Stéphane L'Helguen

Strong stimulation of N 2 fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms

The response of N 2 (dinitrogen) fixation to contrasted (wet and dry) Saharan dust deposition was studied in the framework of the DUNE project (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) during which realistic simulations of dust deposition (10 g m -2 ) into large mesocosms (52 m 3 ) were performed. Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. Here we show that wet and dry dust deposition induced a rapid (24 h or 48 h after dust additions), strong (from 2- to 5.3-fold) and long (at least 4-6 days duration) increase in N 2 fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N 2 fixation. This means in particular that N 2 fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO 3 - m -2 ). The input of new nitrogen associated with N 2 fixation was negligible relative to the atmospheric NO 3 - input associated with the dust. The contribution of N 2 fixation to primary production was negligible (≤ 1%) before and after dust addition in all experiments, indicating that N 2 fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N 2 fixation by dust addition, the rates remained low, and did not significantly change the contribution of N 2 fixation to new production since only a maximum contribution of 10% was observed. The response of N 2 fixation by diazotrophs and CO 2 fixation by the whole phytoplankton community suggests that these metabolic processes were limited or co-limited by different nutrients. With this novel approach, which allows us to study processes as a function of time while atmospheric particles are sinking, we show that new atmospheric nutrients associated with Saharan dust pulses do significantly stimulate N 2 fixation in the Mediterranean Sea and that N 2 fixation is not a key process in the carbon cycle in such oligotrophic environments.

The biogeochemistry of cobalt in the Mediterranean Sea

The soluble (sCo   0.2 µm) fractions of cobalt were investigated along the GEOTRACES-A04 section. Our results show that sCo was the predominant form (90%) of the DCo in the MS and that cCo and pCo generally followed the same distribution suggesting a biogeochemical link between these two fractions. In the Mediterranean Sea, DCo displayed an overall scavenged-like profile in the different sub-basins, with high concentrations (up to 350 pM) in surface and quasi-uniformed low concentrations of DCo (~45 pM) in the deep sea. However, the decoupling between the surface and the deep reservoirs suggested that the transfer of Co from dissolved to particulate pools during the sink of particles may not be the only process governing DCo distribution. High surface Co inputs, stabilization of DCo in a soluble form and the extremely high regeneration rate of biogenic pCo, all lead to the accumulation of DCo in surface. Conversely, low pCo export from the surface waters, low remineralization of biogenic pCo and slow but efficient removal of DCo by scavenging including colloids aggregation into particles, prevented its accumulation in the intermediate and deep sea. Moreover, Mediterranean circulation prevented the exchanges between the DCo-rich surface and the DCo-poor deep layers enhancing the scavenged-like profile of DCo. Finally, tentative DCo budgets were balanced at basin scale and showed the strong imprint of the surface inputs at Gibraltar Strait on the Mediterranean cobalt biogeochemistry.