Il y a des tonnes de plastiques dans les océans mais… pas assez!
Ou en tout cas pas assez selon notre production. Oú disparaît donc ce plastique? En micro billes absorbées par le plancton
Image: Kevin Krejci/Flickr
There’s some 40,000 tons of plastic floating on the surfaces of the world’s oceans, which is leaving researchers wondering: Where the hell is the rest?
That number is nothing to scoff at, of course, but it’s many orders of magnitude lower than the estimated amount of plastic that has been going into the oceans since at least the mid-1970s. Plastic in the ocean isn’t simply disappearing, but it has to be going somewhere. And that’s the scary thing.
In a paper published today in Proceedings of the National Academy of Sciences, Andres Cozar of Spain’s University of Cadiz and an international team of colleagues report that the "quantity of plastic floating in the ocean and its final destination are still unknown."
"A conservative first-order estimate of the floating plastic released into the open ocean from the 1970s (10^6 tons) is 100-fold larger than our estimate of the current load of plastic stored in the ocean," Cozar wrote. "Large loads of plastic fragments with sizes from microns to some millimeters are unaccounted for in the surface loads. The pathway and ultimate fate of the missing plastic are as yet unknown."
Cozar has a couple theories, which we’ll get to in a minute. Chief among them, however, is the idea that fish are eating microplastics (mistaking them for plankton or accidentally eating them along with plankton, which are increasingly calling plastic home) and pooping them out. The feces is then dense enough to sink to the bottom of the ocean, and that’s where all the plastic is.
Gross, yeah, and probably not good news if we want to have any shot of cleaning this stuff up.
The results of Cozar’s survey. Image: PNAS
Just because we don’t know where a lot of this stuff goes doesn’t mean that there isn’t an incredible amount of micro plastics floating on Earth’s oceans. Plastic generally doesn’t sink under normal circumstances, and 88 percent of the more than 3,000 samples from around the world that his team took had micro plastics in it.
As you might expect, roughly 35 percent of the total amount of micro plastics are located in the North Pacific Ocean, home of the gyre that many have begun referring to as a floating island of plastic. There are also substantial gyres in the North and South Atlantic Ocean, and the Southern Indian Ocean.
Back in the 1970s, the National Academy of Sciences estimated that roughly 45,000 tons of plastic made its way into the oceans each year, and that was before the annual production of plastic increased fivefold—in 2010, the world made 265 million tons of plastic, for instance.
That brings us to the crux of the study, and the question that’s probably on your mind—where is it? Cozar has four theories, none of them particularly good news for ocean health.
Shore deposition: Basically, plastics somehow make it out of gyres in the middle of the ocean and make it back onto shore somewhere. This is very unlikely to happen, for pretty obvious reasons, namely that it generally defies the laws of physics. Gyres are essentially very large, circular tides. Absent many large storms, the plastic trapped in the middle of them isn’t making its way back to shore. Cozar wrote, "A selective washing ashore of the millimeter-sized fragments trapped in central areas of the open ocean is unlikely."
Nanofragmentation: This is the idea that micro plastics have become "nano plastics" that are very, very difficult to detect. Plastic naturally breaks into tinier pieces, and the sun has something to do with that, but Cozar says there’s no reason to suggest that "solar-induced fragmentation" has increased since the 1980s, when several studies were done on the phenomenon.
For the plastics to be broken down further, there’s likely some sort of bacteria or plankton that has evolved to do it, or that does so naturally. There is some research to back that up. "Recent scanning electron micrographs of the surface of micro plastic particles showed indications that oceanic bacterial populations may be contributing to their degradation, potentially intervening in the fragmentation dynamics," Cozar wrote.
Biofouling: We’ve seen animals make homes out of plastic, we’ve seen reefs that incorporate plastic—that’s biofouling. Cozar suggests that plankton and other small organisms may be accumulating on the plastic, making the plastics able to sink, probably very slowly because seawater density gets higher with depth.
This is another potentially sound possibility, were it not for the fact that, in field tests, plastic makes a very poor home for much of anything. "Field experiments have shown that biofouled plastic debris undergoes a rapid defouling when submerged, causing the plastic to return to the surface," reads the report.
Ingestion: This is the most likely scenario, Cozart suggests. It’s not a pretty one. Microplastics can end up being roughly the same size as zooplankton, an incredibly important part of the oceanic food chain. Previous studies have found that fish that eat plankton often have plastic in their stomachs, so it’s not a farfetched idea. The idea here is that fish eat the plastic, poop it out, and it sinks to the bottom of the ocean.
"Gut content of [plankton-eating] fish is evacuated as long viscous feces that assume spheroid shapes while sinking at high velocities," he wrote. "Hence, micro plastic fragments could also reach the bottom via defecation, a proposition that requires further quantitative testing."
The overall answer, of course, is probably some combination of the four of these scenarios. We’re going to have to figure it out if we want an outside shot of ever cleaning up the oceans. Maybe we can use what we find to fuel 3D printers.
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