Skeletons that stay put after marine heatwaves can prevent new coral from forming and stop herbivores from clearing away algae
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A new study has found coral skeletons left behind in ocean ecosystems – often due to bleaching events caused by high temperatures – can significantly impact reefs and their ability to survive.
Traditionally, corals face threats due to powerful storms. When this happens, corals are damaged in such a way that their skeletons are either pulverised or removed entirely from the ecosystem. Eventually, these corals regrow – like in the case of a 2010 tropical cyclone storm in Moorea’s reefs, located off a volcanic island in French Polynesia. Within five years of the storm, the quantity of coral was back to previous levels.
But when marine heatwaves happen – an increasingly common occurrence due to rising temperatures on the planet – corals are bleached and killed, yet their skeletons remain intact.
After a bleaching event in 2019 at Moorea’s reefs – the same ones that recovered after a storm – scientists then carried out further analysis of the area in 2023, concluding an accumulation of skeletons actively hinders the growth of colonies.
Why can coral skeletons be harmful?
There are two main reasons why skeletons can harm the marine environment. The first is that they hinder the ability of herbivores within the ecosystem to control macroalgae, another plant that competes for space with coral. Since the skeletons conceal the macroalgae, they cannot be reached and eaten by herbivores. This macroalgae – which spawns multiple times a year compared to just once for coral – is then able to outcompete young coral and eventually colonise particular areas, reducing the quantity of coral.
However, if young coral arrives quickly after a bleaching event, the coral skeletons may offer some protection.
Difficulty in replenishing coral colonies arises with leftover coral skeletons too. Because the macroalgae then can dominate the marine ecosystem, it is difficult for new coral to settle. The macroalgae can even shade existing colonies, leading them to die anyway.
Concerningly, it is difficult to reverse an area overrun by macroalgae once it has taken over. This is why researchers are now considering ways to remove or even repurpose coral skeletons in the world’s oceans.
‘In coral reefs, this is a novel idea and strategy, said Kopecky. ‘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.’
