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Ocean acidity stifles coral-anchored communities

  • Researchers working in the seas around Japan found that higher levels of carbon dioxide, like those found around volcanic vents in the ocean floor, diminish the diversity of corals and other lifeforms.
  • The study took place at the convergence of marine temperate and subtropical climates.
  • Their findings indicate that rising acidity could inhibit coral growth and reduce the number of species living in these ecosystems.

New research confirms the fragility of life living in the world’s oceans, which are growing warmer and more acidic as carbon dioxide levels rise.

The carbon dioxide that ends up dissolved in the sea can cause severe problems for marine life in ways that scientists are just starting to understand, from breaking down the rigid calcium carbonate structure of corals, to tinkering with fishes’ sense of smell.

Recently, a team of scientists set up an experiment off the coast of a volcanic island in Japan called Shikine, where consistent currents keep carbon dioxide levels near the coast relatively low.

“Our research site is like a time machine,” Jason Hall-Spencer, a marine biologist at the University of Plymouth in the U.K. and a co-author of the research, said in a statement.

Corals, such as this table Acroporid, provide habitats for a wide range of fauna at a reference site where the carbon dioxide concentration is currently 300 parts per million. Image by Marco Milazzo.

Near Shikine Island’s shore, carbon dioxide levels mimic what they might have been before the Industrial Revolution. But just offshore, vents in the ocean floor percolate carbon dioxide. These “volcanic seeps” allowed the team to examine the impact of the higher acidity on sea life, simulating a possible future ocean environment.

In previous studies, scientists have used the areas around volcanic seeps as natural laboratories to study ocean acidity. But until now, they hadn’t looked at the transition zone between temperate and subtropical seas. Places like Shikine, which sits along that temperate-subtropical edge, often contain a mishmash of species from both marine climates. Many of these life-forms live at the limits of what they’re able to adapt to, the authors write.

Hall-Spencer and his colleagues set up “stations” in the waters around Shikine at a range of carbon dioxide concentrations pHs. As they expected, surveys conducted by research divers revealed that corals were less prevalent in areas with higher concentrations of carbon dioxide. In its place, they found a surge in the growth of algae. Without as much coral to anchor the community, fewer species of plants and animals survive, resulting in what the authors called a “major simplification of the ecosystem.”

A healthy coral reef in Indonesia. Image by Rhett A. Butler/Mongabay.

Some of the most striking changes occurred between the concentrations of carbon dioxide closest to what we’re currently seeing and those higher concentrations that scientists predict could be coming if humans continue to emit carbon at current rates.

“It shows the extensive damage caused by humans due to [carbon dioxide] emissions over the past 300 years,” Hall-Spencer said, “and unless we can get a grip on reducing [carbon dioxide] emissions we will undoubtedly see major degradation of coastal systems worldwide.”

The team published their research in the journal Scientific Reports on July 27.

The study’s lead author, Sylvain Agostini, a marine ecologist at the University of Tsukuba in Shizuoka, Japan, called the volcanic seeps “a vital window into the future.”

He said ocean acidification could be closing off an escape hatch for corals and the communities of organisms they anchor.

“There was mass mortality of corals in the south of Japan last year, but many people cling to the hope that corals will be able to spread north,” Agostini said in the statement. “Therefore it is extremely worrying to find that tropical corals are so vulnerable to ocean acidification, as this will stop them from being able to spread further north and escape the damage caused by water that is too hot for them.”

At a site with a carbon dioxide concentration corresponding to conditions predicted for 2100, the high carbon dioxide favors the growth of low-profile algae, inhibiting the growth of corals and other habitat-forming species and leading to a drastic decrease in biodiversity. Image by Marco Milazzo.

Still, efforts to cut the amount of carbon dioxide released into the atmosphere offer a measure of hope, said Kazuo Inaba, a study co-author and biologist at the University of Tsukuba.

“If we are able to meet the Paris Agreement targets to limit emissions,” Inaba said, “we should be able to limit further damage to kelp forests, coral reefs and all marine ecosystems.”

Banner image of a clownfish in Indonesia by Rhett A. Butler/Mongabay.


Agostini, S., Harvey, B. P., Wada, S., Kon, K., Milazzo, M., Inaba, K., & Hall-Spencer, J. M. (2018). Ocean acidification drives community shifts towards simplified non-calcified habitats in a subtropical−temperate transition zone. Scientific Reports, 8(1), 11354.

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