Research by the Australian Institute of Marine Science (AIMS) has drawn on the effects of natural carbon dioxide (CO2) seeps in Papua New Guinea to give an insight into what tropical coral reefs could look like if human-induced atmospheric CO2 concentrations continue to rise unabated.

At present rates of increase, the Intergovernmental Panel on Climate Change (IPCC) forecasts atmospheric CO2 levels of about 750ppm or more by 2100. About a third of this extra atmospheric CO2 is absorbed by the world’s oceans. As a consequence, pH levels will drop from 8.1 to 7.8, resulting in increased ocean acidification which impacts on coral reef ecosystems.

AIMS scientist Dr Katharina Fabricius has led two research expeditions, with researchers from six countries including Papua New Guinea (PNG), to study three natural CO2 seeps in Milne Bay Province, PNG. This unique location is the only presently known cool, CO2 seep site in tropical waters containing coral reef ecosystems.

At the seeps, streams of CO2 bubbles emanate from the ocean floor due to volcanic activity.

A scientific paper on the first results of this study has just been published in the international scientific journal Nature Climate Change. It is the first scientific paper to present data on tropical coral reef ecosystems that are naturally adapted and acclimatised to elevated CO2.

While laboratory experiments are important, Dr Fabricius said the natural CO2 seeps in Milne Bay provided a more complete picture of the ecological consequences for coral reef communities when exposed to higher levels of CO2 for many decades. This natural setting allowed scientists to compare coral reef communities along a gradient from normal present day to low pH.

"Our research showed us there will be some winners, but many more losers, when tropical coral reefs are exposed to ocean acidification," she said.

"We found that as pH decreases the number and types of corals making up coral reefs are much reduced. Diversity of corals drops by 40 per cent, and the reef becomes dominated by one form of corals, massive Porites.

"The cover of the more delicate tabulate, foliose and branching corals was reduced three-fold near the CO2 seeps. Similarly, the abundance of soft corals and sponges were also significantly reduced. Most importantly we found that reef development ceased below pH level 7.7."

One of the AIMS co-authors, Dr Janice Lough said: "The study has shown that massive Porites corals are able to tolerate relatively high levels of CO2. However, their growth rates were 30% lower than expected, both at the seep sites and at the surrounding reefs. We attribute this slow growth to recurring heat stress, with nine of the last 12 years having experienced extremely high seawater temperatures".

Amongst the few winners at higher levels of CO2 were seagrasses which showed increased cover with three to four times more shoots and roots than under normal conditions.

Dr Fabricius said the study showed that ocean acidification leads to profound changes in coral reef ecosystems.

"Ultimately, what we observed was that the diversity of reefs progressively declines with increasing CO2. At concentrations similar to those predicted for the end of this century at a ‘business as usual’ emissions scenario, the "coral reef" observed was depauperate and lacked the structural complexity of present healthy tropical coral reefs. These changes are simply due to ocean acidification, i.e., even without the projected +2°C warming of the oceans associated with rising greenhouse gases. The 0.5° warming we have already observed in the tropics in the last 50 years has already caused mass coral bleaching events and declining coral calcification.

Scientists will also be able to use the new $30 million Australian ocean simulator currently under construction at AIMS’ site in Townsville to improve understanding of impacts of CO2 on organisms in Australia’s tropical oceans territory.

The new Australian ocean simulator is a national infrastructure asset that will deliver much improved data and knowledge on the impact of global change, climate adaptation and mitigation issues.

More information is at www.aims.gov.au.