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Reefs at the edge: coral community structure around Rapa, southernmost French Polynesia

机译：边缘的珊瑚礁：法属波利尼西亚最南端的拉帕附近的珊瑚群落结构

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摘要

Rapa (27°36′S, 144°20′W) is a small (~40km2) volcanic island isolated in the Southern Austral Archipelago, where direct anthropogenic stressors are extremely limited. Here, we present the results of the first quantitative survey of coral community structure across habitats and depths around the island. Despite its geographical isolation in the depauperate South Central Pacific, its small size and unfavourable environmental conditions (competition with macroalgae, low sea surface temperatures, reduced reef accretion), the diversity of scleractinian corals at Rapa is particularly high (112 species from 32 genera, including 37 species of Acropora) in comparison to other French Polynesian islands and subtropical Pacific locations. Our results indicate that the abundance (>100 colonies per 10m2 recorded at nine of the 17 sampling stations) and cover (>40% at four stations) of corals are relatively high for a marginal reef location. Strong spatial heterogeneity was found, with high variation in diversity, abundance, cover and community composition among stations. Variation in community composition was related to habitat types, with distinct assemblages among fringing reefs within bays, reef formations at bay entrances, and those on the submerged platform surrounding the island. On the platform, a depth gradient was detected, with generic richness, abundance and cover generally greater at deeper stations (18–20m depth) compared with medium-depth (10–12m) and shallow (1–3m) stations. A gradient was also recorded along bays, with increasing coral diversity and abundance from the bay heads to the bay entrances. The coral community at Rapa was characterized by the presence of several taxa not found in other French Polynesian archipelagos and the rarity of others that are common and abundant in the Society and the Tuamotu islands. Another distinctive feature of reef communities at Rapa is the high cover and dominance of macroalgae, particularly in the shallower parts of the surrounding platform, which probably explains the lower densities of coral colonies recorded there. These characteristics of the diversity and biogeographical composition of coral assemblages at Rapa provide considerable ecological grounds for its conservation.
机译：Rapa（南纬27°36′，西经144°20′）是一个小岛（〜40km2），位于南澳大利亚群岛中，直接人为压力源极为有限。在这里，我们介绍了首次对岛上不同生境和深度的珊瑚群落结构进行定量调查的结果。尽管它在绝望的南太平洋中部地域偏僻，面积小且不利的环境条件（与大型藻类竞争，海面温度低，礁石积聚减少），拉帕的巩膜珊瑚的多样性特别高（32个属的112种，与其他法属波利尼西亚群岛和亚热带太平洋地区相比，包括37种Acropora）。我们的结果表明，对于边缘礁石位置而言，珊瑚的丰度（在17个采样站中的9个采样站上每10m2记录100个以上的殖民地）和覆盖率（在四个站上的40％以上）是相对较高的。发现空间异质性很强，站之间的多样性，丰度，覆盖度和群落组成变化很大。群落组成的变化与生境类型有关，海湾内的边缘礁石，海湾入口处的礁石构造以及岛屿周围淹没平台上的礁石构造具有明显的组合。在平台上，检测到了深度梯度，与中深度站（10-12m）和浅站（1-3m）相比，较深站（18–20m深度）具有一般的丰富度，丰度和覆盖度通常更大。沿海湾地区还记录到一个梯度，从海湾顶端到海湾入口的珊瑚多样性和丰富度不断增加。拉帕（Rapa）的珊瑚群落的特征是存在一些其他法属波利尼西亚群岛中未发现的分类单元，以及该学会和图阿摩图群岛中其他常见且丰富的物种。拉帕珊瑚礁群落的另一个显着特征是大型藻类的高覆盖度和优势地位，特别是在周围平台的较浅部分，这可能可以解释那里记录的珊瑚群落密度较低。拉帕珊瑚群落多样性和生物地理组成的这些特征为其保护提供了可观的生态基础。

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