Dissolved Iron, Aluminium, and Dissolved Inorganic Phosphorus in the (Sub-)Tropical Atlantic Surface Ocean

Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N2). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (~0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north-south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial-temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales.低纬度海洋的多数区域因无机氮匮乏而限制了初级生产力，这为能够固定大气二氮（N₂）的固氮生物（diazotrophic organisms）带来了选择优势。不过，固氮生物的丰度与活性又会受到其他潜在限制性营养盐——包括磷(P)与铁(Fe)——的可获得性调控。 本研究提供了溶解态铁、铝与无机磷的高分辨率数据（约0.3°），证实（副）热带大西洋的表层营养盐浓度存在清晰的南北向生物地球化学边界。结合卫星降水数据与既往研究结果，本研究证明热带辐合带（intertropical convergence zone）区域的湿沉降是表层水体溶解态铁的主要来源。 此外，针对固氮作用与固氮生物束毛藻属（Trichodesmium spp.）分布的对应观测表明：热带辐合带的季节性迁移导致溶解态铁富集区域发生位移，进而驱动固氮作用的纬度分布格局产生改变，并伴随相应的溶解态无机磷匮乏。上述结论与该系统的理想化数值模型结果相一致。 由此可见，（副）热带大西洋两套截然不同的生物地球化学系统之间的边界，似乎由固氮生物对外源铁输入时空变异的响应所决定。因此，本研究除揭示了由大气营养盐输入驱动的独特季节循环外，还提出其背后的生物地球化学机制，或可表征寡营养系统在更长时间尺度上对环境胁迫变化的响应模式。