Abstract
Abstract. The response of N2 (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 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 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 N2 fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N2 fixation. This means in particular that N2 fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO3− m−2). The input of new nitrogen associated with N2 fixation was negligible relative to the atmospheric NO3− input associated with the dust. The contribution of N2 fixation to primary production was negligible (≤ 1%) before and after dust addition in all experiments, indicating that N2 fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N2 fixation by dust addition, the rates remained low, and did not significantly change the contribution of N2 fixation to new production since only a maximum contribution of 10% was observed. The response of N2 fixation by diazotrophs and CO2 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 N2 fixation in the Mediterranean Sea and that N2 fixation is not a key process in the carbon cycle in such oligotrophic environments.
Highlights
Over geological time scales, dinitrogen (N2) fixation is important for the regulation of the fixed nitrogen (N) reservoir in the ocean and thereby sustains ocean productivity (Falkowski, 1997; Tyrrell, 1999)
For P, Q and R experiments, N2 fixation rates were initially low (< 0.25 nmol N L−1 d−1), homogeneous and remained stable over the duration of experiments in the Control-meso. These were consistent with previous surface measurements in the open western Mediterranean Sea during periods of stratification (≤ 0.2 nmol N L−1 d−1 in Ibello et al, 2010; Bonnet et al, 2011; Ridame et al, 2011; Ternon et al, 2011) and were within the range of the lowest rates measured with the gas bubble enrichment method in oligotrophic areas of the Atlantic and Pacific oceans (0.1 to 4 nmol N L−1 d−1 in Mills et al, 2004; Needoba et al, 2007; Bonnet et al, 2009; Fernandez et al, 2010; Rijkenberg et al, 2011)
Our results from original mesocosm experiments demonstrate that atmospheric dust deposition does greatly influence N2 fixation rates in low nutrient low chlorophyll environments impacted by dust deposition
Summary
Dinitrogen (N2) fixation is important for the regulation of the fixed nitrogen (N) reservoir in the ocean and thereby sustains ocean productivity (Falkowski, 1997; Tyrrell, 1999). N2 fixation or diazotrophy is recognized to be the main source of fixed N in the marine environments (Codispoti, 2007; Gruber, 2008; Großkopf et al, 2012) supporting an important part of oceanic primary productivity and organic matter export to the deep ocean (Dore et al, 2002; Karl and Letelier, 2008; Subramaniam et al, 2008). It has been shown that trace elements other than Fe could exert a control on N2 fixation (Ridame et al, 2011; Ho, 2013)
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