Planktonic dinitrogen fixation in the Mediterranean Sea: a major biogeochemical process during the stratified period?

Abstract

Abstract. This study provides extensive data on planktonic N2 fixation fluxes across the whole Mediterranean Sea, representing a variety of trophic conditions. They show that N2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the western basin (10–76 μmol m−2 d−1) to the eastern basin (0–0.4 μmol m−2 d−1. Highest rates are measured in the less oligotrophic areas, between the surface and 75 m-depth, and 45 to 75% of N2 fixation are performed 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 and accounts for up to 25% of the external "new" N supply to the western basin during that period. These data disagree with indirect estimates of N2 fixation based on geochemical tracers and nutrient budgets, who suspected N2 fixation to increase 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 other seasons characterized by less oligotrophic conditions.