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

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.