Recent trends in chemistry and mass budget of a high altitude lake in the southern Alps (Laghetto Inferiore, Canton Ticino, Switzerland)

摘要

Since 1980, surveys have been performed every three years on about 50 small high-altitude Alpine lakes located in the Swiss part of the Lago Maggiore watershed (Southern Central Alps). Since 1996 one of these lakes, Laghetto Inferiore (LI) has been included in the Mountain Lakes Research (MOLAR) program, an extensive European co-operative research project with 23 partners. Laghetto Inferiore, situated at 2074 m a.s.l., has a watershed mainly composed of crystalline silicic rocks, a maximum depth of 33 m and a theoretical renewal time of 41 days. The watershed includes a second lake, Laghetto Superiore (LS), located at 2128 m a.s.l. The surface water pH of LI is around 6.6, while alkalinity is about 30 μeq l-1, increasing to 90 μeq l-1 at the maximum depth. Calculated inputs from atmospheric deposition and the main tributary streams were compared with changes in the lake concentration, and outflow fluxes on a monthly basis. Weathering ranged between 90-130 meq m-2 of alkalinity using a mass budget approach, of which 50% was consumed by the actual (H+) and potential (NH4 +) atmospheric acidity. Chemical trends over the period 1985-1998 show an increase in alkalinity and a decrease in sulphate, due to a reduction in the deposition of atmospheric acidity, and a decrease in nitrate, probably because of increased lake productivity.