Articles | Volume 11, issue 17
Biogeosciences, 11, 4783–4800, 2014

Special issue: Impact of atmospheric inputs on an oligotrophic ecosystem...

Biogeosciences, 11, 4783–4800, 2014

Research article 10 Sep 2014

Research article | 10 Sep 2014

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

C. Ridame1,2, J. Dekaezemacker3,4, C. Guieu5,6, S. Bonnet3, S. L'Helguen7, and F. Malien8 C. Ridame et al.
  • 1CNRS-INSU/IRD/MNHN/UPMC, LOCEAN: Laboratoire d'Océanographie et du Climat: Expérimentation et Approches Numériques, UMR 7159, 4 Place Jussieu – 75252 Paris Cedex 05, France
  • 2Sorbonne Universités, UPMC Univ Paris 06, UMR 7159, LOCEAN, 75005 Paris, France
  • 3Mediterranean Institute of Oceanography, IRD/Aix Marseille Université/CNRS Marseille/Noumea, New Caledonia, France
  • 4Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany
  • 5LOV: Laboratoire d'Océanographie de Villefranche/Mer, CNRS-INSU UMR7093, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France
  • 6Sorbonne Universités, UPMC Univ Paris 06, UMR 7093, LOV, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France
  • 7Université de Brest, CNRS/IRD, UMR 6539, Laboratoire des Sciences de l'Environnement Marin, OSU-IUEM, 29280 Plouzané, France
  • 8GEOMAR Helmholtz Centre for Ocean Research Kiel, Chemical Oceanography, Düsternbrooker Weg 20, 24105 Kiel, Germany

Abstract. 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 mesocosms (52 m3). 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 dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3) (net increases up to +9.8 μM NO3 at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3. By transiently increasing DIP and NO3- 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 phytoplanktonic activity. Due to the higher input of NO3 relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3/DIP ratio, from initially less than 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 strongly 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 based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity.

Final-revised paper