SPATIAL AND TEMPORAL HETEROGENEITY OF LAKE LUGANO SEDIMENT COMPOSITION

The concentrations of eighteen elements (Al, C, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, N, Ni, P, Pb, S, Si, Ti, and Zn) in 76 sediment samples colle cted in Lake Lugano in 1973 and in 83 samples collected in 1985 were m easured and used to draw distribution maps. The chemical compositions of sediments from the three lake subbasins were statistically compared , and used to describe the state of the lake at two time-points twelve years apart. The results obtained seem to be consistent with the avai lable information about the recent evolution of Lake Lugano; both phos phorus and nitrogen are statistically higher in the 1985 samples than in those of 1973, confirming the reported tendency of the lake to evol ve towards higher trophic conditions in this period of time. For both metals and mobile elements, the differences between the two sampling y ears and the literature data reflect fluctuations in the ''normal'' ge ochemical supply from the catchment basin, rather than significant con tamination phenomena. However, elements like Ni, Cr, Zn, Pb and Cu ten d to be more abundant in the most recent layers of the sediments, so f urther studies are needed to assess whether potentially hazardous cond itions will evolve in the future.