Investigating the spatial and temporal distribution of microplastics in the Fraser River and Burrard Inlet (British Columbia, Canada)

Metropolitan areas, such as Metro Vancouver, are major contributors to microplastic (MP) pollution in adjacent receiving water bodies, including the Fraser River and Burrard Inlet (BC, Canada). Through tidal exchange and hydrological events, these MPs may be exported to the Strait of Georgia (SoG) and eventually to the North Pacific Ocean. Yet, MPs spatiotemporal variability within these dynamic riverine systems and shallow-sided fjords is not well understood. To address this knowledge gap, MPs (>10 µm) were examined at multiple depths in the Fraser River (0.5 and 6 m) and Burrard Inlet (0.5, 30, and 60 m) from January to November 2024, using a newly constructed multichannel MPs sampling apparatus and adapted McLane large-volume pumps. To elucidate potential sinks of MPs in Burrard Inlet, the vertical distribution of MPs in a sediment core was determined. To advance current MP processing efforts, a novel method was developed to extract and quantify MPs from various environmental matrices. Findings revealed pronounced seasonal patterns in the distribution of MPs in the Fraser River and Burrard Inlet. Highest MP concentrations were observed in springtime, and attributed in the Fraser River to the freshet, and in Burrard Inlet to the influx of freshwater from the North Shore mountains snowmelt coupled with the Fraser River freshet inputs through First Narrows. MPs in the Fraser River and Burrard Inlet were primarily small (< 100 µm, ~ 63% of the total MPs), and fragments (85% of the total). In Burrard Inlet, MPs concentrations (mean ± SE) were 58 ± 6 particles per cubic meter of water (MP particles m-3), with no significant difference (p > 0.05) among mean MP concentrations in all three depths (0.5, 30 and 60 m). In the Fraser River, mean MP concentrations were 81 ± 19 MP particles m-3, with concentrations significantly higher at 6 than 0.5 m (p < 0.05), associated with faster water velocities and sediment resuspension. The Fraser River exports 9.88 trillion MPs to the SoG annually, with 64% exported during the spring freshet. This study provides valuable insight for predicting MPs behavior in complex and dynamic coastal and riverine systems.