Riou Virginie, Fonseca-Batista Debany, Roukaerts Arnout, Biegala Isabelle C, Prakya Shree Ram, Magalhães Loureiro Clara, Santos Mariana, Muniz-Piniella Angel E, Schmiing Mara, Elskens Marc, Brion Natacha, Martins M Ana, Dehairs Frank
Aix-Marseille Université, Mediterranean Institute of Oceanography (MIO), UM 110 CNRS/INSU, IRD, 13288 Marseille, Université du Sud Toulon-Var, 83957, La Garde, France.
IMAR-Institute of Marine Research, Centre of IMAR at the University of the Azores, Horta, Portugal.
PLoS One. 2016 Mar 9;11(3):e0150827. doi: 10.1371/journal.pone.0150827. eCollection 2016.
To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3--and N2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N2-fixation was observed to sustain 45-85% of the HCO3--fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N2-fixation rates as high as 10.9 μmol N m-3 d-1 and surface nitrate δ15N as low as 2.7‰ were found in the warm (18-24°C), most saline (36.5-37.0) and least productive waters south of the AzF, where UCYN were the least abundant. However, picoplanktonic UCYN abundances up to 55 cells mL-1 were found at 45-200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N2-fixation rates up to 4.5 μmol N m-3 d-1 were detected, associated with depth-integrated H13CO3--fixation rates at least 50% higher than observed south of the AzF. The numerous eddies generated at the NW-AzC/AzF seem to enhance exchanges of plankton between water masses, as well as vertical and horizontal diapycnal diffusion of nutrients, whose increase probably enhances the growth of diazotrophs and the productivity of C-fixers.
为了解亚速尔群岛西北海流锋（NW - AzC/AzF）系统对HCO₃⁻固定、固氮活动以及单细胞固氮蓝藻（UCYN）分布的影响，我们结合了从贫营养表层水到上层中层水的地球化学和生物学方法。在亚速尔群岛西北海流锋南北的叶绿素最大值深处，观察到固氮作用维持了微微型浮游生物部分中45% - 85%的HCO₃⁻固定，占总碳固定量的47%。在亚速尔群岛西北海流锋以南温暖（18 - 24°C）、盐分最高（36.5 - 37.0）且生产力最低的水域中，发现固氮速率高达10.9 μmol N m⁻³ d⁻¹，表层硝酸盐δ¹⁵N低至2.7‰，此处UCYN数量最少。然而，在亚速尔群岛西北海流锋以北最冷且营养丰富的水域中，45 - 200米深度处发现微微型浮游生物UCYN丰度高达55个细胞/毫升。在该区域，检测到固氮速率高达4.5 μmol N m⁻³ d⁻¹，与深度积分的H¹³CO₃⁻固定速率相关，该速率至少比亚速尔群岛西北海流锋以南观测到的高50%。在NW - AzC/AzF处产生的众多涡旋似乎增强了水团之间浮游生物的交换，以及营养物质的垂直和水平斜压扩散，营养物质的增加可能促进了固氮生物的生长和碳固定者的生产力。
