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 N-2 fixation rates across the whole Mediterranean Sea. They show that N-2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10-76 mu mol m(-2) d(-1)) to the ultra oligotrophic eastern basin (0-0.4 mu mol 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 N-2 fixation are found within the picoplanktonic fraction (<3 mu m). While the biogeochemical impact of N-2 fixation in the eastern basin seems negligible, N-2 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 N-2 fixation based on geochemical tracers and nutrient budgets, which indicates that N-2 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 N-2 fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.
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Submitted on : Tuesday, August 21, 2012 - 11:08:16 AM
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Sophie Bonnet, Olivier Grosso, Thierry Moutin. Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise). Biogeosciences, European Geosciences Union, 2011, 8 (8), pp.2257-2267. ⟨10.5194/bg-8-2257-2011⟩. ⟨hal-00724468⟩

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