Time trends in abundance and composition of microplastic particles deposited in profundal sediment of two headwater reservoirs within the Grand River Watershed (Ontario, Canada)

Abstract

Plastic pollution has become pervasive in the environment, raising concern for degradation of aquatic ecosystems by microplastics (MPs). Studies on the supply and abundance of MPs in Canadian freshwaters are rapidly emerging, however temporal trends spanning several decades remain sparse. In this study, we report multidecadal records of MP abundance and composition in dated sediment cores from two headwater reservoirs (Belwood Lake and Conestogo Lake) located within an agricultural region of the rapidly urbanizing Grand River Watershed (GRW; southern Ontario, Canada), a major tributary of Lake Erie. Extracted MPs from contiguous 1-cm thick intervals of sediment cores were enumerated and categorized by shape (fragment, fiber). A subset of samples at approximately decadal intervals were chemically characterized using Laser Direct Infrared (LDIR) Spectroscopy. Results reveal that MP concentrations in both reservoirs varied within a similar range (~50-550 particles g/dw) with no observed increasing trend since the start of each record (1957 for Belwood Lake, 1985 for Conestogo Lake). MP flux in Conestogo Lake increased from ~50 particles/cm^2/year in the mid-1980s to ~100 particles/cm^2/year in recently deposited sediment, whereas MP flux varied without a trend between ~5-60 particles/cm^2/year in Belwood Lake since 1957, apart from a peak in the uppermost sample (~150 particles/cm^2/year). More rapid sediment deposition at Conestogo Lake accounts for the difference in MP flux between the reservoirs, suggesting reduction of sediment transport could reduce the supply of MPs to aquatic ecosystems in the GRW. Analysis by LDIR revealed that polyamide, rubber, and polyethylene were the most abundant polymer types in both reservoirs. The relative abundance of rubber particles has increased since the 1990s, indicating an increase of paved road surface and vehicle traffic as a potential source. The findings suggest MP accumulation in these upstream rural reservoirs of the GRW may be primarily driven by local, regional, and hydrologic factors instead of the rise in global plastic production.