Reefs live in an environment of balance between their construction and their decomposition. As corals grow, a complex calcium carbonate framework is built that serves as habitat for fish and other reef organisms.
At the same time, bioerodors, such as parrotfish or marine worms, break down the reef structure into rubble and the sand that nourishes the beaches.
For reefs to persist, their build rate must exceed their breakdown. This balance is now threatened by the increase of carbon dioxide in the atmosphere, which causes acidification of the ocean, that is, its pH decreases.
Impact of acidification on the oceans
The new research has largely focused on the negative impacts of ocean acidification on reef growth, showing that lower ocean pH enhances reef rupture.
It is "a double whammy for coral reefs in a changing climate," as one of the lead authors, Nisa Silbiger, has noted.
Measurement of biological erosion
To measure biological erosion, the researchers unfolded small blocks of calcium carbonate (the skeleton of a dead coral) on a reef for a year.
Traditionally, these blocks are weighed before and after deployment to the reef. However, this time the scientists used a high-resolution CT scan to create 3D before and after images of each block of coral.
Silbiger noted that this novel technique provides a more accurate measurement of accretion and erosion rates.
Thus, the researchers placed the bioerosion blocks along a 30-meter path over the shallow coral reefs of Kane’ohe Bay (Hawaii), taking advantage of the natural variability of pH in this area.
The study compared the influence of pH, resource availability, temperature, distance from shore, and depth on the accretion-erosion balance. Among all the variables measured, pH was the strongest predictor of accretion-erosion.
Influence of environmental changes
Experts note that this study also highlights the impact of varying coastal ocean chemistry on coral reefs.
Current models from the Intergovernmental Panel on Climate Change (IPCC) predict changes in open ocean pH, but these predictions are problematic for coral reefs, which are embedded in highly variable coastal ecosystems.
The study found dramatic differences in ocean pH and daily pH variability over a short distance.
“It was surprising to find that small-scale changes in the environment can influence reef processes at the ecosystem level. We have seen changes in pH on the order of meters, and those small changes in pH showed patterns in the reefs of accretion-erosion ”, Silbiger pointed out.
Europa Press, RTVE.es