The influence of river and tidal processes and anthropogenic inputs on the distribution of microplastics in the lower Fraser River, Canada

The fluvio–tidal transition in rivers encompasses the region where freshwater flow is influenced by tides and/or mixes with salt water. Fine-grained sediment typically accumulates in these dynamic environments, but the distribution of microplastics (MP) therein is poorly understood. In this study, we map the distribution of MPs in the fluvio–tidal transition of the Fraser River, Canada. The Fraser River is the largest river debouching onto Canada’s west coast, and it flows through both rural and urban landscapes. A total of 39 samples were collected from the freshwater non-tidal, freshwater tidal, and brackish water tidal zones. Substrate characteristics, including grain size, sorting, total organic content, and salinity were measured, and MPs were analyzed using Laser Direct Infrared spectroscopy to confirm polymer composition, particle size and morphology. Results show that the MP concentration increases in a seaward direction from 6974 particles/kg in the freshwater non-tidal zone, to 9345 particles/kg in the freshwater and tidal zone, and then to 10,150 particles/kg in the brackish-water tidal zone. MP abundance was most variable in the freshwater and tidal zone (2079–64,635 particles) and least variable in the freshwater non-tidal zone (4158–9562 particles). No significant correlations were observed between MP abundance and total organic carbon or the percentage of mud, indicating that tidal hydrodynamics play a stronger role than sediment composition in MP distribution. The exceptionally high MP concentration at Derby Reach (64,635 particles) within the freshwater tidal zone likely results from it being the only sample site that is occupied regularly by visitors to the river. These findings indicate that MP accumulation in the fluvial-tidal zone is determined primarily by anthropogenic activity and then by hydrodynamic processes, while factors influencing particle settling, such as grain shape or density, play a comparatively minor role in determining MP distributions.