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Strong stimulation of N2 fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms

July 8, 2013

The response of N2 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) into large mesocoms (52 m3) 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 successive wet depositions) in the north western oligotrophic Mediterranean Sea. Here we show that wet and dry dust deposition induced a rapid (24 h or 48 h after dust additions), strong (2- to 5.3-fold) and long (4 to 6 days duration) increase in N2 fixation indicating that both wet and dry Saharan dust depositions were able to relieve efficiently the nutrient limitation(s) of N2 fixation. This means in particular that N2 fixation activity was not inhibited by the NO3 input associated with the simulated wet deposition. The contribution of N2 fixation to primary production was negligible before (on average 0.4%) and after (on average 1%) dust additions in all experiments indicating that N2 fixation was a poor contributor to the N demand for primary production. Before seedings, new production (NP) was mainly supported by NO3 as a source of N as shown by the low contribution of N2 fixation to NP (on average 3%). Despite the stimulation of N2 fixation by dust, the rates remained low, and did not allow to significantly change the contribution of N2 fixation to NP as a maximum of 10% contribution was evidenced. A comparison of the responses of N2 fixation by diazotrophs and CO2 fixation by the whole phytoplankton community suggests that those metabolic processes were limited or co-limited by different nutrients. The estimated input of new nitrogen (NO3) from simulated wet deposition was much higher than that associated with N2 fixation. We confirm that although the biogeochemical impact of N2 fixation seems negligible in the oligotrophic waters of the western Mediterranean Sea, Saharan dust pulses by bringing new nutrients represent a key controlling factor of the magnitude of N2 fixation rate in the Mediterranean Sea and potentially in all LNLC areas impacted by dust deposition such as the tropical Atlantic and Pacific Oceans.

Ridame C, Guieu C, L’Helguen S, 2013, Strong stimulation of N2 fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms, Biogeosciences Discussions, 10, 10581-10613, doi:10.5194/bgd-10-10581-2013. Article.

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