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Abstract Plastic litter is accumulating in ecosystems worldwide. Rivers are a major source of plastic litter to oceans. However, rivers also retain and transform plastic pollution. While methods for calculating particle transport dynamics in rivers are well established, they are infrequently used to quantify the transport and retention of microplastics (i.e., particles < 5 mm) in flowing waters. Measurements of microplastic movement in rivers are needed for a greater understanding of the fate of plastic litter at watershed and global scales, and to inform pollution prevention strategies. Our objectives were to (1) quantify the abundance of microplastics within different river habitats and (2) adapt organic matter “spiraling” metrics to measure microplastic transport concurrent with fine particulate organic matter (FPOM). We quantified microplastic and FPOM abundance across urban river habitats (i.e., surface water, water column, benthos), and calculated downstream particle velocity, index of retention, turnover rate, and spiraling length for both particle types. Microplastic standing stock was assessed using a habitat‐specific approach, and estimates were scaled up to encompass the study reach. Spatial distribution of particles demonstrated that microplastics and FPOM were retained together, likely by hydrodynamic forces that facilitate particle sinking or resuspension. Microplastic particles had a higher downstream particle velocity and lower index of retention relative to FPOM, suggesting that microplastics were retained to a lesser degree than FPOM in the study reaches. Microplastics also showed lower turnover rates and longer spiraling lengths relative to FPOM, attributed to the slow rates of plastic degradation. Thus, rivers are less retentive of microplastics than FPOM, although both particles are retained in similar locations. Because microplastics are resistant to degradation, individual particles can be transported longer distances prior to mineralization than FPOM, making it likely that microplastic particles will encounter larger bodies of water and interact with various aquatic biota in the process. These empirical assessments of particle transport will be valuable for understanding the fate and transformation of microplastic particles in freshwater resources and ultimately contribute to the refinement of global plastic budgets.
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The Fluid Mechanics of Ocean Microplastics
Microplastic pollution is now ubiquitous in marine environments, posing risks to ecosystem and human health. In order to assess and mitigate this threat, we require accurate prediction of microplastic fate and transport in the ocean. While progress has been made studying global-scale transport pathways, our models often fall short at smaller scales; processes such as vertical transport, horizontal dispersion, particle transformation, and boundary fluxes (e.g., at beaches and the air–sea interface) remain poorly understood. The difficulty lies in the physical features of plastic particles: namely, near-neutral buoyancy in seawater, finite size, and irregular shape. These complexities are compounded by the multiscale forcing from waves and turbulence near the ocean surface where microplastics tend to reside. This review synthesizes recent advances in the fluid dynamics of marine plastic transport, emphasizing the role of fluid–particle interactions in ocean flows and highlighting outstanding challenges.
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- PAR ID:
- 10661748
- Publisher / Repository:
- Annual Review of Fluid Mechanics
- Date Published:
- Journal Name:
- Annual Review of Fluid Mechanics
- Volume:
- 58
- Issue:
- 1
- ISSN:
- 0066-4189
- Page Range / eLocation ID:
- 355 to 382
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1146/annurev-fluid-120423-012604
