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.2018 Nov 8:1:177.
doi: 10.1038/s42003-018-0183-7. eCollection 2018.

Repeat bleaching of a central Pacific coral reef over the past six decades (1960-2016)

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Repeat bleaching of a central Pacific coral reef over the past six decades (1960-2016)

Hannah C Barkley et al. Commun Biol..

Abstract

The oceans are warming and coral reefs are bleaching with increased frequency and severity, fueling concerns for their survival through this century. Yet in the central equatorial Pacific, some of the world's most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16 was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30% bleaching) and two moderate (<30% bleaching) events occurred, each coinciding with El Niño. While the frequency and severity of bleaching on Jarvis did not increase over this time period, 2015-16 was unprecedented in magnitude. The trajectory of recovery of this historically resilient ecosystem will provide critical insights into the potential for coral reef resilience in a warming world.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
A comparative history of thermal stress represented by Degree Heating Weeks (DHWs) and cumulative DHWs or Total Hotspot ona Jarvis Island, central equatorial Pacific,b Palau, western tropical Pacific, andc northern Great Barrier Reef since 1980. DHWs > 4 (red dashed line) are considered conducive for coral bleaching and > 8 for severe bleaching and mortality. Jarvis corals experienced seven > 4 and six > 8 DHW episodes since 1980. Palau corals experienced two episodes > 4 DHW (1998 and 2010). For corals in the northern GBR, the 2016 thermal anomaly was their first encounter with ocean conditions considered conducive for bleaching. Here, DHWs are calculated using a percentile method rather than the traditional mean monthly maximum (MMM) to estimate maximum mean SST experienced by each reef. This approach was taken to enable direct comparison between regions dominated by inter-annual SST variability (central equatorial Pacific) and those dominated by seasonal SST variability (western tropical Pacific). A detailed description of the percentile method and the comparison with traditional NOAA DHWs for these three sites are provided in the Methods and Supplementary Information
Fig. 2
Fig. 2
The impact of the 2015 El Niño on Jarvis Island coral communities recorded by the first visual surveys of the reef during and after peak El Niño.aMontipora-dominated reef communities in April 2015 (NOAA/Paula Ayotte).b The same communities ~100% bleached but still alive in November 2015.c In May 2016, ~95% of corals were dead.d By April 2017, somePorites corals had recovered while cover remained low.e Mean ( ± standard error) live coral cover in November 2015 (the sum of living bleached and healthy non-bleached colonies), May 2016 and Apr 2017. No bleaching was observed in 2016 and 2017. Depths are 5–14 m (white), 15–19 m (light gray), and 20–25 m (dark gray).f Mean ( ± standard error) of coral cover bleached in Nov 2015 at three depth bins
Fig. 3
Fig. 3
The skeletons ofPorites corals record the 2015 bleaching (a–c) and historical bleaching events (d–e) that occurred on Jarvis Island, and for comparison, on Palau in 1998 (f). In the CT scan images, white is high-density, black is low density. In (a) CT scans of the tops of skeletal cores extracted from massive JarvisPorites in 2012 show no sign of stress band formation. 2012 was an El Niño–Southern Oscillation neutral year and average tissue thickness measured in the core tops (left, red arrow) approached 1 cm, indicating the corals were energetically replete. Conversely, in (b), CT scans of 88% of the cores extracted in November 2015 revealed high-density stress band in the process of formation (white arrow). Tissue thickness of bleached colonies was reduced by ~75% (red arrow) indicating starvation. In (c), a core removed in April 2017 reveals recovery of tissue thickness (red arrow) and the 2015 stress band sequestered beneath new skeletal growth (scale bar = 1 cm). Stress bands are confirmed by automated analysis of density variations (d) where Z-scores > 2 (dashed line) in the detrended density time series are considered stress bands. LongPorites cores from Jarvis reveal multiple historical stress bands (e) whereas those from Palau have one or two, consistent with documented bleaching events on Palau in 1998 and 2010 (f)
Fig. 4
Fig. 4
Bleaching, tissue loss and recovery of a massivePorites coral on Jarvis Island, and incorporation of the bleaching signal into the skeleton.aPorites ID 497 at 16.5 m depth on the west side of the island (0.369 °S, 160.008 °W) bleached in 2015, and in May 2016, no live tissue was evident on the colony surface.b,c By April 2017, the coral exhibited almost full recovery. In (d) A 3-D CT scan of a core removed from the recovered colony in 2017 revealed almost 1 cm of new growth above the stress band, a growth rate ~30% lower than pre-bleaching rates (scale bar = 1 cm). A mortality scar (arrow), signaling complete localized loss of tissue for an extended period, is also visible in the scan. Corallite tracks, which are the skeletons of individual polyps, are continuous across the mortality scar, indicating some polyps survived the bleaching deep inside the skeleton, revived and continued to extend their original corallites once ocean conditions returned to normal
Fig. 5
Fig. 5
Historical coral reef bleaching events on Jarvis Island reconstructed from stress bands. In (a) percent ofPorites corals with a stress band in a given year is strongly correlated with the observed level of community bleaching in the same year ( ± one standard error) for Jarvis 2015 (this study) and 2010 (green), Howland 2010 (red), and Palau 1998 and 2010 (open black circles). This relationship allows us to reconstruct the history of bleaching on Jarvis in the absence of visual observations.b Percent of JarvisPorites corals with a stress band in a given year scale with the level of thermal stress experienced by the community that year, indicating that the Jarvis coral community responds predictably to thermal stress. Total Hotspot, an index of the cumulative thermal stress during a specific event, is calculated from weekly satellite SST spanning the time period 1982–2016. In (c) six decades of coral reef bleaching on Jarvis Island (vertical bars, mean ± one standard error), 1960–2016, constructed from stress bands using the calibration in (a). The time series of ERSST anomalies over the same time period, for a 2° × 2° grid centered on Jarvis, is shown in blue. All bleaching events coincide with high SST anomalies
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