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Spatial variation in N 2 -fixation rate and diazotroph activity in the Tropical Atlantic

Abstract. A variety of N2-fixers occur in oligotrophic waters and these diazotrophs make a substantial contribution to the nitrogen budget of the upper water column. A synthesis of previously published and new rate measurements for the North Atlantic provides insight into the role of two different groups of N2-fixers (Trichodesmium and small diazotrophs) in supporting N2 fixation in the tropical Atlantic. The highest rates of N2 fixation occurred in the western part of the basin, but the full data set showed no significant difference between the eastern and western parts of the basin in overall rates of N2-fixation by the two groups of diazotrophs. N2-fixation by Trichodesmium was strongly dominant in the western part of the basin while small diazotrophs played a much larger role to the east of 40° W. The reasons for this shift in dominance are unclear, as is the identity of the small organisms fixing N2 in the water column.