Laverick Jack H, Andradi-Brown Dominic A, Rogers Alex D
Department of Zoology, University of Oxford, Oxford, United Kingdom.
Operation Wallacea, Old Bolingbroke, Spilsby, Lincolnshire, United Kingdom.
PLoS One. 2017 Aug 15;12(8):e0183075. doi: 10.1371/journal.pone.0183075. eCollection 2017.
Shallow water zooxanthellate coral reefs grade into ecologically distinct mesophotic coral ecosystems (MCEs) deeper in the euphotic zone. MCEs are widely considered to start at an absolute depth limit of 30m deep, possibly failing to recognise that these are distinct ecological communities that may shift shallower or deeper depending on local environmental conditions. This study aimed to explore whether MCEs represent distinct biological communities, the upper boundary of which can be defined and whether the depth at which they occur may vary above or below 30m. Mixed-gas diving and closed-circuit rebreathers were used to quantitatively survey benthic communities across shallow to mesophotic reef gradients around the island of Utila, Honduras. Depths of up to 85m were sampled, covering the vertical range of the zooxanthellate corals around Utila. We investigate vertical reef zonation using a variety of ecological metrics to identify community shifts with depth, and the appropriateness of different metrics to define the upper MCE boundary. Patterns observed in scleractinian community composition varied between ordination analyses and approaches utilising biodiversity indices. Indices and richness approaches revealed vertical community transition was a gradation. Ordination approaches suggest the possibility of recognising two scleractinian assemblages. We could detect a mesophotic and shallow community while illustrating that belief in a static depth limit is biologically unjustified. The switch between these two communities occurred across bathymetric gradients as small as 10m and as large as 50m in depth. The difference between communities appears to be a loss of shallow specialists and increase in depth-generalist taxa. Therefore, it may be possible to define MCEs by a loss of shallow specialist species. To support a biological definition of mesophotic reefs, we advocate this analytical framework should be applied around the Caribbean and extended into other ocean basins where MCEs are present.
浅水共生虫黄藻珊瑚礁在透光带深处逐渐演变成生态上截然不同的中光层珊瑚生态系统（MCEs）。MCEs被广泛认为始于绝对深度30米处，可能没有认识到这些是不同的生态群落，其深度可能会根据当地环境条件变浅或变深。本研究旨在探讨MCEs是否代表不同的生物群落，其上限是否可以定义，以及它们出现的深度是否可能在30米以上或以下变化。使用混合气体潜水和闭路再呼吸器对洪都拉斯乌蒂拉岛周围从浅海到中光层礁梯度的底栖生物群落进行定量调查。对高达85米的深度进行了采样，涵盖了乌蒂拉周围共生虫黄藻珊瑚的垂直范围。我们使用各种生态指标来研究垂直礁带划分，以确定群落随深度的变化，以及不同指标对定义MCE上限的适用性。在造礁石珊瑚群落组成中观察到的模式在排序分析和利用生物多样性指数的方法之间有所不同。指数和丰富度方法表明垂直群落过渡是一个渐变过程。排序方法表明有可能识别出两个造礁石珊瑚组合。我们可以检测到一个中光层群落和一个浅海群落，同时说明认为存在静态深度极限在生物学上是不合理的。这两个群落之间的转变发生在深度仅10米至50米的测深梯度范围内。群落之间的差异似乎是浅海特有物种的减少和深度广适性类群的增加。因此，可以通过浅海特有物种的减少来定义MCEs。为了支持对中光层珊瑚礁的生物学定义，我们主张应在加勒比海周围应用这个分析框架，并扩展到其他存在MCEs的海洋盆地。
