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Sci Rep
2019 Jun 10;91:8402. doi: 10.1038/s41598-019-44809-9.
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Changes in the population and community structure of corals during recent disturbances (February 2016-October 2017) on Maldivian coral reefs.
Pisapia C
,
Burn D
,
Pratchett MS
.
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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.
Figure 1. Map of the study sites in the Central Maldives Archipelago, Indian Ocean inclusive of (a) the global map of the Indian Ocean, (b) an enlargement of the Maldives and (c) the area of study. White squares are the sites surveyed in both 2016 and 2017, while black squares refer to sites surveyed only in 2017. Site and atoll names are reported. The symbol * refers to sites also affected by outbreaks of A. planci and/or sedimentation. The oceanic sites (KudaKandu, Rasfari, and Emboodhu) were all on the fore-reefs. This figure was generated using Google Earth Digital Globe (https://earth.google.com), and Adobe Illustrator and was modified after73.
Figure 2. Daily temperatures (°C) from January 2016 to August 2017 in the Central Maldives Archipelago. The continuous line indicates maximum monthly mean SST, while the dotted line indicates local bleaching threshold SST. Data were downloaded from: https://coralreefwatch.noaa.gov/vs/gauges/maldives.php. The NOAA estimate of degree heating weeks (DHW) peaked is 10.443 at June 26, 2016.
Figure 3. Variation in mean (±SE) live coral cover at (a) 5 and (b) 10 meters in all study sites before and after the bleaching; the * refers to site affected by outbreaks of A. planci, while the + refers to sites affected by sedimentation; (c) nMDS graph used as visualization tool to show shifts in coral assemblages at each site between February 2016 and October 2017 at 5 m (stress = 0.081) and (d)10 m (stress = 0.096). The black arrows connect the 2016–2017 pairs of data points for each site to show changes in adult composition after eighteen months. nMDS stress is specified for each graph (c,d).
Figure 4. Variation in mean (±SE) live coral cover of different coral taxa at (a) 5 m and (b) 10 m sites before and after the bleaching.
Figure 5. Log-transformed size-frequency distributions of Acropora muricata, tabular Acropora, Acropora humilis, Pocillopora spp and Porites spp at 5 m and 10 m before and after the mass bleaching event.
Figure 6. Relative abundance of juvenile Acropora, Pocillopora, Porites, Pavona and other corals among the study sites at (a) 5 m and (b) 10 m in 2017. (c) nMDS graph used as visualization tool to show composition of juvenile corals in October 2017 following major disturbances (stress = 0.039). nMDS and stress are merely used as visualization tools.
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