Microplastics collect in vast quantities even on the seafloor

6 Mei 2020

Many of the greatest threats to our health come in the form of invisible microbes, the novel coronavirus that originated in Wuhan, China, being the latest example. The same goes for the health of the environment: imperceptibly small microplastics can pose grave risks to ecosystems both on land and in water.

Yet microplastics have been saturating nature to an alarming extent. Blown by winds, these tiny particles have even made it to the top of mountains. Driven by ocean currents and facilitated by gravity, they have also been deposited at the bottom of the oceans.

Recently, a team of researchers at the Scripps Institution of Oceanography found that there could be a million times more pieces of plastic in the ocean than previously thought. According to their new estimates ocean water contains as many as 8.3 million pieces of mini-microplastics per cubic meter, which is five to seven orders of magnitude more than previous estimates.

“For years we’ve been doing microplastics studies the same way, by using a net to collect samples,” explained one of the researchers, Jennifer A. Brandon. “But anything smaller than that net mesh has been escaping.”

Now comes more proof that microplastic pollution has reached endemic levels in the oceans. New research by an international team of scientists has found that up to 1.9 million pieces of plastic have formed in a thin layer within just 1 square meter at certain spots on the seafloor.

This is among the highest levels of microplastic contamination ever recorded in oceanwater. These microplastics deposited on the seafloor, the scientists say, mostly come from fibers that are used in textiles and clothing. These particles are widely left unfiltered by domestic wastewater treatment plants, from where they enter rivers and oceans, spreading far and wide.

The scientists have also shown that currents near the bottom of the ocean, known as near-bed thermohaline currents, play a major part in the way microplastics drift down to the seafloor. “These currents are known to supply oxygen and nutrients to deep sea benthos, suggesting that deep sea biodiversity hotspots are also likely to be microplastic hotspots,” the researchers explain.

These underwater currents can lead to microplastics accumulating in great quantities in sediments at select underwater microplastic hotspots. These deep-sea hotspots are the equivalents of the giant garbage patches that float as vast eyesores on the oceans’ surface driven by surface currents.

“Almost everybody has heard of the infamous ocean ‘garbage patches’ of floating plastic, but we were shocked at the high concentrations of microplastics we found in the deep-seafloor,” says Ian Kane, a geologist at the University of Manchester and lead author of the new study.

“We discovered that microplastics are not uniformly distributed across the study area; instead they are distributed by powerful seafloor currents which concentrate them in certain areas,” Kane adds.

Deep-water currents serve a key function in carrying oxygen and nutrients from the surface to the seafloor where unique ecosystems often thrive. Yet they are now also assisting in the accumulation of tiny plastic particles on the ocean floor, which is bound to interfere with marine lifeforms there. The effects of massive plastic pollution deep down in the oceans have yet to be studied and fully understood.

“It’s unfortunate, but plastic has become a new type of sediment particle, which is distributed across the seafloor together with sand, mud and nutrients,” laments Florian Pohl, a scientist at the Department of Earth Sciences of Durham University in the United Kingdom.

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