Microplastics are settling into Ontario cottage country lakes

Until recently, microplastics research was focused on the ocean environment, where plastic litter conspicuously accumulates. We know that microplastics have a variety of negative impacts on marine organisms when ingested, in some cases leading to mortality. Microplastics have also been found in seafood products, suggesting exposure to humans through consumption.

At a global scale, microplastics research had largely overlooked fresh waters because they were assumed to act as mere conduits, with lakes and rivers conveying terrestrial microplastics to marine environments. In reality, microplastics are more like an invasive species in our fresh waters.

An investigation on the fate of microplastics within three rural lake catchments in Ontario’s cottage country found that, on average, 57 per cent of incoming microplastics were retained in the terrestrial environment surrounding the lake. Of the remaining 43 per cent that entered the lake, 37 per cent stayed put.

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This is the first study to link atmospheric inputs — when microplastics arrive in fresh water from the air — to lake catchment outputs, or the microplastics that leave the lake environment. We did this using a novel microplastic budget, which is similar to an annual financial budget: inputs (deposits) into a lake (account) and outputs (withdrawals) are tracked during the year. This allowed us to show microplastics are not just passing through these lakes but can reside for more than a decade.

We showed for the first time that atmospheric inputs are the dominant pathway of microplastics to rural catchments in Ontario and inputs increase with a rise in anthropogenic (human) activity. Lakes with more residents and higher recreational activity had a higher input of atmospheric microplastics compared with lakes that had limited access and no permanent dwellings.

The movement of microplastics

We know microplastics are present in every environment across the globe but, despite all of the media attention and growing research on these particles, we still know little about their abundance and characteristics — their size, shape and type of plastic polymer — in different ecosystems. Understanding how microplastics move within and between different ecosystems is crucial to managing them.

Microplastics are light enough to be suspended in the atmosphere and transported long distances by the wind. Not surprisingly, they have been detected in rainfall across the globe, with research suggesting the atmosphere is an important pathway for dispersing microplastics globally between urban and remote locations.

Several studies have demonstrated the multidirectional movement of microplastics between air, water and soil, which has highlighted the importance of incorporating terrestrial, freshwater and atmospheric systems into the cycle. Catchment budgets have been widely used to understand the fate of pollutants across air, soil and water but, until now, have not been used to describe the movement of microplastics.

Using this novel approach for microplastics, we looked at fluxes between air, soil and water. In collaboration with the Ontario Ministry of Environment, Conservation and Parks at the Dorset Environmental Science Centre, we conducted comprehensive field sampling of lake inflows and outflows, as well as sediment and atmospheric inputs at their monitored lake catchments.

We found a large proportion of microplastics were retained on land, and almost half of the microplastics in streams fed by rainfall were subsequently retained in the lakes. We also showed microplastics can reside in rural lakes for more than a decade.

Microplastics on a global scale

Our research builds on a recent large-scale international study using standardized methods to assess microplastics in lakes across the globe.  As plastic production and disposal has exponentially increased since the 1950s, we expect the concentration of microplastics in fresh water will do the same within the coming decades. From a management perspective, we need to better understand the ‘microplastic cycle,’ as this will aid in the development of policy aimed at reducing emissions and mitigate the movement of microplastics among terrestrial, freshwater and atmospheric systems.

Our work has raised additional questions about how microplastic retention rates vary between lakes in urban and remote settings, the importance of snow melt and spring runoff on annual microplastic budgets and the influence climatic conditions, such as the formation of snow and ice, may have on microplastic concentrations in lakes. We hope the recent studies on microplastics in fresh water will spur more research in this area to better inform solutions for protecting and conserving these ecosystems around the world.