The missing ocean plastic sink: Gone with the rivers

Estimates of the flux of microplastics from rivers, in the context of the mass of plastic that has been observed in the ocean, have made it appear that a large, unidentified sink of plastics must exist there. Weiss et al. show that there may not be a missing sink after all. By reformulating how mass fluxes are calculated from observations of particle numbers, they demonstrate that those mass fluxes were overestimated by two to three orders of magnitude. This explains why the residence time of plastics in the ocean seemed so puzzlingly short and implies that ocean plastics may persist and degrade over longer periods than previously thought. Science , abe0290, this issue p. 107 The flux of microplastics to the ocean from rivers is much lower than was thought. 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.