Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

International audienceThe response of the phytoplanktonic community (primary production and algal biomass) to contrasted Sa-haran 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 simu-lated realistic dust deposition events (10 g m −2) into large mesocosms (52 m 3). Three distinct dust addition experiments 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-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dis-solved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO − 3) (net in-creases up to +9.8 µM NO − 3 at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO 3 . The dry deposi-tion event was shown to be a negligible source of NO − 3 . By transiently increasing DIP and NO − 3 concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phy-toplanktonic activity. Due to the higher input of NO − 3 relative to DIP, and taking into account the stimulation of the bio-logical activity, a wet deposition event resulted in a strong increase in the NO − 3 /DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggest-ing 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 strongly increased after wet dust de-position events, from initially 15 % to 60–70 % 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE ex-periments show that wet and dry dust deposition events in-duce contrasting responses of the phytoplanktonic commu-nity due to differences in the atmospheric supply of bioavail-able new nutrients. Our results from original mesocosm ex-periments demonstrate that atmospheric dust wet deposition Published by Copernicus Publications on behalf of the European Geosciences Union. 4784 C. Ridame et al.: Phytoplanktonic response to Saharan dust events greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO − 3 /DIP ratio, leading to a switch in the nu-trient limitation of the phytoplanktonic activity