Environmental forcing of nitrogen fixation in the Eastern Tropical and Sub-Tropical North Atlantic Ocean

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 ^-1 nifH gene copies, unicellular group A cyanobacteria with up to 10 ⁵ L ^-1 nifH gene copies and gamma A proteobacteria with up to 10 ⁴ L ^-1 nifH gene copies. N ₂ fixation rates were low and ranged between 0.032-1.28 nmol N L ^-1 d ^-1 with a mean of 0.30±0.29 nmol N L ^-1 d ^-1 (1σ, n = 65). CO ₂-fixation rates, representing primary production, appeared to be nitrogen limited as suggested by low dissolved inorganic nitrogen to phosphate ratios (DIN:DIP) of about 2±3.2 in surface waters. Nevertheless, N ₂ fixation rates contributed only 0.55±0.87% (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 N ₂ fixation rates and the distribution of the dominant diazotrophic phylotypes. Our results suggest that lower predicted oxygen concentrations and higher temperatures due to climate warming may increase N ₂ fixation rates. However, the balance between a decreased supply of DIP and DFe from deep waters as a result of more pronounced stratification and an enhanced supply of these nutrients with a predicted increase in deposition of Saharan dust may ultimately determine the consequences of climate warming for N ₂ fixation in the North Atlantic.