Constraining Thermal Thresholds and Projections of Temperature Stress on Pacific Coral Reefs Over the 21st Century: Method Refinement and Application

在过去40年里，全球大部分热带地区的海洋表面温度(SST)上升了0.5-1摄氏度，珊瑚白化事件和珊瑚礁死亡的频率和地理范围也随之增加。随着大气二氧化碳水平的持续上升，人们越来越担心二氧化碳引发的气候变化将对珊瑚礁的生存构成最大的威胁。针对中期二氧化碳排放情景的21个IPCC气候模型的平均产出预测，到本世纪末，热带海温将再上升1.5-3摄氏度。结合目前对珊瑚白化温度阈值的估计，全世界珊瑚礁生态系统的未来前景似乎很黯淡。有几个关键问题限制了对SSTs上升的全面和持久影响的准确预测。这些指标包括：(1)对预测变暖的空间和时间模式的置信度；(2)对不同造礁珊瑚物种的温度阈值的了解；(3)珊瑚通过反复暴露于高温事件而提高对热应激的抵抗力的潜力。新的骨骼标记已经被开发出来，它限制了跨越三维空间和时间的多个独立珊瑚群体的温度阈值和适应潜力。该方法基于珊瑚骨骼的三维CAT扫描重建，已经生成了红海的两个珊瑚物种--大堡礁和凤凰岛--的初步数据。结果表明，在NOAA的度加热周指数(DHWs)定义的累积热应激阈值处，骨骼生长会出现大幅、突然的下降。此外，宿主脂肪储备(一种独立衡量压力和死亡风险的指标)与骨骼生长率之间存在显著的相关性。由于珊瑚骨架记录了每一种珊瑚对连续的记录在案的热异常的反应和恢复的历史，这种方法确定了亚致命影响的温度阈值、恢复到正常海洋条件后的恢复时间(如果有的话)，并测试在连续事件后的受潮反应，表明适应。这项研究计划建立在初步工作的基础上，重点是方法的改进和对太平洋中部两个珊瑚礁上的珊瑚的应用。由于热史的反差，这些珊瑚礁被认为面临着未来海洋变暖的最大风险。同时，将在海洋环流模式上进行新的实验，该模式非常适合热带太平洋，具有足够高的水平和垂直分辨率，以最大限度地预测未来几十年特定的中太平洋珊瑚礁地点的热应力。OGCM的输出还将具有足够的时间分辨率来计算DHW，从而解决了直接应用全球气候模式输出(如IPCC AR4存档)用于珊瑚礁研究的一个主要限制。具体地说，这项研究将：(1)在热带太平洋中部吉尔伯特群岛和贾维斯群岛1-30米深的两个主要造礁物种中收集多个新的中长(15-30年)岩芯和分支；(2)应用三维CAT扫描和图像分析技术，系统地量化每个物种在每个珊瑚礁地点和深度的温度阈值、恢复率和恢复力；(3)量化热恢复能力较强和恢复能力较差的群体中的能量储备和共生体基因，在钙化压力和死亡风险之间建立定量联系，并确定钙化对热应激反应的生理基础；(4)使用专门为热带太平洋量身定做的全球气候模式，为目标珊瑚礁地点的近期气候变化产生一套动态一致的预报；以及(5)将珊瑚数据与模型输出相结合，并改进对21世纪这一中部太平洋岛群珊瑚的预测热应力预测，单位为升温度周。该项目包括通过WHOI的暑期学生奖学金计划资助一名年轻的研究员、一名博士后研究员、一名研究生和几名本科生研究人员。由于量化温度阈值和恢复率的方法可以相对有效和廉价地用于在大的空间尺度上生成许多不同珊瑚物种的数据，预期的成功结果将促进从其他太平洋珊瑚礁收集类似数据。这个数据“网络”与每周解析的OGCM输出相结合，将使珊瑚对21世纪热带太平洋个别珊瑚礁的温度压力反应的物种和深度特定预测成为可能。所有研究将在国家和国际会议上以及在公共场所公布；结果将在网站上汇总，并将由联合绩效指标纳入本科生和研究生课程。

Sea surface temperature (SST) across much of the global tropics has increased by 0.5-1 degrees C in the past 4 decades and, with it, the frequency and geographic extent of coral bleaching events and reef mortality. As levels of atmospheric CO2 continue to rise, there is mounting concern that CO2-induced climate change will pose the single greatest threat to the survival of coral reefs. Averaged output of 21 IPCC climate models for a mid-range CO2 emissions scenario predicts that tropical SSTs will increase another 1.5-3 degrees C by the end of this century. Combined with current estimates of thermal thresholds for coral bleaching, the outlook for the future of coral-reef ecosystems, worldwide, appears bleak. There are several key issues that limit accurate predictions of the full and lasting impact of rising SSTs. These include (1) level of confidence in the spatial and temporal patterns of the predicted warming, (2) knowledge of thermal thresholds of different reef-building coral species, and (3) the potential for corals to increase resistance to thermal stress through repeated exposure to high temperature events. New skeletal markers have been developed that constrain the thermal thresholds and adaptive potential of multiple, individual coral colonies across 3-D space and through time. The method, based on 3-D CAT scan reconstructions of coral skeletons, has generated initial data from two coral species in the Red Sea, Great Barrier Reef and Phoenix Islands. Results showed that large, abrupt declines in skeletal growth occur at thresholds of accumulated heat stress defined by NOAA's Degree Heating Weeks Index (DHWs). In addition, there was a significant correlation between host lipid reserve, an independent measure of stress and mortality risk, and rates of skeletal growth. Because the coral skeleton archives the history of each coral's response to and recovery from successive, documented thermal anomalies, this approach pinpoints the thermal thresholds for sub-lethal impacts, the recovery time (if any) following a return to normal oceanographic conditions, and tests for a dampened response following successive events, indicative of acclimation. This research program builds on initial work, focusing on method refinement and application to corals on two central Pacific reefs. With contrasting thermal histories, these reefs are considered at greatest risk from future ocean warming. In parallel, new experiments will be run on an ocean general-circulation model (OGCM) that is well suited to the tropical Pacific and of sufficiently high resolution, both horizontal and vertical, to maximize projections of thermal stress on specific central Pacific Reef sites over the next few decades. The OGCM output will also be of sufficient temporal resolution to compute DHWs, thus addressing a major limitation of the direct application of global climate model output (as archived for the IPCC AR4) toward coral-reef studies. Specifically, this study will: (1) collect multiple new, medium-length (15-30 yrs) cores and branches from two dominant reef-building species at 1-30m depth in the Gilbert and Jarvis Islands, central tropical Pacific; (2) apply 3-D CAT scanning and image analysis techniques to quantify systematically thermal thresholds, rates of recovery and resilience for each species, at each reef site and with depth; (3) quantify energetic reserve and symbiont genotype amongst thermally more- and less- resilient colonies, establishing a quantitative link between calcification stress and mortality risk, and determining the physiological basis for calcification responses to thermal stress; (4) use an OGCM specifically tailored to the tropical Pacific to produce a dynamically consistent set of forecasts for near-term climate change at the target reef sites; and (5) combine coral data with model output and refine the projected thermal stress forecast, in degree heating weeks, for corals in this central Pacific Island group over the 21st century. This project includes funding for a young investigator, a postdoctoral investigator, a graduate student, and several undergraduate researchers through WHOI's summer student fellowship program. Because the method of quantifying thermal thresholds and recovery rates can be applied relatively efficiently and inexpensively to generate data for many different coral species across large spatial scales, an anticipated successful outcome will promote collection of similar data from other Pacific reefs. This data "network," when combined with the weekly-resolved OGCM output, will enable species and depth-specific projections of coral responses to thermal stress on individual reefs across the tropical Pacific over the 21st century. All research will be presented at national and international meetings and at public venues; results will be summarized on websites, and will be integrated into undergraduate and graduate courses by the Co-PIs.