Transport of Suspended Matter in the Hypolimnion of Lake Lugano: a Comparison of Field Observations and Model Predictions

An unusual feature was observed in the anoxic hypolimnion of the north basin of Lake Lugano during a campaign of regular measurements of turbidity profiles by forward light scattering nephelometry: turbidity increased with depth and decreased with time over the period of investigation (November 1992 to January 1994). Correlation of the chemical composition of the major elements present in lake particles with turbidity allowed the association of the measured turbidity with inorganic, mainly alumino-silicate particles. Particles in the turbidity structure probably originated from a gravity current triggered by a landslide that occurred in the port of Lugano in March 1992. Seasonal events such as fluxes of authochthonous calcite or diatom frustules had little influence on the observed hypolimnion turbidity. Allochthonous particles from flood events did not markedly affect the sedimentation structure either. Calculations showed that the evolution of the suspended particles cannot be explained by taking only particle sedimentation into account. To test whether particle aggregation could have been the dominating process leading to rapid settling, a simple coagulation-sedimentation model for hydrophobic particles was used to simulate the sedimentation event. The model provided clear evidence that particles involved in the sedimentation process were fine sized (1 to 2 μm mean diameter) and that coagulation played a major role in the sedimentation of mineral particles. Difficulties encountered in fitting the second phase of particle removal (after June 1993) have been tentatively explained by the increase in particle stability caused by the mineralization of the abundant organic matter generated during the lake productivity period.