Pollution concentrates

Highest level of plastics found on seafloor

Ocean currents create ‘microplastic hotspots’ with up to 1.9 million pieces per square metre



An international research project has revealed the highest levels of microplastic ever recorded on the seafloor, with up to 1.9 million pieces in a thin layer covering just one square metre.

Over 10 million tons of plastic waste enters the oceans each year. Floating plastic waste at sea has caught the public’s interest thanks to the ‘Blue Planet Effect’ seeing moves to discourage the use of plastic drinking straws and carrier bags. Yet such accumulations account for less than 1% of the plastic that enters the world’s oceans.

The missing 99% is instead thought to occur in the deep ocean, but until now it has been unclear where it actually ended up. Published this week in the journal Science, the research conducted by: The University of Manchester, National Oceanography Centre (UK), University of Bremen (Germany), IFREMER (France) and Durham University (UK) showed how deep-sea currents act as conveyor belts, transporting tiny plastic fragments and fibers across the seafloor.

These currents can concentrate microplastics within huge sediment accumulations, which they termed ‘microplastic hotspots’. These hotspots appear to be the deep-sea equivalents of the so-called ‘garbage patches’ formed by currents on the ocean surface.

The lead author of the study, Dr Ian Kane of The University of Manchester said: “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. 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.”

Microplastics on the seafloor are mainly comprised of fibers from textiles and clothing. These are not effectively filtered out in domestic waste water treatment plants, and easily enter rivers and oceans. In the ocean they either settle out slowly, or can be transported rapidly by episodic turbidity currents – powerful underwater avalanches – that travel down submarine canyons to the deep seafloor.

Once in the deep sea, microplastics are readily picked up and carried by continuously flowing seafloor currents (‘bottom currents’) that can preferentially concentrate fibers and fragments within large drifts of sediment.

These deep ocean currents also carry oxygenated water and nutrients, meaning that seafloor microplastic hotspots can also house important ecosystems that can consume or absorb the microplastics. This study provides the first direct link between the behavior of these currents and the concentrations of seafloor microplastics and the findings will help to predict the locations of other deep-sea microplastic hotspots and direct research into the impact of microplastics on marine life.


Abstract of “Seafloor microplastic hotspots controlled by deep-sea circulation”
Science, April 30, 2020

While microplastics are known to pervade the global seafloor, the processes that control their dispersal and concentration in the deep sea remain largely unknown. Here we show that thermohaline-driven currents, which build extensive seafloor sediment accumulations, can control the distribution of microplastics and create hotspots of up to 1.9 million pieces m−2. This is the highest reported value for any seafloor setting, globally. Previous studies propose that microplastics are transported to the seafloor by vertical settling from surface accumulations; here we demonstrate that the spatial distribution and ultimate fate of microplastics is strongly controlled by near-bed thermohaline currents (bottom currents). 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.

 

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