Functional Ecology ( IF 4.6 ) Pub Date : 2022-07-13 , DOI: 10.1111/1365-2435.14141 Felipe de Vargas Ribeiro 1, 2, 3 , Albert Pessarodona 3 , Chenae Tucket 3 , Yannick Mulders 3 , Renato Crespo Pereira 1, 2, 4 , Thomas Wernberg 3, 5, 6
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1 INTRODUCTION
Marine communities are experiencing acute changes in their structure and distribution in response to ongoing accelerated environmental changes (Beaugrand et al., 2019; Hyndes et al., 2016; Pecl et al., 2017; Vergés et al., 2014). In temperate regions, ocean warming and marine heatwaves have driven the poleward migration and establishment of warm affinity species, while dominant species locally declined and/or retracted towards cooler waters at higher latitudes (Vergés et al., 2014). The loss of dominant habitat-forming species directly affects the availability of food resources and spatial refuges, as well as local biogeochemical cycles and the reproductive output of the associated biodiversity (Vergés et al., 2019).
A striking example of climate-driven changes in habitat-forming species is the establishment of corals in temperate latitudes (Vergés et al., 2014). Novel coral-dominated ecosystems (Graham et al., 2014) are emerging and expanding in multiple temperate–subtropical biogeographic transition zones (Sommer et al., 2014) such as in Japan (Kumagai et al., 2018; Yamano et al., 2011), South Korea (Denis et al., 2014), Australia (Baird et al., 2012; Booth & Sear, 2018; Tuckett et al., 2017) and the Mediterranean Sea (Cutajar et al., 2020; Serrano et al., 2012; Serrano et al., 2013).
Despite the increase of corals in temperate regions, the processes and mechanisms enabling the expansions remain poorly resolved. Corals inhabiting higher latitudes face marginal conditions, yield less calcareous accretion due to low temperatures and low aragonite saturation, resulting in lower growth rates (Kleypas et al., 1999; Veron et al., 2015). Increases in temperature may remove some of these physiological limitations (McIlroy et al., 2019) and positively affect expanding species by promoting establishment in subtropical and temperate locations (Price et al., 2019). Dispersal potential, tolerance to environmental gradients such as the resilience to cold stress (Higuchi et al., 2020) and competitive ability are additional key features for overcoming biogeographic barriers (Keith et al., 2015; Sommer et al., 2014). Competitive interactions may also play a paramount role in the establishment of coral recruits in temperate areas, constraining expansions through competitive exclusion (Miller & Hay, 1996; Thomson et al., 2012).
In tropical reefs, macroalgae exert negative effects on corals by physical and chemical mechanisms (Fong et al., 2020; Jompa & McCook, 2003; Morrow et al., 2017). In temperate communities, it has been demonstrated that macroalgae such as kelps may damage corals by physical abrasion and overgrowth (Coyer et al., 1993; Miller & Hay, 1996). Abrasion takes place whenever the soft tissue of corals is exposed to brushing by kelp lamina (Coyer et al., 1993), and the removal of live coenosarc may impair several physiological functions such as growth, nutrition and excretion (Veron, 2000). Aside from the characterization of community patchiness (Thomson et al., 2012), interactions between kelp and corals have received little attention, and mechanisms of kelp-coral competition remain poorly resolved.
Declines of habitat-forming kelps due to warming have been proposed to provide a competitive release for corals, resulting in increased coral abundances (Tuckett et al., 2017). The observed concurrent decline in large habitat-forming kelps and increase in corals suggest a pathway to coral-dominated states in some regions (Kumagai et al., 2018), which can then promote the establishment of other tropical species (Yamano et al., 2011). Therefore, unravelling the driving mechanisms is central to understanding the dynamics of these regime shifts.
Across subtropical and temperate Australia, the laminarian kelp Ecklonia radiata is a major provider of habitat on reefs down to 40-m depth (Marzinelli et al., 2015; Wernberg et al., 2011; Wernberg et al., 2019). In recent decades, Ecklonia kelp forests in mid- and high-latitude regions in Australia have been losing ground to turf macroalgae and corals due to progressive warming, marine heatwaves and the migration of tropical grazers increasing consumer pressure on kelp biomass (Bennett, Wernberg, De Bettignies, et al., 2015; Connell et al., 2008; Vergés et al., 2016; Wernberg et al., 2016). The temperate coast of Western Australia (WA), where coastal and offshore reefs are particularly influenced by the southward-flowing Leeuwin Current, has experienced one of the most significant kelp losses recorded. Marine heatwaves, in combination with tropical herbivores facilitated by the warm waters of the Leeuwin Current (Feng et al., 2010; Wernberg et al., 2013), have caused kelp loss along the coast, at its extreme resulting in 90% loss of kelp forests over 100 km around 29°S (Wernberg et al., 2016). Across this mid-west section of the WA coastline, coral counts more than doubled after an intense heatwave event in 2011 (Tuckett et al., 2017).
Here, we aim to investigate whether the trend of increasing coral abundance has continued in the years following the marine heatwave and unravel some of the mechanisms behind coral proliferation in temperate reefs. To do so, we resurvey sites from a longitudinal study on the abundance of seaweeds and corals along the midwestern WA coast, and measure the effects of physical (abrasion and shading) and chemical (allelopathy) interactions between kelps and corals through manipulative field and laboratory experiments. Specifically, we test the hypotheses that the abundance of corals in WA's mid-west coastal reefs (a) increased over time and (b) is negatively affected by macroalgal canopies.
中文翻译：
盾墙：海带是热带珊瑚礁中对抗珊瑚的最后一道防线
1 简介
为了应对持续加速的环境变化，海洋群落的结构和分布正在经历剧烈变化（Beaugrand 等人， 2019 年；Hyndes 等人， 2016 年；Pecl 等人， 2017 年；Vergés 等人， 2014 年）。在温带地区，海洋变暖和海洋热浪推动了向极地迁移和温暖亲和物种的建立，而优势物种在高纬度地区局部下降和/或退缩到较冷的水域（Vergés et al., 2014）。主要栖息地形成物种的丧失直接影响食物资源和空间避难所的可用性，以及当地的生物地球化学循环和相关生物多样性的繁殖产出（Vergés 等人， 2019 年）。
气候驱动的栖息地形成物种变化的一个显着例子是在温带地区建立珊瑚（Vergés 等人， 2014 年）。新的以珊瑚为主的生态系统 (Graham et al., 2014 ) 正在多个温带-亚热带生物地理过渡区 (Sommer et al., 2014 ) 出现和扩展，例如在日本 (Kumagai et al., 2018 ; Yamano et al., 2011 年）、韩国（Denis 等人， 2014 年）、澳大利亚（Baird 等人， 2012 年；Booth & Sear， 2018 年；Tuckett 等人， 2017 年）和地中海（Cutajar 等人， 2020 年；Serrano 等人）等， 2012; 塞拉诺等人， 2013 年）。
尽管温带地区的珊瑚有所增加，但促成扩张的过程和机制仍然很难解决。栖息在高纬度地区的珊瑚面临边际条件，由于低温和文石饱和度低，产生的钙质增生较少，导致生长速度较低（Kleypas 等人， 1999 年；Veron 等人， 2015 年）。温度升高可能会消除其中一些生理限制（McIlroy 等人， 2019 年），并通过促进在亚热带和温带地区的建立（Price 等人， 2019 年）对扩张的物种产生积极影响。分散潜力，对环境梯度的耐受性，例如对冷应激的恢复能力（Higuchi et al., 2020) 和竞争能力是克服生物地理障碍的额外关键特征 (Keith et al., 2015 ; Sommer et al., 2014 )。竞争性互动也可能在温带地区珊瑚新兵的建立中发挥重要作用，通过竞争排斥来限制扩张（Miller & Hay, 1996 ; Thomson et al., 2012）。
在热带珊瑚礁中，大型藻类通过物理和化学机制对珊瑚产生负面影响（Fong 等人， 2020 年；Jompa 和 McCook， 2003 年；Morrow 等人， 2017 年）。在温带社区，已经证明海带等大型藻类可能会因物理磨损和过度生长而损害珊瑚（Coyer 等人， 1993 年；Miller 和 Hay， 1996 年）。每当珊瑚的软组织受到海带层的刷洗时，就会发生磨损（Coyer 等人， 1993 年），并且去除活体腔肉可能会损害一些生理功能，例如生长、营养和排泄（Veron， 2000 年）。除了社区斑块的表征（Thomson 等人， 2012），海带和珊瑚之间的相互作用很少受到关注，海带-珊瑚竞争的机制仍然很难解决。
已经提出由于变暖导致栖息地形成海带的减少为珊瑚提供竞争性释放，导致珊瑚丰度增加（Tuckett 等人， 2017 年）。观察到的大型栖息地形成海带的同时减少和珊瑚的增加表明在某些地区通往珊瑚主导状态的途径（Kumagai 等人， 2018 年），然后可以促进其他热带物种的建立（山野等人， 2011 年）。因此，解开驱动机制对于理解这些政权转变的动态至关重要。
在亚热带和温带澳大利亚，海带Ecklonia radiata是深达 40 米的珊瑚礁栖息地的主要提供者（Marzinelli 等人， 2015 年；Wernberg 等人， 2011 年；Wernberg 等人， 2019 年）。近几十年来，由于逐渐变暖、海洋热浪和热带食草动物的迁徙增加了消费者对海带生物量的压力，澳大利亚中高纬度地区的Ecklonia海带森林已经逐渐被大型藻类和珊瑚所取代（Bennett，Wernberg， De Bettignies 等人， 2015 年；Connell 等人， 2008 年；Vergés 等人， 2016 年；Wernberg 等人， 2016 年）。西澳大利亚 (WA) 的温带海岸，沿海和近海珊瑚礁特别受到向南流动的露纹洋流的影响，经历了有记录以来最严重的海带损失之一。海洋热浪与由露纹洋流温暖水域促进的热带草食动物相结合（Feng 等人， 2010 年；Wernberg 等人， 2013 年），导致沿海海带损失，在极端情况下导致 90% 的损失南纬 29° 周围超过 100 公里的海带林（Wernberg 等人， 2016 年）。在 2011 年的强烈热浪事件之后，在西澳海岸线的中西部地区，珊瑚数量增加了一倍以上（Tuckett 等人， 2017 年）。
在这里，我们旨在调查在海洋热浪之后的几年中珊瑚丰度增加的趋势是否持续，并揭示温带珊瑚礁珊瑚增殖背后的一些机制。为此，我们对西澳大利亚州中西部海岸海藻和珊瑚丰度的纵向研究进行了重新调查，并通过操作场和实验室测量了海带和珊瑚之间的物理（磨损和阴影）和化学（化感作用）相互作用的影响实验。具体来说，我们测试了西澳中西部沿海珊瑚礁中的珊瑚丰度（a）随时间增加和（b）受到大型藻类冠层负面影响的假设。
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