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The structure of diatom communities constrains biogeochemical properties in surface waters of the Southern Ocean (Kerguelen Plateau)
Journal of Marine Systems ( IF 2.8 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.jmarsys.2020.103458
Augustin Lafond , Karine Leblanc , Justine Legras , Véronique Cornet , Bernard Quéguiner

Abstract In the context of climate change, understanding the ecological processes controlling the functioning and the efficiency of the biological pump is of primary importance. Plankton community structure and species-specific properties are often invoked as likely to affect biogeochemistry and the export of organic and biogenic material to the ocean interior. Although a major player in this respect, diatoms are still viewed as a single functional type whose diversity is generally overlooked. Here we examine that question, building on the results achieved during the MOBYDICK expedition, which occurred in the vicinity of the Kerguelen Islands (Southern Ocean) in late summer, a time window corresponding to the demise of the annually recurrent phytoplankton blooms already known to be controlled by iron availability. The Si/C/N stoichiometry of the particulate matter was studied in conjunction with the different diatom community structures, their physiological states, as well as their species-specific carbon contents and silicification degrees. Our results show that diatoms outside the iron-fertilized plateau were more heavily silicified, due to the combined effects of both taxonomic composition of the resident community and a direct physiological response to iron stress, resulting in higher Si:C elemental ratios in diatoms as well as in the bulk particulate matter. Despite low silicic acid concentrations, large chains of weakly silicified Corethron inerme were able to grow in the upper mixed layer above the plateau, while in adjacent high nutrient low chlorophyll (HNLC) waters, communities were dominated by Fragilariopsis spp., Cylindrotheca closterium and the centric genera Actinocyclus/Thalassiosira spp. Depth was also an important factor shaping diatom communities, with the presence of a deep and inactive assemblage located within the pycnocline gradient, both on- and off-plateau, which likely resulted from the differential sinking and accumulation of species previously grown at the surface. In HNLC waters, below the mixed layer, detrital frustules of the heavily silicified species Fragilariopsis kerguelensis carried mostly Si, while above the plateau, Eucampia antarctica and Chaetoceros spp. (resting spores and vegetative stages) were efficient vectors of both Si and C to the deeper layers. Our study shows that the stoichiometry of the biological pump cannot be considered solely as a simple response to a single limiting factor (here iron) highlighting the importance of a species-centered approach in order to finely resolve biogeochemical fluxes and improve our understanding of the biological pump.

中文翻译:

硅藻群落结构限制了南大洋(凯尔盖朗高原)地表水的生物地球化学特性

摘要 在气候变化的背景下,了解控制生物泵功能和效率的生态过程至关重要。浮游生物群落结构和物种特异性特性通常被认为可能影响生物地球化学以及有机和生物材料向海洋内部的出口。虽然硅藻在这方面是主要参与者,但仍被视为单一功能类型,其多样性通常被忽视。在这里,我们以 MOBYDICK 探险期间取得的结果为基础来研究这个问题,该探险于夏末在凯尔盖朗群岛(南大洋)附近发生,该时间窗口对应于已知的每年反复出现的浮游植物大量繁殖的消亡。受铁的可用性控制。结合不同的硅藻群落结构、生理状态以及物种特定的碳含量和硅化程度,研究了颗粒物的 Si/C/N 化学计量。我们的研究结果表明,由于居民群落的分类组成和对铁胁迫的直接生理反应的综合影响,铁肥高原以外的硅藻硅化程度更高,导致硅藻中的 Si:C 元素比例更高就像在散装颗粒物一样。尽管硅酸浓度较低,但在高原上方的上层混合层中,大链的弱硅化 Corethron inerme 能够生长,而在邻近的高营养低叶绿素 (HNLC) 水域中,群落以 Fragilariopsis spp.、Cylindrotheca closterium 和中心属 Actinocyclus/Thalassiosira spp。深度也是塑造硅藻群落的一个重要因素,在高原内外的pycnocline梯度内存在一个深而不活跃的组合,这可能是由于先前在地表生长的物种的差异下沉和积累造成的。在 HNLC 水域,在混合层下方,严重硅化物种 Fragilariopsis kerguelensis 的碎屑硅藻壳主要携带 Si,而​​在高原上方,Eucampia antarctica 和 Chaetoceros spp。(静止孢子和营养阶段)是 Si 和 C 到更深层的有效载体。
更新日期:2020-12-01
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