A first-of-its kind, field-controlled experiment carried out for 22 days between September 16, 2014 and October 10, 2014 in a natural coral-reef community in the Great Barrier Reef has allowed scientists to unequivocally show the detrimental effects of ocean acidification on coral reefs across the world. According to the study published on February 25 in the journal Nature, the net coral-reef growth would have been seven per cent more in the absence of ocean acidification.

The unique design of the study allowed the researchers to pinpoint the role of ocean acidification in diminishing the coral-reef growth even without altering or removing other compounding factors such as elevated sea surface temperature due to global warming, land-based pollution and overfishing. Coral reefs, which provide marine ecosystems comparable to tropical rain forests, are most vulnerable to ocean acidification.

Ocean acidification arises when nearly 25 per cent of carbon dioxide released into the atmosphere and absorbed by the oceans reacts with water to form carbonic acid.  The carbonic acid thus produced leads to ocean acidification by decreasing the pH of the ocean, reducing the concentration of carbonate ion (which is essential for organisms such as corals and clams to build their shells and skeletons) and a decreased aragonite mineral saturation state (“a measure of the availability of dissolved carbonate and calcium ions”).

According to Nature, today’s oceans are already 30 per cent more acidic than they were before the Industrial Revolution.

Increasing the pH of the ocean to make it more alkaline than acidic will provide an ideal condition for coral reefs to grow.  While it is nearly impossible to do so on a large scale and study its impact on coral growth, the unique conditions seen in the One Tree Reef that encloses three lagoons in the southern Great Barrier Reef provided an ideal location to test this.

All the three lagoons in the One Tree Reef get cut off from the ocean during low tide, and water tends to flow from the first lagoon to the third lagoon as a result of elevation difference of 30 cm. A reef made of live corals separates the two lagoons and water flows over the reef in one direction for one full hour a day during peak low tide.

The team artificially increased the alkalinity of the water flowing from one lagoon to another over the reef flat by introducing sodium hydroxide solution once a day for 15 days.  An active tracer in the form non-reactive dye solution was introduced into the reef flat during the entire duration of the study — 22 days. The first seven days when no sodium hydroxide was added along with the tracer served as control.

“By measuring the concentrations of dye and sodium hydroxide the scientists were able to calculate the overall increase in calcification across the reef when the seawater chemistry was altered,” notes Nature.  What they found was truly dramatic — net coral-community increased by 7 per cent. In addition, they found that around 17 per cent of the added sodium hydroxide was taken up by the reef community and the increase in alkalinity led to 0.6 per cent increase in aragonite saturation state.

The experiment helped prove that restoring the ocean chemistry to pre-industrialisation conditions led to an increase in coral growth.  “We provide evidence that net community calcification is depressed compared with values expected for preindustrial conditions, indicating that ocean acidification may already be impairing coral reef growth”, writes Rebecca Albright, the first author of the paper from the Carnegie Institution for Science, Stanford, California. The reduction in calcification as a result of acidification is “projected to shift coral reefs from a state of net accretion to one of net dissolution this century”.

Global warming has not only increased the acidity of the oceans but has also elevated the sea surface temperature. While a warming ocean would have initially favoured coral reefs and led to more growth, the continued increase has proved harmful. Hence, coral reefs suffer from the combined onslaught of both acidification and elevated sea surface temperature. Future studies need to look at the combined effect on coral reefs.

While the researchers have been able to temporarily reverse the ocean chemistry at a very local scale, the only way to achieve this globally is by reducing the amount of carbon dioxide emitted. Even the Paris climate agreement has able to reach a consensus to keep the global temperature “well below the 2 degree C above the pre-industrial levels would mean little for the health of coral reefs; they run the highest risk of acidification and ocean warming.

“Caldeira’s team is planning a second experiment — scheduled for September — that will help them to take a peek at possible future ocean conditions. Rather than adding an antacid to seawater, the researchers will add carbon dioxide to boost acidity to levels that would be expected in 2100 if emissions continue to rise,” notes Nature.

Published in The Hindu on February 28, 2016