Australasian Science: Australia's authority on science since 1938

Underwater Acid Lab

By Stephen Luntz

The discovery of carbon dioxide seeps surrounded by coral reefs has given Dr Katharina Fabricius a chance to investigate our oceanic future. The news is not good.

Ocean acidification is gaining a profile as the second major danger from our fossil fuel addiction. Many marine species depend on seawater that is slightly alkaline, and carbon dioxide makes water acidic. Rising atmospheric carbon dioxide therefore makes the ocean less alkaline.

Researchers worldwide have been trying to predict the damage to coral reefs associated with acidification (AS, May 2008, pp.31–33), typically by setting up large experimental tanks with different CO2 concentrations. However, the studies are hampered by their temporary nature. “In the lab most experiments last just a few weeks,” says Dr Katharina Fabricius of the Australian Institute of Marine Sciences (AIMS). Consequently it is impossible to tell if species will eventually adapt.

Ten years ago Fabricius was exploring coral reefs off Papua New Guinea and discovered three carbon dioxide seeps. “At the time acidification was not on my radar,” says Fabricius. “I was looking at water quality, so I noted them, and when I started to be concerned about acidification remembered and organised the first expedition.”

Carbon dioxide vents exist elsewhere, but come with poisonous heavy metals or hydrogen sulfide, creating a poor model for future global conditions. Fabricius established, however, that the PNG vents were seeping pure carbon dioxide, something unknown in any equivalent location.

“Locals tell us the seeps have been there since they were children, so at least 70 years, and we have tested corals to confirm the seeps are at least 70 years old, but we can’t go back further,” says Fabricius.

Some Mediterranean seeps are still operating after being noted by the ancient Romans, but even if Fabricius’ finds are of more recent vintage they represent a long-term experiment unmatched in the lab.

Hopes that species adaptation might occur have been unfulfilled. The closer one gets to the vents, the lower the pH of the water and the more limited the ecosystem. “Ocean acidification slowly selects boulder-like massive coral over structurally complex branching and foliose (leaf-like) corals, which are the home of many species like crabs, shrimps and sea stars,” says Fabricius. “As a result, ocean acidification has a domino effect: as the habitat structure decreases, the animals that live and hide in their nooks and crannies find it far harder to survive, simply because they cannot hide from predators.”

The exact reason boulder corals survive acidification better than branching corals is not clear, but Fabricius says: “Boulder corals tend to be very tough and less susceptible to bleaching in hot weather or high turbidity. They’re the cockroaches of the seas.”

A few thousand square metres of ocean are affected by the seeps, with healthy communities around them, “We are working in conditions similar to what the world will experience in 50–100 years,” says Fabricius. When the pH drops below 7.8, as anticipated in around 2100 if current emissions continue, corals almost entirely disappear.

The surrounding reefs provide an opportunity for corals to recolonise areas that are lost, but this has generally not happened. “I’m convinced adaptation will occur,” says Fabricius, “but this could take thousands of years. Many corals take 10–20 years to start producing large numbers of larvae.”

For all their value, seeps complement rather than replace laboratory work. “In the field we can only talk about association and gradients. We don’t want to interfere,” Fabricius says. She uses the observations to influence what she tests in the lab, including the growth rates of branching and boulder corals.

Laboratories also provide an opportunity to learn when the damage is done. “Larvae drift in water for 6 days, not calcifying, so are probably OK, but we don’t know what happens once larvae settle,” she says. “We know the density of young corals is low at seeps compared to controls, recruitment seems to be impaired but we don’t know where the bottleneck is.”

Fabricius says she was keen on biology and ecology as a child, but marine studies came much later. “Growing up in Munich I was a long way from the sea.” After an undergraduate degree at the University of Munich in ecology she did her Masters at AIMS on feather stars, and came back to conduct much of her PhD at the Institute.

Coral reefs have been her focus ever since, primarily looking at the impact of run-off on reef health, both in the form of sunlight-blocking sediment and fertilisers that give algae and crown-of-thorns starfish the upper hand. “I’ve spent over 1000 days at sea diving in coral reefs,” says Fabricius, “studying coral reefs from the Great Barrier Reef to Palau.”