As oceans get more acidic, sediments that constitute coral reefs could begin dissolving by the end of this century, suggests a study published in the journal Science.
Coral reefs are formed by not just the calcium carbonate skeletons that tiny animals called coral ‘polyps’ create, but also carbonate sediments which accumulate on them over thousands of years. Ocean acidification – lowering of sea water’s pH when it absorbs the excess, human-caused carbon dioxide from the atmosphere – prevents polyps from building their stony skeletons.
In several reef systems, acidification also dissolves corals’ carbonate sediments. Scientists from several institutes including the Southern Cross University in Australia studied this less-explored aspect of sediment dissolution at 57 locations across five reefs in the Pacific and Atlantic Oceans. They found that the link between sediment dissolution and acidification is stronger than that between acidification and coral formation.
Predictions
Based on this, the team also predicted changes in coral systems by incorporating several factors including current rates of coral formation and sediment dissolution. According to their calculations, coral sediments will begin dissolving by 2050; by 2080, they will dissolve faster than they are formed.
“It would be extremely worrying if this does start to happen,” says scientist Rohan Arthur of Nature Conservation Foundation, who studies the coral reef systems of India's Lakshadweep Islands. “This could add to the problem of coral decline that we are already seeing because many of the reefs in the Indian Ocean are already net eroding.”
This means ocean acidification is causing coral reef systems to erode rather than grow. Currently, the processes of coral formation are also under threat. In 1998, Lakshadweep’s reefs experienced bleaching: increased ocean temperatures caused algae that live as symbionts within corals to leave, stressing the corals. Two more bleaching events followed in 2010 and 2016. With repeated bleaching, frequent storms due to climate change and now, ocean acidification that causes sediment dissolution apart from slowing down coral-building, Lakshadweep’s reefs could be facing a triple whammy, says Arthur.
“Local factors like over-fishing in reefs too play a role,” he adds.
Indian reefs
Coral reefs span 3,062 sq. km in India. Many coral species are afforded protection at par with tigers: they are included in Schedule I of the Wildlife Protection Act (1972). While coral systems support a diversity of fish species that local communities depend on for sustenance, many like those in Lakshadweep also provide protection from storms and prevent coastal erosion.