The north basin of Lake Lugano (Ticino, Switzerland), with a maximum depth of 288 meters (m), is permanently stratified. The water column is oxygenated above 80 m, suboxic between 80 and 100 m and anoxic below 100 m. The stratification built up during the twentieth century in response to anthropogenicaly induced eutrophication of the lake. The main focus of the thesis is the study of the dynamics of particles within this system. Through sampling of suspended particles and through sediment trap deployment, particle transport processes, precipitation-, mineralization- and dissolution processes between (1) the oxic- and anoxic layers, (2) the epilimnion, the hypolimnion and the bottom sediment, and (3) within the hypolimnion, were characterised. Sampling and measurements were performed on a regular basis between June 1992 and January 1994 at a pelagic sampling station (Gandria).

Most important in this study were the following results:1) The geochemical cycle of Fe and Mn, confined to the suboxic zone, was markedly influenced by the spring input of labile organic matter originating from a diatom bloom. Oxyhydroxide precipitation was stimulated.2) Particle aggregation was deduced from analysis of sediment trap fluxes and turbidity profiles. Observations concerned different particle size classes and water depths. In one case, model calculations could be applied. Results indicated high coagulation coefficients (0.1) for particles in the 1-2 micro m range.3) Resuspension of particles was evaluated through Al fluxes in sediment traps. Focusing sensu stricto was identified.4) Mineralization of organic matter, dissolution of authochtonous calcite and biogenic silica in the watercolumn were deduced from total fluxes of organic carbon, calcium and biogenic silica by taking in account resuspension fluxes. Maximal mineralization and dissolution fluxes were reached between 265 m and the lake bottom, due to longer residence times of particles in the benthic nepheloid layer (resuspension).5) Mineralization of organic matter and dissolution of calcite in the hypolimnion seem to induce a slight progression of the chemical stratification of the watercolumn despite the considerable reduction of anthropogenic wastewater input to the lake since 1976.