Does phosphate adsorption onto Saharan dust explain the unusual N/P ratio in the Mediterranean Sea? L’absorption des phosphates de poussières sahariennes explique-t-elle le taux N/P inhabituel en Méditerranée ?

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform radio-labelled phosphate adsorption experiments using PO4 : leached particles were exposed to poisoned western Mediterranean seawater for varying lengths of time. The measured adsorption capacity of Saharan dust for phosphate was 0.13 μmol g. Considering this value and an annual Saharan dust deposition of 12.5 t km year, we show that Saharan particles do not represent a significant sink for seawater phosphate in the western Mediterranean Sea. This result is in agreement with that determined from a similar approach conducted in the eastern basin. As a consequence, the unusual N/P ratio measured in the whole Mediterranean Sea (up to 29) cannot be explained by the adsorption process of seawater phosphate onto Saharan dust. © 2003 Published by Éditions scientifiques et médicales Elsevier SAS.