First basin‐wide experimental results on N₂ fixation in the open Mediterranean Sea

Abstract

The Mediterranean Sea presents several biogeochemical anomalies compared to the global ocean. An unbalanced N budget, high nitrate/phosphate ratios in subsurface waters and low 15N/14N ratios in particulate and dissolved nitrogen suggest a significant occurrence of N2 fixation. This study presents, for the first time, a basin‐wide overview of direct measurements of N2 fixation, with values in the North Atlantic for comparison, during late spring 2007. Very low N2 fixation rates (0.052 ± 0.031 nmols N l−1d−1) were observed in all sub‐regions of the Mediterranean, in contrast to the higher values measured in the North Atlantic (0.300 ± 0.115 nmols N l−1d−1). Higher phosphorus (inorganic or organic) concentrations were not associated with higher N2 fixation rates. Low 15N/14N ratios in particulate organic nitrogen (from −2.10 to +4.11‰), associated with low N2 fixation rates, suggest that other N sources, such as atmospheric inputs, fuel the Mediterranean ecosystem.

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

“…In the eastern basin, N 2 fixation rates were low, especially at station C where they reached 0.4 ± 0.1 µmol m −2 d −1 , which is in agreement with recent data obtained by Ibello et al (2010) who also reported extremely low rates (0.5 to 2 µmol m −2 d −1 ) at three stations in the Levantine basin in May-June 2007. These low rates , , Webb et al, 2009).…”

“…However, molecular studies conducted over coastal stations in the eastern and western basin have clearly reported the presence of diazotrophic microorganisms related to Archaea, Proteobacteria and/or Cyanobacteria (Man-Aharonovich et al, 2007;Le Moal and Biegala, 2009), and of the diatom symbiont Richelia intracellularis (Bar Zeev et al, 2008). The potential of these organisms to bloom and fix dinitrogen at high rates in the P-depleted Mediterranean waters is still in debate; recent studies have indeed reported during the summer stratification period either high N 2 fixation rates at one isolated station in the Levantine basin (Rees et al, 2006), or extremely low rates at six stations distributed across the basin (Ibello et al, 2010). This leads to a difficulty in concluding about the biogeochemical importance of diazotrophy in this environment.…”

“…At the scale of the Eastern Mediterranean Sea, if we assume that N 2 fixation occurs 6 months per year (approximately the length of the strongest stratification period) with mean areal rates of 0.4-3.0 µmol N m −2 d −1 (This study, Ibello et al, 2010;Yogev et al, 2011), the total N supply provided by N 2 fixation to the eastern Mediterranean Sea is 0.1-0.7 × 10 9 N yr −1 (Table 4), which accounts for only 0.1-0.4 % of the total external annual N inputs (excluding exchanges at the straits and winter convection) over the eastern basin (160 × 10 9 mol N yr −1 ), which are mainly driven by atmospheric deposition and river discharge (Table 4; Krom et al, 2010). This result is two orders of magnitude lower than estimates from Sachs and Repeta (1999), who calculated, using a two end member source model that 46-70 % of the nitrate pool in the eastern Mediterranean Sea originates from N 2 fixation.…”

Silica cycling in the ultra-oligotrophic eastern Mediterranean Sea

“…The nutrient budget for N and P has been used to explain why the EMS has an unusually high nitrate : phosphate ratio (28 : 1) in deep water and predict that there would be very limited N 2 fixation (Krom et al, 2004), which was subsequently confirmed by field measurements (Ibello et al, 2010;Yogev et al, 2011). Only two silica budgets have been attempted in the Mediterranean.…”

Intriguing diversity among diazotrophic picoplankton along a Mediterranean transect: a dominance of rhizobia

“…The processes that induce such atypical ratios are under debate, and two hypotheses have been proposed, involving either significant diazotrophic activity, or low denitrification rates in combination with external inputs of nutrients (Béthoux and Copin-Montégut, 1986;Krom et al, 2010). Direct N 2 fixation measurements conducted in the eastern and western Mediterranean basins have shown mainly low diazotrophic activity with punctual peaks at different sites or seasons (0.01-129 nmol N L −1 d −1 , Rees et al, 2006;Sandroni et al, 2007;Ibello et al, 2010). This heterogeneity in rates of N 2 -fixation implies the need of high frequency surveys to fully integrate, over the long term, the role of diazotrophy in Mediterranean biogeochemical cycles.…”