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.
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.
Seascape at control site. Image: Katharina Fabricius.
Seascape showing moderate seeps. Image: Katharina
AIMS scientist Dr Katharina Fabricius has led two
research expeditions, with
researchers from six countries, including scientists from Port
Moresby University to study three natural CO2 seeps
at Milne Bay, PNG. This unique location is the only 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
subterranean geological activity.
Seascape showing intense
vents. Image: Katharina Fabricius.
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
"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."
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.
Seascape showing high
vents. Image: Katharina Fabricius.
"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 four-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", Dr Lough
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 reefs ecosystems.
"The decline of the structurally complex corals means the
reef will be much simpler and there will be less habitat for
the thousands of species we associate with today’s coral
"They would not be the richly diverse and beautiful
habitats we currently see in places such as the Great Barrier
"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
"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 that is already
causing mass coral bleaching events."
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 therefore 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
Dr Fabricius said it was important for the researchers to
continue their study in the unique location in PNG.
She said the new $30 million Australian Tropical Oceans
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 ATOS is a national infrastructure asset that will
attract scientists from throughout the world, working on
cutting edge climate change research. It is aimed at delivery
of 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.