Natural carbon dioxide (CO2) seeps in Papua New Guinea have given scientists rare insights into what tropical coral reefs could look like if human-induced atmospheric CO2 concentrations continue to rise unabated.
Australian Institute of Marine Science (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. The study has given scientists unprecedented insights into what coral reefs would look like if greenhouse gas emissions and resulting ocean acidification continues to increase at present rates. 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 prestigious, 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.
In the past, we have relied on short-term laboratory experiments to tell us what happens to marine organisms exposed to ocean acidification," Dr Fabricius said. "Those experiments indicated deleterious effects on the performance of many species.
Dr Katharina Fabricius
While laboratory experiments are important, Dr Fabricius said the natural CO2 seeps in Milne Bay provided a more complete picture about 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."
Porites are more tolerant
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.
"The decline of the structurally complex corals means the reef will be much simpler and there will be less habitat for the hundreds of thousands of species we associate with today’s coral reefs.
"They would not be the richly diverse and beautiful habitats we currently see in places such as the Great Barrier Reef."
"There are also fewer juvenile corals in areas with high CO2 levels, therefore coral reefs in those environments face greater challenges recovering from disturbances such as tropical storms.
"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."
Dr Fabricius said: "The rate of increase of atmospheric CO2 continues to accelerate due to human activities. The range of exposures at the Milne Bay seep sites are comparable to end-of-century CO2 projections.
"It would be catastrophic if pH levels dropped below 7.8."
"This study proves we must urgently transition to a low CO2 emissions future or we face the risk of profound losses of coral ecosystems."
Dr Fabricius said it was important for the researchers to continue their study in the unique location in PNG.
She said the $30 million new Australian ocean simulator currently under construction at AIMS’ site in Townsville would also contribute significantly to researchers understanding of impacts of CO2 on organisms in Australia’s valuable and complex tropical oceans territory.
The new Australian ocean simulator is a national infrastructure asset that will attract scientists from throughout the world, working on cutting edge climate change research. It is aimed to deliver much improved data and knowledge on the impact of global change, climate adaptation and mitigation issues.
The new facility will boost collaborative, world-class scientific and technological research, and will coordinate efforts in tropical marine science.
Our research showed us there will be some winners, but many more losers, when tropical coral reefs are exposed to ocean acidification
—Australian Institute of Marine Science (AIMS) scientist Dr Katharina Fabricius