Contrasted geographical distribution of N₂ fixation rates and <i>nif</i>H phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

Abstract

Abstract 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 µmol N m −2 d −1 ) compared to those measured in the Coral Sea (2 to 109 µmol N m −2 d −1 ). Rates measured in the Solomon Sea were in the upper range (100–1000 µmol 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 γ ‐24774A11 dominated in the Coral Sea and were correlated with N 2 fixation rates ( p &lt; 0.05), Trichodesmium and UCYN‐B dominated in the Solomon Sea and were correlated ( p &lt; 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.