Characterizing microplastic pollution in aquatic sediments from Southwestern British Columbia, Canada

Microplastics are an emerging environmental pollutant of concern because of their potential effects on biota, their ubiquity, and their persistence in the environment. Characterizing their occurrence, composition, and spatial and temporal trends in different aquatic sedimentary environments will provide insight to the likely sources of microplastics and help diagnose relationships with sedimentary processes. Our study analyzed microplastics and sedimentary properties from six cores from different sedimentary environments in southwestern British Columbia (BC), spanning urban and remote locations. These sites included estuarine environments from the protected area of Clayoquot Sound, lacustrine sediments in Orchid Lake from the protected Metro Vancouver watershed, and estuarine sediments from urban Boundary Bay. We also examined temporal changes in microplastic accumulation rates in Clayoquot Sound using 210Pb dated sediment cores. We detected microplastics within all sediment cores, supporting previous research suggesting the ubiquity of microplastics in aquatic sedimentary environments. We examined the sediments for all forms of microplastics and found only microfibers that were predominantly black and blue. After applying corrections for contamination, we calculated concentrations and accumulation rates for the surface 10 cm in each core (the depth over which microplastics were found). Mean (+/- SE) microplastic concentrations (#particles/kg) in Clayoquot Sound were 288 ± 92 particles/kg, approximately 2.5x greater than in Boundary Bay (114 ± 61 particles/kg) and 1.3x greater than Orchid Lake (223 ± 188 particles/kg). When compared with 42 sites from marine sedimentary environments across the globe, we found microplastic concentrations at our sites were of a similar order of magnitude to 79% of surveyed sites, with mean/median microplastic concentrations ranging from 0 to 500 particles/kg. Microplastic accumulation rates in Clayoquot Sound increased from mean (+/- SE) accumulation rates of 7 ± 3 particles/100cm2/year in 1950 to 33 ± 12 particles/100cm2/year in 2016. While atmospheric transport is the most likely source of microplastic deposition at Orchid Lake, multiple sources are possible at the other two locations. In Clayoquot Sound, the proximity of our sites to wastewater effluent and aquaculture sites points to these activities as potential sources. At both Clayoquot Sound and Boundary Bay, tidal currents introducing marine sources of microplastics is also likely.