| Summary: | During the winter of 2006 we measured nifH gene abundances, dinitrogen (N 2) fixation rates and carbon fixation rates in the eastern tropical and sub-tropical North Atlantic Ocean. The dominant diazotrophic phylotypes were filamentous cyanobacteria, which may include Trichodesmium and Katagnymene, with up to 10 6 L 21 nifH gene copies, unicellular group A cyanobacteria with up to 10 5 L 21 nifH gene copies and gamma A proteobacteria with up to 10 4 L 21 nifH gene copies. N 2 fixation rates were low and ranged between 0.032–1.28 nmol N L 21 d 21 with a mean of 0.3060.29 nmol N L 21 d 21 (1s, n = 65). CO2-fixation rates, representing primary production, appeared to be nitrogen limited as suggested by low dissolved inorganic nitrogen to phosphate ratios (DIN:DIP) of about 263.2 in surface waters. Nevertheless, N2 fixation rates contributed only 0.5560.87 % (range 0.03–5.24%) of the N required for primary production. Boosted regression trees analysis (BRT) showed that the distribution of the gamma A proteobacteria and filamentous cyanobacteria nifH genes was mainly predicted by the distribution of Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. In addition, BRT indicated that multiple a-biotic environmental variables including nutrients DIN, dissolved organic nitrogen (DON) and DIP, trace metals like dissolved aluminum (DAl), as a proxy of dust inputs, dissolved iron (DFe) and Fe-binding ligands as well as oxygen and temperature influenced N2 fixation rates and the distribution of the dominant diazotrophic phylotypes. Our results suggest that lower predicted oxygen concentrations and higher temperatures due to climate
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