Isotopic composition of suspended particulate nitrogen (δ15Nsus) in surface waters of the Atlantic Ocean from 50°N to 50°S -: art. no. 1059

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