Nitrogen isotopic evidence for micronutrient control of fractional NO3- utilization in the equatorial Pacific

During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (delta N-15) and their control by relative NO3- drawdown were examined in detail. Near-surface delta (NO3-)-N-15 data clearly conform to Rayleigh isotopic fractionation during phytoplankton utilization of NO3-, with a fractionation factor of about 5 parts per thousand. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large-scale characteristic of the system. Decreasing near-surface [NO3-] with distance from the equator strongly correlates with increasing delta N-15 in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Nino) and 2 (non-El Nino) cruises. Despite a doubling of [NO3-] accompanying relaxation of El Nino conditions, delta N-15 values in near-surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since delta N-15 varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled NO3- was consumed in the immediate vicinity of the equator, as estimated from the delta N-15 data. The moored sediment trap time series provided a more highly resolved temporal view of the N-15 dynamics of this system. At the equator and to the south, there is little temporal variation in delta N-15 and, hence, in relative NO3- utilization over the 1-yr duration of the program. At 2 degrees and 5 degreesN, there is a large 5 parts per thousand decrease in delta N-15 during the transition to non-El Nino conditions in response to the northward movement of NO3--rich waters, which probably reflects the intensification of the north equatorial current. Overall, % NO3- 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 NO3- utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative NO3- utilization is likely determined by the product of the ratio of Fe to NO3- in upwelled waters and the NO3-:Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Nino, will affect surface [NO3-] new and export production, but it will not affect relative NO3- utilization. In this light, downcore delta N-15 records in the equatorial Pacific should reflect past changes in the limitation of NO3- drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.