Israeli and American researchers have found that the growth of part of Australia’s Great Barrier Reef has slowed by as much as half in recent decades — threatening the reef and the ecosystem it supports.
Coral reefs grow as coral polyps secrete exoskeletons made of calcium carbonate. On an expedition, researchers from the Hebrew University of Jerusalem and from the Carnegie Institute of Science measured a stretch the Great Barrier Reef’s “calcification rate” and compared it to a similar measurement done more than 30 years earlier. They found the rate had declined by up to 49 percent.
The reduction matches predictions of coral growth rate based on carbon dioxide levels, suggesting a connection. The findings were published this month in the journal Geochimica et Cosmochimica Acta.
As carbon dioxide levels in the atmosphere increase, driving global warming, the ocean absorbs more of the greenhouse gas and so becomes more acidic. Higher acidity makes it harder for coral polyps to grow their exoskeletons. Meanwhile, reefs naturally dissolve and erode over time.
“The results of this study show a dramatic decrease in the calcification of the reef, and that it was likely caused by ocean acidification,” said Prof. Jonathan Erez, a geologist at Hebrew University, who helped lead the expedition. “When the rate of calcification becomes lower than the rate of dissolution and erosion, the entire coral ecosystem could collapse and eventually be reduced to piles of rubble. The collapse of this habitat would ultimately lead to the loss of its magnificent and highly diverse flora and fauna.”
Despite spanning less than 0.1 percent of the ocean’s surface and growing mostly in tropical waters, coral reefs support more than a quarter of all ocean life. Some coral reefs are major tourist attractions, including the Great Barrier Reef, the world’s largest coral reef system, located in the Coral Sea, off the coast of Queensland, Australia.
Coral reefs are threatened by recent environmental changes, like coastal runoff, global warming, and ocean acidification. A 2012 study found that the Great Barrier Reef had lost half of its live coral coverage since 1985. Other research has found declines in growth rates for individual corals.
To better understand the role of acidification in coral growth decline, the researchers studied the calcification rate of all the corals on Lizard Island on the Great Barrier Reef between 2008 and 2009. Comparing the rate to similar measurements made between 1975 and 1976, they found a decrease of 27 to 49 percent.
Contrary to the 2012 study, the researchers found little change in the Great Barrier Reef’s coral coverage. But they note that the method they and their predecessors in the 1970s used to estimate the coral coverage is inherently uncertain.
The researchers say the lower coral growth rate they found is consistent with predictions that take into account the carbon dioxide increase since the previous measure, suggesting that ocean acidification is the main cause.
The results, they say, may mean that coral reefs around the world are now making exoskeletons that are less dense and more fragile. While they still look the same, these coral reefs are less able to resist physical and biological erosion.
“Routine measurements of net community calcification should be continued not only at Lizard Island but at other reefs around the world in order to monitor their well-being in a high carbon dioxide world,” said Prof. Boaz Lazar, a geochemist at Hebrew University, who helped lead the study.
The InterAcademy Panel, a network of 106 science academies that advises the public on global issues, in 2009 said reducing human carbon emissions is the “only practicable solution to mitigating ocean acidification.” The panel said levels would need to be halved by 2050 compared to the 1990 level.