COLLABORATIVE RESEARCH: Nitrogen Fixation and its Coupling with Denitrification in the Eastern Tropical North Pacific

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Abstract Nitrogen fixation plays a now well-recognized role in enhancing primary production and export in oligotrophic regions of the subtropical ? tropical ocean. However, recent evidence suggests that nitrogen fixation may be even more globally significant, perhaps occurring commonly in regions of the surface ocean proximate to zones of intense subsurface denitrification. In this project, a team of researchers from Oregon State University, University of Hawaii, and University of South Carolina will examine how the interactions between denitrification and nitrogen fixation enhance primary and export production in one such area, the Gulf of California and adjacent waters of the eastern tropical North Pacific (ETNP). This oceanographic region provides an excellent field laboratory in which to study these processes due to its rich biological productivity and the predominance of denitrified intermediate waters. Summertime surface waters in this region of the Pacific, characterized by effectively zero nitrate-to-phosphate (N:P) values with replete P concentrations (0.3-0.8M) and pronounced thermal stratification, provide potentially prime habitat for the occurrence of significant nitrogen fixation and contribution of such primary production to export production. Given this prospect and its relevance to advancing knowledge of the overall marine carbon and nitrogen cycles, this multidisciplinary biogeochemical study will address an overarching hypothesis: Upwelling of denitrified waters in the Gulf of California and adjacent ETNP, if followed by stratification and Redfield-type nutrient drawdown, primes surface waters for nitrogen fixation and thus leads to potential feedbacks (positive and negative) for export production, the maintenance of the suboxic conditions that favor denitrification, and the magnitude of dissolved N:P ratios that are generated in the denitrification zone. The project has three key objectives: (1) to demonstrate that nitrogen fixation is a common, quantitatively important primary production process in summer stratified surface waters of this region; (2) to identify the specific organisms responsible for the N2 fixation and determine the carbon production rate for each identified N-fixing organism; and (3) to quantify the export of the newly fixed nitrogen and assess the possible contribution that different members of the N-fixing community directly contribute to export. To achieve these objectives the team will use a combination of microscopy, genetic identification, lipid biomarkers and both stable isotopic and radioisotopic measurements on samples collected during a 2008 summer research cruise in the Gulf of California region. Observations will be made to: (1) determine the spatial pattern and rates of gross and net N2 fixation in a diagnostic region of the Gulf of California and ETNP; (2) identify what specific organisms are responsible for the nitrogen fixation fixation and, use a novel, newly established isotopic tracer technique to directly determine the carbon production rate for each identified organism; (3) assess the rates of organism-specific carbon and nitrogen export production; and (4) evaluate the extent to which ?rain? of this newly fixed nitrogen is decomposed within the upper zone of the denitrification layer and subsequently nitrified. In terms of broader impacts, this project will include outreach through collaboration with a Mexican colleague and his undergraduates. Involvement of high school, undergraduate (including work-study, Honor?s College and REU students), graduate and postdoctoral researchers in the project will encourage national and international collaboration at all levels of education.