The missing ocean plastic sink: Gone with the rivers

Abstract

Plastic floating at the ocean surface, estimated at tens to hundreds of thousands of metric tons, represents only a small fraction of the estimated several million metric tons annually discharged by rivers. Such an imbalance promoted the search for a missing plastic sink that could explain the rapid removal of river-sourced plastics from the ocean surface. On the basis of an in-depth statistical reanalysis of updated data on microplastics—a size fraction for which both ocean and river sampling rely on equal techniques—we demonstrate that current river flux assessments are overestimated by two to three orders of magnitude. Accordingly, the average residence time of microplastics at the ocean surface rises from a few days to several years, strongly reducing the theoretical need for a missing sink.

Addressing the importance of microplastic particles as vectors for long-range transport of chemical contaminants: perspective in relation to prioritizing research and regulatory actions

“…Their reanalysis results in the average residence time of MPs at the ocean surface increasing from days to years, which they suggests strongly reduces "the theoretical need for a missing sink" [40], an observation that is also consistent with Pabortsava and Lampitt [36]. Nevertheless, Weiss et al [40] acknowledge that their estimates are based on underlying data that contain significant uncertainties, but that their observations are perceived to be better aligned with an understanding that surface oceanic MP stocks, particularly within oceanic gyres, result from slow accumulation processes, an observation that is inconsistent with suggestions that there exists relatively rapid environmental fate processes that might help to sequester surface oceanic MPs. An additional notable observation from Wiess et al [40] relates to differences in the properties of MPs between those observed in rivers and those in accumulation areas, whereby the observed differences are suggested to potentially represent a selective sequestration of smaller size MPs.…”

“…However, there continues to be a debate regarding the relative importance of the various processes with respect to the environmental fate and transport of MPs, which results in varying interpretations of data presented throughout the literature. Most recently, Wiess et al [40] provide a statistical re-analysis based on updated data on MPs, the output of which suggests that previous estimates of river fluxes potentially represent overestimates, of up to three-orders of magnitude. Their reanalysis results in the average residence time of MPs at the ocean surface increasing from days to years, which they suggests strongly reduces "the theoretical need for a missing sink" [40], an observation that is also consistent with Pabortsava and Lampitt [36].…”

“…Most recently, Wiess et al [40] provide a statistical re-analysis based on updated data on MPs, the output of which suggests that previous estimates of river fluxes potentially represent overestimates, of up to three-orders of magnitude. Their reanalysis results in the average residence time of MPs at the ocean surface increasing from days to years, which they suggests strongly reduces "the theoretical need for a missing sink" [40], an observation that is also consistent with Pabortsava and Lampitt [36]. Nevertheless, Weiss et al [40] acknowledge that their estimates are based on underlying data that contain significant uncertainties, but that their observations are perceived to be better aligned with an understanding that surface oceanic MP stocks, particularly within oceanic gyres, result from slow accumulation processes, an observation that is inconsistent with suggestions that there exists relatively rapid environmental fate processes that might help to sequester surface oceanic MPs.…”

“…Nevertheless, Weiss et al [40] acknowledge that their estimates are based on underlying data that contain significant uncertainties, but that their observations are perceived to be better aligned with an understanding that surface oceanic MP stocks, particularly within oceanic gyres, result from slow accumulation processes, an observation that is inconsistent with suggestions that there exists relatively rapid environmental fate processes that might help to sequester surface oceanic MPs. An additional notable observation from Wiess et al [40] relates to differences in the properties of MPs between those observed in rivers and those in accumulation areas, whereby the observed differences are suggested to potentially represent a selective sequestration of smaller size MPs. An implication from Weiss et al [40], could therefore relate to the potential role of a global fractionation process that may cause some types of MPs to be more susceptible to LRET relative to others, analogous to processes illustrated in Fig.…”

“…An additional notable observation from Wiess et al [40] relates to differences in the properties of MPs between those observed in rivers and those in accumulation areas, whereby the observed differences are suggested to potentially represent a selective sequestration of smaller size MPs. An implication from Weiss et al [40], could therefore relate to the potential role of a global fractionation process that may cause some types of MPs to be more susceptible to LRET relative to others, analogous to processes illustrated in Fig. 1 in relation to the LRET of POPs.…”