In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin-wide nitrogen (N) inputs. However, the factors regulating coastal N₂ fixation remain poorly understood, in part due to sharp physico-chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂ fixation on the western North Atlantic shelf, at the confluence of Mid- and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the ¹⁵N₂ bubble release method and used to build empirical models of regional N₂ fixation via a random forest machine learning approach. N₂ fixation rates were then predicted from high-resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂ fixation rates (> 100 nmol N L⁻¹ d⁻¹), which were likely driven by the haptophyte-symbiont UCYN-A. Above and below this hotspot, N₂ fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂ uptake rates, a biomass-independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂ fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

中文翻译：

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哈特拉斯角前部沿海N2固定热点：通过监督机器学习阐明重氮营养活动的空间异质性

在北大西洋，在西部大陆架上固定的二氮（N ₂）代表了整个盆地范围内氮（N）输入的很大一部分。然而，调节沿海N ₂固着的因素仍然知之甚少，部分原因是由于尖锐的理化梯度和动态水质相互作用，而这些相互作用很难通过传统海洋学方法加以限制。这项研究旨在表征北大西洋西部架子上N ₂固定的空间异质性，2016年8月在中大西洋和南大西洋海岸线架子水和墨西哥湾流的交汇处。使用¹⁵ N ₂气泡释放来量化速率方法并用于建立区域N的经验模型₂通过随机森林机器学习方法进行固定。然后从高分辨率CTD和卫星数据预测N ₂固定率，以分别推断其深度和表面分布的变化。我们的发现表明，额叶混合区创造了有利于极高N ₂固着率（> 100 nmol N L ^-1 d ^-1）的条件，这很可能是由触藻共生菌UCYN-A驱动的。在该热点之上和之下，由于架子上的高颗粒氮浓度，N ₂固定率在架子上最高。相反，特定N ₂贫营养斜坡水中的吸收速率（重氮营养活性的生物量独立指标）得到了提高。从广义上讲，这些观察结果表明，N ₂固定在近海受到青睐，但在整个架子上连续发生。然而，我们的模型结果表明，随着浮游植物种群的减少，可能是由于沿海养分的耗尽，沿海地区的重氮营养生物存在一个利基。