Microplastic deposition velocity in streams follows patterns for naturally occurring allochthonous particles

Abstract

Accumulation of plastic litter is accelerating worldwide. Rivers are a source of microplastic (i.e., particles <5 mm) to oceans, but few measurements of microplastic retention in rivers exist. We adapted spiraling metrics used to measure particulate organic matter transport to quantify microplastic deposition using an outdoor experimental stream. We conducted replicated pulse releases of three common microplastics: polypropylene pellets, polystyrene fragments, and acrylic fibers, repeating measurements using particles with and without biofilms. Depositional velocity (v dep ; mm/s) patterns followed expectations based on density and biofilm ‘stickiness’, where v dep was highest for fragments, intermediate for fibers, and lowest for pellets, with biofilm colonization generally increasing v dep . Comparing microplastic v dep to values for natural particles (e.g., fine and coarse particulate organic matter) showed that particle diameter was positively related to v dep and negatively related to the ratio of v dep to settling velocity (i.e., sinking rate in standing water). Thus, microplastic v dep in rivers can be quantified with the same methods and follows the same patterns as natural particles. These data are the first measurements of microplastic deposition in rivers, and directly inform models of microplastic transport at the landscape scale, making a key contribution to research on the global ecology of plastic waste.

Is the Hyporheic Zone Relevant beyond the Scientific Community?

“…Hotspots with considerable concentrations were commonly found in the HZ. This result coincides with a recent experimental study showing that microplastics are transported similarly to naturally occurring allochthonous particles [200], which are known to develop hotspots in river corridors. Finally, deposit feeders seem to affect microplastic transport into the HZ [201] and flood events can partially remobilize microplastics retained in river sediments [202].…”

A Global Perspective on Microplastics

“…Identification of microplastics in the environment presents a multidimensional challenge that is yet unmet. Their complexity parallels that of naturally occurring, particulate organic matter (Hoellein et al, ). Plastics (and thus microplastics) vary in chemical composition, physical form, size, texture, and shape.…”

“…However, it is likely that principles gleaned from studies of naturally occurring particulates of comparable densities are applicable, especially as microplastics weather and accumulate biofilms. In terms of behavior after release, Hoellein et al () observed that microplastics mimicked natural particles in terms of deposition in an experimental stream. They argued that existing studies on particulate organic matter could provide insights on microplastic behavior and fate.…”

Macroplastic Storage and Remobilization in Rivers

“…The processes of transport, deposition and remobilization of mineral and organic sediments are controlled by their physical properties (e.g., density, size) and their relation to river size and hydrodynamics [42][43][44][45]. Recent insights into interactions of macroplastic debris with fluvial processes documented by laboratory experiments [46,47] and the monitoring of floating [24,26,27] and deposited macroplastic debris [48] suggest that transport, storage and remobilization of macroplastic debris are also partly related to fluvial processes. The main similarities between macroplastic and larger particles of organic debris (e.g., seeds, vegetation propagules, large wood pieces) encompass their relatively low density and large surface area determining their transport by water in flotation (see [45]).…”