Most coral ecosystems are struggling to adapt to the recent influx of carbon dioxide in the atmosphere. The oceans are naturally alkali, however, CO2 reacts with water molecules to in a process known as ocean acidification. As more CO2 enters the ocean, the pH level decreases. Overall, ocean acidity has increased by 30 per cent since the beginning of the industrial revolution, a figure which is projected to climb to nearly 150 per cent by the end of the century. By 2100, the pH level of the open ocean will be the lowest it has been for 20 million years.
The waters of Palau are naturally less alkaline. In fact, they currently have a pH level that the open oceans are predicted to have by the end of the century. The secret is a labyrinth of rock formations. The rocks shelter the water and hold it in place for a long residence time, allowing for the build up of acidity. For this reason, scientists at the Woods Hole Oceanographic Institution (WHOI) have spent the last five years investigating why Palau’s coral is thriving.
‘Surprisingly, in Palau where the pH is lowest, we see a coral community that hosts more species and has greater coral cover than in the sites where pH is normal,’ says Anne Cohen, co-author of the study with Hannah Barkley, which proposes that the sites in Palau are unique in their unaffected growth.
‘The vast majority of laboratory pH experiments and field studies of naturally low-pH sites suggest that most coral reefs will not be able to survive predicted ocean acidification at their current level of form and function,’ explains Barkley. ‘The highly diverse, coral-dominated communities that we observe at low-pH levels in Palau are, to date, the only exception to this otherwise grim prognosis for coral reefs.’
Acidic environments usually result in stunted growth and low biodiversity. ‘That’s not to say the coral community is thriving because of the low pH,’ adds Cohen, ‘rather it is thriving despite the low pH, and we need to understand how.’
While increased acidity does not effect growth in Palau, a lower pH isn’t completely harmless. Core samples of the corals revealed that as acidity increases, so does bioerosion – the general wear and tear by fish, crustaceans, sponges, molluscs, and other sea life. Patterns emerged which showed that greater and more frequent scars from bioerosion occurred in waters that were lower in pH. ‘We see coral skeletons that are eaten up and have holes on the top and sides,’ says Barkley. ‘The coral almost looks like Swiss cheese because of the volume that’s been removed.’
Why biorerosion increases with acidity is still up for debate. As yet, there are two potential explanations. ‘The first possibility is that coral skeletons growing under low pH conditions are less dense, which in turn could make it easier for bioeroding organisms (such as molluscs, sponges, and worms) to remove skeletal material,’ explains Barkley. ‘The second possible explanation is that low pH conditions make it easier for bioeroding organisms to chemically dissolve coral skeletons.’
If corals are being eroded faster than they are growing they will decrease overall. ‘The balance between calcium carbonate production and removal by bioerosion and dissolution is very tight,’ explains Cohen. This could suggest that, even in healthy reefs that are able to maintain their biodiversity and growth, increased degradation might be unavoidable. ‘Nonetheless, this does give us hope that some coral communities may be able to withstand changes in ocean pH,’ says Barkley. ‘It also suggests that all coral reefs may not respond to ocean acidification in the same way.’