Dead coral skeletons prevent reefs from recovering

The intricate structure of coral reefs creates a vibrant underwater ecosystem, bustling with life. Ironically, this same complexity can hinder coral recovery after disturbances, as researchers studying reefs in Moorea, French Polynesia, have discovered. 

Network of coral skeletons 

The experts found that the network of dead coral skeletons left behind by bleaching events disrupts critical processes, ultimately preventing the reefs from regenerating. 

These dead structures protect seaweed from herbivores, allowing it to rapidly colonize and outcompete young coral. 

The findings, published in the journal Global Change Biology, highlight how the aftermath of disturbances can alter a reef’s recovery trajectory.

Coral reef disturbances

Coral reefs are dynamic systems that undergo constant change, and disturbances such as storms, coral predation, or bleaching events can significantly impact their health. 

Historically, tropical storms and cyclones have caused the most damage to Moorea’s reefs. 

“They tend to scrape all the coral off the reef and leave behind a flat surface,” said Kai Kopecky, lead author and a former doctoral student at the University of California Santa Barbara’s Department of Ecology, Evolution, and Marine Biology. 

However, more recent disturbances, such as coral bleaching and predation, have become increasingly prevalent. Unlike storms that physically remove coral, bleaching events leave the reef’s skeletons intact.

Bleaching and other stressors

Bleaching occurs when stress, usually from heat, causes coral to expel the symbiotic algae that supply it with food. 

While coral can sometimes recover if favorable conditions return, more often than not, the colony dies, particularly when other stressors like pollution are present. 

For instance, a major cyclone struck Moorea’s reefs in 2010, clearing almost all the coral from the fore reef. “But within about five years, it recovered back to the amount of coral it had before the storm had hit,” Kopecky noted. 

However, following a bleaching event in 2019, Kopecky and his team observed a starkly different outcome. 

The dead coral structure remained, but instead of recovering, the reef continued to decline, and seaweed (macroalgae) began to proliferate. Kopecky wanted to understand how these different disturbances affected the reef’s recovery.

Modeling a reef system

To explore this, the researchers conducted an experiment as part of their work at the Long Term Ecological Research (LTER) site at Moorea Coral Reef. 

The goal was to analyze the processes that were being disrupted after the bleaching event. In 2023, the scientists developed a mathematical model of the reef system, while their field study focused on examining the underlying mechanisms.

The research team cleared small patches of the reef to create experimental areas. They cemented controlled numbers of dead coral skeletons in these patches and added young, healthy coral that could be periodically removed and measured. 

Trays of macroalgae were also placed on the reef to compare how herbivores interacted with algae in the presence of dead coral skeletons versus open areas.

Coral skeletons limits new growth

“We found that dead coral skeletons prevent herbivores from being able to remove macroalgae, enabling growth and preventing new corals from being able to settle and survive on the reef,” Kopecky explained. 

These skeletons might theoretically offer some protection to young coral recruits if they arrived shortly after a bleaching event. 

Unfortunately, coral tends to spawn only once a year, whereas algae reproduce continuously, giving seaweed the upper hand in colonizing newly available spaces.

Competition for resources

Macroalgae directly compete with coral for resources like space and light. Algae grow much faster than coral, so when herbivores are unable to control seaweed growth, the algae can quickly overrun the reef. 

This prevents new coral from settling, and even shades out existing colonies, making recovery difficult. 

Young coral is particularly vulnerable to this competition, and once a reef flips from coral to algae dominance, it is challenging to reverse the process. 

The researchers’ previous studies have shown how hard it can be to restore coral cover once this shift occurs.

Dramatic differences in reef recovery

By comparing their experimental results with long-term data from Moorea, the researchers noted dramatic differences in reef recovery following different types of disturbances. 

“Coral cover shot up on the reefs after the cyclone, while macroalgae cover went down,” Kopecky said. “After the bleaching event, it was just the opposite.”

These findings fit into the broader concept of ecological memory, which examines how past events shape an ecosystem’s response to current challenges. 

As the nature of disturbances shifts, the way ecosystems like coral reefs respond also changes. 

“As these disturbance regimes change, ecological memory is also changing,” Kopecky said. 

Removing coral skeletons 

Unfortunately, the legacy of dead coral skeletons may not align with what these ecosystems are adapted to handle, which could alter long-standing relationships between coral, algae, and herbivores.

Kopecky is now considering whether removing dead skeletons could help coral recover or at least reduce the negative effects of bleaching. 

“In coral reefs, this is a novel idea and strategy. But if you look to other ecosystems – like prescribed burns in forests to remove dead wood – people have been increasingly thinking about manipulating dead stuff in ecosystems for management purposes,” he concluded.

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