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

在过去的40年里，全球大部分热带地区的海表温度（SST）上升了0.5-1摄氏度，随之而来的是珊瑚白化事件和珊瑚礁死亡的频率和地理范围。随着大气中二氧化碳含量的持续上升，人们越来越担心二氧化碳引起的气候变化将对珊瑚礁的生存构成最大的威胁。21个IPCC气候模式对中期二氧化碳排放情景的平均产出预测，到本世纪末，热带海温将再增加1.5-3摄氏度。结合目前对珊瑚白化的热阈值的估计，全球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海温上升的全面和持久影响的准确预测。这些包括(1)对预测变暖的时空格局的置信度，(2)对不同造礁珊瑚物种的热阈值的了解，以及(3)珊瑚通过反复暴露于高温事件而增加对热应力的抵抗力的潜力。新的骨骼标记物已经被开发出来，这些标记物可以限制多个单独珊瑚群落在三维空间和时间上的热阈值和适应潜力。该方法基于珊瑚骨骼的三维CAT扫描重建，已经从红海、大堡礁和凤凰群岛的两种珊瑚中获得了初始数据。结果表明，在由NOAA的温度加热周指数（dhw）定义的累积热应激阈值处，骨骼生长会出现大幅突然下降。此外，宿主脂质储备（应激和死亡风险的独立衡量指标）与骨骼生长速率之间存在显著相关性。由于珊瑚骨架记录了每个珊瑚对连续记录的热异常的反应和恢复的历史，因此该方法确定了亚致死影响的热阈值，恢复正常海洋条件后的恢复时间（如果有的话），以及连续事件后的抑制反应测试，表明适应。这项研究计划建立在最初的工作基础上，重点是改进方法并将其应用于两个太平洋中部珊瑚礁的珊瑚。通过对比热历史，这些珊瑚礁被认为是未来海洋变暖的最大风险。与此同时，将在海洋环流模式（OGCM）上进行新的实验，该模式非常适合热带太平洋，并且具有足够高的水平和垂直分辨率，以便在未来几十年内最大限度地预测太平洋中部特定珊瑚礁地点的热应力。OGCM输出也将具有足够的时间分辨率来计算dhw，从而解决直接将全球气候模式输出（如IPCC第4次评估报告存档）应用于珊瑚礁研究的一个主要限制。具体而言，本研究将：(1)在热带太平洋中部吉尔伯特群岛和贾维斯群岛的1-30m深度收集两个优势造礁物种的多个新的中等长度（15-30年）核心和分支；(2)应用三维CAT扫描和图像分析技术，系统地量化每个物种、每个珊瑚礁地点和深度的热阈值、恢复率和恢复力；(3)量化热适应性强和热适应性差菌落的能量储备和共生体基因型，建立钙化胁迫与死亡风险之间的定量联系，确定热胁迫下钙化反应的生理基础；(4)使用专门为热带太平洋定制的OGCM，对目标珊瑚礁地点的近期气候变化进行动态一致的预测；(5)将珊瑚数据与模型输出结合起来，细化21世纪太平洋中部岛群珊瑚在升温周内的预估热应力预报。该项目包括通过WHOI的暑期学生奖学金计划资助一名青年研究员、一名博士后研究员、一名研究生和几名本科生研究人员。由于量化热阈值和恢复速率的方法可以相对有效和廉价地应用于在大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。该数据“网络”与每周解析的OGCM输出相结合，将使21世纪热带太平洋单个珊瑚礁的物种和深度特定珊瑚对热应力反应的预测成为可能。所有研究都将在国家和国际会议和公共场所提出；研究结果将在网站上汇总，并由合作项目负责人整合到本科和研究生课程中。

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.