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Plastic is "missing" in the oceanSingle-use plastics are, as the name suggests, manufactured for one use before being disposed of. Examples include the plastic cutlery that accompanies your takeout meal, straws, wrappers or general food packaging. Of the 300 million tons of plastic we produce yearly, 50% is attributed to single-use plastics.

Between 117-320 million tons of plastics are currently present in the ocean. However, the amount of plastic debris that has been quantified on the ocean surface is less than one percent of this total figure. Where is the rest of the plastic?

Scientists acknowledge that a large amount of plastic in the ocean is effectively missing. It "goes off the radar", moving from the ocean surface and shallow waters to the deeper ocean. It is likely that some plastic will be degraded by one of three pathways: mechanical, photodegradation or biological degradation; however, research suggests that it can take years, perhaps even centuries, for different plastics to undergo complete degradation by such methods if they are able to occur at the deepest points of the ocean.

"Many scientists believe floating plastics eventually sink into the deeper ocean, yet we havent reached a consensus regarding the amount of plastic debris that has accumulated on the deep seafloor and how these debris were transported there," explains Ryota Nakajima, biological oceanographer at the Japan Agency for Marine Science and Technology (JAMSTEC).

Nakajima is one of the scientists behind a new body of work published in Marine Pollution Bulletin that presents the first study of microplastics on the abyssal seafloor beneath the Kuroshio Extension (KE) and the KE recirculation gyre.

"Why Japanese deep-sea? Because most of the plastic is coming from the Asian continent. A previous study (Jambeck et al. 2015) estimated the top 20 contributors of mismanaged plastic waste available to enter the ocean in 2010.2 Nations on the Asian continent represented 12 of the top 20 countries, with China way out ahead of the pack [] Japan is located in the area where a large amount of plastic waste is transported from these massive waste producers via ocean currents such as Kuroshio, so the seas around Japan could be a hotspot of plastic debris," Nakajima says.

Nakajima and colleagues conducted video observations and physical sampling of plastic debris from multiple deep-sea floor sites beneath the KE and its recirculation gyre, a depth of approximately 5700-5800m, using a human occupied submersible, the Shinkai 6500.

The researchers found that the majority of the debris was single-use plastics, such as plastic bags and food packaging. The mean number of items per km2 was approximately 4561, the highest on record for an abyssal plain.

The specific items collected by the team were varied, including clothes, a toothpaste tube and steak packaging that was dated September 1984 meaning it was 35 years old at the time of collection. The packaging can be seen as visually intact in the image below. "This is an interesting example of the persistence of plastic debris in the marine environment. UV radiation and thermal oxidation are the primary factors to degrade plastics, but these factors are completely missing in the deep-sea environment. Plastic debris on the deep seafloor will most likely persist for at least a century," Nakajima explains.

Chicken steak packaging discovered by the research team. Credit: Nakajima.

The overall findings were, as Nakajima and team had hypothesized, that the deep-sea floor beneath the KE and its recirculation gyre act as a significant reservoir of plastic debris.

The researchers believe that the journey to find missing plastics in the ocean has barely begun. Next summer, the team will target the deep-sea floor beneath the other recirculation gyre of the Kuroshio Current, named the Kuroshio recirculation gyre, which is located to the south of the Japanese archipelago.

Ryota Nakajima was speaking with Molly Campbell, Science Writer for Technology Networks.

References:

1. Nakajima R, Tsuchiya M, Yabuki A, et al. Massive occurrence of benthic plastic debris at the abyssal seafloor beneath the Kuroshio Extension, the North West Pacific. Marine Pollution Bulletin. 2021:112188. doi:10.1016/j.marpolbul.2021.112188.

2. Jambeck JR, Geyer R, Wilcox C, et al. Plastic waste inputs from land into the ocean. Science. 2015;347(6223):768. doi:10.1126/science.1260352.

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Single-Use Plastics Found at the Deepest Points of the Ocean - Technology Networks