Coral reefs are complex and vibrant underwater ecosystems, but ironically, their very complexity can impede their recovery after disturbances. Researchers have found that the network of dead coral skeletons left behind by bleaching events can protect seaweed, enabling it to outgrow young coral and prevent reef regeneration.
The Resilience Conundrum
This ecosystem dynamism can make it more challenging for coral reefs to persist, as major disturbances like storms, pulses of predators, or bleaching events cause significant damage. Storms scrape off the coral, but also tend to remove reef heterogeneity, leading to a flat surface as opposed to bleaching and predation events that can kill the coral while leaving the reef structure of the storm intact.
Herein lies the problem. Analysis of the researchers revealed that in Moorea’s reefs, the ecosystem recovered within approximately five years following a cyclonic event in 2010 and restored to its initial conditions. The 2019 bleaching killed the alive coral and most of the plating coral, but did not kill anything else…in other words, it left behind dead coral skeletons. Instead of healing, the reef was getting sicker, becoming dominated by nasty macroalgae (seaweed) rather than coral.
The Deadly Shelter
For an insight into the mechanisms driving this, a group of researchers used the reef to perform a series of experiments. They made small squares, gluing a few dead corals to each and letting more of their children grow. In addition, they deployed trays of macroalgae to investigate herbivory inside the bleached skeletons versus outside.
The results were eye-opening. Those pits, created when the herds of damselfish and other herbivores gouged coral skeletons for food or shelter, allowed macroalgae to establish themselves among the living corals; research documented how well those mats of seaweeds resisted removal by smaller herbivores – which now thrived in a system with fewer bigger fish – allowing them to outcompete young coral polyps for space, light, and resources. It is an important step in fragmented, which ultimately means that the reef does not recover. If algae blooms grow out of control and the ecosystem becomes dominated by algae, it can be hard for new coral to get established due to competition.
Conclusion
These results underline the complexity of factors affecting coral reefs and the lasting effects disturbances can have. Given their increasing frequency and severity as a result of climate change, it is important to develop approaches for reversing the negative impacts of dead coral skeletons on reef recovery. According to the researchers, the movements of these skeletons could represent a new management option for enhancing the recovery of corals giving us hope for a healthier future for our life-supporting aquatic realms.
