Several independent lines of geochemical and remote sensing evidence suggest that dinitrogen (N2) fixation may be associated with surface waters downstream of major oxygen minimum zones (OMZs) and in particular in the Eastern Tropical South Pacific (ETSP). However, little direct evidence supports these inferences. Besides substantiating these indirect assessments, documenting significant N2 fixation in the ETSP would provide insight into two longstanding controversies: Is the marine N budget balanced, as implied by modeling and paleoceanographic data, and if so, how are the processes that add and remove N spatially, and thus temporally coupled?
In this project researchers at the University of Southern California and the University of Miami will test the hypothesis that fixation occurs in the ETSP at areal rates that equal or exceed those previously documented in more well-studied regions such as the oligotrophic waters of the sub/tropical North Atlantic. If scaled to the surface area of ETSP waters, this could add an additional 10-50 Tg N per year of inputs to the global marine N budget. They will undertake two cruises in the ETSP during early and late summer in two consecutive years to assess the quantitative significance of N2 fixation as a source of new N to surface waters using complementary biological and geochemical tools. N2 fixation rates will be evaluated on two temporal/spatial scales: daily/local (bottle 15N2 incubations and floating sediment traps); and seasonal/regional (d15N budget using moored sediment traps and water column TDN d15N). These estimates provide detailed observations of potential N2 fixation during station occupation in two summer seasons, when rates are expected to be greatest, as well as prolonged observation over lower expected N2 fixation periods. A combination of these different estimates will aim to determine if N2 fixation in this region can help balance the marine N budget. If all goes as planned, this study will determine the quantitative importance of N2 fixation in the ETSP, and whether these previously undocumented rates can help resolve the marine N budget. Implications include the ability of the marine N cycle to maintain homeostasis, and thus the global C cycle on glacial/interglacial time scales.
Broader Impacts: This project will provide research experience and opportunities for a postdoctoral associate as well as a graduate student. There will be direct interactions and opportunities for mentoring with graduate students in the laboratory and on cruises. Also, the project will routinely involve undergraduates as work-studies and participate in the USC Geobiology REU Program. The proposed activities will be integrated with the work of the NSF funded COSEE-West program to USC and UCLA. This Center involves a network of oceanographic researchers, K-12 educators, education centers and the general public in the greater Los Angeles area.
FINAL REPORT OUTCOMES: Documenting N2 Fixation in the Eastern Tropical South Pacific Oceanic life is ultimately controlled by the various inputs of nutrients, which provide sustenance for the base of the food chain, oceanic plants. Our study focused on a very unusual nutrient acquisition strategy used by marine phytoplankton, the conversion of inert nitrogen gas into utilizable, fixed nitrogen. We studied this process in a rather unique, but very poorly studied, portion of the global ocean, the Eastern Tropical South Pacific, a region in which other nutrients may be available, but fixed nitrogen is generally lacking. Here we asked the question: In these waters, do plants acquire fixed nitrogen when deeper water rich in fixed nitrogen is mixed up to the surface, or are there bacteria living in the surface ocean and fixing nitrogen? A theoretical analysis suggested that nitrogen fixation should be very important. Our answer, following two month long research cruises, is that it is primarily nutrients mixed up from the subsurface that fuels plant growth in this part of the ocean. We also learned what other N transformations take place in this unique region and how nitrogen fixation rates could be increased by the addition of certain trace elements. The nitrogen fixing organisms that we did detect were unlike those observed in other nutrient poor regions of the world oceans. We mapped this region in terms of plant growth rate and coupled this view of the surface ocean with the view from the deep benthos, to see how much food arrives to fuel life in the deep sea. We were an international collection of scientists, collaborating on aspects of this broad topic and are still in the process of disseminating our results in the scientific literature and at public meetings.
