Ocean acidification and coral reefs

Natural CO2 seeps show what could happen to coral reefs in a world of increasing greenhouse gas emissions

June 01, 2011

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.

Site exposed to very high concentrations of CO2 where coral developement ceases to exist.

One of the MPI co-authors, Martin Glas said: "Not only did coral abundance change, also other calcifiers like foraminifera and calcifying algae were strongly reduced under elevated CO2 levels. This is disturbing news as they represent key-species for the formation of a healthy coral reef and contribute significantly to the reefs calcium carbonate production."

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. 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 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°C warming we have already observed in the tropics in the last 50 years has already caused mass coral bleaching events and declining coral calcification.

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 CO2emissions future or we face the risk of profound losses of coral ecosystems."
Fabricius went to say that it was important for the researchers to continue their study in the unique location in Papua New Guinea and that future expeditions are in preparation.

For further information contact:

Dr Katharina Fabricius
AIMS Principal Research Scientist,
+49 15259173182
k.fabriciusaims.gov.au (at present in Germany)

Participating institutions
Australian Institute of Marine Science (AIMS) www.aims.gov.au/
Max Planck Institute for Marine Microbiology www.mpi-bremen.de
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Florida, USA www.rsmas.miami.edu/

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