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

C. Ridame, C. Guieu, S. L'Helguen
2013 Biogeosciences  
<p><strong>Abstract.</strong> The response of N<sub>2</sub> (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<sup>&amp;minus;2</sup>) into large mesocosms (52 m<sup>3</sup>) were performed. Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet
more » ... on 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<sub>2</sub> fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N<sub>2</sub> fixation. This means in particular that N<sub>2</sub> fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO<sub>3</sub><sup>&amp;minus;</sup> m<sup>&amp;minus;2</sup>). The input of new nitrogen associated with N<sub>2</sub> fixation was negligible relative to the atmospheric NO<sub>3</sub><sup>&amp;minus;</sup> input associated with the dust. The contribution of N<sub>2</sub> fixation to primary production was negligible (&amp;le; 1%) before and after dust addition in all experiments, indicating that N<sub>2</sub> fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N<sub>2</sub> fixation by dust addition, the rates remained low, and did not significantly change the contribution of N<sub>2</sub> fixation to new production since only a maximum contribution of 10% was observed. The response of N<sub>2</sub> fixation by diazotrophs and CO<sub>2</sub> 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<sub>2</sub> fixation in the Mediterranean Sea and that N<sub>2</sub> fixation is not a key process in the carbon cycle in such oligotrophic environments.</p>
doi:10.5194/bg-10-7333-2013 fatcat:jrofqk4udrablnjwciyeezjt2a