Coral bleaching is the most devastating and discussed issue that plagues coral reefs in 2021. Prior research made it clear that climate change and warming oceans are the primary factor that triggers and profligates this reaction. The United Nations UN Chronicle details the harmful effects of warming oceans and how bleaching is devastating to coral populations across the world’s oceans.

The increase in water temperature wreaks havoc in the coral’s food cycle by destroying the symbiotic zooxanthellae, a tiny photosynthetic being that helps feed the coral. The temperature fluctuations were observed to trigger bleaching events after the zooxanthellae withered away. The coral gets rid of the dead zooxanthellae which causes them to lose all pigmentation. They also become weak due to the lack of nutrition and die. The ones that do survive often show signs of a compromised immune system, according to this report by the United Nations. 

Now, a team of researchers from the University of Guam, have observed and documented another crucial trigger for coral bleaching. The team, studying staghorn corals in Guam observed that direct sunlight, along with an increase in water temperature can also trigger bleaching. A discovery of immense significance to conservation efforts. 

To understand why this is an important finding and also how this can be used in coral conservation, we need to understand how temperature and light trigger coral bleaching, it’s impact and how this finding can help mitigate some of the negative effects.

Study and Findings

The team conducted this study on staghorn coral in the coast off Guam. This particular coral reef species of coral was selected for the study because they are the primary reef building species in the area and also, they are easy to raise in captivity for the purpose of research.

By subjecting transplanted staghorn coral (Acropora cf. pulchra) in a controlled environment (in tanks where water temperature can be easily monitored and altered).This species of coral resides in a region where water temperatures naturally climb up to 97 degrees Fahrenheit during the summer. Corals in this region are more prone to bleaching episodes as witnessed in the last 20 years. Severe summers and a climbing mean ocean temperature makes the waters of Guam a bleaching hotspot.

The study published in Marine Biology Research details the method of study. To understand the role of light in the bleaching process and understand if light exposure impacts recovery, the team used a method called pulse amplitude modulated (PAM) fluorometry. Using this process, they were able to monitor chlorophyll fluorescence, thus allowing the team to measure light damage and monitor recovery. 

The test corals were “subjected to a week-long heat treatment some 2–3°C above average ambient seawater temperatures of 30°C.”  After this stress, the corals were allowed to rest and recover for a month in water suitable for healthy coral growth. The team also placed some of the test corals in shaded regions and exposed some to direct sunlight. 

“One group was subjected to consistent baseline temperatures observed on Guam’s reef flats,” said UOG Assistant Professor Bastian Bentlage, the supervisor and co-author of the study in the press release, “and another was set to temperatures that are projected to become the new normal over the next couple of decades.” 

The researchers also made a fascinating finding. The corals that were in the shade (not exposed to direct light) throughout the period of stress showed much higher elasticity in their ability to bounce back after the water temperature returned to optimal levels.

“We saw the corals recover rather slowly,” Justin T. Berg, a UOG graduate student studying biology said. “The length of recovery indicates that corals are vulnerable during this time and management efforts may be particularly necessary during this period to reduce coral mortality.”

The team also documented the recovery process and the time it took for the corals to return to pre-stress function levels and found that corals exposed to light also needed a much longer recovery period. The corals exposed to light took three times longer than the duration of the stressful temperatures  so if these corals were exposed to one week of elevated water temperatures, they would take 3 weeks (approx) to return to baseline photosynthesis levels. 

“We found that when we put the shading over coral with increased seawater temperatures, it greatly increased photosynthetic yield of the symbiotic algae. Shade made a huge difference for coral health when you have high temperatures,” Berg added.

Significance of Findings 

Before this study, the impact of light on coral bleaching had never been studied to this capacity. The findings document that light, along with heat contributed to stress. It has been widely accepted that global warming is the main driver of coral bleaching. But, the study shows that warming ocean water is just one aspect of coral bleaching, which when combined with direct sunlight, causes severe negative effects to coral health. 

The potential of the finding too, are immense. In the recent past, episodes of coral bleaching have become common and widespread. Just in the last 10 years, several thriving reef beds have been decimated by this terrible affliction. Research has gone into trying to understand how bleaching happens and ways to mitigate it.

The team understands that shielding vulnerable corals from the sun is not a viable method to prevent coral bleaching. But the findings show that certain steps can be taken to protect areas that are extremely vulnerable.

But there could be easy and economical ways to ensure that vulnerable coral beds are protected from direct sunlight. The team suggests turbulence on the water surface generated using a simple motor device could help reflect back some of the light. Also, temporary roofs could be erected in areas with shallow pools of water. Corals in shallow areas receive more sunlight and could end up badly affected by an increase in water temperatures. This could help scientists create zonal systems where coral bleaching is common and help protect the most vulnerable reef systems from direct sunlight first. This finding has the potential to drastically improve conservation efforts and can create a huge difference in marine ecosystems. Remember, knowledge is power and this knowledge can help improve our marine ecosystems.