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How does your coral garden grow?

December 13, 2010

Panama: I don’t usually cover new research paper releases here, there are plenty of people already doing a fine job of that. But a coral experiment published this past weekend caught my eye – probably because I am here in Panama where it was carried out.

Researchers here have been looking at how ocean warming and acidification will affect one of the most important survival strategies reefs have: the settlement of coral larvae on a suitable rock, followed by the growth of baby coral polyps. Aaron O’Dea, Holger Anlauf and their colleagues at the Smithsonian Tropical Research Institute, scooped up orangey-pink larvae of a tropical eastern-Pacific coral, Porites Panamensis, and grew them under very mild conditions of global warming to see what happened.

The baby corals, called polyps, send out tentacles that they can use to catch plankton prey (although the water was filtered for this experiment)

This kind of experiment may seem an obvious one if you’re interested in knowing climate change impacts on a living system, but until recently, no one’s really been doing them. We visited something similar for fish at INPA in Manaus, but this is the first to look at baby corals in this way before.

Holger babysat 40 of the baby corals for 42 days under four different conditions: In the first tank, the researchers simulated 1C of ocean warming; in the next, they simulated ocean acidification by bubbling carbon dioxide through the tank to lower the pH by 0.25; the third combined this warming and acidification; and a fourth tank maintained current ocean conditions as a control.

The experiment was set up so temperature and acidification could be regulated throughout

“The different conditions had absolutely no effect on the ability of the larva to settle – to stick to the rock surface – which may be good news for people who are trying to grow coral gardens,” Aaron says.

But post-settlement, some of the young coral polyps were showing the effects ‘global warming’.

“The biggest surprise was that neither temperature alone, nor acidification alone had a big effect on the growth or survival rate [95%] of the coral, even though the warming prompted zooxanthallae expulsion as expected,” Aaron says. “Once we combined this moderate warming and acidification, though, we saw significant impacts: growth rate of the polyps – for both the skeletal and soft pulpy mass – plummeted to almost half of the rate seen under the other three conditions, and they were twice as likely to die [90% survival rate].”

Baby corals need to calcify if we are to continue to see reefs

Aaron thinks that the experiment may have been too short to show the separate detrimental effects of acidification and warming. But when the two scenarios were combined, the pressures became evident, as previous studies have shown.

Under more acidic conditions the coral needs to use greater energies from its metabolism to calcify, but the higher temperatures cause the coral to expel the zooxanthallae algae that live inside the coral skeleton and provide most of its nutrients – ie. less food (sugars) and oxygen are available to the corals when they need it most, O’Dea says. “The cumulative effects are disastrous for reefs,” he says.

The prognosis for coral reefs is dire, with some biologists predicting as many as 98% of corals will be extinct by 2050. Human-induced global warming has a double whammy effect on corals: higher ocean temperatures lead to bleaching, in which the marine organisms to expel so much zooxanthallae that they become white; and higher carbon dioxide levels make oceans more acidic, slowing down the rate at which corals can build their calcium carbonate skeletons. The past 20 years have already seen several major coral bleaching events, which many corals never recovered from .

I called up Sophie Dove, director of Coral Reef Ecosystems Lab, at the Global Change Institute, Queensland, Australia, which is starting similar experiments to look at reef survival under climate change scenarios. She says that it was interesting that the Panama team found ‘global warming’ made no difference to lava settlement, but pointed out that “if settlement isn’t followed by rapid calcification then all you’ve got is a living coral – it won’t be a coral reef”

Also, she cautions, the extent of ocean warming and acidification is likely to be far greater than those simulated in this experiment – IPCC scenarios look at warming of 4C and pH reductions of 0.4. “Organisms don’t respond linearly to temperature or acidification. They reach a lethal threshold at which their proteins deteriorate or they cannot manage acids,” Sophie says.

And, even if the coral babies survive these mild global warming conditions they then have to battle additional reef stressors, such as pollution, anchor damage and overfishing.

If you want to look at the study, it’s published here: Anlauf, H., et al., J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.11.009

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