Corals escape bleaching in regions that recently and historically experienced frequent thermal stress

Abstract

The response of coral-reef ecosystems to contemporary thermal stress may be in part a consequence of recent or historical sea-surface temperature (SST) variability. To test this hypothesis, we examined whether: (i) there was a relationship between the historical frequency of SST variability and stress experienced during the most recent thermal-stress events (in 1998 and 2005-2006) and (ii) coral reefs that historically experienced frequent thermal anomalies were less likely to experience coral bleaching during these recent thermal-stress events. Examination of nine detrended coral delta(18)O and Sr/Ca anomaly records revealed a high- (5.7-year) and low-frequency (>54-year) mode of SST variability. There was a positive relationship between the historical frequency of SST anomalies and recent thermal stress; sites historically dominated by the high-frequency mode experienced greater thermal stress than other sites during both events, and showed extensive coral bleaching in 1998. Nonetheless, in 2005-2006, corals at sites dominated by high-frequency variability showed reduced bleaching, despite experiencing high thermal stress. This bleaching resistance was most likely a consequence of rapid directional selection that followed the extreme thermal event of 1998. However, the benefits of regional resistance could come at the considerable cost of shifts in coral species composition.

Figure 1.

Sites of the nine detrended δ¹⁸O and Sr/Ca records (channels) used in M-SSA to determine the significant modes of SST variability: 1. Ras Umm Sidd (−0.812); 2. Madagascar (−0.907); 3. the Seychelles (−0.726); 4. the Houtman Abrolhos Islands (−0.732); 5. Ningaloo Reef (−0.798); 6. Guam (−0.677); 7. Maiana Atoll (−0.637); 8. Rarotonga (−0.783); 9. Palmyra Island (−0.797). The values next to each site represent the correlation coefficients (r_s) between the δ¹⁸O or Sr/Ca records and local HadISST SSTs.

Figure 2.

Flow chart summarizing the methodology used to analyse the association between the relative strength of the high- and low-frequency modes of sea-surface temperature (SST) variability in the past and bleaching severity during the 1998 and 2005–2006 bleaching events, where RCs are the reconstructed components from the M-SSA of coral δ¹⁸O and Sr/Ca records.

Figure 3.

Correlation of the annual mean of the low-frequency (>54-year) component (reconstructed component 1, RC1, which explained 5.5% of the variance) with the detrended, annually resolved 1° by 1° globally gridded HadISST1.1 sea-surface temperature data over the common time period (1886–1993), where shading represents the strength of the correlation and contours mask out correlations that were not significant at the 95 per cent confidence level (map created using climate explorer, http://climexp.knmi.nl/start.cgi?someone@somewhere).

Figure 4.

Correlation of the annual mean of the high-frequency (>5.7-year) component (reconstructed component 2_3 RC2_3, which explained 7.2% of the variance) with the detrended, annually resolved 1° by 1° globally gridded HadISST1.1 sea-surface temperature data over the common time period (1886–1993), where shading represents the strength of the correlation and contours mask out correlations that were not significant at the 95 per cent confidence level (map created using climate explorer, http://climexp.knmi.nl/start.cgi?someone@somewhere).

Figure 5.

(a) Observed and expected (random, Chi-square) frequencies of sites displaying no (0% of corals bleached), low (1–10%), medium (>10–30%) and high (>30%) coral bleaching during the 1998 event in regions dominated by (i) low- and (ii) high-frequency SST variability. (b) Same as (a) but for the 2005–2006 bleaching event.

Figure 6.

(a) Scatterplot displaying the significant positive association between the stress experienced by the reef during the 1998 coral bleaching event and the relative strength of the high- and low-frequency modes of SST variability (|r_{s,RC2_3}| − |r_s,RC1|) (r_s = 0.373, n = 76, P < 0.05). Stress was defined as the product of the cumulative photosynthetic active radiation (PAR) integral two months prior to the observed bleaching and the cumulative SST anomaly integral one month prior to bleaching (denoted as Temp *PAR). (b) Same as (a) but for stress experienced during the 2005–2006 coral bleaching event (r_s = 0.396, n = 49, P < 0.05).