Ocean acidification and coral reefs
Natural CO2 seeps show what could happen to coral reefs in a world of increasing greenhouse gas emissions
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 will severely impact marine ecosystems. The coral reefs, having the highest biodiversity of all marine ecosystems, are probably the most vulnerable.
Katharina Fabricius, scientist at the Australian Institute of Marine Science (AIMS) has led two research expeditions, with researchers from six countries including Germany, USA and Papua New Guinea. Among the international team of scientists was a group from the Max Planck Institute for Marine Microbiology in Bremen, experts in the field of microsensors, a valuable tool for studying coral reefs.
The team of scientists studied three natural CO2 seeps in Milne Bay Province, Papua New Guinea. This unique location was discovered by chance during a field trip to document biodiversity and 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.
This week a scientific paper on the first results of this study is 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. "Our research showed us there will be some winners, but many more losers, when tropical coral reefs are exposed to ocean acidification," said principal investigator Fabricius. "In the past, we have relied on short-term laboratory experiments to tell us what happens to marine organisms exposed to ocean acidification," she said. "Those experiments indicated deleterious effects on the performance of many species."
While laboratory experiments are important, 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.
"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 boulder corals (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."