Abstract Bathymetric gradients in the deep sea are known to affect key benthic community characteristics such as diversity. However, most studies investigate large-scale bathymetric variation, while habitat heterogeneity related to modest bathymetric variation has generally been overlooked because of limitations to sampling technology. We investigate the role of modest bathymetric variation (∼10 m water depth intervals) on an abyssal hill, and horizontal variation at the 0.1 – 10 km scale, in the structuring of abyssal megafaunal assemblages. We assess numerical density, biomass density, diversity, and assemblage composition using seabed photographs captured with an autonomous underwater vehicle and sediment characteristics determined from cores. We detect significant differences in sediment particle size and organic carbon content, in relation to modest topographic elevation, with a greater fraction of fine particles and organic carbon on the abyssal plain than the hill. Total megafaunal numerical and biomass density, diversity, and the numerical densities of feeding groups were significantly different with modest topographic elevation; similarly, megafaunal composition varied significantly between ∼10 m depth intervals. In relation to mesoscale horizontal variation, we also record significant differences between megabenthic communities in two abyssal plain areas with no significant differences in measured sedimentary characteristics and only a 2 m difference in water depth. Differences in these communities were detected in terms of dominance, assemblage composition by density and biomass, and numerical densities of feeding groups. These observations strongly indicate that previous general concepts of the abyssal environment greatly underestimate this mesoscale heterogeneity, such that beta- and gamma-diversity in the abyss may be higher than estimated. Importantly, these results also have clear implications for the design and interpretation of environmental survey and monitoring programmes in the abyss.

中文翻译：

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深海地形的细微变化导致底栖巨型动物丰度、多样性和群落结构发生显着变化

摘要 众所周知，深海中的测深梯度会影响关键的底栖群落特征，例如多样性。然而，大多数研究调查了大规模的水深变化，而由于采样技术的限制，与适度水深变化相关的栖息地异质性通常被忽视。我们研究了深海山丘上适度的测深变化（~10 m 水深间隔）和 0.1 – 10 公里范围内的水平变化在深海巨型动物群结构中的作用。我们使用自主水下航行器拍摄的海底照片和从岩心确定的沉积物特征来评估数值密度、生物量密度、多样性和组合成分。我们检测到沉积物粒径和有机碳含量的显着差异，相对于适度的地形海拔，深海平原上的细颗粒和有机碳的比例高于山丘。总的巨型动物数量和生物量密度、多样性和摄食群体的数量密度在中等地形高程下有显着差异；同样，巨型动物组成在约 10 m 的深度间隔之间变化很大。关于中尺度水平变化，我们还记录了两个深海平原地区巨型底栖动物群落之间的显着差异，测量的沉积特征没有显着差异，水深只有 2 m 差异。这些群落的差异是在优势、密度和生物量的组合组成以及饲养组的数量密度方面检测到的。这些观察结果有力地表明，先前关于深海环境的一般概念大大低估了这种中尺度异质性，因此深渊中的 β 和 γ 多样性可能高于估计值。重要的是，这些结果对深渊环境调查和监测计划的设计和解释也有明确的意义。