The intricate structure of coral reefs resembles a lively underwater metropolis, teeming with a wide variety of life forms. However, this very complexity can also hinder the recovery of coral following disturbances.
The intricate structure of coral reefs resembles a lively underwater metropolis, teeming with a wide variety of life forms. However, this very complexity can also hinder the recovery of coral following disturbances.
Scientists researching the reefs of Moorea, located in French Polynesia, discovered that the remnants of dead coral skeletons resulting from bleaching events disrupted vital processes, ultimately stalling the reefs’ recovery. The complex environment helps protect seaweed from herbivores, allowing it to rapidly spread across the reef and overshadow newly growing coral. Their findings are published in the journal Global Change Biology.
Dynamic ecosystems
Coral reefs are dynamic ecosystems characterized by ongoing changes. Occasionally, a major event, such as a storm, an increase in coral predators, or a bleaching incident, can significantly impact the reef. While all these factors can pose threats to the ecosystem, subtle differences can greatly influence how the reef recuperates.
Traditionally, tropical storms and cyclones have posed the greatest challenges to Moorea’s reefs. “They typically strip all the coral from the reef, leaving a flat surface behind,” explained lead author Kai Kopecky, a former Ph.D. student in UCSB’s Department of Ecology, Evolution, and Marine Biology. However, bleaching and predation are increasing in frequency, causing coral mortality while leaving the reef’s structure intact.
Bleaching occurs when environmental stress, normally from high temperatures, forces corals to expel the algae that provide them nourishment. Corals can recuperate if conditions quickly improve, but often, the colonies perish, especially when facing additional stressors like pollution.
A cyclone severely impacted Moorea’s reefs in 2010. “It removed practically all coral colonies from the fore reef,” Kopecky said. “Yet within about five years, the coral rebounded to its pre-storm levels.”
In 2019, the reef underwent a significant bleaching event, just a year after Kopecky began his research on the island. “It effectively cooked and killed about half the corals on the reef,” he noted. Unlike the cyclone, this occurrence left behind the structure of dead corals.
Kopecky and his team at the NSF-funded Long Term Ecological Research (LTER) site in Moorea Coral Reef observed that the reef did not bounce back with the same speed in the following years. Rather, coral continued to perish, with macroalgae, often referred to as seaweed, proliferating rapidly. Kopecky sought to understand how the differences between the two events influenced recovery processes. In 2023, he and his collaborators published a mathematical model of the ecosystem, focusing on the mechanisms at play.
“This integration of long-term data on ecosystem responses, mathematical modeling, and field experiments significantly enhances our scientific knowledge and ability to develop practical solutions,” said co-author Professor Russ Schmitt, the lead principal investigator at the Moorea Coral Reef LTER site.
“The LTER network’s multi-decade, site-based approach is both unique and extremely valuable in our fast-changing world,” added LTER co-principal investigator Professor Sally Holbrook, another author of the study.
“This current project was spearheaded by Kai, who was a Ph.D. student at the time, along with contributions from UCSB undergraduate researchers, in addition to senior ecologists. It highlights how the Moorea Coral Reef project cultivates and mentors the next group of environmental scientists,” Schmitt noted.
Investigating the reefscape
The researchers set up controlled patches of the reef to establish a test environment. They secured a specific number of dead coral skeletons in each area and introduced healthy young corals, allowing them to be periodically removed and assessed for growth. They also placed trays filled with macroalgae to compare the consumption between the bleached skeletons and those exposed in open areas.
“Our findings indicated that dead coral skeletons hinder herbivores from removing macroalgae, allowing it to thrive and blocking new corals from settling and surviving on the reef,” Kopecky expressed.
While dead coral skeletons might theoretically offer some protection for young corals if new recruits arrive shortly after a bleaching event, the reality is that corals typically spawn only once a year, whereas many types of algae reproduce continuously, giving seaweeds the upper hand in taking over newly freed surfaces.
Macroalgae vie with coral for essential resources like space and light. They tend to grow faster than corals; thus, the absence of herbivory can lead to a situation where they dominate the reef, preventing new corals from establishing and overshadowing those that do. Young coral recruits are especially susceptible to this competition, and once a reef transitions from coral to algae cover, reverting back is challenging, as previous studies have demonstrated.
Considering long-term shifts
The researchers correlated their small-scale experimental outcomes with the long-term data from the site, revealing starkly contrasting trends following different disturbances. “Coral cover significantly increased on the reefs after the cyclone, while macroalgae cover decreased,” noted Kopecky. “In contrast, after the bleaching event, the opposite occurred.”
The findings relate to the idea of ecological memory, which explores how historical events can shape an ecosystem’s future trajectory. These changes can create mismatches between an ecosystem’s historical conditions and its current ones. “As disturbance patterns evolve, so does ecological memory,” Kopecky elaborated. Unfortunately, the ecosystem may struggle to adapt to this new pattern, characterized by the abundance of dead coral skeletons left in the aftermath of disturbances. This situation can disrupt long-standing interactions, such as those between herbivores, algae, and coral.
Kopecky is eager to explore whether removing dead coral skeletons could help stimulate coral recovery or at least alleviate the effects of bleaching. “This is a new idea and strategy for coral reefs,” he pointed out. “However, in other ecosystems—like using prescribed burns in forests to clear out dead wood—there’s a growing trend toward manipulating dead materials in ecosystems for management purposes.”
