Fight to save coral reefs undermined by new threat
BRISBANE The world’s coral reefs, already enduring multiple threats from bleaching to nutrient runoff from farming, face another challenge – this time from below.
New research, published in the journal Science yesterday, has found that the sediments on which many reefs are built are 10 times more sensitive to the acidifying oceans than the living corals themselves.
Some reef bases are already dissolving.
The study used underwater chambers at four sites in the Pacific and Atlantic oceans, including Heron Island in the Great Barrier Reef, and applied modelling to extrapolate results for 22 reefs in three ocean basins.
As the oceans turn more acidic, corals produce less of the calcium carbonate that forms their base. Instead of growing, the reef bases start to dissolve.
‘‘The public is less aware of the threat of ocean acidification [than warming waters],’’ said Bradley Eyre, a professor of biogeochemistry at Australia’s Southern Cross University and the paper’s lead author.
‘‘Coral reef sediments around the world will trend towards dissolving when seawater reaches a tipping point in acidity – which is likely to occur well before the end of the century,’’ he said.
At risk would be coral reef ecosystems that supported tourism, fisheries and many other human activities, he said.
The ocean’s acidity has increased about 30 per cent since the start of the Industrial Revolution, as seas absorb about onethird of the buildup of greenhouse gases in the atmosphere.
‘‘It is vital that we put pressure on governments globally to act in concert to lower carbon dioxide emissions, as this is the only way we can stop the oceans acidifying and dissolving our reefs,’’ Eyre said.
The Hawaiian reef studied was already showing signs of its sediment dissolving, with higher organic nutrient levels likely to be contributing, he said.
Living corals themselves appear to be able to resist the acidification process, with mechanisms and strategies to resist some of the impacts.
Still, the study says the transition of the dissolution of reef sedi- ment ‘‘will result in the loss of material for building shallow reef habitats such as reef flats and lagoons, and associated coral cays’’.
It is unknown if the reefs will face ‘‘catastrophic destruction’’ once the erosion begins, the paper says.
Over time, as coral bases began to dissolve, they were more likely to become more vulnerable to cyclones and other threats, Eyre said.
He said further study was needed to understand how reefs would be affected by the combination of higher temperatures, rising organic and nutrient levels and more acidic waters.
The impact of bleaching – such as the two mass events in the 2015-16 and 2016-17 summers on the Great Barrier Reef – would most likely accelerate the breakdown of reefs, the paper said. Fairfax