Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise) INSU-CNRS, Laboratoire d'Océanographie Physique et Biogéochimique (LOPB), UMR6535, CNRS-IRD-Université de la Méditerranée, Centre d'Océanologie de Marseille, Faculté des Sciences de Luminy, Case 901, 13288 Cedex 09 Marseille, France
19 Aug 2011
Received: 03 February 2011 – Published in Biogeosciences Discuss.: 10 February 2011 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.
Revised: 17 June 2011 – Accepted: 20 June 2011 – Published: 19 August 2011
Citation: Bonnet, S., Grosso, O., and Moutin, T.: Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise), Biogeosciences, 8, 2257-2267, doi:10.5194/bg-8-2257-2011, 2011.