In recent years, the spring diatom bloom in eutrophic Lake Lugano (Switzerland, Italy) has been dominated by the small centric diatom Stephanodiscus minutulus. Sediment traps, deployed from 1992 to 1994 in the permanently stratified, deep north basin of this lake, revealed high settling velocities of, at least, 8 m/d for the bulk of the diatom bloom. Geochemical and ecological considerations point to nutrient resource competition and nutrient-limited development of the algal bloom and not to decimation through zooplankton grazing. To test whether physical aggregation could have been the dominating process leading to the observed fast settling, a coagulation-sedimentation model combining classical coagulation theory for hydrophobic colloids with Stokes' law was used to simulate the sedimentation event. High values of the sticking coefficient (a = 0.5-1) were necessary to obtain significant sedimentary fluxes. Besides a, two additional factors appeared important to obtain sedimentary fluxes comparable to experimental field data: (i) S. minutulus is equipped with setae or hair-like extensions that may increase the collision frequency of the cells. Taking this effect into account in the simulation considerably increases the calculated settling fluxes, (ii) S. minutulus may interact with the larger, colony-forming diatoms present in the bloom in minor number. Inclusion of such cross group aggregation considerably increases simulated fluxes and seems to play a role in the total diatom sedimentation flux. Through rapid settling of the diatom aggregates, large amounts of fresh, aquagenic organic matter were transported to the hypolimnion. This was documented through microscope analysis of the organic matter present in the sediment traps. The rapid settling of organic-siliceous aggregates seems to have a major influence on trace metal scavenging in the epilimnion, as revealed by 7Be residence times, and also on the cycle of Mn- and Fe-oxyhydroxides.