News | December 04, 2014

CT scans of coral skeletons reveal ocean acidity increases reef erosion

Scientists at the University of Hawai'i-Manoa used microCT to generate before and after images

December 4, 2014 — New research from scientists at the Hawai’i Institute of Marine Biology (HIMB) uses micro computed tomography (microCT) to demonstrate that ocean acidification (decreasing ocean pH) not only negatively impacts coral reef growth, but also enhances reef breakdown.

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. Prior research has largely focused on the negative impacts of ocean acidification on reef growth, but the new research from HIMB — based at the University of Hawai'i - M?noa (UHM) — demonstrates enhancement of reef breakdown.

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 K?ne'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.

Silbiger and colleagues are learning all they can from the microCT scans, as this is the first time before-and-after microCT scans were used as a measure of accretion-erosion on coral reefs. In ongoing work, they are using this technology to distinguish between accretion and erosion and to single out erosion scars from specific bioeroder groups (e.g., holes from boring worms versus bioeroding sponges). The researchers are also using microCT to investigate the drivers of the accretion-erosion balance over the much larger area of the Hawaiian Archipelago.

For more information: www.soest.hawaii.edu

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