A critical review of environmental factors influencing the transport dynamics of microplastics in riverine systems: implications for ecological studies

Microplastics (MPs) in rivers present an ecological risk. In this paper, we review hydro-geomorphological, biological, and allochthonous factors that may influence the distribution and transport of MPs in riverine systems. We also review MPs characteristics that may impact their distribution and transport. At the reach scale, hydraulic biotopes and their key features such as flow velocity, bed roughness, depth, and channel morphology are important features that shape the distribution and transport of MPs in riverine systems and should be considered in the design of MPs studies. Microbial-MPs interaction may impact MPs density, aggregation and thus transport dynamics. Instream vegetation may act as a physical trap of MPs, which may impact their horizontal transport and aggregation. Lateral transport of MPs is impacted mostly by precipitation, run-off, point and non-point discharges. The polymer density, size and shapes of MPs are critical factors that influence their transport dynamics in riverine systems. Microplastic sampling protocols should be designed to reflect hydro-geomorphological considerations. The unique interaction of MPs physical characteristics and hydraulic biotopes creates differential exposure of riverine organisms to MPs and should be used to unravel potential impacts. Biomonitoring studies should integrate the complex MPs-hydraulic interaction for ecologically meaningful investigation into organismal exposure to MPs in their preferred biotopes. Overall, our review indicates the influences of hydro-geomorphological features on the transport dynamics of MPs and their ecological significance for the study of MPs in rivers.