Hydrochemistry measured on water bottle samples during Sarmiento de Gamboa BEO 2011 cruise@en

The ability of the subtropical North Atlantic to sustain export production despite the lack of available nutrients is fascinating. Subtropical gyres are expected to expand under a global warming scenario, so it is important to understand the mechanisms supplying the required nutrients. Current issues for the region concern the nutrient and metabolic balance, the origin of excess nitrogen and phosphorus shortage, and the maintenance of nitrogen fixation. We report data on the allocation of nitrogen and phosphorus in dissolved and suspended pools, the isotopic δ15N of suspended nitrogen, and the lability of dissolved organic phosphorus (DOP) along a section crossing the eastern seasonally stratified North Atlantic (SSNA) and the western subtropical North Atlantic (NASW). We find extreme P-deficiency in the NASW, with the highest dissolved inorganic N:P ratios located within the upper-thermocline isopycnals (σϴ = 26.3-26.8). Our data indicate an important role of the mid-latitude northeast SSNA bringing dissolved organic matter (DOM) to the thermocline of the North Atlantic. The mineralisation of N-rich DOM contributes to the N excess (P deficit) of the upper-thermocline of NASW. We find lower concentrations of more reactive DOP in the western than in the eastern part of the transect, indicating an active role of DOP in the nutrition of microbial communities. Our results support recent hypotheses concerning the environmental controls of marine nitrogen fixation identifying the key role of DOP utilisation.北亚热带大西洋在有效营养盐匮乏的条件下仍能维持输出生产的能力，令人颇感兴趣。在全球变暖情景下，亚热带环流预计将出现扩张，因此厘清支撑其所需营养盐的供给机制，具有重要研究价值。该海域当前的研究热点与待解问题聚焦于营养盐与代谢平衡、氮盈余与磷匮乏的成因，以及固氮作用的维持机制。本研究报道了横跨东部季节性层化北大西洋（Seasonally Stratified North Atlantic, SSNA）与西部亚热带北大西洋（Western Subtropical North Atlantic, NASW）的断面数据，涵盖溶解态与悬浮态库中的氮、磷分配特征、悬浮态氮的同位素δ¹⁵N比值，以及溶解有机磷（Dissolved Organic Phosphorus, DOP）的生物可利用性。研究团队在NASW海域发现了极端的磷匮乏现象，最高的溶解态无机氮磷比出现于上层温跃层的等密度面（σθ=26.3~26.8）中。本研究数据表明，东北SSNA的中纬度海域将溶解有机质（Dissolved Organic Matter, DOM）输送至北大西洋温跃层，发挥了重要作用。富氮DOM的矿化作用，是导致NASW上层温跃层出现氮盈余（磷匮乏）的重要原因。本研究发现，断面西部的活性DOP浓度低于东部区域，这表明DOP在微生物群落的营养供给中发挥了活跃作用。本研究结果佐证了近期关于海洋固氮作用环境调控的相关假说，该假说指出了DOP利用过程的关键作用。