In recent years, the prevalence of plastics in the world’s oceans has become a major environmental issue. From drinking straws and water bottles to single-use plastic bags and much smaller particles, up to 12.7 million metric tons of plastic waste are dumped into the oceans each year, with devastating consequences for seabirds and marine wildlife and potentially harmful effects on human health.
Because ocean currents can carry this waste thousands of kilometers from where it enters the water, plastic pollution has quickly become a global issue. Debris, including microplastics (particles <5 millimeters across), has been found at the poles, in sea ice, and—most visibly—in enormous sea surface garbage patches trapped by recirculating oceanic gyres.
Although transport models have successfully described the distribution of microplastics observed floating on the ocean surface, these buoyant bits account for a small fraction of the total expected plastics volume. Turbulent mixing, biofouling, and a decrease in particle size appear to cause a large proportion of microplastics to sink, but what happens to these particles deeper in the water column, and whether they can account for the prevalence of plastic in the polar regions, is not yet known.
Now Wichmann et al. have assessed how subsurface currents disperse microplastics. Using the Parcels framework, the team modeled the trajectories of 1 million virtual microplastic particles around the globe over a 10-year period. After running the same simulations for four types of particles located at depths ranging from the surface to 120 meters, the researchers compared the resulting microplastic distributions and their transport pathways between different oceanic regions.
The results indicate that submerged microplastics are controlled by different dynamics and therefore follow very different routes than particles found floating on the surface. The simulations suggest that near-surface currents carry large quantities of microplastics from subtropical and subpolar regions toward the poles—a finding that may explain why this material is commonly detected even in those remote regions.
In addition to presenting a plausible explanation for how microplastics can be transported from low latitudes to Earth’s polar regions, this study offers novel insights into subsurface oceanic transport as well as the distribution of plastic particles in marine ecosystems. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2019JC015328, 2019)
—Terri Cook, Freelance Writer