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The Benguela Upwelling System (BUS) is the most productive of all eastern boundary upwelling ecosystems and it hosts a well-developed oxygen minimum zone. As such, the BUS is a potential hotspot for production of N2O, a potent greenhouse gas derived from microbially driven decay of sinking organic matter. Yet, the extent at which near-surface waters emit N2O to the atmosphere in the BUS is highly uncertain. Here we present the first high-resolution surface measurements of N2O across the northern part of the BUS (nBUS). We found strong gradients with a threefold increase in N2O concentrations near the coast as compared with open ocean waters. Our observations show enhanced sea-to-air fluxes of N2O (up to 1.67 nmol m(-2) s(-1)) in association with local upwelling cells. Based on our data we suggest that the nBUS can account for 13% of the total coastal upwelling source of N2O to the atmosphere.

Plain Language Summary Nitrous oxide (N2O), commonly known as laughing gas, is a potent greenhouse gas that contributes both to Earth's warming and to the depletion of ozone in the stratosphere. Typically, N2O is produced in the water column as a result of microbial decay of organic matter (under low oxygen conditions) and then it is transferred to the atmosphere at the air-sea interface. Hence, high productive regions associated with low oxygen waters could create hotspots for N2O production. Yet, emission estimates might have underestimated the potential of coastal regions due to data sparcity. We focus on the northern Benguela region, which is the most productive of four major coastal upwelling ecosystems. Here we aim to answer: what is the large-scale distribution and variability of air-sea fluxes of N2O? What is the impact of small-scale oceanographic features in N2O variability? and Do regional emissions of N2O need to be revisited? Based on the analysis of continuous highly resolved measurements, we suggest that the share of this region to the total N2O emitted by the global ocean is higher than previously thought. Considering that this region represents only 0.06% of the ocean surface, we argue that the marine budget of this gas might need revision.