More acidic water may be a sign of healthy corals, says a new study, muddying the waters still further on our understanding of how coral reefs might react to climate change.

Andreas Andersson of the Scripps Institution of Oceanography in San Diego, California, and his colleagues carefully monitored a coral reef in Bermuda for five years, and found that spikes in acidity were linked to increased reef growth.

“At first we were really puzzled by this,” says Andersson. “It’s completely the opposite to what we would expect in an ocean-acidification scenario.”


The researchers observed the chemistry of the water on the reef between 2007 and 2012. During that time, there were two sharp spikes in acidity – once in 2010 and again in 2011.

The team found that coral growth itself made the water more acidic as the corals sucked alkaline carbonate out of the water to build their skeletons. The corals also ate more food during these high-activity periods and pumped more CO 2 into the water, increasing acidity further.

So the researchers tried to figure out what was driving the changes in coral growth and water acidity. Luckily, there is a station 80 kilometres further offshore from this reef that also measures ocean chemistry. They found that the two spikes in coral growth and acidity coincided with peaks in blooms of phytoplankton – single-celled plants that corals feed on.

All the pieces then seemed to fit together: phytoplankton blooms seemed to be washing in and feeding the corals, resulting in a higher growth rate and greater acidity levels in the water around the reef.

Coral conundrum

The results complicate the question of how coral reefs will respond to climate change, which is raising the acidity of the oceans. “Do corals care about ocean pH if they have plenty of food and light? At this point, we don’t fully know the answer to that question,” says Andersson.

These corals didn’t seem to mind the fluctuations in local acidity that they created, which were much bigger than those we expect to see from climate change. This may mean that corals are well equipped to deal with the lower pH levels.

Malcolm McCulloch of the University of Western Australia in Perth says this is backed by a recent study in which he and his colleagues put boxes around corals at Heron Island on the Great Barrier Reef and bubbled carbon dioxide into them, increasing acidity. Those corals didn’t seem affected at all in a simulation of the acidity expected by 2100.

Journal reference: PNAS, DOI: 10.1073/pnas.150721112

Read more: “Coral comeback: Reefs have secret weapon against climate change”

Image credit: Georgette Douwma/Getty

Correction: Since this article was first published, the title and some of the text has been changed to clarify that although the water the corals live in is more acidic than it was, but it isn't actually acidic.