Surface diatom community composition and species-specific contribution to carbon biomass, live and empty cell abundances over the Kerguelen region in the Southern Ocean (KEOPS 2 program)

In the naturally iron-fertilized surface waters of the northern Kerguelen Plateau region, the early spring diatom community composition and contribution to plankton carbon biomass were investigated and compared with the High Nutrient Low Chlorophyll (HNLC) surrounding waters (October-November 2011, KEOPS 2). The large iron-induced blooms were dominated by small diatom species belonging to the genera Chaetoceros (Hyalochaete) and Thalassiosira, which rapidly responded to the onset of favorable light-conditions in the meander of the Polar Front. In comparison, the iron-limited HNLC area was typically characterized by autotrophic nanoeukaryote-dominated communities and by larger and more heavily silicified diatom species (e.g. Fragilariopsis spp.). Our results support the hypothesis that diatoms are valuable vectors of carbon export to depth in naturally iron-fertilized systems of the Southern Ocean. Comparison with the diatom assemblage composition of a sediment trap deployed in the iron-fertilized area suggests that the dominant Chaetoceros (Hyalochaete) cells were less efficiently exported than the less abundant yet heavily silicified cells of Thalassionema nitzschioides and Fragilariopsis kerguelensis. Our observations emphasize the strong influence of species-specific diatom cell properties combined with trophic interactions on matter export efficiency, and illustrate the tight link between the specific composition of phytoplankton communities and the biogeochemical properties characterizing the study area.

本研究以凯尔盖朗高原北部天然铁肥化表层水域为研究对象，于2011年10—11月（KEOPS 2）开展调查，分析早春硅藻群落组成及其对浮游植物碳生物量的贡献，并与周边高营养盐低叶绿素（High Nutrient Low Chlorophyll, HNLC）水域进行对比。该区域大规模铁诱导水华以隶属于角毛藻属（海毛藻亚属，Chaetoceros (Hyalochaete)）和海链藻属（Thalassiosira）的小型硅藻物种为优势类群，此类物种可在极锋弯曲区域快速响应适宜光照条件的出现。相较之下，受铁限制的HNLC水域通常以自养纳米真核生物占主导的群落为特征，同时伴随个体更大、硅质化程度更高的硅藻物种（如脆杆藻属未定种，Fragilariopsis spp.）。本研究结果支持如下假说：在南大洋天然铁肥化系统中，硅藻是向深层输出碳的重要载体。通过与铁肥化区域部署的沉积物捕集器（sediment trap）所获硅藻组合组成进行对比，结果显示优势类群角毛藻属（海毛藻亚属）细胞的碳输出效率，低于丰度较低但硅质化程度更高的菱形海线藻（Thalassionema nitzschioides）与凯尔盖朗脆杆藻（Fragilariopsis kerguelensis）细胞。本研究结果强调了硅藻物种特异性细胞特性与营养相互作用共同对物质输出效率的显著影响，并阐明了浮游植物群落的特定组成与研究区域的生物地球化学特性之间的紧密关联。