The lowering of the ocean’s pH is making it harder for corals to grow their skeletons and easier for bioeroding organisms to tear them down. Erosion rates increase tenfold in areas where corals are also exposed to high levels of nutrients, according to a study published January 2015 in the journal Geology. As sea level rises, these reefs may have a harder time growing toward the ocean surface, where they get sunlight they need to survive.
The lowering of the ocean’s pH is making it harder for corals to grow their skeletons and easier for bioeroding organisms to tear them down. Erosion rates increase tenfold in areas where corals are also exposed to high levels of nutrients, according to a study published January 2015 in the journal Geology. As sea level rises, these reefs may have a harder time growing toward the ocean surface, where they get sunlight they need to survive.
The study, led by scientists at Woods Hole Oceanographic Institution (WHOI), highlights the multiple threats to coral reef ecosystems, which provide critical buffers to shoreline erosion, sustain fisheries that feed hundreds of millions of people, and harbor 25 percent of all marine species. And it points to a key management strategy that could slow reef decline: reducing the input of nutrient pollution to the coastal ocean from human activity such as runoff from sewers, septic tanks, roads, and fertilizers.
Corals make their skeletons out of calcium and carbonate ions from seawater, constructing massive colonies as large as cars and small houses. As the ocean absorbs excess carbon dioxide from fossil-fuel burning, it spurs chemical reactions that lower the pH of seawater, a process known as ocean acidification. The process removes carbonate ions, making them less available for corals to build skeletons.
“A healthy coral reef ecosystem exists in a constant and often overlooked tug-of-war. As corals build their skeletons up toward the sea surface, other organisms—mollusks, worms, and sponges—bore into and erode the skeletons to create shelters,” said lead author Thomas DeCarlo, a graduate student in the WHOI-MIT Joint Program in Oceanography, working in Anne Cohen’s lab at WHOI.
This process, called bioerosion, reduces skeletons to rubble, which is transported offshore during fierce storms or gradually dissolved in the sediments. On healthy reefs today, calcium carbonate production barely exceeds the loss by erosion, dissolution, and offshore transport. As a result of this delicate balance, coral reefs grow very slowly, if at all, when sea level is stable.
The new study shows that additional nutrients provide a dramatic boost for bioeroders that, combined with lower pH conditions, will tip this balance in favor of erosion. The bioeroders are filter feeders, sifting particles of food out of seawater. Nutrients spur the growth of plankton, supplying food for large populations of bioeroders that burrow into coral skeletons.
Continue reading at Woods Hole Oceanographic Institution.
Coral reef image via Shutterstock.
