1. Key points

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There is limited information about the extent of plastic debris pollution in the Arctic Ocean on the Pacific side (Chukchi Sea). Microplastics were detected even offshore of the Chukchi Sea, far from human habitats.

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The mean water-column microplastic inventory in the Chukchi Sea was 5,236 pieces/km², and the total amount of microplastic particles in the Chukchi Sea water was estimated to be 3.3 × 10⁹ pieces. By contrast, microplastic inflow from the Pacific Ocean was estimated to be 1.8 × 10¹⁰ pieces/year, of which only a portion was found in the Chukchi Sea water.

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Most of the microplastics entering the Arctic Ocean from the Pacific Ocean may be accumulating outside of the Chukchi Sea water-column (e.g., sea ice and seafloor sediments) or in the downstream regions of the Pacific-origin water, such as the Beaufort Sea. It is important to clarify this missing sink in the future.

1. Overview

Takahito Ikenoue, a researcher at the Research Institute for Global Change (RIGC), Marine Biodiversity and Environmental Assessment Research Center (BioEnv), Marine Plastics Research Group (M-Plastics), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and his colleagues have clarified the water-column microplastic inventory in the Chukchi Sea of the Arctic Ocean on the Pacific side (*1) and the annual flux of microplastics from the Pacific Ocean into the Chukchi Sea.

The recent influx of microplastics into the Arctic Ocean may increase environmental stress on the marine ecosystem, which is experiencing significant sea-ice loss owing to global warming. Quantitative data on microplastics in the Arctic Ocean on the Pacific side are very limited, and the microplastic budget of the water column is completely unknown.

Therefore, in this study, we observed surface concentrations (number of particles per unit volume of seawater) of microplastics using a neuston net (*2) in the Chukchi Sea, Bering Strait, and Bering Sea. From these observations, we estimated the water-column microplastic inventories (total number of microplastics in the entire water column) (*3). The mean water-column microplastic inventory in the Chukchi Sea was 5,236 pieces/km², which was one-thirtieth of those for the Arctic Ocean on the Atlantic side and less than one-tenth of the average for the global ocean. Because the total area of the Chukchi Sea is 620,000 km², the total number of microplastics floating in the entire Chukchi Sea was estimated to be 3.3 × 10⁹ pieces.

However, the annual flux of microplastics from the Pacific Ocean into the Chukchi Sea, estimated from microplastic concentrations in the Bering Strait, was estimated to be 1.8 × 10¹⁰ pieces/year. This value was approximately 5.5 times greater than the total amount of microplastic in the seawater of the entire Chukchi Sea. This suggests that microplastic inflows from the Pacific Ocean are accumulating in large amounts in reservoirs other than the Chukchi Sea water-column (e.g., sea ice and seafloor sediments) or in the downstream regions of the Pacific-origin water, such as the Beaufort Sea.

This finding was published in Science of the Total Environment on November 1st, 2022 (JST).

Title：

Horizontal distribution of surface microplastic concentrations and water-column microplastic inventories in the Chukchi Sea, western Arctic Ocean

DOI：

10.1016/j.scitotenv.2022.159564

Authors：

Takahito Ikenoue¹, Ryota Nakajima¹, Amane Fujiwara¹, Jonaotaro Onodera¹, Motoyo Itoh¹, Junko Toyoshima², Eiji Watanabe¹, Akihiko Murata¹, Shigeto Nishino¹, Takashi Kikuchi¹

Affiliations：

1. Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan
2. The Ocean Policy Research Institute (OPRI), Sasakawa Peace Foundation, Japan

[Term meaning]

*1

Arctic Ocean on the Pacific side: The Chukchi Sea, Beaufort Sea, and East Siberian Sea

*2

Neuston net: A net used to collect juvenile fishes, fish eggs, zooplankton, and other living organisms that suspend in the ocean surface layer (neuston). The net is towed by partially submerging the opening and scooping the ocean surface layer. In this study, the net was used to collect microplastics suspended in the ocean surface layer (0–0.5 m).

*3

Water-column microplastic inventory: When the ocean is rough, the number of microplastics suspended in the ocean surface layer varies with the depth of vertical mixing because the microplastics dive below the sea surface with the seawater. However, by observing wave height and wind speed when microplastics are collected, the depth of vertical mixing can be determined, and the vertical distribution of microplastics below the sea surface can be estimated (Isobe et al., 2021, Micropl. Nanopl. 1, 16). In this study, the total number of microplastics in the entire water column per km2 (Water-column microplastic inventory) was estimated by adding the number of microplastics below the sea surface to infinity, in the direction of water depth.