Project Funding: 2008 - 2011

NRA: 2007 NASA: New Investigator Program   

Funded by NASA

Abstract:
Biological nitrogen fixation is the microbially mediated conversion of dinitrogen gas (N2) into a bioavailable nutrient. It is an intriguing process as much for the organisms that perform it, as for the power it can hold over elemental cycling at the local, regional, and global scales. Historically, the primary N2-fixer in the ocean has been the cyanobacterium Trichodesmium, which has the ability to form dense blooms near the sea surface, visible even in photography from space shuttle missions. These blooms are ephemeral in both space and time, yet the high rates of nitrogen fixation measured in them suggest they contribute significantly to the global nitrogen budget. Until recently, their spatial and temporal distribution was not well known and thus, their significance at the global scale was uncertain. Novel ocean color remote sensing efforts using NASA products have now demonstrated that we can successfully distinguish these blooms globally from satellite. This ability has revealed patterns that partly reflect historical observations, but which also show previously undocumented, yet persistent bloom features in traditionally undersampled regions of the ocean. However, in terms of prediction, we still have no mechanistic constraints on bloom formation, persistence, and decay. Here, we hypothesize a framework for Trichodesmium bloom formation that results from in situ growth, physical accumulation of existing populations, or both. Using globally gridded datasets from satellite and model-based sources, we will examine environmental variability in sea surface temperature, wind speed, and mixed layer depth for persistent patterns that precede or accompany observed Trichodesmium blooms. In addition, and of particular interest, we will also explore potential relationships between bloom occurrence and atmospheric dust deposition (and resultant iron addition) and sea surface macronutrient concentration (e.g., nitrate and phosphate). These latter two properties have been suggested to exert environmental control on abundance and N2-fixing capacity of Trichodesmium and other diazotrophs that can have far-reaching consequences for earth system climate (Falkowski, 1997; Broecker and Henderson, 1998). Patterns will be diagnosed using both correlative and Bayesian probabilistic approaches. Our efforts will focus on 5 specific regions, chosen for the frequency and persistence of Trichodesmium blooms as seen from satellite ocean color data, and for their prevalence of bloom observations in literature. Both the correlative and probabilistic approaches will be used in a hindcast manner throughout the common lifetime of all the data sources (~2000-2006), but also lend themselves to prediction as well. Last, we will partner with the Science and Math Investigative Learning Experience (SMILE) program at Oregon State University to insure an efficient interface between the basic science and novel results from this proposal and a broader community. SMILE is an established program which integrates perfectly into the NASA Education Strategic Coordination Framework and which provides opportunities for historically underserved youths to gain research skills and to design and carry out ecosystem science investigations.