Why 99% of ocean plastic pollution is "missing"

Wednesday, April 28, 2021

/ by mansuralisaha


The News Cover: Between Hawaii and California, --in an area about the size of Australia-- Is the Great Pacific Garbage Patch. For decades, tons of our plastic debris have accumulated there because of swirling ocean currents. It looks like a cloudy soup: and that’s because the plastic objects are spaced far apart, and they range in size from large debris to microscopic.

There are at least 4 other garbage patches like this in the world, And after scientists discovered them, starting in the 90s. They thought that this might be where a lot of the plastic ended up, out there floating on the surface. But recently, scientists brought large nets to the Great Pacific Garbage Patch, and took a closer look at the objects they pulled out. They found water bottles And hard hats And bottle caps And toothbrushes And toilet seats And laundry baskets And using what they found, they were able to calculate how much debris was in all of the garbage patches. 

There were about a few hundred thousand tons of plastics at the surface of the ocean, which is a huge number. It is a big number. But a few hundred thousand metric tons of plastic is only about 1% of the estimated 8 million tonnes of plastic scientists believe is emitted into the ocean every year. So scientists have been left investigating a mystery: Where does the rest of the ocean’s plastic go? This is clue number 1 in the case of the missing plastic: a sea floor sediment sample It was taken from the bottom of the Santa Barbara Basin, off the coast of California. 

It represents a measure of time, from 1870 at its deepest layer of sediment, up until 2009. But this period, from 1945 to 2009, is where the study authors were focused on. It’s the era of plastic production. In these layers, the study authors found plastic fibers and fragments that were 1 millimeter or smaller in size. They found more and more plastic particles as the years went on, doubling every 15 years. That rate is nearly identical to the rate of global plastic production. Ss soon as you see a layer of microplastic you pass the 1950’s and that's the legacy of our generation. 

This is Laurent Lebreton—he works at the Ocean Cleanup, and led the study of the objects in the Great Pacific Garbage Patch. And this will be there forever. We know that. The sea sediment study looked at microplastics—particles smaller than 5 millimeters. These either come from clothing fibers, or they are the result of larger plastics breaking down. We’ve found these tiny particles floating throughout the ocean, And even in the guts of the ocean’s tiniest creatures, like plankton. But the sediment study shows that some of our plastic is likely hiding, buried in the seafloor, too. 

But here’s another clue: This is a plastic bag, captured 2,500 meters below the surface of the Arctic deep sea. It’s one of over 2,100 photographs taken with this deep-sea camera. Part of the work that we're doing is to look at the impact of climate change in the Arctic. We use towed camera surveys to look at the impact on large animals like starfish and snails, sponges. And while I was doing these surveys, I saw that more and more plastic debris was on the seafloor.

 Melanie Bergmann’s research shows that large plastic objects don’t just float on the surface or degrade into microplastic — some of them sink without breaking down. One study found that about 50% of plastic in landfills is denser than seawater, which means these objects could sink on their own. But even those other 50 percent may actually travel to the seafloor with time because what we see is that the debris which is floating on the ocean surface becomes colonized with biota over time… barnacles, mussels, all sorts of different organisms… then it becomes heavier and heavier and at certain at a certain point and then starts to sink. 

Bergmann’s research is difficult to replicate throughout the ocean, because of the challenge of surveying the deep-sea environments. But it suggests that some of that missing plastic might be sitting on the seafloor, intact. Another clue complicates the mystery, though: this plastic crate, from Taiwan. It’s one of the objects excavated during that harvest in the Great Pacific Garbage Patch. And what struck researchers was its production date: 1971. 

When they looked at the production dates of other objects, they saw a trend: a lot of it was old trash. This was a new lead: because if the majority of plastic pollution degraded into microplastic or fell to the ocean floor, then what you’d see in garbage patches would be new plastic. And that changed the story...the plastic that is accumulated at the surface of the ocean is actually very persistent. The plastic we find in subtropical oceans may actually be there for decades, if not centuries. It turns out the new plastic is far closer than the open ocean or the bottom of the deep sea. 

Lebreton’s research found that plastic objects on coastlines have more recent production dates than plastic in the open ocean. This clue led scientists to think a lot of debris actually stays close to shorelines around the world—hidden in plain sight. Some of that will end up in the middle of the ocean and garbage patch, but actually, a lot of it stays fairly near shore and hop from beach to beach to beach.

 Erik Van Sebille is an oceanographer and is building an ocean model that predicts where our missing plastic ends up. The completed model will be finished in 2022, but in the meantime, he and his team publish initial results to a Twitter feed. We use simulations of the ocean currents and they're a bit like weather models for the ocean.

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