Seafloor microplastic hotspots controlled by deep-sea circulation

While microplastics are known to pervade the global seafloor, the processes that control their dispersal and concentration in the deep sea remain largely unknown. In this research, Ian A. Kane and colleagues 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; this study demonstrates 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.

Bottom currents are efficient conveyors of nutrients and oxygen, and consequently dictate the location of important biodiversity hotspots. This study shows that the same seafloor currents can also transport and emplace microplastics. The highest concentrations of microplastics on the seafloor occur in contourite drifts formed by bottom currents, and their distribution is controlled by spatial variations in current intensity. How effectively microplastics are buried, or become re-exhumed (and hence become more available for trophic transfer) depends on temporal fluctuations in current intensity. Although there are ongoing efforts to reduce the release of plastics into the environment, our oceans will continue to be impacted by the legacy of past waste mismanagement. Seafloor currents will play a crucial role in the future transfer and storage of microplastics in the deep ocean.

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