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.2013 Jul 29;8(7):e70443.
doi: 10.1371/journal.pone.0070443. Print 2013.

Changes in bleaching susceptibility among corals subject to ocean warming and recurrent bleaching in Moorea, French Polynesia

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Changes in bleaching susceptibility among corals subject to ocean warming and recurrent bleaching in Moorea, French Polynesia

Morgan S Pratchett et al. PLoS One..

Abstract

Background: Climate-induced coral bleaching poses a major threat to coral reef ecosystems, mostly because of the sensitivities of key habitat-forming corals to increasing temperature. However, susceptibility to bleaching varies greatly among coral genera and there are likely to be major changes in the relative abundance of different corals, even if the wholesale loss of corals does not occur for several decades. Here we document variation in bleaching susceptibility among key genera of reef-building corals in Moorea, French Polynesia, and compare bleaching incidence during mass-bleaching events documented in 1991, 1994, 2002 and 2007.

Methodology/principal findings: This study compared the proportion of colonies that bleached for four major genera of reef-building corals (Acropora, Montipora, Pocillopora and Porites), during each of four well-documented bleaching events from 1991 to 2007. Acropora and Montipora consistently bleached in far greater proportions (up to 98%) than Pocillopora and Porites. However, there was an apparent and sustained decline in the proportion of colonies that bleached during successive bleaching events, especially for Acropora and Montipora. In 2007, only 77% of Acropora colonies bleached compared with 98% in 1991. Temporal variation in the proportion of coral colonies bleached may be attributable to differences in environmental conditions among years. Alternately, the sustained declines in bleaching incidence among highly susceptible corals may be indicative of acclimation or adaptation.

Conclusions/significance: Coral genera that are highly susceptible to coral bleaching, and especially Acropora and Montipora, exhibit temporal declines in their susceptibility to thermal anomalies at Moorea, French Polynesia. One possible explanation for these findings is that gradual removal of highly susceptible genotypes (through selective mortality of individuals, populations, and/or species) is producing a coral assemblage that is more resistant to sustained and ongoing ocean warming.

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Conflict of interest statement

Competing Interests:The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Annual thermal conditions in Moorea from 1991 to 2007.
Data were derived from the NOAA Pathfinder v 5.0 dataset. a) Annual accumulated heating stress (in °C-weeks) was calculated by summing positive anomalies above the maximum monthly mean of 29.0 °C. Years where bleaching was observed are shown in white. b) The long-term trend in daily temperature reveals an increase during the study period at a rate of 0.16 °C per decade.
Figure 2
Figure 2. Taxonomic differences in bleaching susceptibility in Moorea, French Polynesia.
Taken in late summer 2007, these photographs show bleachedAcropora adjacent to colonies ofPocillopora andPorites, which are seemingly unaffected.Pocillopora are generally considered to be amongst the most susceptible genera to climate-induced coral bleaching (e.g., McClanahan et al. 2004), butPocillopora corals exhibit unusually high resistance to high temperatures at Moorea.
Figure 3
Figure 3. Size-specific bleaching susceptibility for four key genera (Acropora,Montipora,Pocillopora, andPorites) in 2007.
Bleaching susceptibility was calculated using a bleaching index (BI) that weights the proportion of colonies that bleached by the severity of bleaching, following Guest et al. (2012).
Figure 4
Figure 4. Proportional bleaching in a)Acropora, b)Montipora, c)Pocillopora, and d)Porites during well-documented bleaching events in Moorea, French Polynesia.
Graphs distinguish the proportion of colonies for each genus that had any evidence of bleaching (in white) from those that did not bleach (coloured) in 1991, 1994, 2002 and 2007. All surveys were conducted on the outer reef slope (6-18m depth) along the northern coast of Moorea.
Figure 5
Figure 5. Summer temperature and cloud conditions at Tiahura Reef, Moorea, from 1991 to 2007.
Average sea surface temperature (red circles) and the proportion of early-afternoon summer cloud cover observations less than the summer median (blue squares). Open symbols represent the four bleaching events considered in 1991, 1994, 2002 and, 2007.
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References

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This research was supported by the Australian Research Council Centre of Excellence for Coral Reef Studies. The work was supported by grants to MP from the Australian Research Council (Australian Research Fellowship) and from and from Centre National de la Recherche Scientifique and Centre de Recherches Insulaires et Observatoire de l’Environnement to JM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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