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 basi n of Lake Lugano during a campaign of regular measurements of turbidity pro files by forward light scattering nephelometry: turbidity increased with de pth and decreased with time over the period of investigation (November 1992 to January 1994). Correlation of the chemical composition of the major ele ments present in lake particles with turbidity allowed the association of t he measured turbidity with inorganic, mainly alumino-silicate particles. Pa rticles in the turbidity structure probably originated from a gravity curre nt triggered by a landslide that occurred in the port of Lugano in March 19 92. Seasonal events such as fluxes of authochthonous calcite or diatom frus tules had little influence on the observed hypolimnion turbidity. Allochtho nous particles from flood events did Mot markedly affect the sedimentation structure either. Calculations showed that the evolution of the suspended p articles cannot be explained by taking only particle sedimentation into acc ount. To test whether particle aggregation could have been the dominating p rocess 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 mu m mean diameter) and that coagulation p layed a major role in the sedimentation of mineral particles. Difficulties encountered in fitting the second phase of particle removal (after June 199 3) have been tentatively explained by the increase in particle stability ca used by the mineralization of the abundant organic matter generated during the lake productivity period.