We've all heard about sea turtles, dolphins or seabirds dying from entanglement in six-pack rings, plastic bags or other detritus — or from bellies full of mistakenly swallowed plastic. But some marine researchers are concerned about the effect that much smaller bits of plastic may be having on the seas.
So-called "microplastics" may concentrate pollutants, be ingestible by the ocean's tiny denizens — from zooplankton to filter feeders like clams and mussels — and move up the food chain.
A group of scientists gathered this month to identify what's known about this problem and where more research is needed.
"We know that stuff breaks down, and as it breaks down, it forms smaller and smaller pieces of plastic," said workshop organizer Joel Baker of the University of Washington, Tacoma. "But there's another story, and that is that there are some processes that either purposefully or inadvertently create microplastic particles in their own right."
One such source is nurdles, the little plastic pellets used as the raw material that's molded or extruded into plastic products. A growing source is tiny plastic spheres — less than a millimeter across, and in some cases just microns in diameter — used in new industrial abrasives or in cosmetics as exfoliants, Baker said.
"Because they're used as abrasives, presumably they're pretty hard and pretty resilient to breakup," he said. "The general rule of thumb is, if it doesn't break down pretty quickly, it ends up in the ocean. We don't have any way of monitoring for them. We have no idea, really, if they're having any impact on any organisms."
Estimates of exactly how many particles are in the ocean give a wide range.
"You tend to have numbers that are much less than one per cubic meter," Baker said. "But if you do that in terms of the number of pieces per square kilometer of sea surface, it's tens of thousands."
Amphipods, lugworms, barnacles and mussels take up microplastic in aquarium experiments. Fish and birds in the wild have been found with microplastic pieces in their bodies. But the extent and effect of this ingestion is not yet known.
Plastic specks in the oceans appear to adsorb poorly water-soluble pollutants, such as polychlorinated biphenyls (PCBs), and persistant pesticides like DDT. This might give creatures that ingest pellets a superdose of toxins that can accumulate up the food chain.
"There's some indication that when the animal ingests those, they not only get the physical damage to the gut, but those pollutants can desorb into the animal," said workshop participant Douglas Helton of the National Oceanic and Atmospheric Administration's Marine Debris Program, in Silver Spring, Md.
On the other hand, the pellets might act like pollutant sponges that mop up the contaminants and sequester them out of harm's way, Baker said. One study presented at the meeting suggested that the amount of pollutant accumulated by one type of marine worm decreased when more plastic was added to sediment in an aquarium, suggesting the latter mechanism may work in that case.
One of the outcomes of the workshop was to identify areas where the greatest effects are likely to be seen. "There are probably areas where it floats on the surface, and those are lagoons and marshes," Baker said. "The other place is coastal urban sediments, where it has settled to the bottom."
These are good starting points for additional research, because if microplastics are causing problems, such locations should show the effects most directly, he added.
In the meantime, taking steps to reduce plastic debris — large and small — is a good idea, Helton said. "I don't think there's any right amount of plastic to dump in the ocean."