Acidic 'champagne sea' nothing to celebrate for corals
Earth 8 June 2008
By Catherine Brahic
Video: Acidifying oceans of the future
Researchers found snails whose shells were dissolving in low pH water
(Image: Maoz Fine)
Corals were healthy in "normal" water but fared poorly in acidic water
(Image: Riccardo Rodolfo-Metalpa)
Lush, grassy, and populated by invasive algae and molluscs with paper-thin shells – that is what the acidic oceans of the future could look like.
An exploration of natural "bubble streams" of carbon dioxide in shallow Mediterranean waters off the coast of Italy is the first to document the effects of ocean acidification in a real ocean setting.
The notion that the oceans will become more acidic as CO2 concentrations rise is well understood. By the year 2100, ocean acidity is predicted to be 7.8 pH, compared to 8.2 pH in 1900.
But all the studies of how this will affect marine ecosystems have been carried out in laboratories, many involving organisms with shells being placed in low-pH seawater and watching them slowly dissolve.
The experiments give little indication of the degree to which this would happen in the open oceans, affected by currents and the population dynamics which regulate ecosystems.
Champagne sea
Jason Hall-Spencer and colleagues at the University of Plymouth, UK, have now looked at just this process. They found a site off the island of Ischia in southern Italy where geologic CO2 naturally seeps through the seafloor.
"It is visually stunning, like a 300-meter-long Jacuzzi," says Hall-Spencer, who says diving on the site felt like swimming through champagne.
Natural CO2 seeps are usually associated with hot vents. The Ischia site offers an usual opportunity to study cool, acidified ecosystems that are not modified by the toxic effects of sulphur.
It also allowed the researchers to study how the Mediterranean ecosystem changed across a pH gradient. On the outer edges of the "Jacuzzi", the pH was a "normal" 8.2; in water immediately above the seep, it dropped to 7.4.
At a pH of 7.8, the team noticed a marked change. Populations of coralline algae, which hold reefs together, suddenly crashed. Sea-urchins also disappeared.
And there was a transition from a coral ecosystem to one dominated by lush sea grasses. These, along with invasive algae originally from Asia, thrived in the acidic waters. The researchers warn that as waters become more acidic, opportunistic invasive species that are better able to survive at low pH could move in.
Another marked effect of the acidified water was something which has been described in laboratory settings – the hard shells of animals such as limpets softened. "They were paper-thin," says Hall-Spencer. "You could push your thumb through them."
Hall-Spencer believes the ecosystem changes he and his colleagues documented in Ischia are relevant to other ocean ecosystems because the types of organisms they studied – sea urchins, corals and sea-grasses – are found around the world.
In the US, an initiative known as the Free Ocean CO2 Enrichment experiment is underway to pump CO2 into the sea-floor off the coast of California and look at how this changes the environment.
Hall-Spencer believes that other natural seeps could be found and studied around the planet. Having looked at geological maps of the Gulf of Mexico, he believes cold CO2 seeps could be found there.
Journal reference: Nature (DOI: 10.1038/nature07051)
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