Nitrogen isotopic evidence for micronutrient control of fractional NO 3 − utilization in the equatorial Pacific

During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (d 15 N) and their control by relative NO 3 − drawdown were examined in detail. Near‐surface d 15 N data clearly conform to Rayleigh isotopic fractionation during phytoplankton utilization of NO , with a fractionation factor of about 5‰. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large‐scale characteristic of the system. Decreasing near‐surface [NO 3 − ] with distance from the equator strongly correlates with increasing d 15 N in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Niño) and 2 (non‐El Niño) cruises. Despite a doubling of [NO 3 − ] accompanying relaxation of El Niño conditions, d 15 N values in near‐surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since d 15 N varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled NO 3 − was consumed in the immediate vicinity of the equator, as estimated from the d 15 N data. The moored sediment trap time series provided a more highly resolved temporal view of the 15 N dynamics of this system. At the equator and to the south, there is little temporal variation in d 15 N and, hence, in relative NO 3 − utilization over the 1‐yr duration of the program. At 28 and 58°N, there is a large 5‰ decrease in d 15 N during the transition to non‐El Niño conditions in response to the northward movement of NO 3 − ‐rich waters, which probably reflects the intensification of the north equatorial current. Overall, % NO 3 − utilization at the equator remained between 40 and 60% over the observation period, despite large hydrographic and dynamical changes. This observation implies a tight control of relative NO 3 − utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative NO 3 − utilization is likely determined by the product of the ratio of Fe to NO 3 − in upwelled waters and the NO 3 − : Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Niño, will affect surface [NO 3 − ] new and export production, but it will not affect relative NO 3 − utilization. In this light, downcore d 15 N records in the equatorial Pacific should reflect past changes in the limitation of NO 3 − drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.