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Life on a coral reef is a delicate balance between destruction and creation — a balance under threat as oceans become more acidic, according to research at the Hawaiian Institute of Marine Biology. Corals build a calcium carbonate skeleton beneath them for support as they grow but, all the while, creatures such as parrotfish and marine worms are eating away at the new growth. This balance between growth and erosion is a perfectly natural process — the work of these bio-eroders gives us the fine white sand found on tropical beaches — but it may be endangered by the rising acidity of our oceans, due to CO2 emissions from human activities.
A study published in Marine Ecology Progress Series shows that the erosion of coral reefs is given a boost in an acidic environment. “In order for reefs to exist, the rate of reef growth must be higher than the rate of reef erosion,” says Nyssa Silbiger, lead researcher. “The problem is not that reefs are eroding but that erosion rates could become excessive in an altered environment.”
The team placed blocks of calcium carbonate in Kaneohe Bay, Hawaii, for a year and used a form of high-resolution X-ray scanning called microCT to build up 3D models of the blocks on their return. These enabled researchers to visualise the erosion that occurred.
Analysis of a variety of factors, including depth, temperature and distance from the shore, showed that the acidity of the water was linked most strongly to reef erosion. Silbiger believes that the increased acidity could make it easier for bio-eroders to break down the skeleton.
The finding is doubly bad news for coral reefs, as we already know that growth is hampered by acidity as well: when under stress, corals don’t lay down new skeleton. “This could be a double whammy for the future of coral reefs,” Silbiger says.
It’s a worrying thought for the 360 million people living in Southeast Asia’s Coral Triangle. The Triangle is home to 76 per cent of the world’s coral species. The World Wild Fund Nature (WWF) estimated in 2012 that coral reefs brought a net annual benefit of more than $2bn to Indonesia alone.
“The resources directly sustain the lives of people living in coastal areas . . . through provision of food and medicines, livelihoods, income generation, [and] protection of coastal zones,” says Jackie Thomas, leader of the WWF’s programme to protect the region. “The World Resources Institute predicts that, by 2030, virtually all coral reefs in the Coral Triangle region will be threatened by a combination of local human activities, ocean warming and acidification.”
Photographs: Alamy; Dreamstime