Biological nitrogen fixation is a key process balancing the loss of combined nitrogen in the marine nitrogen cycle. Its relevance in upwelling or high nutrient regions is still unclear, with the few available studies in these regions of the ocean reporting rates that vary widely from below detection limit to > 100 nmol N L⁻¹ d⁻¹. In the eastern tropical Atlantic Ocean, two open ocean upwelling systems are active in boreal summer. One is the seasonal equatorial upwelling, where the residual phosphorus associated with aged upwelled waters is suggested to enhance nitrogen fixation in this season. The other is the Guinea Dome, a thermal upwelling dome. We conducted two surveys along 23° W across the Guinea Dome and the Equator from 15° N to 5° S in September 2015 and August–September 2016 with high latitudinal resolution (20–60 nm between stations). The abundance of Trichodesmium colonies was characterized by an Underwater Vision Profiler 5 and the total biological nitrogen fixation in the euphotic layer was measured using the ¹⁵N₂ technique. The highest abundances of Trichodesmium colonies were found in the area of the Guinea Dome (9°–15° N) with a maximum of 3 colonies L⁻¹ near the surface. By contrast, colonies were almost absent in the Equatorial band between 2° N and 5° S. The highest nitrogen fixation rate was measured at the northern edge of the Guinea Dome in 2016 (ca. 31 nmol N L⁻¹ d⁻¹). In this region, where diazotrophs thrived on a sufficient supply of both phosphorus and iron, a patchy distribution was unveiled by our increased spatial resolution scheme. In the Equatorial band, rates were considerably lower, ranging from below detection limit to ca. 4 nmol N L⁻¹ d⁻¹, with a clear difference in magnitude between 2015 (rates close to zero) and 2016 (average rates around 2 nmol N L⁻¹ d⁻¹). This difference seemed triggered by a contrasting supply of phosphorus between years. Our study stresses the importance of surveys with sampling at fine-scale spatial resolution, and shows unexpected high variability in the rates of nitrogen fixation in the Guinea Dome, a region where diazotrophy is a significant process supplying new nitrogen into the euphotic layer.

生物固氮是平衡海洋氮循环中结合氮损失的关键过程。其与上升流或高营养物区域的相关性仍不清楚，这些海洋区域的可用研究报告的速率变化很大，从低于检测限到> 100 nmol N L ⁻¹  d ⁻¹。在热带大西洋东部，两个公海上升流系统在北半球夏季活跃。一是季节性的赤道上升流，其中与老化上升流水相关的残留磷被认为可以增强该季节的固氮作用。另一个是几内亚圆顶，一个热上升圆顶。我们于 2015 年 9 月和 2016 年 8 月至 2016 年 9 月沿西经 23° 横跨几内亚圆顶和赤道，从北纬 15° 到南纬 5° 进行了两次高纬度分辨率（站间 20-60 海里）的调查。Trichodesmium菌落的丰度通过 Underwater Vision Profiler 5 进行表征，并使用¹⁵ N ₂技术测量透光层中的总生物固氮量。在几内亚圆顶地区（北纬 9°–15°）发现了最丰度的Trichodesmium菌落，靠近地表最多有 3 个菌落 L ^{-1 。}相比之下，北纬 2° 至南 5° 之间的赤道地带几乎没有菌落。2016 年在几内亚穹顶北缘测得最高固氮率（约 31 nmol N L -1 ^d  -1 ^）。在这个地区，固氮生物依靠充足的磷和铁供应而蓬勃发展，我们增加的空间分辨率方案揭示了不均匀的分布。在赤道带，比率要低得多，范围从低于检测限到大约。4 nmol N L ^-1  d ^{-1 ，2015 年（比率接近于零）和 2016 年（平均比率约为 2 nmol N L -1}  d ^-1）之间存在明显的差异。这种差异似乎是由年份之间磷供应量的对比引起的。我们的研究强调了以精细尺度空间分辨率采样进行调查的重要性，并显示了几内亚穹顶固氮速率的意外高变异性，在该地区，固氮营养是向透光层提供新氮的重要过程。