An estimated eight million tons of plastics enter our oceans each year, yet only one percent can be seen floating at the surface. This is the second in a three-part article series about how researchers at Woods Hole Oceanographic Institution are trying to understand the fate of “hidden” microplastics and their impacts on marine life and human health.
Millions of tons of plastics end up in the global ocean each year, but where does all that material go once it gets there?
It’s a fair question, and an important one given that scientists don’t really know how harmful plastics are to marine animals or humans that consume them, or what ends up happening to plastics in the ocean.
The answer really depends on the type of plastics you’re talking about. Scientists have a good sense of where larger plastics end up—most float at the surface of the ocean. Typically, wind-driven currents push them smack dab into the middle of subtropical gyres—large systems of circulating currents that span oceans. Smaller spinning currents called eddies can stir and spread the plastics out a bit to make these so-called “garbage patches” larger, but most of the floating material stays put within the gyres.
The problem is, floating plastics only make up a small percentage of the eight million tons believed to be in the ocean. The balance is thought to be made up of microplastics—plastic fragments less than a millimeter in size.
Microplastics get tossed around in snow-globe-like fashion by fast-moving currents and shifting circulation patterns. Even worse, most of them become submerged so they’re no longer visible at the surface. This makes the job of tracking their final destinations even tougher.
Difficult or not, pinpointing where most of marine plastics go is a challenge scientists are starting to tackle. Sam Levang, an MIT-WHOI Joint Program graduate student, develops computerized models that simulate how large ocean circulation patterns transport larger plastics around the global ocean. He says that knowing where the smaller particles concentrate will be critical if we ultimately find out they do pose a health risk to marine life.
“Our knowledge about the physiological effects of microplastics on different species is limited so far,” he said, “but recent lab studies do suggest that strange things can happen to an organism with a belly full of plastic. If the toxicology effects are significant, we want to know where in the ocean to look for any ecological fallout.”
WHOI physical oceanographer Carol Anne Clayson points out that because most microplastics sink into a swirling array of currents in the depths, it presents some new challenges and makes finding them a “3-D problem.”
But in her mind, it also opens up some interesting new research questions. “We know that sand at the bottom of the ocean can change the flow of currents, so it would be interesting to know if tiny grains of plastic have a similar effect,” she said.
Microplastics may also offer a bit of a silver lining. They can serve as tracers, like Hansel and Gretel’s bread crumbs, to help scientists map out the intricate highway of swirling currents and eddies that move the particles about and understand more about larger ocean circulation patterns, said WHOI scientist Jake Gebbie.
Larger plastics have already been used this way. Scientists have discovered that most end up in the middle of subtropical gyres, which validates the idea that plastics generally follow many of the same pathways as seawater. And, in recent years, pristine places in the world such as the Barents Sea north of Russia have become hot spots for floating plastics. That suggests that large-scale ocean circulation systems such as the Gulf Stream, North Atlantic Current, and Norwegian Current are conveying the litter all the way up to the Arctic.
Drifting microplastics could reveal similar types of secrets.
“If we have a sense for where larger plastics enter the ocean, and where the broken-down fragments end up,” said Gebbie, “we may be able to better understand the various pathways they took to get from point A to point B.”
Above all, Levang says that knowing more about the distribution of microplastics and where they concentrate will help uncover the longstanding mystery of where the larger, less visible portion of plastics go once they reach the ocean.
“Do the really small fragments get carried down to the depths and end up in sediments, or do they break down chemically?” he said. “Knowing the ultimate fate of these tiny particles will be quite important to understanding the nature and magnitude of the problem.”