Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise)

Abstract. This study provides extensive data on planktonic N₂ fixation rates across the whole Mediterranean Sea. They show that N₂ fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10–76 μmol m⁻² d⁻¹) to the ultra oligotrophic eastern basin (0–0.4 μmol m⁻² d⁻¹). Highest rates are measured in the less oligotrophic western basin, between the surface and 75 m-depth, where 45 to 75 % of N₂ fixation are found within the picoplanktonic fraction (<3 μm). While the biogeochemical impact of N₂ fixation in the eastern basin seems negligible, N₂ fixation is able to sustain up to 35 % of new primary production during the stratified period in the western basin. These data disagree with indirect estimates of N₂ fixation based on geochemical tracers and nutrient budgets, which indicates that N₂ fixation increases with increasing N:P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO₃⁻ from west to east. These results finally point out the need to assess N₂ fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.