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Acidic seas really do harm coral reefs — but the harm can be reversed

First experiment on a natural reef uses antacid to help coral grow

Experimental seawater flowing over the reef flat study site. A pink dye tracer was used to track the movement of seawater.
Experimental seawater flowing over the reef flat study site. A pink dye tracer was used to track the movement of seawater.
Rebecca Albright

Ocean acidification is making it more difficult for coral reefs to grow, a study shows for the first time. Though scientists have speculated for the last decade that more-acidic seas have been harming coral, it took a dose of antacids for them to figure it out for sure.

Over the course of 22 days in September and October 2014, scientists doused Australia’s Great Barrier Reef with an antacid. They found that by lowering the water’s acidity, they helped coral grow, according to a report in the journal Nature.

The oceans are becoming more acidic because they absorb carbon dioxide — around one-quarter of what’s released into the atmosphere, according to the study. When carbon dioxide interacts with sea water, the water becomes more acidic. The Blue Ribbon Panel on Ocean Acidification conducted in Washington State in 2012 found that oysters and crabs have had thinning shells , a product of acidification. Corals are also built from calcium carbonate, and they grow by laying down bands; scientists have noticed that band growth was declining. That’s because acidic water corrodes the calcium that makes the shells and structures in these creatures. But until now, there wasn’t definitive proof the water’s acidity was causing decalcification.

Coral calcification increased by nearly 7 percent

"What makes this study groundbreaking is that it utilized experimental manipulations on an actual reef," said Derek Manzello, principal investigator of NOAA’s National Coral Reef Monitoring Program, in an email interview. This was the first experiment to reproduce lab results in a natural reef, according to Manzello, who did not participate in the study.

Acidity is measured using pH, a scale ranging from zero on the acidic end to 14 on the basic — or alkaline — end. Pure water is neutral, with a pH of seven; anything less than seven is acidic. Historically, the pH of the ocean has hovered around 8.2, or slightly alkaline. But over the past two centuries, the water has tipped toward the acidic end of the scale. Currently, the ocean’s pH is 8.1 , a 25 percent increase in acidity. While a drop of 0.1 pH units might not seem drastic, it‘s nonetheless proving deadly for marine life.

Today’s study was conducted by a team led by Rebecca Albright and Ken Caldeira of the Carnegie Institution for Science on One Tree Reef in Australia’s Great Barrier Reef. The location was chosen because of its isolation: an island encircling three lagoons that were closed off from the ocean at low tide. This let the team work in natural conditions but with greater control than a reef in open water.

Albright described the study as a gamble. "In our case it really paid off because it really allows us to kind of move beyond laboratory experiments and understand responses in the natural environment," she said.

The scientists pumped a control dye and sodium hydroxide, which Caldeira referred to as "an antacid," into the lagoons once a day for an hour during low tide. They wanted to reduce the ocean’s acidity to pre-industrial levels. At the end of the hour, they compared seawater upstream and downstream from the pumping station, looking for changes in the level of both the antacid and dye. Changes in the amount of dye would help the team understand how much the surrounding ocean water was diluting their mixture, while changes in the amount of the antacid would tell them how much was being absorbed by the lagoons’ reefs.

"Really the only solution is cutting carbon emissions."

The scientists first thought that decreasing the seawater’s acidity would have no effect on the coral. However, they found that that the reef absorbed an average of 17.3 percent of the antacid over the duration of the experiment. Absorbing antacid helps offset the calcium-corroding acid in the seawater. Thanks to that absorption, they found that coral calcification increased by nearly 7 percent over the course of the study — proving that reversing ocean acidification increases coral growth.

This was the third such experiment for Caldeira. The first two experiments were focused on refining the methodology. In the second study, conducted in 2013, the group’s results were similar to those published today. But they were never published, as Caldeira wanted to confirm the findings before releasing them.

Albright considers this study the "tropical counterpart" to the seminal Blue Ribbon Panel on Ocean Acidification conducted in the Pacific Northwest in 2012. Their findings, published in November 2012 by NOAA, found that the billions of oyster larvae deaths in Washington hatcheries between 2005 and 2009 were due to ocean acidification. The declining pH of the seawater had "created conditions corrosive to shell-forming organisms like young oysters," the panel wrote.

"What it found was that ocean acidification is already impairing [growth]. This is not a concern for the future; it’s happening and it’s happening now," Albright said. Her study is the first strong evidence scientists have of similar effects on marine calcifiers in tropical waters.

While today’s study proves the impact of our carbon emissions on coral reefs, it points to no easy solutions. Damage to coral could be reversed by pumping antacid across reefs, but the authors note that this is so technically challenging and costly that it wasn’t feasible "at anything but highly localized scales."

"I think everybody kind of wants a bandaid for what’s going on right now with global warming and ocean acidification," Albright said. "And really the only solution is cutting carbon emissions."