Island south of Tokyo could hold clues to future acidified oceans

An underwater carbon dioxide (CO2) vent site near an island 160 kilometers south of central Tokyo could provide vital information for University of Tsukuba researchers who are studying the impact of ocean acidification.

The ocean is being acidified with the rising CO2 concentration in the atmosphere, and there is a growing interest in the scientific community in how that will affect ecosystems.

An Asahi Shimbun reporter in June accompanied researchers on the team on a scientific diving tour to Shikinejima island, which falls in the jurisdiction of the village of Niijima under the Tokyo metropolitan government.

Bubbles were seen bursting out and swirling sand up from the seabed at a depth of about 10 meters near Mikawa Bay on the southern side of the island. The reporter saw shimmering curtains of bubbles rising here and there, and she sensed the heat as she moved her hand closer to one of the vents.

Local divers refer to the site as an “underwater hot spring.”

Shigeki Wada, who is on the research team, said the gas spurting out is mostly composed of CO2 and contains very little hydrogen sulfide.

Seawater is generally weakly alkaline near the surface, with the pH, a measure of the hydrogen ion concentration, hovering around 8.1, but the seawater is more acidic at the Shikinejima vent site, with a pH of around 7.

There are reports of similar environments only from a limited number of sites, including the Italian island of Ischia, Papua New Guinea and the Japanese island of Io Torishima. No details are known about why CO2 gushes out at similar sites, said Wada, who is assistant professor of biological oceanography with the University of Tsukuba’s Shimoda Marine Research Center.

Members of the University of Tsukuba team have been conducting about 10 scientific diving sessions a year at Shikinejima island since 2014. They are comparing an area of pH 7.5 to 7.9 some 100 meters from the vent site with another, unacidified area to study the impact on the photosynthesis and growth of seaweed.

No coral is to be seen near the vent site, where seaweed is seen sprawling over the seafloor. Calcareous algae, or seaweed with calcic crust, is also scarce there. The species composition of the fish population in the area is not the same as in other parts of the ocean, Wada added.

“Coral and seaweed, which serve as the cradle of diverse creatures including fish, could be lost under the influence of acidification,” Wada said. “We are studying this testing ground for an acidified ‘ocean of the future’ in hopes of telling beforehand what will happen.”

Yumi Nakayama, The Asahi Shimbun, 14 August 2018. Article.

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