Graduation date: 2007The biological transformation of dinitrogen gas (N2) into combined forms(termed N2 fixation) by certain genera of oceanic cyanobacteria represents the largest incoming flux of nitrogen to the global ocean. As such, biological nitrogen fixation plays a significant role in the regulation of oceanic productivity and the export of carbon and nitrogen out of the sun-lit surface waters. Currently, our knowledge of the biogeochemical and ecological significance of N2-fixing organisms is restricted by our relative inability to define mechanistically the relevant chemical, biological and physical controls of the production and abundance of biological N2 fixation in the marine environment. The four chapters that form the main body of this dissertation touch upon specific aspects of the controls of the production and abundance of N2-fixing organisms, particularly that of the cyanobacterial genus Trichodesmium. Chapter II defines the range of intracellular elemental composition exhibited by Trichodesmium and identifies phosphorus (P) as a key factor limiting N2 fixation by populations of this organism residing in specific regions of the Atlantic and Pacific oceans. Chapter III explores vertical migration as a physiological adaptation relevant to the growth of this same cyanobacterium, Trichodesmium. Results from this work indicate that vertical migration may allow a subset of the population to exploit the separation of light and nutrients under stratified oligotrophic conditions, thus potentially supplementing their P requirements for growth. Chapter IV examines time-series records in an attempt to define the physical characteristics of the environment that may regulate surface blooms of cyanobacteria. These analyses indicate that season, sea surface temperature and mixed layer depth are the most constrained predictors of blooms of N2-fixing cyanobacteria in the North Pacific. Lastly, Chapter V identifies a novel region of the ocean inhabited by N2-fixing organisms.