Award: OCE-1537314

The outcomes from this project are primarily a better understanding of the magnitude and spatial distribution of biological di-nitrogen (N2) fixation in the southwest Pacific Ocean. This project confirmed the hypothesis that N2 fixation supports a significant fraction of export production in the southwest Pacific Ocean (Knapp et al., 2018, Biogeosciences). This finding is significant because placing better quantitative constraints on the spatial distribution of N2 fixation in the ocean helps identify the chemical and physical sensitivities of this process in the ocean. In this case, little prior work had been done quantifying the role of N2 fixation for supporting export production in this region. However, the results from this project support the hypotheses of recent modeling work which suggests that the southwest Pacific Ocean, which is relatively warm, has favorable macro-nutrient concentrations (i.e., nitrate and phosphate), and appears to have sufficient sources of iron to surface waters, supports rates of N2 fixation comparable to or greater than those found in the tropical North Atlantic. Indeed, results from this project found that N2 fixation supported the highest fraction (>50%) of export production ever measured in the global ocean when evaluated using modern methods (Knapp et al., 2018, Biogeosciences). Consequently, this project has shifted our understanding of where the highest global rates of N2 fixation are occurring and suggests than in addition to the tropical North Atlantic, the southwest Pacific Ocean supports significant global rates of N2 fixation. Additional outcomes from this work include evidence for low-d15N release by N2 fixing organisms in a large volume mesocosm experiment that subsequently supported export production by non-N2 fixing organisms (Knapp et al., 2016). This result was particularly exciting because it provided the first geochemical evidence for this process that has been hypothesized to occur based on biological evidence. Other work supported by this project includes participation by the PI in the Future of Chemical Oceanography (?COME ABOARD?) meeting, which produced a paper summarizing the discussions at this meeting (Fassbender et al., 2017, Marine Chemistry). Other outcomes include papers constraining nitrogen cycling in the Gulf of Mexico (Redalje et al., 2019, in the Gulf of Mexico Origin, Waters, and Biota, Vol. 5, Chemical Oceanography), the North Atlantic (Marconi et al., 2017, Global Biogeochemical Cycles) and North Pacific (Wilson et al., 2019, Science) Oceans. In particular, this project supported the rapid-response analysis of samples collected in response to a lava-induced phytoplankton bloom observed after the lava from the Kilauea eruption of 2018 entered the ocean, with the key interpretation that the nitrate supporting the phytoplankton bloom originated from upwelling and not N2 fixation (Wilson et al., 2019, Science). Finally, additional outcomes from this work that are soon to be submitted for publication include a manuscript describing the first measurements of the isotopic composition of water column nitrate from the Gulf of Mexico (Howe et al., in preparation). This manuscript concludes that nitrate from the Mississippi and Atchafalaya River Systems is not entrained in the Loop Current, nor is it exported from the Gulf of Mexico. This finding is consistent with recent modeling work indicate >90% of riverine nutrients are retained in the near-shore region. Additionally, a paper will soon be submitted that describes methods for isolating dissolved organic nitrogen for chemical characterization using solid phase extraction (SPE) for Fourier Transform-Ion Cyclotron Resonance analysis. The Broader Impacts from this project include that this project supported one early-career faculty member, and the sole-PI NSF grant was cited as a reason for granting the PI tenure. Additionally, this project supported one female graduate student and four undergraduates, one of whom is currently in medical school and another, who completed her undergraduate honors thesis on the isotopic composition of nitrate in the Gulf of Mexico, is applying to graduate schools in earth sciences. All datasets generated by this project have been archived at the Biological and Chemical Oceanography Data Management Office (BCO DMO). Further outreach activities supported by this award included speaking with Florida State University?s ?Women in Math, Science, and Engineering? undergraduate scholars, who toured the PI?s lab and learned about the science conducted as part of this project. The PI also visited local summer camps and K-12 classrooms in association with this project, as well as a middle school ?Sci Girls? group. Last Modified: 10/29/2019 Submitted by: Angela N Knapp