Unusually low oxygen and acidified water found in a high seas fishable area of the Pacific Arctic Ocean – A priority area of marine environmental and ecosystem monitoring for potential high seas fisheries –

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 25th June 2021.

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.

This study was supported by the Arctic Challenge for Sustainability II (ArCS II), Program Grant Number JPMXD1420318865, funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT); Korea Institute of Marine Science and Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (20210605, Korea-Arctic Ocean Warming and Response of Ecosystem (K-AWARE), KOPRI); and the Joint Ocean Ice Study (JOIS), funded by Fisheries and Oceans Canada, in collaboration with the Beaufort Gyre Exploration Project, funded by the National Science Foundation Office of Polar Programs.

The findings of this study are scheduled to be published in Nature Communications on November 2, 2023 (Japan time).

Fig. 1．Maps of (a) the Arctic Ocean and (b) the study area. In (a), blue and red arrows represent flows from the Pacific and Atlantic oceans, respectively, into the study area (surrounded by black lines in a fan shape). A blue circular arrow represents Beaufort Gyre circulation. A yellow arrow indicates the flow from the shelf-slope north of the East Siberian Sea (ESS). This flow transports low oxygen and acidified water from the ESS shelf-slope to the Chukchi Plateau. In (b), an enlarged map of the study area in (a), red, green, and blue dots denote the hydrographic stations conducted by the R/V Araon (Korea), R/V Mirai (Japan), and CCGS Louis S. St-Laurent (Canada), respectively, under the international Synoptic Arctic Survey project in 2020. Black dots indicate other hydrographic stations between 2002 and 2019.

Fig. 2．Schematic of the transport of low oxygen and acidified water from the shelf-slope off Siberia to the Chukchi Plateau. Terrestrial/marine organic matters derived from rivers, coastal erosion, permafrost thawing, and biological production are deposited on the seafloor off Siberia. A high quantity of organic matter decomposition produces low oxygen and acidified water. This water is transported to the Chukchi Plateau with a northward flow caused by a change in large-scale ocean circulation related to the sea ice loss (see text for details). The organic matter supply from each source is expected to increase in the future, resulting in wider areas of production and spread of the low oxygen and acidified water.