Seafloor currents may direct microplastics to biodiversity hotspots of the deep

April 30, 2020

Microplastic particles entering the sea surface were thought to settle to the seafloor directly below them, but now, a new study reveals that slow-moving currents near the bottom of the ocean direct the flow of plastics, creating microplastic hotpots in sediments of the deep sea. Importantly, these "bottom" currents supply oxygen and nutrients to deep sea creatures, and so, by following the same route, these toxic microplastics may be settling into biodiversity hotspots, increasing the chance of ingestion by creatures of the deep. Currents are known to control the global distribution of plastics on the sea surface, creating patches of floating garbage. However, sea surface plastic concentrations account for only 1% of the estimated global marine plastic budget. Most of the plastic ends up in the deep sea, though it's largely unknown how they're distributed there. Using high-resolution data from sediments sampled from the Tyrrhenian Sea (off the western coast of Italy), as well as numerical modeling, Ian Kane and colleagues demonstrate that the ultimate fate of microplastics (those smaller than one millimeter) is strongly controlled by bottom currents. They found microplastics were focused within depths of 600 to 900 meters, where these currents have the greatest interaction with the seafloor, transporting fine-grained particles that result in large sediment accumulations, called contourite drifts. All seafloor samples were found to contain microplastics - most of which were fibers - and the highest concentrations occurred on the mounds of contourite drifts. On average, bottom currents created microplastic hotspots of up to 1.9 million pieces per square meter, one of the highest reported values for any seafloor setting, globally. The authors note that above a certain threshold of the currents' shear force on the deep seabed, microplastics no longer became concentrated at the seafloor.
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American Association for the Advancement of Science

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