Oysters, scallops, urchins, shrimps and crabs: All are popular among seafood lovers and all are at risk of disappearing from restaurant menus if the ocean keeps absorbing carbon dioxide from the atmosphere, causing it to become more acidic.
Carbon dioxide emissions are known to cause climate change, but they also affect the ocean. Dubbed “the other carbon dioxide problem,” ocean acidification is getting increasing attention worldwide, as it impacts a wide variety of marine species, including corals and other sea creatures that depend on calcium carbonate to survive.
In Japan — which on Monday marks Marine Day, a celebration of the ocean's importance to the country — efforts to study the phenomenon have recently gained momentum. But there’s still a long way to go before the government, academics and nonprofit organizations are properly working together to comprehensively monitor its impact, share know-how and respond to the change, experts say.
“Ocean acidification is something we humans don’t feel directly,” said Masahiko Fujii, associate professor of environmental sciences at Hokkaido University. “Climate change often comes to mind when we wonder, ‘Why is it so hot these days?’ and ‘Why are there so many downpours?’”
But awareness of ocean acidification has been slower to develop because people cannot experience it, he said.
Thus the importance of relating the issue to something people can better understand: food.
But why are these creatures hit hardest? The answer lies in the makeup of their shells and exoskeletons.
When carbon dioxide dissolves in water, it breaks down into hydrogen ions and bicarbonate ions. Much of marine life — ranging from phytoplanktons and shellfish to corals — forms their bones and shells from calcium ions and carbonate ions, which are abundant in the sea. But hydrogen ions and carbonate ions bind together, causing a shortage of the latter when there is a lot of the former.
The pH level of global surface waters — a measure of ocean acidification — has decreased from 8.2 to 8.1 since the preindustrial era, a trend attributable to rising atmospheric carbon dioxide, according to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), which was released in February.
A pH of 8 means the water is slightly alkaline, roughly the same as baking soda, compared with a neutral pH of 7 for pure water. A decrease by 0.1 pH units may seem subtle, but it means the acidity has increased by 30%.
The Japan Meteorological Agency, meanwhile, has conducted observations of carbon dioxide concentrations in the western North Pacific since the 1990s, with the results showing a clear long-term trend of pH decrease.
The ocean acidification process poses a threat to the future of the fishery and aquaculture industries, researchers say. Estimates vary greatly, but the cumulative economic loss experienced by those industries in Japan could reach up to ¥2 trillion ($14.4 billion) by 2100, according to Fujii’s 2018 paper.
In 2020, Fujii, together with other members of the Okayama Prefecture-based Satoumi Research Institute, started studying how the aquaculture of Pacific oysters is being impacted by monitoring oyster farm sites with different climate and water conditions in the town of Hinase, which faces the Seto Inland Sea, and Minamisanriku in Miyagi Prefecture. Since 2021, the researchers have added an oyster farm in the city of Hatsukaichi, Hiroshima Prefecture, as a third site to study.
The project is being supported by the Nippon Foundation, with the organization having committed to providing about ¥130 million over three years so far.
None of the hundreds of oyster larva samples collected from observation spots have yet to show any abnormalities, but acidification of the ocean water was found to be far more serious than researchers had predicted, said Takehiro Tanaka, director of the Satoumi Research Institute.
In addition to the pH of ocean water, the group has monitored the “aragonite saturation state,” a commonly used measurement of carbonate ion concentration. When it falls below 3, marine organisms become stressed, and when the saturation state is less than 1, shells and other aragonite structures begin to dissolve, according to the U.S. National Oceanic and Atmospheric Administration.
“The aragonite saturation levels we saw were stunning,” Tanaka said. “Though temporary, some observation points registered readings as low as 0.8, which is much lower than a level that is internationally known to impede the development of oyster and sea urchin larvae.”
If acidification has already progressed to the extent that it should start harming marine animals, why have samples not shown signs of stress? One possible explanation for this is that the oysters themselves may have adapted to the acidification of the ocean, Fujii said.
“But it’s just a hypothesis at this point,” he said. “We need to study this further.”
Haruko Kurihara, professor of marine science at the University of the Ryukyus in Okinawa Prefecture, said ocean acidification may already be affecting shellfish species. The farmers she has worked with have reported that their oysters are not growing as big and healthy as before. Her research shows that ocean acidification may be one of the reasons for this, and may become a bigger contributor within a few decades.
Kurihara has compared the development of oyster eggs in seawater with different pH levels. She has found that oyster eggs fertilized in carbon dioxide-enriched seawater did not grow as big as ones fertilized in less acidic seawater. If acidification accelerates further in the future, their shells won't form properly, thereby exposing their soft body parts, she said.
“Such oysters are likely to be captured and eaten by other sea animals, threatening their survival,” she said.
Kurihara also pointed out that ocean acidification is caused both by the absorption of carbon dioxide from the atmosphere and by the excessive richness of nutrients, called eutrophication, which can be worsened by rises in ocean temperatures. She called it “a double whammy.”
Eutrophication causes an increase in phytoplanktons, or very small plants that float near the surface of water, which leads to the creation of more organic matter. That matter produces carbon dioxide as it sinks to the bottom of the ocean and decomposes. Warmer ocean water accelerates this process, resulting in an increase in carbon dioxide in the deeper water, she said.
Oceans have absorbed the vast majority of the extra heat resulting from the greenhouse gas effect, according to the IPCC.
Furthermore, with a rise in the ocean’s surface temperatures, the surface water layer and the bottom water layer, which is much cooler, become harder to mix, she explained. That means it is harder for carbon dioxide at the bottom of the ocean to come to the surface, where it can be released back into the atmosphere, she said.
So what measures should be taken? Fujii says two approaches are necessary to counter the effects of ocean acidification. One is, of course, to reduce carbon dioxide emissions. The other is to come up with ways to adapt to it by changing the way the aquaculture and fishery industries operate.
For example, oyster larvae — which are most susceptible to acidic water — can be cultured in an artificial, less acidic environment before they are brought back to the sea. Another idea, the benefits of which are still being debated by experts, is to create seagrass meadows at the mouth of rivers to help raise pH levels during the day thanks to their absorption of carbon dioxide, Fujii said.
Kurihara argues that, on top of reducing carbon dioxide emissions, we need to better manage natural resources in watersheds and rethink the size of the aquaculture industry.
“Aquaculture is often said to be a solution to overfishing of the ocean. But overcrowded aquaculture can cause accumulation of organic matter (which causes ocean acidification),” she said. “So excessive aquaculture is not good either.”
While there are no quick and easy fixes, Tanaka stressed the importance of stepping up surveillance and networking among people with different areas of expertise. The group’s project currently involves 25 organizations, including national research institutes, universities and fisheries associations, he said.
“We would like to create a framework where fishers and scientists can talk and work closely with each other,” Tanaka said.
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