Nitrogen fixation is critical for ocean productivity, yet mechanistic controls remain poorly resolved. Here we investigated rates and drivers of nitrogen fixation in the Southeast Atlantic from the Benguela upwelling zone to the subtropical gyre, including high resolution observations across filaments. We find low or undetectable rates in elevated nitrate waters of the upwelling zone/filaments and in the nitrate-depleted gyre. In contrast, elevated rates were found at the low-nitrate outer boundaries of filaments in the upwelling-gyre transition. We propose this pattern was regulated by iron-to-nitrogen supply ratios: low ratios in the upwelling zone/filaments and gyre led to diazotrophs being outcompeted, whilst they were competitive at the outer boundaries of filaments where ratios were elevated. Although iron availability was likely the limiting factor both within filaments and in the gyre, experimental iron supply only enhanced rates within filaments, suggested to be driven by stronger competitive exclusion of diazotrophs in the gyre.