1. Key points

• Japan-Korea-Canada/US joint cruises in 2020 under the international Synoptic Arctic Survey (SAS; *1) project found unusually low oxygen and acidified water on the Chukchi Plateau (CP), a high seas fishable area, for the first time. The data shows that Atlantic-origin water extension into the Pacific Arctic associated with the recent sea ice loss triggered a frontal northward flow along the CP that transported the low oxygen and acidified water from the shelf-slope north of the East Siberian Sea (ESS).
• The transport of the low oxygen water from the ESS shelf-slope to the CP was well simulated by a physical sea ice–ocean general circulation model. The model suggests that the low oxygen water transport to the CP has become more significant since 2017, but the appearance of the water on the CP is intermittent on a timescale of several months.
• Since the CP is experiencing the fastest ocean hypoxia and acidification in the high seas of the Arctic Ocean, continuous observation (monitoring) of the ocean environment and marine ecosystem of this area will help to develop policies facilitating effective management related to the Agreement to Prevent Unregulated High Seas Fisheries in the Central Arctic Ocean (commonly referred to as the Central Arctic Ocean Fisheries Agreement, or CAOFA; *2) that entered into force on 25^th June 2021.

[Supplemental information]

*1 International Synoptic Arctic Survey: A coordinated multiship and multination pan-Arctic ship-based sampling campaign that was planned to be implemented in 2020 to obtain a synoptic view of the totality of hydrographic and ecosystem changes occurring in the Arctic Ocean. However, several of these research cruises were canceled due to COVID-19. Even in such a situation, Japan, Korea, and Canada/US managed to conduct field expeditions that covered a wide area of the western Arctic Ocean (Fig. 1).

*2 The Agreement to Prevent Unregulated High Seas Fisheries in the Central Arctic Ocean: This is a legally binding agreement signed on 3^rd October 2018 by the five Arctic Ocean coastal States (Canada, Denmark (in respect of Greenland and the Faroe Islands), Norway, Russia, and US) together with China, the European Union (EU), Iceland, Japan, and Korea; and it entered into force on 25^th June 2021. The objective of the agreement is to prevent unregulated fishing in high seas of the central Arctic Ocean based on precautionary conservation and management measures. The parties have agreed to ban commercial fishing in the high seas at least for 16 years. The ban will be continued until scientists can properly assess the ecosystem that underpins sustainable fisheries and until the parties agree on mechanisms to ensure the sustainability of fish stocks.

2. Overview

The international research team led by Shigeto Nishino, Senior Researcher at Institute of Arctic Climate and Environment Research (IACE), Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Jinyoung Jung, Principal Research Scientist at Division of Polar Ocean Science, Korea Polar Research Institute (KOPRI) found unusually low oxygen and acidified water on the CP, a high seas fishable area of the Pacific Arctic Ocean, for the first time during Japan-Korea-Canada/US joint cruises in 2020 by the R/V Mirai (Japan), R/V Araon (Korea), and CCGS Louis S. St-Laurent (Canada) (Fig. 1). The joint cruises were conducted as part of the international SAS project.

The combined data of the joint cruises show that the Beaufort Gyre in the Pacific Arctic shrinks to the east of the CP and forms a front between the Pacific-origin water within the gyre and the Atlantic-origin water extending from the Atlantic Arctic associated with the recent sea ice loss. That phenomenon triggers a frontal northward flow along the CP. This flow likely transports the low oxygen and acidified water toward the CP; similar biogeochemical properties had previously been observed only on the shelf-slope north of the ESS, where large amounts of terrestrial/marine organic matters are deposited on the bottom (Fig. 2). A high quantity of organic matter decomposition, which consumes oxygen and produces CO₂, results in the formation of low oxygen and acidified water, if the water is not ventilated.

A tracer experiment was conducted using a physical sea ice–ocean general circulation model to evaluate whether the low oxygen water on the ESS shelf-slope was transported to the CP. A virtual passive tracer, which represents the low oxygen water, was continuously provided on the ESS shelf-slope. The tracer distribution shows that the low oxygen water on the ESS shelf-slope is transported to the CP by a northward flow. Associated with this transport, the appearance of low oxygen water passing through the CP has become more pronounced since 2017. The appearance of the water is not continuous throughout 2017 and beyond; instead, it is intermittent on a timescale of several months. Although we observed the low oxygen water on the CP only in October 2020, it might also have been present between 2017 and 2019 when observations were not conducted.

The CP is part of a shallow water area in the high seas of the Pacific Arctic Ocean. It is considered that areas shallower than 2000 m are fishable in future ice-free high seas of the Arctic Ocean. However, according to the present study, the CP was already occupied by the acidified water. In addition, there will be a threat of deoxygenation. Therefore, this site should be monitored as a bellwether of ecosystem degradation caused by ocean deoxygenation and acidification in the Arctic high seas fishable areas.