The supply of excess phosphate across the Gulf Stream and the maintenance of subtropical nitrogen fixation

The subtropical North Atlantic is considered a hot spot for biological nitrogen fixation, with estimated rates between 1 and 20 × 10 ¹¹ mol nitrogen fixed annually. However, the region's nutrient reservoir beneath the euphotic zone is so enriched in nitrate relative to phosphate that it is perplexing how fixation might be sustained there. Here, we investigate whether the physical transport of excess phosphate into the subtropical gyre is sufficient to sustain nitrogen fixation in the gyre. Specifically, we assess the Ekman advection and isopycnal mixing of excess phosphate to the subtropical North Atlantic, using detailed hydrographic and nutrient sections occupied across the Gulf Stream combined with satellite wind data. Ekman advection and along-isopycnal mixing provide a source of approximately 2 × 10 ¹⁰ mol yr ^-1 of excess phosphate in the northwestern subtropics, a physical mechanism that has the potential to support more than 3 × 10 ¹¹ mol yr ^-1 of biological nitrogen fixation, after accounting for alternative sinks of excess phosphate. This excess phosphate supply across the gyre's northern boundary and high nitrogen fixation there offers a mechanism that can explain both the maintenance of subtropical North Atlantic nitrogen fixation in a phosphate-poor environment and help account for the weak gradients in the proxies of fixation observed along interior circulation pathways of the gyre.