Abstract
Climate change is the greatest threat to coral reef ecosystems. In particular, increasing ocean temperatures are causing severe and widespread coral bleaching, contributing to extensive coral loss and degradation of coral reef habitats globally. Effects of coral bleaching are not however, equally apportioned among different corals, leading to shifts in population and community structure. This study explored variation in bleaching susceptibility and mortality associated with the 2016 severe mass bleaching in the Central Maldives Archipelago. Five dominant coral taxa (tabular Acropora, Acropora humilis, Acropora muricata, Pocillopora and massive Porites) were surveyed in February 2016 and October 2017 to test for changes in abundance and size structure. Substantial taxonomic differences in rates of mortality were observed; the most severely affected taxa, Acropora, were virtually extirpated during the course of this study, whereas some other taxa (most notably, massive Porites) were relatively unaffected. However, even the least affected corals exhibited marked changes in population structure. In February 2016 (prior to recent mass-bleaching), size-frequency distributions of all coral taxa were dominated by larger size classes with over-centralized, peaked distributions (negatively skewed with positive kurtosis) reflecting a mature population structure. In October 2017, after the bleaching, coral populations were dominated by smaller and medium size classes, reflecting high levels of mortality and injury among larger coral colonies. Pronounced changes in coral populations and communities in the Maldives, caused by coral bleaching and other disturbances (outbreaks of crown-of-thorns starfish and sedimentation), will constrain recovery capacity, further compounding upon recent coral loss.
Highlights
Climate change is among the foremost threats to natural ecosystems, contributing to species transformations and global degradation across many different aquatic and terrestrial habitats[1,2,3]
Coral bleaching may greatly alter the size-structure of coral populations, both due to differential susceptibility of large versus small colonies and incomplete or partial mortality that effectively reduces the size of colonies
Was at least partially attributable to, extreme ocean temperatures and severe coral bleaching recorded from March 2016
Summary
Climate change is among the foremost threats to natural ecosystems, contributing to species transformations and global degradation across many different aquatic and terrestrial habitats[1,2,3]. Bleaching susceptibility greatly varies among different coral taxa[22,23,24,25], whereby the portion of colonies that bleach and die is invariably higher for some coral taxa (e.g., Acropora and Stylophora), whereas other taxa (e.g., Galaxea and Cyphastrea) rarely exhibit coral bleaching, except during the most extreme heatwaves[22,23,24,25] These taxonomic differences will contribute to changes in the community structure of coral assemblages[23,26], though it is both differential susceptibility and recovery that will determine the future structure (and vulnerability) of coral assemblages[27]. While survival of juvenile corals is beneficial for recovery[38], loss of larger colonies may constrain reproduction following bleaching[36,39]
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