Particle composition and sedimentation rates in two Italian subalpine lakes

Two Italian subalpine lakes, Pusiano and Mezzola, with comparable surfaces and anthropogenic pressures and differences in the trophic state, water mat rix, water volume and theoretical renewal time, were investigated over an a nnual period to evaluate sedimentation rates and fluxes of major elements t o the sediments. The composition of settling particles revealed a dominant influence in lake Pusiano of autochthonous matter (about 80 %). This consis ted of biogenic organic material and autochthonously precipitated calcite g iving approximately similar contributions on an annual basis. In lake Mezzo la, on the contrary, about 86 % of settling materials were estimated to be allochthonous and most of this external load was discharged during the thaw . Estimated mass fluxes of organic C, N, P, Fe and Mn were 133; 17; 3; 10; 3 and 113; 12; 11; 370; 13 g/m(2) yr, respectively in lakes Pusiano and Mez zola and the corresponding sedimentation rates were 1300 and 11400 g/m(2) y r. The flux of inorganic carbon was 90 g/m(2) yr in lake Pusiano while it w as practically absent in Mezzola. Redfield type ratios in settling particle s agreed well with the POC/PN ratio expected for biogenic matter while they showed deviations in the case of POC/PP and PN/PP ratios. These were proba bly due to the overlapping of the sorption/desorption processes to biologic al binding in the case of lake Pusiano and to the dominance of allochthonou s particles in the Mezzola.