Biogeochemical behavior and mass balance of dissolved aluminum in the western Mediterranean Sea

The biogeochemical cycle of dissolved aluminum in the western Mediterranean has been thoroughly investigated during the EROS 2000 program. Measurements of the concentration of this element were carried out for 5 years both in the water column and in the pore waters of the sediment. Special attention was paid to the distribution of dissolved Al in the river plumes of the Rhone and Ebro. The vertical profiles of this element in the vicinity of the straits of Gibraltar and Sicily also were determined in order to quantify the exchange fluxes of Al at these two boundaries. Budget calculations indicate that the mass balance of the input and output fluxes is fairly well constrained. They also show that atmospheric dust of Saharan origin is by far the most important external source of dissolved Al for the western Mediterranean and thus is responsible for the anomalously high concentrations of this element observed in the water column. Data obtained in this study strongly support the hypothesis that the unusual nutrient-like vertical distribution is controlled mainly by biological activity. Calculations of renewal times of dissolved Al imply that its concentration and distribution can be affected significantly at time scales of a few decades if the atmospheric input is modified. Presently the net input flux of dissolved Al to the western Mediterranean is entirely exported to the Atlantic.