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This study investigates the diversity and nutrient responses of diazotrophs, nitrogen-fixing microorganisms critical for marine productivity, across a transect from the California Current System to the North Pacific Subtropical Gyre. Using nifH gene analysis as a proxy for diazotroph presence, we documented shifts in community composition from coastal to oligotrophic waters, with UCYN-A and Pseudomonas dominating transition zones while more diverse communities including Trichodesmium emerged in oligotrophic regions. Nutrient addition experiments tested UCYN-A1 responses to various nitrogen forms (NH□□, NO□□, urea), iron, and phosphorus over 48-hour incubations. Results challenge traditional assumptions that diazotrophs prefer low dissolved inorganic nitrogen environments, as UCYN-A1 abundances showed variable but non-negative responses to nitrogen additions. Time-course analyses revealed region-specific utilization patterns: continuous nitrogen utilization from 24-48 hours in the California Current transition zone versus more rapid utilization within the first 24 hours in the North Pacific Subtropical Gyre. These findings contribute to our understanding of nitrogen fixation biogeography and suggest that diazotroph distributions may be more strongly influenced by competition for phosphorus and iron rather than inhibition by fixed nitrogen availability.