Assessing the geo-ecological consequences of the severe 2023 bleaching event on Mexican Caribbean coral reefs

气候变化，特别是重大海洋热异常事件的发生，对热带珊瑚礁的持久性构成重大威胁。这一威胁来自于这样一个事实，即当珊瑚受到热胁迫（比年最高温度高1摄氏度以上），并且暴露时间超过约4周时，它们的反应是排出共生藻类（虫黄藻），使它们完全变成白色。如果压力事件非常高和/或延长，珊瑚就会死亡。过去几年中制定的未来漂白预测预测，根据未来排放情景，在未来60-80年内，许多珊瑚礁将发生高频率的漂白事件。然而，在过去的几个月里，加勒比地区的许多珊瑚礁暴露在与这些未来预测相匹配的温度下，其强度和寿命基本上比预期的要早得多，从而使人们能够更早地了解珊瑚礁可能越来越多地暴露在其中的未来变暖状况。2023年5月初开始出现严重变暖，测得的水温超过33摄氏度（比正常季节最高值高出3摄氏度）。在撰写本文时（23年9月初），珊瑚漂白诱导温度暴露的时间已经是自1985年卫星监测开始以来该地区最高记录的两倍。大范围的珊瑚白化现象现在很明显，对未来的生态和该地区珊瑚礁结构的持久性造成的后果尚不清楚。然而，许多墨西哥加勒比海珊瑚礁的珊瑚覆盖率已经很低，使许多珊瑚礁处于潜在的临界点-任何对剩余的假定更具复原力的生物群的灭绝都将使珊瑚礁在大的空间尺度上广泛过渡到净侵蚀状态。因此，对珊瑚礁未来功能的影响是严重的。我们正处于一个独特的情况下，以评估这两个生态属性的漂白事件的影响，但也独特的碳酸盐预算状态，在这一地区的珊瑚礁。碳酸盐收支状态数据是一个特别有用的珊瑚礁健康监测指标，从中我们可以量化珊瑚礁维持珊瑚礁框架建设和垂直珊瑚礁增生潜力的能力，生物多样性和珊瑚礁衍生服务，如沿海波浪保护的持久性至关重要的功能。我们在2017年至2019年期间开展的过去的工作涉及在整个墨西哥中美洲珊瑚礁系统（从坎昆北部到伯利兹边境）和韦拉克鲁斯州沿着的详细珊瑚礁碳酸盐预算评估的主要计划。我们建议对这些地点进行全面的重新评估，以提供有关白化影响的最全面的珊瑚礁规模数据，使我们能够：1）量化每个地点珊瑚组合的性质和变化幅度，包括确定地点之间物种敏感性的任何差异; 2）量化珊瑚礁碳酸盐生产和生物侵蚀速率的变化幅度，并辨别分类群在多大程度上仍然对生产与侵蚀平衡做出贡献;以及3）量化对这些生物礁的垂直加积潜力的影响。

Climate change generally, but especially the occurrence of major ocean thermal anomaly events, represents a major threat to the persistence of tropical coral reefs. This threat arises from the fact that when corals are exposed to heat stress (>1 degree C above the annual maximum) and the exposure period exceeds about 4 weeks they respond by expelling their symbiotic algae (zooxanthellae), causing them to turn completely white. If the stress event is very high and/or prolonged the corals will die. Future bleaching projections developed over the past few years have predicted the onset of high frequency bleaching events on many reefs through the next 60-80 years depending on future emission scenarios. However, over the past few months many reefs in the Caribbean have been exposed to temperatures that match these future projections in terms of magnitude and longevity - in essence providing far earlier than expected insights into the future warming regimes reefs may be increasingly exposed to.The Mexican Caribbean region has been no exception. Severe warming started in early May 2023 and measured water temperatures have exceeded 33 degrees C (3 degrees C above the normal seasonal maximum). At the time of writing (early Sept '23) the period of coral bleaching-inducing temperature exposure has already been double the maximum recorded in the region since satellite monitoring began in 1985. Widespread coral bleaching is now evident with unclear consequences for the future ecology and the persistence of reef structures in this region. However, the already very low coral cover on many Mexican Caribbean reefs leaves many reefs at a potentially critical tipping point - whereby any extirpation of what were the remaining and assumed more resilient taxa will see a widespread transition of reefs to states of net erosion over large spatial scales. The implications for the future functionality of the reefs are thus severe. We are in a unique situation to assess the impacts of this bleaching event on both the ecological attributes, but also uniquely the carbonate budget states, of the reefs in this region. Carbonate budget state data is an especially useful reef health monitoring metric from which we can quantity the capacity of reefs to sustain reef framework building and vertical reef accretion potential, functions essential to the persistence of biodiversity and reef-derived services such as coastal wave protection. Our past work, undertaken between 2017 and 2019, involved a major programme of detailed reef carbonate budget assessments at sites along the entire Mexican Mesoamerican reef system (from north of Cancun to the Belize border) and in Veracruz. We propose to undertake a full re-assessment of these sites to provide the most comprehensive reef-tract scale data on the impacts of bleaching that will allow us: 1) to quantify the nature and magnitude of change in coral assemblages at each site, including identification of any between-site variations in species susceptibility; 2) to quantify the magnitude of change in rates of reef carbonate production and bioerosion, and to discern the extent to which taxa are still contributing to the production versus erosion balance; and 3) to quantify the impacts on the vertical accretion potential of these reefs.