What happens to coral colonies after death? Exploring erosion of dead coral colonies

Juliano Morais, Renato Morais, Sterling B. Tebbett, David R. Bellwood

This is a plain language summary of a Functional Ecology research article which can be found here.

One of the main things people may have heard about corals in recent years is that they are dying due to coral bleaching events. Coral bleaching occurs when corals are exposed to high water temperatures for too long. Because of climate change, ocean temperatures are reaching these high temperatures more often, and for longer periods of time, resulting in more severe coral bleaching events. However, while we know that bleaching can result in coral death, important questions about what happens to coral skeletons after death remain largely unexplored. For example, how long does the colony structure on the reef last and what are the factors involved in the erosion of coral colonies?

Dead coral colonies after the 2016 bleaching event on Lizard Island, Australie (credit: Sterling B. Tebbett)

To help answer such questions, we monitored the fate of individual coral colonies, for up to five years after their death, on reefs around Lizard Island in the northern Great Barrier Reef, Australia. During this time there were severe bleaching events that resulted in mass-coral death. By monitoring colonies after death this allowed us to investigate how long coral skeleton structure persisted for as well as quantify the erosion rates of individual coral colonies. We also examined how factors commonly believed to underpin erosion rates on reefs related to variability in erosion rates of the dead corals.

We found that dead coral colonies eroded rapidly. After 40 months half of all the dead corals examined had completely disappeared. Interestingly, we also found no effect of key factors such as parrotfish erosion, wave exposure, nor coral growth form, on observed coral erosion rates. Overall, our results call for a re-evaluation of a) the role of corals with complex growth forms in reef growth and b) the current approach to assessing the role of parrotfishes in reef erosion.

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