Modelling coral reef connectivity in the Western Indian Ocean under climate change

世界各地的珊瑚礁面临着前所未有的风险。政府间气候变化专门委员会（IPCC）公布，全球海洋表面变暖最强烈，印度洋表面温度以每十年0.11摄氏度的速度上升，比大西洋和太平洋快。（Hoegh-Guldberg et al., 2014； IPCC, 2018; van Vuuren et al., 2011）与1950 -2009年相比，印度洋变暖0.65℃也超过了其他海洋（Hoegh-Guldberg et al., 2014）。在过去的30 -40年里，几乎所有地方的平均活珊瑚覆盖面积都减少了50% - 75% （Bruno et al., 2019）。预计这种情况会随着未来的大规模漂白事件而变得更糟（Hughes et al., 2017）。在IUCN红色名录中，三分之一的造礁珊瑚物种被认为是受威胁的，另外三分之一被认为是近危的（Bridge等人，2020年）。这些分类群对于构成健康鱼类群落基础的珊瑚礁弹性和复杂性至关重要（Eakin et al., 2019）。全球变化下的海岸保护和粮食安全取决于有效的保护管理，该管理考虑了当前条件和未来气候变化情景下珊瑚的遗传连通性（Graham et al., 2015; Hughes et al., 2017）。目前的气候变化下珊瑚或暗礁鱼类分布模型通常不包括幼虫扩散，而幼虫扩散是在白化等干扰事件后影响珊瑚礁轨迹的关键因素（Graham et al., 2015; Magris et al., 2016）。气候变化对印度洋的影响尤其严重，西部地区的珊瑚礁是世界上一些最贫穷社区的重要资源。目前，只有三个关键的连接珊瑚礁受到保护，需要将保护重点放在其他高度连接的珊瑚礁上，以便使它们成为幼虫和基因传播的垫脚石（Gamoyo等人，2019）。然而，尽管威胁和人为压力不断增加，但关于该地理区域未来气候情景下珊瑚连通性和珊瑚礁恢复力的知识有限。随着气候的进一步影响，了解目前的循环模式、珊瑚和鱼类种群动态如何变化，以及这如何影响珊瑚礁的恢复力和粮食安全，是至关重要的。海洋保护区（MPAs）成为支持珊瑚礁的关键措施之一，但它们的成功取决于它们如何通过鱼类的运动或幼虫的扩散在大范围内联系起来（Crochelet et al., 2016）。英属印度洋领地（BIOT）是世界上最大的海洋保护区之一，位于印度洋的中心，由英国于2010年指定。自20世纪70年代以来，除了最大的环礁迪戈加西亚上有一个美国军事基地外，该群岛无人居住，因此除了一些非法活动外，没有捕鱼压力。尽管受到保护，地处偏远，人为影响最小，但群岛上的珊瑚礁遭受了严重的白化和死亡，珊瑚覆盖率从2012年的30%下降到2016年的12%。Acropora是生态系统的主要建筑师之一，在白化事件后，它的数量下降了86%，被Porites所取代。（Head et al., 2019）由于其位于非洲和印度尼西亚之间的印度洋中部，这个大型海洋保护区可以作为全球变化下连接西印度洋和东印度洋的重要垫脚石（Robinson et al., 2017）。我的研究旨在模拟当前条件下和IPCC气候情景下西印度洋和BIOT的珊瑚礁连通性。关于造礁珊瑚连通性的知识可以为观测研究提供基础，并改善气候变化情景下物种分布的预测（Wood et al., 2014）。

Coral reefs worldwide arefacing more risks than ever before.The Intergovernmental Panel on Climate Change(IPCC)published that global ocean warming is strongest at the surface and the Indian Ocean surface temperatures have increased at a rate of0.11 C per decade, fasterthan for the Atlantic and Pacific Oceans.(Hoegh-Guldberg et al., 2014; IPCC, 2018; van Vuuren et al., 2011)The Indian Ocean warming of 0.65 C compared to 1950 -2009 exceeds the other oceans as well(Hoegh-Guldberg et al., 2014). Average live coral cover has decreased by 50 -75 % almost everywherein the last 30 -40 years(Bruno et al., 2019).This situation is predicted to get worse with future mass bleaching events (Hughes et al., 2017). A third of reef-forming coral species are considered threatened and another third as near threatened in the IUCN Red List (Bridge et al., 2020).These taxa are critical for reef resilience and complexity which form the base of healthy fish communities(Eakin et al., 2019). Coastal protection and food security under global change depend on efficient conservation management that accounts for genetic connectivity of corals under present conditions and future climate change scenarios (Graham et al., 2015; Hughes et al., 2017).Current models of coral or reef fish distribution under climate change often do not include larval dispersal which is a critical factor that influences the trajectory of reefs after a disturbance event such as bleaching(Graham et al., 2015; Magris et al., 2016).The Indian Ocean is especially heavily impacted by climate change and reefs in the Western region are essential resourcesfor some of the world's poorest communities.Currently, only three key connected reefs are under protectionand conservation focus on the other highly connectedreefs is requiredin order for them to act as stepping stones for larval and gene dispersal(Gamoyo et al., 2019). However,knowledge about coral connectivity and reef resilience under future climate scenarios in this geographical region is limited despite increasing threats and anthropogenic pressure. With further climate impacts it is vital to understand how present circulation patterns and coral and fish population dynamics might change and how this affects reef resilience and food security. Marine protected areas (MPAs)emerged as one of the key measures to support coral reefs but their success depends on how connected they are on large scales by movement of fish or larval dispersal (Crochelet et al., 2016). One of the world's largest no-take marine reserves is the British Indian Ocean Territory(BIOT) MPAin the centre of the Indian Ocean which was designated by the UK in 2010. Since the 1970s, the archipelago is uninhabited except for a US military base on the largest atoll,Diego Garcia,and thus, has no fishing pressure besides some illegal activities. Despite its protection and remote location with minimal anthropogenic impacts the reefs in the archipelago endured severe bleaching and mortality with coral cover decreasingfrom 30% to 12 %between 2012 and 2016. Acropora, one of the key ecosystem architects, showed an 86% declineand was replaced by Porites after the bleaching event. (Head et al., 2019) With its central Indian Ocean location halfway between Africa and Indonesia this large MPA could act as an important stepping stone linking the Western and Eastern Indian Ocean under global change (Robinson et al., 2017). My research aims to model coral reef connectivity under present conditions and the IPCC climate scenarios in the Western Indian Oceanand the BIOT.Knowledge on connectivity of reef-building corals can underpin observational studies and improve predictions of species distributions under climate change scenarios (Wood et al., 2014).