The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE "a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem" project. We simulated realistic dust deposition events (10 g m −2 ) into large 5 mesocosms (52 m 3 ). 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

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... sition) and 2010 (DUNE-2-R1, -R2: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentration (Chl a) were observed after a dry deposition event 10 while a wet deposition event resulted in a rapid (24 h after dust additions), strong (up 2.4 fold) and long (at least a week duration) increase in PP and Chl a. We show that in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of NO − 3 (net increases up to +9.8 µM NO − 3 at 0.1 m depth) to the nutrient depleted surface waters due to cloud processes 15 and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO − . By transiently increasing DIP and NO − 3 concentrations in P-N starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO − 3 relative to DIP, a wet deposition event resulted in a strong increase 20 in the NO − 3 /DIP ratio from initially < 6 to over 150 at the end of the DUNE-2-R1 experiment suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP increased after wet dust deposition events from initially 15% to 60-70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production 25 based system. DUNE experiments show that wet and dry dust deposition events induce contrasted responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm 755