Ocean acidification is already significantly altering marine ecosystems, and if CO2 concentrations continue to rise, the effects of ocean acidity could be even more profound.

By studying the impacts of volcanic CO2 seeps off the coast of Japan, scientists have gained a better understanding of how increasing CO2 concentrations are likely to reshape marine ecosystems.

"These CO2 seeps provide a vital window into the future," Sylvain Agostini, researcher at the University of Tsukuba Shimoda Marine Research Center, said in a news release. "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."

Surveys of the underwater seeps showed smaller aquatic weeds and algae benefited most from increasing CO2 concentration, marine plants that tend to blanket the seafloor and smother corals. Researchers also found some smaller species were benefiting from the shifting ocean chemistry, but that rising CO2 levels -- and resulting ocean acidification -- were associated with diminished biodiversity.

"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," Agostini said.

Because strong ocean currents ensure the surrounding waters remain well-mixed, the chemistry reflects pre-industrial conditions, allowing scientists to better compare and contrast ecosystems exposed to lower and higher levels of acidity.

"Our research site is like a time machine. In areas with pre-Industrial levels of CO2 the coast has an impressive amount of calcified organisms such as corals and oysters," said Jason Hall-Spencer, professor of marine biology at the University of Plymouth. "But in areas with present-day average levels of surface seawater CO2 we found far fewer corals and other calcified life, and so there was less biodiversity."

The findings -- detailed in the journal Scientific Reports -- suggest rising CO2 levels in the ocean are likely to encourage less diverse more monolithic ecosystems.

"It shows the extensive damage caused by humans due to CO2 emissions over the past 300 years and unless we can get a grip on reducing CO2 emissions we will undoubtedly see major degradation of coastal systems worldwide," Hall-Spencer said.

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