The 2014 coral bleaching and freshwater flood events in Kāneʻohe Bay, Hawaiʻi
The 2014 coral bleaching and freshwater flood events in Kāneʻohe Bay, Hawaiʻi
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2014 年夏威夷卡内奥赫湾珊瑚白化和淡水洪水事件
作者：
K. Bahr;P. Jokiel;K. Rodgers
直到最近，亚热带夏威夷群岛逃过了破坏许多热带地区的主要漂白事件，但全球长期平均温度的持续上升和太平洋年代际振荡（PDO）冷却阶段的明显结束增加了漂白事件的风险。气候模型和观测预测，在未来几十年，夏威夷的白化现象将以越来越频繁和越来越严重的方式发生。2014年7月，在Kāne Kagohe湾的北方发生了淡水“死亡”事件，使直接受洪水影响的地区的珊瑚覆盖率减少了22.5%。随后在2014年9月发生的一次重大漂白事件导致整个海湾的珊瑚广泛漂白和死亡，并进一步减少了淡水杀死区域的珊瑚覆盖率60.0%。高温漂白事件仅导致整个海湾部分活珊瑚减少1.0%，而淡水事件未直接影响。因此，低盐度事件和热漂白事件的综合影响似乎不仅仅是简单的相加。2014年9月白化事件期间的温度状况在持续时间和强度上与1996年8月发生的大型白化事件相似，但导致更大面积的白化和珊瑚死亡。显然，季节性的时间以及加热的持续时间和幅度很重要。主要珊瑚物种的珊瑚产卵发生在初夏，因此珊瑚中储存的脂质库在2014年9月事件之前已经因产卵而耗尽。高于27 °C的温暖月份导致珊瑚生长较低，可能会进一步减少脂质储备，导致漂白事件比夏季早些时候发生的高温更严重。夏威夷礁珊瑚在27 °C以上的温度下会减少骨骼的生长，所以也许“压力期”实际上早在29 °C的漂白阈值达到之前就开始了。夏威夷群岛直接受到PDO的影响，PDO可能成为影响亚热带夏威夷群岛白化事件的一个因素，就像厄尔尼诺南方涛动（ENSO）的变化影响热带太平洋低纬度地区的白化事件一样。记录显示，卫星测量的近海温度并不总是能预测近海漂白，因为其他因素（高云量、高风浪作用、潮汐交换率）可能限制近海加热，防止海湾温度达到漂白阈值。由于云层覆盖或高浊度造成的低光照水平也可以防止漂白。
Until recently, subtropical Hawaiʻi escaped the major bleaching events that have devastated many tropical regions, but the continued increases in global long-term mean temperatures and the apparent ending of the Pacific Decadal Oscillation (PDO) cool phase have increased the risk of bleaching events. Climate models and observations predict that bleaching in Hawaiʻi will occur with increasing frequency and increasing severity over future decades. A freshwater “kill” event occurred during July 2014 in the northern part of Kāneʻohe Bay that reduced coral cover by 22.5% in the area directly impacted by flooding. A subsequent major bleaching event during September 2014 caused extensive coral bleaching and mortality throughout the bay and further reduced coral cover in the freshwater kill area by 60.0%. The high temperature bleaching event only caused a 1.0% reduction in live coral throughout the portion of the bay not directly impacted by the freshwater event. Thus, the combined impact of the low salinity event and the thermal bleaching event appears to be more than simply additive. The temperature regime during the September 2014 bleaching event was analogous in duration and intensity to that of the large bleaching event that occurred previously during August 1996, but resulted in a much larger area of bleaching and coral mortality. Apparently seasonal timing as well as duration and magnitude of heating is important. Coral spawning in the dominant coral species occurs early in the summer, so reservoirs of stored lipid in the corals had been depleted by spawning prior to the September 2014 event. Warm months above 27 °C result in lower coral growth and presumably could further decrease lipid reserves, leading to a bleaching event that was more severe than would have happened if the high temperatures occurred earlier in the summer. Hawaiian reef corals decrease skeletal growth at temperatures above 27 °C, so perhaps the “stress period” actually started long before the bleaching threshold of 29 °C was reached. Hawaiʻi is directly influenced by the PDO which may become a factor influencing bleaching events in subtropical Hawaiʻi in much the same manner as variations in the El Niño Southern Oscillation (ENSO) influences bleaching events at low latitudes in the tropical Pacific. Records show that offshore temperatures measured by satellite will not always predict inshore bleaching because other factors (high cloud cover, high wind and wave action, tidal exchange rate) can limit inshore heating and prevent temperatures in the bay from reaching the bleaching threshold. Low light levels due to cloud cover or high turbidity can also serve to prevent bleaching.