ORIGINAL A mesocosm experiment coupled with optical measurements to assess the fate and sinking of atmospheric particles in clear oligotrophic waters Matthieu Bressac & Cécile Guieu & David Doxaran & François Bourrin & Grigor Obolensky & Jean-Michel Grisoni Received: 15 April 2011 /Accepted: 24 November 2011 # Springer-Verlag 2011 Abstract It has recently been postulated that lithogenic par- ticles such as Saharan dust strongly influence particulate organic carbon export to the deep ocean by acting as mineral ballast. However, our understanding of the processes involved remains scant. In the present study, optical measurements were performed to monitor variations in the concentration, composition and size distribution of particles in suspension within the water column after simulating a Saharan dust event in very clear Mediterranean waters off Corsica in June 2010. A new methodology set up in large mesocosms proved very successful in this regard. Values obtained simultaneously from three instruments (WetLabs ECO-BB3, WetLabs ac-9, Se- quoia Scientific LISST-100) provided evidence that (1) part of the Saharan dust pool has a rapid settling velocity (∼24– 86 m day -1 ), (2) particulate export following a dust event is a nonlinear multi-step process and (3) export is controlled in part by the formation of organic-mineral aggregates. This experimental study provides the first insight of the complex export processes occurring after a dust event involving both physical and biogeochemical forcings in clear oligotrophic waters. Introduction Sporadic high-magnitude Saharan dust deposition occurs over large areas in the Mediterranean Sea (Loÿe-Pilot and Martin 1996; Ternon et al. 2010), and it has long been known that particulate concentrations and fluxes in the upper water column can increase significantly following dust transport and deposition events with a short response time (Buat-Ménard et al. 1989). Recent studies have dem- onstrated that Saharan dust bring elements of biogeochem- ical significance to the Mediterranean Sea, such as phosphorus and iron that may alleviate biological limitation during summer (e.g. Bonnet et al. 2005; Ternon et al. 2011). Being the only external source of new nutrients during the period of water column stratification, these inputs can be seen as fertilizing events. Moreover, through the process of aggregation and the ballast effect, atmospheric particles have been postulated to enhance particulate organic carbon Responsible guest editor: O. Mikkelsen Electronic supplementary material The online version of this article (doi:10.1007/s00367-011-0269-4) contains supplementary material, which is available to authorized users. M. Bressac (*) ACRI-ST, BP 234, 06904 Sophia-Antipolis, France e-mail: bressac@obs-vlfr.fr M. Bressac : C. Guieu : D. Doxaran : G. Obolensky CNRS-INSU, Université Pierre et Marie Curie-Paris 6, UMR 7093, Laboratoire d’Océanographie de Villefranche/Mer, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France F. Bourrin Université de Perpignan, CEFREM, CNRS-UPVD, UMR 5110, 52 avenue Paul Alduy, 66860 Perpignan, France J.-M. Grisoni Observatoire Océanologique de Villefranche/Mer, CNRS, Université Pierre et Marie Curie-Paris 6, 06230 Villefranche-sur-Mer, France Geo-Mar Lett DOI 10.1007/s00367-011-0269-4