On the fate of dead coral colonies

Authors: Juliano Morais, Renato Morais, Sterling Tebbett & David Bellwood

Abstract:

Carbonate budgets dynamically balance production and loss of calcium carbonate (CaCO3) on coral reefs. To sustain or expand the coral reef framework, CaCO3production by calcifying organisms must be higher than erosion. However, global climate change has been negatively impacting carbonate production, with bleaching events causing widespread coral mortality. While bleaching and coral mortality are well documented, the fate of coral colonies after their death, including their erosion rates, are still poorly known.

We followed the fate of 143 recently dead individual coral colonies with complex growth forms (arborescent, caespitose, corymbose, digitate, and tabular), whose mortality was triggered by two consecutive bleaching events. These colonies, spread over 16 km2 of the Lizard Island reef complex, were tracked for up to 5 years, allowing detailed examination of erosion rates and post-mortality structural persistence. We also tested how variables that are commonly used in coral reef erosion studies relate to spatial and temporal variability in the erosion rates of dead coral colonies.

We revealed rapid erosion of dead coral colonies, with an average of 79.7% of dead colonies completely disintegrating within 60 months. The predicted half-life of a dead coral colony was 40 months, with limited variation among wave exposure levels. Remarkably, we found no effect of estimated parrotfish bioerosion, wave exposure, nor coral growth form, on observed erosion rates.

Our results suggest that our understanding of the erosion of dead corals may be more limited than previously thought. The rapid loss of coral colonies on our study sites calls for a re-evaluation of the role of corals with complex growth forms in reef growth and of parrotfishes in reef erosion.