Coral reefs, vital to marine ecosystems, face increasing threats from extreme events like marine heatwaves and cyclonic storms. The aftermath of these disturbances—particularly the legacy of dead coral skeletons—may significantly affect the ability of coral reefs to recover.
Dying corals. Photo by Ila France Porcher
In particular, these dead skeletons can foster macroalgae growth, which compete with corals for space and resources and ultimately hinder coral regrowth.
A disruptive legacy
Marine heatwaves, which cause coral bleaching, and powerful cyclones, which physically damage reefs, have both become more frequent. While both disturbances harm coral tissue, they leave behind different types of reef structures. Cyclones typically pulverise coral skeletons, creating open reef surfaces, while heatwaves leave a "forest" of intact dead skeletons. This difference plays a crucial role in post-disturbance recovery.
A new study has found that dead coral skeletons can reduce herbivore activity on nearby macroalgae. Normally, herbivores such as fish help control macroalgae growth, but the presence of dead coral skeletons appears to offer macroalgae a form of protection. This is especially true for less palatable types of macroalgae, which are harder for herbivores to consume, thus allowing them to thrive and outcompete coral for space.
Major barrier to coral recovery
Dead coral skeletons not only protect macroalgae from herbivores, but they also increase the surface area available for algae to grow. This creates a "refuge" for macroalgae, allowing it to establish itself quickly after a disturbance. The study showed that where dead skeletons remained, macroalgae covered up to 45 percent of the reef after two years, compared to reefs without skeletons, where macroalgae struggled to grow.
In areas where macroalgae were allowed to proliferate, the growth of new corals was significantly suppressed. Coral recruits, or young corals, struggled to establish themselves in areas where macroalgae had already taken hold, showing that the timing of coral recruitment plays a critical role in determining whether the reef can recover to its former state.
Timing in coral recovery
Herbivores play a crucial role in controlling macroalgae, but their ability to do so can be compromised by the legacy of dead coral skeletons. The study suggests that if corals can recruit before macroalgae become firmly established, they may have a better chance of growing to a size where they can compete with algae. However, if coral recruitment happens too late, after macroalgae have already taken over, recovery becomes much less likely.
Interestingly, the study also revealed that young corals growing among dead skeletons before macroalgae had a chance to dominate showed significant growth advantages. These recruits seemed protected from predators and competition with macroalgae, suggesting that the material legacy of dead coral can sometimes benefit coral recruitment if the timing is right.
A vicious cycle
The study's findings indicate that dead coral skeletons create a feedback loop that favours macroalgae. As macroalgae grow and thrive on dead skeletons, they suppress coral recruitment, and the system becomes locked into a "macroalgal state." This shift is difficult to reverse, especially in areas where herbivore populations are not able to control algae growth.
The research highlights how the material legacies left by coral deaths—whether from heatwaves or storms—can shift ecosystems toward a more persistent macroalgal state, which is much harder to restore to coral-dominated reefs.
Implications
These findings have important implications for coral reef conservation and management. The study suggests that intervention may be needed shortly after a bleaching event or cyclone to prevent macroalgae from taking over. For example, removing dead coral skeletons could help maintain herbivore activity and allow coral to recruit and grow before algae can dominate. Additionally, enhancing herbivore populations through conservation efforts could help reduce macroalgae cover and promote coral recovery.
As climate change continues to alter disturbance regimes in the oceans, understanding the long-term effects of dead coral skeletons on reef dynamics will be essential. This research offers a crucial insight into how material legacies from disturbances can either support or undermine the resilience of ecosystems, depending on the context.
