A one-dimensional model of iron speciation and biogeochemistry is coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model and applied for the Tropical Eastern North Atlantic Time-series Observatory (TENATSO) site. The doubel role of dust in the iron cycle is studied: dust deposition brings not only iron into surface waters but also inorganic particles which control the scavenging of Fe(III)' and colloidal iron in surface waters and contribute to particle aggregation. Sensitivity studies show that redissolution of colloidal and particulate iron is important to prevent DFe from scavenging and keep DFe concentration in deep waters constant. Our assumptions, valid for inorganic colloids, lead to an underestimate of colloidal:soluble iron ratio, indicating that organic colloids should be considered in future work.