Longitudinal variability of the biogeochemical role of Mediterranean aerosols in the Mediterranean Sea
1INSU-CNRS, UMR 7093, Laboratoire d'Océanographie de Villefranche/Mer, Observatoire Océanologique de Villefranche-sur-mer, France
2UPMC Université Paris 06, UMR 7093, LOV, Observatoire Océanologique de Villefranche-sur-mer, France
3LOCEAN, UMR 7159, Université Pierre et Marie Curie, Paris, France
4Laboratoire des Sciences de l'Environnement Marin, UMR UBO/CNRS/IRD 6539, IUEM, 29280 Plouzané, France
5UPMC Université Paris 06, UMR 7621, LOMIC, Observatoire Océanologique, 66650, Banyuls/mer, France
6CNRS, UMR 7621, LOMIC, Observatoire Océanologique, 66650, Banyuls/mer, France
Abstract. The Mediterranean Sea is a semi-enclosed basin characterized by a strong thermal stratification during summer during which the atmosphere is the main source of new nutrients to the nutrient-depleted surface layer. From aerosol sampling and microcosm experiments performed during the TransMed BOUM cruise (June–July 2008) we showed that: (i) the Mediterranean atmosphere composition (Al, Fe, P) was homogeneous over ~28° of longitude and was a mixture with a constant proportion of anthropogenic contribution and a variable but modest contribution of crustal aerosols. This quite stable composition over a one month period and a long transect (~2500 km) allowed to define the Mediterranean atmospheric "background" that characterizes the summer season in the absence of major Saharan event and forest fires, (ii) primary production significantly increased at all tested stations after aerosols addition collected on-board and after Saharan dust analog addition, indicating that both additions relieved on-going (co)-limitations. Although both additions significantly increased the N2 fixation rates at the western station, diazotrophic activity remained very low (~0.2 nmol N L−1 d−1), (iii) due to the presence of anthropogenic particles, the probable higher solubility of nutrients associated with mixed aerosols (crustal + anthropogenic contribution), conferred a higher fertilizing potential to on-board collected aerosol as compared to Saharan dust analog. Finally, those experiments showed that atmospheric inputs from a mixed atmospheric event ("summer rain" type) or from a high-intensity Saharan event would induce comparable response by the biota in the stratified Mediterranean SML, during summer.