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Winners and losers in ocean acidification

CO2 seep

Underground releases of CO2 like this are giving scientists insight into the impact of ocean acidification (Source: Katharina Fabricius)

pH change A natural experiment in tropical waters off Papua New Guinea suggests that with increasing CO2 emissions and ocean acidification some marine organisms will benefit, but many more will lose out.

Climate scientist, Dr Janice Lough of the Australian Institute of Marine Sciences in Townsville and colleagues report their findings today in the journal Nature Climate Change.

"Even aside from thermal stress and bleaching we're going to get progressive acidification of the oceans and this study demonstrates that is not good for maintaining healthy coral reef ecosystems," says Lough.

"Although there are a few winners as you get higher CO2 there are many more losers."

Increasing atmospheric CO2 levels lead to an increase in ocean acidity as the ocean absorbs some of this CO2.

"About a third of the extra CO2 that we've put into the atmosphere has been absorbed by the oceans and that changes the ocean chemistry," says Lough.

The IPCC predicts that with atmospheric levels of at least 750 parts per million by 2100, ocean pH levels will drop from 8.1 to 7.8.

Short-term laboratory experiments suggest this will change the chemistry of the ocean and make it harder for coral and other organisms to form calcium carbonate skeletons.

But now scientists have real world evidence of how increasing acidity affects marine communities.

CO2 seeps

Lough and colleagues studied three areas of tropical coral reef in Papua New Guinea's Milne Bay Province where CO2 naturally seeps into the water from volcanic activity beneath the ocean floor.

Unlike at other known CO2 seeps, the CO2 released here is at the cool ambient temperature of the surrounding water.

This means that any impact on the coral there is due to CO2 rather than warmer temperatures, says Lough.

The researchers found acidity increased closer to the CO2 seeps and decreased further away from them.

"There is a natural physical gradient due to CO2 where we were able to compare the characteristics of the coral reef communities between high and low CO2," says Lough.

The researchers found coral cover was maintained between pH 8.1 (the pH of normal ocean water) and 7.8. But by 7.7 there was no coral development at all.

And despite coral cover being maintained until a pH of 7.8 the diversity of corals suffered with less branching corals and more boulder-like porites.

"So there were still corals at the high CO2 sites, but it was not the coral reef that we know and love to look at," says Lough.

"It's a very dramatic illustration of what can happen to coral reef communities as a result of ocean acidification."

Seaweed and seagrass were also "winners" with increasing acidity, says Lough.

Biodiversity threat

Hundreds of thousands of different organisms rely on the complexity of coral reefs for their habitat.

The implication of lower diversity on reefs doesn't just mean a less "pretty" reef, but also a less productive one, says Lough.

"We have the luxury in Australia of being able to appreciate the Great Barrier Reef for what it is, but there are a lot of communities in the Pacific and Southeast Asia that rely on healthy coral reefs for their food and wellbeing," she says.

"So if we're going to start compromising that complexity, we'll end up with much simpler reef systems and a lot of the goods and services they provide are going to diminish."

The researchers also found that there were fewer juvenile corals in higher acidity water, reducing the resilience of the corals to tropical storm and bleaching events.

Tags: climate-change, oceans-and-reefs