Stable nitrogen isotopic determination of particulate organic matter over the eastern North Atlantic in spring 2000 reveal a region of low natural abundance of 15N relative to 14N between 26¿N and 32¿N along 20¿W. This light isotopic signal, together with phytopigment data and persistently elevated nitrate to phosphate ratios in the upper thermocline, suggest that nitrogen fixation provides a local dominant supply of nitrogen to phytoplankton over part of the eastern North Atlantic. These independent biogeochemical proxies are coincident with a region of enhanced atmospheric dust deposition, as suggested by an atmospheric transport model. Hence, the atmospheric dust events may spatially and temporally constrain the distribution of N2 fixers.