Corals can’t fully acclimate to low pH conditions in nature, according to a University of California, Santa Cruz study that was recently published in the Proceedings of the National Academy of Sciences. This marks the first time that a study on the effects of ocean acidification that was not conducted in a lab shows that low pH has adverse effects on corals.
Scientists from the university studied areas where corals are exposed to low pH levels during their entire lives and determined that while the corals don’t die off, they do not produce structures to the scale that they would have had pH levels been more robust. The scientists, led by graduate student Elizabeth Crook, examined seawater chemistry in areas around natural springs in the ocean and removed skeletal cores from the coral Porites astreoides, a Caribbean reef-building coral. They then performed CT scans on the core samples to measure coral densities that helped them to determine annual calcification rates. These measurements were conducted in the lab of co-author Anne Cohen at the Woods Hole Oceanographic Institution.
“People have seen similar effects in laboratory experiments,” said coauthor Adina Paytan, a research scientist in the Institute of Marine Sciences at UC Santa Cruz. “We looked in places where the corals are exposed to low pH for their entire life span. The good news is that they don’t just die. They are able to grow and calcify, but they are not producing robust structures.” Paytan co-authored another study that examined the effects pH has on a coral reef located near a freshwater spring. In that study, Paytan said that while the smaller corals were able to calcify, albeit at a lower rate, the larger corals that form the foundation for the area’s reefs were not. They found that the closer to the spring the corals got, the colonies declined in size and structure.
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They found that coral calcification rates decreased quite a bit along a natural gradient in seawater pH. This leads to the corals laying down fewer structures, which reefs are built upon. With lower pH, the corals have to work harder to accumulate carbonate ions internally, the building blocks of corals. Without robust carbonate ions, entire reef frameworks are weakened, which results in further degradation of complex coral reef ecosystems.
This reduced density makes the corals more vulnerable to damage caused by storms, organisms that bore into corals, further weakening their structures, and parrotfish, which feed on corals.
What the study ultimately points to is a weakening of reefs worldwide as they are further exposed to acidification, including loss of coral cover that many fish species rely on for survival.