In the Southern Ocean, diatom blooms have attracted a lot of attention, while other small nonsilicified phytoplankton groups have been less studied. Here, small phytoplankton (< 20 μm, including small diatoms and nonsilicified small phytoplankton) are focused on in two contrasting areas: the productive Kerguelen plateau and its surrounding low productivity waters. To assess the diversity and spatial structuration of phytoplankton, discrete plankton samples (0–300 m layer) of two size fractions (< 20 and 20–100 μm) were analyzed with 18S rDNA amplicon sequencing in late summer. Phytoplankton seasonal succession was described using flow cytometry, pigments, and environmental data, from two previous cruises (during the onset and decline of the diatom bloom). In the mixed layer, small nonsilicified phytoplankton represented less than 10% of chlorophyll a (Chl a) during the onset and late diatom bloom on the plateau, but they increased on and off the plateau after the bloom (53–70% of Chl a). Phaeocystis antarctica was relatively abundant at all stations after the bloom, but other small phytoplanktonic groups featured marked differences on and off the plateau. Higher concentrations on the plateau appeared to stimulate the presence of Micromonas, while Pelagophytes were enhanced off the plateau. A diverse assemblage of small diatoms was also promoted off the plateau, where silicate concentration was still high. Interestingly, P. antarctica represented up to 25% of all reads at 300 m depth off the plateau in the larger size fraction suggesting a significant contribution to carbon export through aggregation in low productive waters.

中文翻译：

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在南大洋（克格伦地区）的对比水域中，小型浮游植物结构的时空变化明显

在南大洋，硅藻的绽放引起了人们的广泛关注，而其他小型非硅化浮游植物群的研究较少。在这里，小浮游植物（<20  μ m，包含小硅藻和nonsilicified小浮游植物）被集中在两种截然不同的区域：生产凯尔盖朗海台和其周围的低生产率的水域。为了评估2级大小的级分（<20和20-100的多样性和浮游植物的空间结构化的，离散的浮游生物样品（0-300米层）  μm）在夏末用18S rDNA扩增子测序进行了分析。使用流式细胞术，色素和环境数据描述了之前两次巡游中的浮游植物的季节性演替（在硅藻开花的开始和结束期间）。在混合层中，小的非硅化浮游植物在高原的发病和晚期硅藻开花期间仅占叶绿素a（Chl  a）的不到10％  ，但在开花后在高原上和离开高原时它们增加（叶绿素的53-70％）  ）。在开花之后，南极Phaeocystis南极的所有站都相对丰富，但其他小型浮游植物群在高原内外均表现出明显差异。更高高原上的浓度似乎刺激了微单胞菌的存在，而高原上的Pelagophytes则增强了。在硅酸盐浓度仍然很高的高原地区，也促进了各种小硅藻的聚集。有趣的是，在距离高原300 m深度处，南极假单胞菌占所有读数的25％，占较大比例，这表明低产水域中的聚集对碳出口具有重要贡献。