Data from: Vertical gradients in species richness and community composition across the twilight zone in the North Pacific Subtropical Gyre

Although metazoan animals in the mesopelagic zone play critical roles in deep pelagic food webs and in the attenuation of carbon in midwaters, the diversity of these assemblages is not fully known. A metabarcoding survey of mesozooplankton diversity across the epipelagic, mesopelagic and upper bathypelagic zones (0-1500m) in the North Pacific Subtropical Gyre revealed far higher estimates of species richness than expected given prior morphology-based studies in the region (4,024 OTUs, 10-fold increase), despite conservative bioinformatic processing. OTU richness of the full assemblage peaked at lower epipelagic-upper mesopelagic depths (100-300m), with slight shoaling of maximal richness at night due to diel vertical migration, in contrast to expectations of a deep mesopelagic diversity maximum as reported for several plankton groups in early systematic and zoogeographic studies. Four distinct depth-stratified species assemblages were identified, with faunal transitions occurring at 100m, 300m, and 500m. Highest diversity occurred in the smallest zooplankton size fractions (0.2-0.5mm), which had significantly lower % OTUs classified due to poor representation in reference databases, suggesting a deep reservoir of poorly understood diversity in the smallest metazoan animals. A diverse meroplankton assemblage also was detected (350 OTUs), including larvae of both shallow and deep living benthic species. Our results provide some of the first insights into the hidden diversity present in zooplankton assemblages in midwaters, and a molecular reappraisal of vertical gradients in species richness, depth distributions, and community composition for the full zooplankton assemblage across the epipelagic, mesopelagic and upper bathypelagic zones.

尽管中层带（mesopelagic zone）的后生动物在远洋深层食物网及中层水域碳衰减过程中发挥着关键作用，但目前学界对这类群落的多样性仍未完全认知。本研究针对北太平洋副热带环流（North Pacific Subtropical Gyre）内上层带（epipelagic）、中层带及上深渊带（upper bathypelagic zone，水深0~1500m）的中型浮游动物（mesozooplankton）多样性开展元条形码（metabarcoding）测序调查，结果显示：尽管采用了保守的生物信息学分析流程，该区域的物种丰富度估计值远高于此前基于形态学研究的预期（共得到4024个操作分类单元（Operational Taxonomic Unit, OTU），数量提升10倍）。全群落的OTU丰富度在浅上层-中上层水深（100~300m）处达到峰值，夜间因昼夜垂直迁移（diel vertical migration）导致物种丰富度峰值出现小幅上移；这与早期分类学及动物地理学研究中，多个浮游类群的多样性峰值出现在深层中层带的预期相悖。研究共识别出4个显著的深度分层物种群落，群落更替界面分别位于100m、300m与500m水深处。物种最高多样性出现在最小粒径分级的浮游动物组分（0.2~0.5mm）中，但该组分中被成功分类的OTU占比显著更低，这源于参考数据库（reference databases）对这类生物的覆盖度不足，暗示小型后生动物中存在大量尚未被认知的多样性储备。研究同时检测到多样的阶段性浮游生物（meroplankton）群落（共350个OTU），涵盖浅海与深海栖息的底栖生物（benthic species）幼虫。本研究结果首次揭示了中层水域浮游动物群落中潜藏的多样性，并针对上层带、中层带及上深渊带的完整浮游动物群落，从物种丰富度垂直梯度、深度分布及群落组成方面开展了分子水平的重新评估。