Ocean Currents Found to Embed Microplastics in Seafloor

While most people think of the swirling patch of garbage in the Pacific Ocean when they think of plastic pollution, what’s on the surface is just 1% of all the plastic trash in Earth’s seas.

(Photo by Silke Stuckenbrock)

SAN FRANCISCO (CN) — As scientists continue to study the impact of risings levels of plastic pollution in waterways, new research reveals how a large portion of microplastics get lodged in sediment deep below the ocean surface.

When it comes to plastic pollution, a grotesque patch of floating garbage in the middle of the ocean is often the first image that comes to mind, but what hovers on the surface accounts for just 1% of plastic rubbish in the sea.

Slow-moving currents near the ocean floor that supply vital oxygen and nutrients to deep sea creatures also control the flow of plastics, causing them to settle in large collections of sediment far below the surface, according to a study published in the journal Science Thursday.

“These plastics are hidden from view,” said Ian Kane, a geologist from the University of Manchester and lead author of the study. “What we want to do is expose where they are and make them real.”

While prior studies have found microplastics lodged in the seafloor, no past research has shown how plastics move near the bottom of the sea.

It is estimated that more than 165 million tons of plastic waste exists in the ocean today, and close to 8.8 million tons of plastic continues to leak into the ocean each year. That number is expected to grow exponentially with global plastic production projected to increase from 342 million tons in 2014 to 1.9 billion tons in 2050.

About 80% of plastic enters the ocean from land-based sources, most which results from a lack of effective recycling and waste management. Another 20% comes from ocean-based sources, such as ropes and nets from fishing boats or waste from cruise ships.

Research has confirmed that humans are ingesting plastic, but how it affects human health is still under investigation. A June 2019 study by Australia’s University of Newcastle found humans may be ingesting 5 grams of plastic per week, equal to one credit card. Another Austrian study in 2018 found plastic particles in the stool of people in eight countries.

Plastics that remain in bodies of water absorb toxins and pollutants over time, making them more harmful. Small species of underwater animals consume or absorb microplastics and can pass them up the food chain to predators, including humans.

Some underwater toxins are known to have negative effects on reproduction and early stages of development for fish and other marine organisms. Another recent study found exposure to microplastics impaired the ability of hermit crabs to select shells, disrupting an essential behavior for the creature’s survival.

“Although we don’t understand all those effects right now, it’s very important to understand where the plastics are going and where they are ending up,” Kane said.

There are two kinds of microplastics, which are defined in Kane’s study as plastics measuring 1 millimeter, or 0.04 inches, or less in width, length and height. Primary microplastics are those manufactured to be small, such as microbeads used in cosmetics and paint stripping products or plastic fibers used in synthetic fabrics. Secondary microplastics are fragments created when bigger pieces of plastic, such as bottles, break down.

Kane and his research team studied samples of sediment clawed from an approximately 62-square-mile area of the Tyrrhenian Sea off the western coast of Italy at depths of 2,000 to 3,000 feet below the surface.

Using a high-resolution map of the sea floor, the researchers were able to compare microplastic concentration in sediment samples from different areas. They found higher levels of plastic in mounded drifts that resemble underwater sand dunes where slow-moving bottom currents flow. In adjacent areas called moats, which sit lower than the drifts and where currents flow in a parallel direction, the team found about 75% less plastic.

“There’s no relationship between distance from coast and the level of microplastics, but there is a relationship between current strength and the level of microplastics,” Kane said.

The researcher said he hopes this study makes the largely unseen bulk of plastic pollution in the world’s oceans more visible to the public, world leaders and policy makers.

“Just saying that 99% of the plastic is missing is not really tangible so our results show where the plastics are actually going,” Kane said. “It makes it all the more real.”

To follow up on these findings, Kane said researchers may dig deeper into the ocean floor to find out how permanently plastic gets stored in underwater sediment. Another avenue to explore is tracing these microplastics back to their sources, he said.

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