Strong stimulation of N<sub>2</sub> fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms
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&g...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2013-11-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/10/7333/2013/bg-10-7333-2013.pdf |
| Summary: | 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>−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 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>−</sup> m<sup>−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>−</sup>
input associated with the dust. The contribution of N<sub>2</sub> fixation to
primary production was negligible (≤ 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. |
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| ISSN: | 1726-4170 1726-4189 |