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

Abstract

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 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 N 2 fixation are found within the picoplanktonic fraction (<3 µm). 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 NO − 3 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.

Characterization of turbulence from a fine-scale parameterization and microstructure measurements in the Mediterranean Sea during the BOUM experiment

“…Regarding the impact on nutrients fuxes, however, Bonnet et al (2011) have shown that the vertical nitrogen turbulent fuxes determined from K z values obtained in eddies still balance only a small fraction of the nitrogen fluxes resulting from primary production, suggesting a main contribution of regenerated production. Vertical advection was not considered in this study and may also provide significant vertical transport of nutrients, as suggested in previous studies (Ledwell, 2008).…”

“…A large part of the BOUM experiment was dedicated to the precise characterization of biogeochemical processes within three oligotrophic environments, namely anticyclonic eddies (eddies A, B, C) (Moutin and Prieur, 2012). During the experiment special effort was made to determine the physical forcing, and specifically the vertical mixing in these environments, which impacts most of biogeochemical processes studied and modeled within the BOUM project (Bonnet et al, 2011;Mauriac et al, 2011). Another point of interest is the impact of vertical turbulent mixing on the water masses evolution and water massed circulation in the Mediterranean Sea.…”

Influence of anticyclonic eddies on the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM cruise)

“…This K z value, with an estimated error of about 40 % ( Table 2 in Cuypers et al, 2012), was considered constant. N-input by N 2 fixation is the mean integrated value measured in the photic zone, and atmospheric deposition is a mean value taken in the absence of rain (Bonnet et al, 2011 and further references herein, Table 1). The estimated errors could be 30-40 % (Bonnet et al, 2011, their Table 1).…”

“…N-input by N 2 fixation is the mean integrated value measured in the photic zone, and atmospheric deposition is a mean value taken in the absence of rain (Bonnet et al, 2011 and further references herein, Table 1). The estimated errors could be 30-40 % (Bonnet et al, 2011, their Table 1). New N-input corresponds to the sum of these three N-fluxes, because other possible inputs T. Moutin and L. Prieur: Influence of anticyclonic eddies on BOUM 3833 Table 3.…”

“…The MS is also characterized by a high N:P ratio in both the mineral and organic pools (Mc Gill, 1961, 1965Coste et al, 1984;Raimbault and Coste, 1990;, which are higher in the eastern than in the western basin (Krom et al, 1991). Three fundamentally different processes have been proposed to explain the typical N:P ratios in the MS. Firstly, there is the biological process of nitrogen (N 2 ) fixation (Bethoux and Copin-Montegut, 1986;Bonin et al, 1989;Sachs and Repeta, 1999;Kerhervé et al, 2001;Pantoja et al, 2002) which may lead to nitrogen accumulation; however, recently acquired data indicate very low nitrogen fixation rates (Ibello et al, 2010;Yogev et al, 2011;Bonnet et al, 2011). Secondly, there is the chemical process of phosphate adsorption onto iron-rich particles which leads to further P-depletion in the Mediterranean Sea (Krom et al, 1991); however, it has been demonstrated that phosphate removal by adsorption from the water column does not represent a significant enough sink for phosphate in the MS to explain the high N:P ratios (Pan et al, 2002;Ridame et al, 2003).…”