Dinitrogen (N2) fixation enables primary production and, consequently, carbon dioxide drawdown in nitrogen (N) limited marine systems, exerting a powerful influence over the coupled carbon and N cycles. Our understanding of the environmental factors regulating its distribution and magnitude are largely based on the range and sensitivity of one genus, Trichodesmium. However, recent work suggests that the niche preferences of distinct diazotrophic (N2 fixing) clades differ due to their metabolic and ecological diversity, hampering efforts to close the N budget and model N2 fixation accurately. Here, I explore the range of N2 fixation across physico-chemical gradients (e.g., light, nutrients, oxygen) in nearshore environments of significance in global biogeochemical cycling: the major pelagic oxygen deficient zones (ODZs) in the Eastern Tropical South (ETSP) and North (ETNP) Pacific Ocean, and the broad continental shelf of the Western North Atlantic Ocean (WNA). The ODZs are hypothesized to play an important role in N cycle homeostasis by generating conditions thought to promote diazotrophy; recent work suggests that broad continental shelf environments may contribute substantially to new reactive N inputs globally. N2 fixation rates were measured using a robust 15N tracer method that accounts for the slow dissolution of N2 gas. To explore niche partitioning and better characterize spatial heterogeneity on the WNA shelf, I built an empirical model of N2 fixation and investigated diazotroph identity using amplicon sequencing and qPCR. In the ETSP, N2 fixation was only detected in a subset of low-oxygen samples. N2 fixation within the ETNP ODZ was patchy and driven by organic carbon availability; however, significant rates were observed at coastal stations near the Gulf of California. Frontal mixing on the WNA shelf resulted in exceptionally high rates of N2 fixation, associated with high UCYN-A activity. My findings suggest that (1) diazotrophy is more energetically favorable (relative to dissolved inorganic N) in low-oxygen waters but may be carbon-limited, and (2) continental inputs and dynamic conditions at coastal margins can favor significant N inputs via diazotrophy
