Isotopic composition of suspended particulate nitrogen (δ15Nsus) has been studied in near-surface waters along a transect extending from 50¿N to 50¿S in the Atlantic Ocean. The δ15Nsus values ranged from -0.8 to +5.4? and showed a significant correlation with depth of nitracline (δ15Nsus = -0.047 ¿ DNO3 + 5.706; R2 = 0.81, n = 15) in nutrients-depleted regions (45¿N--40¿S). This inverse relationship implied that higher uptake of new NO3- during mesotrophic conditions, characterized by shallower DNO3, derived relatively enriched δ15Nsus. On the other hand, lower 15N signature of the remineralized NH4+ would be mainly imprinted on particulate nitrogen pool in the oligotrophic regions characterized by deep DNO3. The observed inverse δ15Nsus-DNO3 relationship can be modeled by a simple isotopic mass balance equation concerning with the N utilizability of phytoplankton (in terms of f-ratio). The model results that the variations in δ15Nsus reflect the regional differences in nitrogen cycle in the upper ocean. In the region between equator and 15¿N, where N2-fixing cyanobacteria (Trichodesmium spp.) were found, the measured δ15Nsus values were about 2? lower than those expected from the regression with the DNO3 indicating that over 38% of the suspended particulate nitrogen came through N2-fixation in this region. Regional variations in δ15Nsus in surface waters also correlated with those in water column productivity implying that the δ15Nsus signature can provide an estimate of in situ algal production.