An observational study of microplastics sinking into the deep sea off the Boso Peninsula

1. Key Points

• For the first time, marine sinking particles were analyzed for microplastics^※1 in the Kuroshio Extension recirculation gyre off the Boso Peninsula using archive samples.

• We were able to demonstrate that the impact of microplastic contamination from sample bottles and preservation solutions is negligible in marine sinking particle archive samples, and that they can be used for microplastics research.

• The amount of sinking microplastics was found to increase or decrease with seasonal changes in the amount of marine sinking particles^※2.

• The sinking of microplastics per square meter per day was similar to that in the subtropical gyre of the northeast Atlantic based on the number of individual pieces.

• The total annual transport of microplastics in the Kuroshio Extension recirculation gyre was estimated to be 0.028 million metric tons per year from the surface to a water depth of 4,900 meters.

Figure 1 Schematic diagram of the study

2. 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, for the first time, estimated the amount of sinking microplastics (hereafter referred to as "MPs") from the surface to the deep sea through actual observation. Specifically, they analyzed marine sinking particles within the Kuroshio Extension recirculation gyre in the subtropical western North Pacific Ocean (Fig. 1). Previous observations have suggested the accumulation of large amounts of MPs and macroplastic debris^※3 on the deep-sea floor of the Kuroshio Extension recirculation gyre off the Boso Peninsula. However, few studies have quantitatively determined exactly how much MPs have sunk into the deep sea.

In this study, archived samples of marine sinking particles from station KEO, within the Kuroshio Extension recirculation gyre, were used to calculate the amount of sinking MPs. The samples used were collected in a time series every 18-21 days from July 1, 2014, to October 2, 2016, using a sediment trap^※4 moored at a water depth of 4,900 m at station KEO. There were concerns about the contamination of the archived samples due to the release of MPs from the polyethylene bottles in which the samples were stored and the MPs in the formalin seawater used as a preservative. However, analysis of the formalin seawater stored in polyethylene bottles for approximately two and a half years demonstrated that it contained only 0.25 MPs per volume used for MPs analysis, which was negligible compared to the amount of MPs detected in the marine sinking particles.

Marine sinking particle samples were analyzed and MPs were detected in all samples, with 17 different types of plastics were identified. Ninety percent of the MPs detected were less than 100 µm in size. The amount of sinking MPs per square meter ranged from 111 to 889 pieces per day. On average there were 352 pieces, similar to the sinking MPs in the northeast Atlantic subtropical gyre. On mass basis, they varied from 4.5 × 10^-3 to 3.8 × 10^-1 mg per day during the observation period, with a mean value of 5.4 × 10^-2 mg. Seasonal variations in sinking MPs indicated that it was driven primarily by an increase in marine sinking particles, owing to increased primary production^※5 in the surface layer. The amount of sinking MPs per square meter per year at station KEO was 20 mg. By extrapolating this value to the entire area of the Kuroshio Extension recirculation gyre, it was estimated that 0.028 million metric tons of MPs were transported annually to a water depth of 4,900 m. These results suggest that a large amount of MPs are transported by the Kuroshio Current and Kuroshio Extension to this area and accumulate in the deep sea.

This finding will be published in Environmental Science ＆ Technology on August 26, 2024 (JST). This study was supported by Grants-in-Aid for Scientific Research (23K11414, JP15H05822, JP18H04144, JP19H05667, and JP22H05207).