior.

Finally, the study found no statistically significant difference between the paired net samples, indicating that single net sampling should produce a representative estimate of microplastic particle abundance and distribution within a body of water.

Cox, K., Brocious, E., Courtenay, S.C., Vinson, M.R. & Mason, S.A. (2021). Distribution, abundance and spatial variability of microplastic pollution on the surface of Lake Superior,  Journal of Great Lakes Research, 47(5), 1358-1364. https://doi.org/10.1016/j.jglr.2021.08.005

For more information about WaterResearch, contact Julie Grant.

Microplastics photo by Florida Sea Grant is licensed under CC BY-NC-ND 2.0.