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Changing ocean conditions pose threat to coral reefs

Coral reefs are complicated underwater ecosystems found in the shallow waters of the ocean, yet they are delicate, and with specific needs in order to remain healthy and reproduce. Coral reefs are built by colonies of tiny marine animals found in ocean waters that contain very little nutrients. The reefs are the “rainforests of the sea,” and while comprising less than 0.1 percent of the world’s ocean surface, they are home to one-fourth of all marine species.

Coral reefs are home to over 4 000 species of fish.

Coral reefs are home to over 4,000 species of fish.
Richard Ling


Scientists from the Woods Hole Oceanographic Institution (WHOI) led the latest study published in the January issue of the journal Geology. Their research highlights the multiple threats that are impacting the world’s coral reefs.

Not only do reef ecosystems act as buffers to shoreline erosion, they also sustain fisheries that provide food to millions of the Earth’s people. The corals make their skeletons from calcium and carbonate ions in seawater. Some of their colonies are huge, sometimes as large as a house or an automobile.

The ocean absorbs excess carbon dioxide from fossil-fuel burning, putting into motion chemical reactions that lower the pH of the seawater. This is called ocean acidification (OA), something we have been hearing a lot about lately. This process has a direct impact on coral because the increased acidity of the seawater reduces the availability of carbonate ions.

Bioerosion and ocean acidification
“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.

Reef of the Maldivian island of Kandholhudhoo showing healthy corals.

Reef of the Maldivian island of Kandholhudhoo showing healthy corals.
Bruno de Giusti


As Mr. DeCarlo explains, bioerosion reduces the coral skeletons to a rubble that is carried out to sea by storm currents where the rubble is then slowly dissolved in the sediments. Under healthy, or normal conditions, the balance between calcium carbonate build-up of the coral skeletons and the bioerosion by sea creatures is a delicate balance, and is why corals grow very slowly when the sea level is stable. As long as this balance is maintained, the corals remain healthy.

Study shows the impact of ocean acidity levels on coral
The research team investigated coral reefs in the Pacific Ocean from Panama to Palau. The reefs included naturally occurring pH and nutrient conditions in the ocean waters, including a number of coral reefs with pH levels today as low as those expected by 2100. The scientists were then able to study bioeroders under the combined influences of pH and nutrient conditions.

Water quality improvements  particularly controlling nutrient inputs  can bolster reef resilience to...

Water quality improvements, particularly controlling nutrient inputs, can bolster reef resilience to bleaching and implementation of existing laws may help mitigate ocean acidification effects on nearshore habitats.
Jerry Reid/U.S. Fish and Wildlife Service


What the researchers discovered was that reefs with a lower pH in the surrounding seawater had a higher degree of bioerosion in comparison to reefs with higher pH levels in the waters. While this was interesting enough, an even more astounding discovery was made. The scientists found that in waters with high nutrient levels and lower-pH, bioerosion in the coral reefs was 10-times higher than in reefs located in waters with lower pH levels and lower nutrient levels.

“The ocean will certainly absorb more CO2 over the next century, and ocean acidification is a global phenomenon that reefs cannot escape,” DeCarlo said. “But the encouraging news in our findings is that people can take action to protect their local reefs. If people can limit runoff from septic tanks, sewers, roads, farm fertilizers, and others sources of nutrient pollution to the coastal ocean, the bioeroders will not have such an upper hand, and the balance will tip much more slowly toward erosion and dissolution of coral reefs.”

We have a responsibility to help in mitigating the effects of anthropogenic climate change, and this means working toward reducing runoff from sewers farm fertilizers and other sources if we want to keep our coral reefs alive. Their ecosystem is just one part of the bigger system we call life.

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We are deeply saddened to announce the passing of our dear friend Karen Graham, who served as Editor-at-Large at Digital Journal. She was 78 years old. Karen's view of what is happening in our world was colored by her love of history and how the past influences events taking place today. Her belief in humankind's part in the care of the planet and our environment has led her to focus on the need for action in dealing with climate change. It was said by Geoffrey C. Ward, "Journalism is merely history's first draft." Everyone who writes about what is happening today is indeed, writing a small part of our history.

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