Mass coral bleaching and mortality on a remote atoll: evidence of climate change impacts?

Tom DeCarlo, Geology & Geophysics
Advisor: Anne Cohen, Geology & Geophysicss

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Research Summary

Introduction Worldwide, the value of goods and services provided by coral reef ecosystems, including fisheries, shoreline protection, and tourism, is $10 trillion per year1. Coral reefs also harbor a disproportionately large 25% of marine species even though they cover less than 1% of Earth’s surface2.  Yet, coral reefs are considered the “canary in the coalmine”, among the ecosystems most sensitive to climate change and that serve as an early warning of habitat degradation worldwide3. Reef-building corals are imminently threatened by rising sea surface temperatures (SST) because anomalously warm SST has the potential to induce coral bleaching, the loss of the symbiotic zooxanthellae algae that live within the host coral cells and provide much of the energy required by the coral, often leading to coral mortality4.
 
Of great concern for coral reef futures, projected SST changes worldwide are expected to impose annual bleaching events on more than 90% of all coral reefs by the year 20805. These projections, while critical for estimating the impacts of climate change on coral reefs, are based on estimates of the threshold SST for coral bleaching that are extrapolated globally from sparse observations of bleaching in the field.  Long-term bleaching histories are very scarce and for the vast majority of coral reefs, direct observations of the presence or absence of bleaching, if they exist at all, extend back only a few decades6. Further, evidence is mounting that coral bleaching is not solely a function of temperature, but depends in part on the local physical and chemical environment in which corals live7-9 as well as historical exposure to SST variability10-11. Therefore, it is critical to establish historical bleaching records in order to determine the sensitivity of bleaching to changing SST and how this sensitivity varies around the world and with other environmental (e.g. local currents) or biological (e.g. adaptation) factors.
 
Proposed activities We possess a novel and unique opportunity to evaluate the historical sensitivity of bleaching to SST changes based on observations made on Dongsha Atoll, a remote coral reef in the South China Sea. Dongsha is located in a region that is experiencing especially rapid warming as summertime SST has increased 0.7 °C since satellite observations began in 1982. The May-July SST around Dongsha in 2015 was the hottest on record, and a mass coral bleaching event began on Dongsha in June 2015. Ecological surveys that we conducted on Dongsha revealed that the bleaching event resulted in mortality of approximately half of the corals on the reef flat, with live coral cover decreasing from 22% to 11% in just two months. Approximately half of the massive Porites colonies, many 100+ years old, were found recently dead and covered with algae during the post-bleaching surveys (Fig. 1), preliminarily supporting the hypothesis that the 2015 bleaching event is unprecedented in the past century.
However, testing this hypothesis requires a historical bleaching record, which is unavailable on Dongsha because direct observations of the reef community are limited to the past few years. We have the ability to fill this gap in knowledge of historical bleaching on Dongsha using cores of skeleton collected from living coral colonies. When corals recover from bleaching (i.e. they are repopulated with symbionts), they are thought to form distinct high-density “stress bands” within their skeleton7,12, although this hypothesis has never been tested directly. On Dongsha, we observed a transient bleaching event last year, in June 2014, when many Porites colonies bleached and subsequently recovered, at which time we tagged 10 colonies with underwater labels. During our 2015 expedition to Dongsha, we collected a skeletal core from each of these colonies, half of which died two weeks afterwards during the bleaching event.
 
Our set of coral skeletal cores can be used to provide a first-of-its-kind view of the skeletal signature of bleaching, and the historical record of bleaching in corals that did not survive a massbleaching event. Specifically, I will test the following hypotheses:
1) Coral colonies that bleached in 2014 formed stress bands. If true, this will allow us to use stress bands to test for past bleaching events on Dongsha. Under this framework, I will test a second hypothesis:
2) Bleaching on Dongsha in 2014 and 2015 is anomalous relative to the past ~50 years.
I will test these hypotheses by scanning the coral cores that I collected on Dongsha with computerized tomography (CT). I previously developed an automated computer program that analyzes the growth history, including identification of high-density stress bands, in CT scans of coral skeletal cores13. Using this program, I will evaluate the proportion of corals that formed 2014 stress bands (hypothesis 1), and I will investigate the presence/absence of stress bands each year over the lifetime (50- 100 years) of these corals (hypothesis 2). To evaluate the sensitivity of bleaching to SST, I will compare the presence of stress bands to summertime SST data derived from satellite observations (since 1982) and/or gridded SST products based on ship-board measurements (prior to 1982). 
 
The results of this study will provide in situ observations linking the historical presence/absence of bleaching with SST variability in a region that is experiencing particularly rapid warming, and will potentially reveal the threshold of SST anomaly that leads to bleaching and mortality. These data are critical for constraining projections of future climate change impacts on coral reefs globally, and our framework of investigating bleaching on Dongsha with stress bands may serve as a model for studying the sensitivity of bleaching to SST on other coral reefs systems. 
 
References
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