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Shading the Great Barrier Reef may slow coral bleaching

Over the past 20 years, the world has witnessed a concerning decline in coral reefs, a crucial ecosystem that supports about 25 percent of all marine life. Extreme weather events have primarily driven this loss of reefs, particularly due to widespread coral bleaching.

The Great Barrier Reef, the world’s most extensive coral reef system, has not been spared, with a shocking 91% of its reefs undergoing bleaching in the years 2016 and 2017.

As the frequency and intensity of mass bleaching events is expected to increase in the near future, scientists have been urgently searching for countermeasures. 

Reef restoration 

The Reef Restoration and Adaptation Program (RRAP) is dedicated to creating strategies to help coral reefs withstand and bounce back from the rising ocean temperatures. 

A recent study led by Dr. Peter Butcherine from Southern Cross University has revealed a potential new solution to help corals recover by shading them. As part of the RRAP, the researchers examined the effectiveness of shading on two coral species.

The study primarily focused the yellow scroll coral (Turbinaria reniformis) and whisker coral (Duncanopsammia axifuga).

Promising results

By reducing sunlight by 30% for a span of four hours around solar noon, the onset of coral bleaching in certain shallow corals can be slowed down, according to the study. This approach to intermittent shading was shown to ease the impact of light stress.

The experts found that shaded corals bleached significantly less than unshaded ones, and corals that were shaded for 24 hours bleached less than those shaded for four hours. Higher water temperatures also favored bleaching. 

Shade as a refuge

The research also introduced the concept of degree heating weeks (DHW). Once water temperatures surpass the maximum monthly average by more than 1°C, corals begin to accumulate DHW. 

Crossing a threshold of four consecutive degree heating weeks can lead to considerable coral bleaching. Interestingly, shading proved effective in delaying the bleaching response by up to three DHW.

Complex interactions

Not all corals react to shading in the same manner. For instance, while the yellow scroll coral benefited from just four hours of daily shading, the whisker coral needed a 24-hour shading period to showcase a positive response. 

“The complex nature of coral interactions with their environment means there are likely to be a range of responses to shading. We showed that coral species can respond differently when shaded, but these differences were not necessarily detrimental, just different from each other,” explained Dr. Butcherine.

While shading offers some reprieve, it might not be enough during intense and prolonged marine heatwaves, noted the researchers.

Shading reefs

“This work directly informs the development of cooling and shading interventions to help protect the Great Barrier Reef during future bleaching events,” said Dr Daniel Harrison, a researcher at Southern Cross University and program lead of RRAP Cooling and Shading. 

“Coral reefs are a critically important ecosystem so it is vital to investigate all the possible ways we can help them survive climate change.”

There are several ways to shade reefs, including the use of artificial coverings and seawater fogging systems. 

“The focus of the fogging technologies we are developing is for deployment at an individual reef site of some tens of hectares in size,” said Harrison. 

The team’s efforts are currently aimed at localized cooling and shading of small high-value reef environments.

“Our trials show some promising results, but there is still more research and development required before the current technologies are ready for scaled up deployment in the field,” Harrison concluded.

The research is published in the journal Frontiers in Marine Science.

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