Contrasted geographical distribution of N$_2$ fixation rates and $nif$H phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

International audience; Biological dinitrogen (N$_2$) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N$_2$ fixation rates were measurable at every station, but integrated (0-150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 $\mu$mol N m$^{-2}$ d$^{-1}$ compared to those measured in the Coral Sea (2 to 109 $\mu$ mol N m$^{-2}$ d$^{-1}$. Rates measured in the Solomon Sea were in the upper range (100-1000 mu mol N m$^{-2}$ d$^{-1}$ or higher than rates compiled in the global MARine Ecosystem biomass DATa database, indicating that this region has some of the highest N$_2$ fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN-A1 and UCYN-A2) and the proteobacteria $\gamma$-24774A11 dominated in the Coral Sea and were correlated with N$_2$ fixation rates (p < 0.05), $Trichodesmium$ and UCYN-B dominated in the Solomon Sea and were correlated (p < 0.05) with N$_2$ fixation rates. UCYN-A were totally absent in the Solomon Sea. The biogeographical distribution of diazotrophs is discussed within the context of patterns in measured environmental parameters.