CT scans of coral skeletons reveal ocean acidity increases reef erosion

Coral reefs persist in a balance between reef construction and reef breakdown. As corals grow, they construct the complex calcium carbonate framework that provides habitat for fish and other reef organisms. Simultaneously, bioeroders, such as parrotfish and boring marine worms, breakdown the reef structure into rubble and the sand that nourishes our beaches. For reefs to persist, rates of reef construction must exceed reef breakdown. This balance is threatened by increasing atmospheric carbon dioxide, which causes ocean acidification (decreasing ocean pH). Prior research has largely focused on the negative impacts of ocean acidification on reef growth, but new research this week from scientists at the Hawai'i Institute of Marine Biology (HIMB), based at the University of Hawai'i - Manoa (UHM), demonstrates that lower ocean pH also enhances reef breakdown: a double-whammy for coral reefs in a changing climate.

An image of µCT scan showing the outside (top) and inside (bottom) of an experimental
 coral block after a one-year deployment in Kane'ohe Bay, Hawai'i. Note the bioerosion 
scars inside the block [Credit: Nyssa Silbiger at UH Manoa and Mark Riccio at
 the Cornell Unversity µCT Facility for Imaging and Preclinical Research]

To measure bioerosion, researchers deployed small blocks of calcium carbonate (dead coral skeleton) onto the reef for one year. Traditionally, these blocks are weighed before and after deployment on the reef; however, HIMB scientists used microCT (a high-resolution CT scan) to create before and after 3-D images of each block. According to Nyssa Silbiger, lead author of the study and doctoral candidate at HIMB, this novel technique provides a more accurate measurement of accretion and erosion rates.

The researchers placed the bioerosion blocks along a 100-ft transect on shallow coral reef in Kane'ohe Bay, Hawai'i, taking advantage of natural variability of pH in coastal reefs. The study compared the influence of pH, resource availability, temperature, distance from shore, and depth on accretion-erosion balance. Among all measured variables, pH was the strongest predictor of accretion-erosion. Reefs shifted towards higher rates of erosion in more acidic water - a condition that will become increasingly common over the next century of climate change.