Hidden plastics of Lake Ontario, Canada and their potential preservation in the sediment record
Introduction
Plastic debris pollution remains a significant environmental issue because of its persistence on a global scale. Although the sources of plastic items are anthropogenic and thus originate on land, the extent of plastics pollution only became apparent once plastic debris reached Earth's oceans and became more visible in surface waters and along shorelines (e.g. Carpenter and Smith, 1972, Colton et al., 1974, Gregory, 1977, Morris, 1980, Dixon and Dixon, 1983, Ryan and Moloney, 1993, Moore et al., 2001). The dangers of plastic debris in marine environments have been well-documented. Recent examples demonstrating the effects of plastic on marine organisms point to ingestion (e.g. Denuncio et al., 2011, Possatto et al., 2011, Bond et al., 2014, Bravo Rebolledo et al., 2013, Van Cauwenberghe and Janssen, 2014) and entanglement (e.g. Laist, 1997, Sazima et al., 2002, Votier et al., 2011, Yorio et al., 2014) as the major threats. Plastics also assist in the transfer of persistent organic pollutants (POPs) that may travel up the food chain (Endo et al., 2005; Rios et al., 2007; Colabuono et al., 2010, Rochman et al., 2013, Koelmans et al., 2014). In addition, floating plastic debris acts as transport media for encrusting organisms that may become invasive species (e.g. Winston, 1982, Barnes, 2002, Gregory, 2009). In contrast, relatively little is known about plastics pollution in fresh- or mixed-water settings. Characterization and quantification of plastic debris items in rivers (Lechner et al., 2014, Morritt et al., 2014, Rech et al., 2014, Sanchez et al., 2014, Castañeda et al., 2014) and estuaries (Browne et al., 2010, Lima et al., 2014, Yonkos et al., 2014) indicate that these are significant pathways for polymers travelling to larger bodies of water. Plastics accumulation in lakes remains poorly understood because only a minor amount of investigations have been conducted (Zbyszewski and Corcoran, 2011, Faure et al., 2012, Imhof et al., 2013, Zbyszewski et al., 2014, Free et al., 2014, Driedger et al., 2015, Hoellein et al., 2015), and factors such as seasonal changes in surface water currents, locations of urban centres, and river and wastewater input are amplified by the relatively small size of a lake compared with an ocean.
The types and distribution of plastics in open water and shoreline regions of the Great Lakes system of North America are relatively unknown. Available results show that polyethylene (PE) and polypropylene (PP) are the most common polymer types (Zbyszewski and Corcoran, 2011, Zbyszewski et al., 2014), the majority of the plastic items are <5 mm in size (Zbyszewski and Corcoran, 2011, Eriksen et al., 2013), and POPs were found sorbed to the surfaces of plastics (International Pellet Watch, 2005–2013; L. Rios, unpublished data). This information was provided through surveys of Lakes Superior, Huron, Erie and St. Clair. However, until 2014, the only available data concerning plastics pollution of Lake Ontario was provided through the Great Canadian Shoreline Cleanup and the Alliance for the Great Lakes Adopt-a-Beach Program. The latter indicates that from September, 2012 to August, 2014, 46% of visible debris items collected from shorelines was composed of plastic (Alliance for the Great Lakes, 2012–2014). Dreidger et al. (2015) combined the Adopt-a-Beach and Great Canadian Shoreline Cleanup data and found that 77–90% of all shoreline debris collected in 2012 was composed of plastic items.
To date, Castañeda et al. (2014) in their investigation of the St. Lawrence River, Canada, are the only researchers who have described microplastics in bottom sediments of a non-oceanic body of water. The primary objective of this paper is to provide quantitative and compositional results of microplastic (<5 mm) debris items sampled from shoreline and lake-bottom sediments of Lake Ontario, Canada (Fig. 1), and to assess their potential for preservation in the current sediment and future rock record.
Section snippets
Study areas
Lake Ontario is the smallest of the Laurentian Great Lakes with an average depth of 86 m, and a land drainage area of 64,030 km2 (U.S. Environmental Protection Agency, 2012). The summer (May–October) surface water circulation pattern is mainly cyclonic, whereas during the winter months (November–April), the lake exhibits a two-gyre circulation pattern with cyclonic flow in the south and east, and anti-cyclonic flow in the northwest (Beletsky et al., 1999).
The Humber Bay region, located along
Humber Bay plastics abundance, accumulation and composition
A total of 6172 pieces of plastic debris were collected from the Humber Bay Park West beach site over the course of the sampling period (Fig. 3A). A large quantity of expanded polystyrene was sampled, but was only quantified in terms of mass. Due to the fragility of this type of polymer and the large quantity collected, quantification in terms of number of individual pieces was not practical. Excluding polystyrene, the most common type of debris on the three sampling dates was industrial
Tributaries as transport pathways
The quantity of plastic debris per unit area along the Humber Bay beach site is the second highest observed on any Great Lakes beach surveyed to date, with the initial survey yielding 21.8 items/m2 (excluding polystyrene). The 16.3 pellets/m2 collected were exceeded only by the 34 pellets/m2 collected from Sarnia Beach along the Lake Huron shoreline (Zbyszewski et al., 2014). An average of 10.5 pellets/m2 was collected from the Humber River site, and while sampling, we observed several pellets
Conclusion
This study showed the abundance, accumulation rates, and compositions of microplastic particles from 4 sampling sites in Lake Ontario. The accumulation of industrial pellets along the shoreline of Humber Bay was dependent on weather conditions and the presence of beached organic debris along the strandline. More pellets accumulated on the beach following a period of increased rainfall events than following fair-weather conditions. Increased precipitation led to higher flow volume in the
Acknowledgements
We thank the University of Western Ontario, the Ontario Ministry of the Environment, and Environment Canada for providing funding and samples for the investigation of microplastics in the Humber River area and Lake Ontario bottom sediment. We also thank Mansour Al- Hashim for his assistance in the field, as well as Alexandra Pontefract, Gordon Osinski and Sean Shieh (Earth Sciences, University of Western Ontario) for providing equipment and assistance with the Raman analysis.
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