Seasonal, annual and long-term variability in the water chemistry of a remote high mountain lake: Acid rain versus natural changes

Seasonal fluctuations as well as long-term trends in water chemistry were studied in Schwruzsee ob Solden (Tyrol, Austria), an oligotrophic softwater lake situated at 2796 m a.s.l. The catchement is composed of granite, plagioclase and micaschists containing considerable amounts of sulphur, with little soil cover. The lake is ice covered for about nine months, during this time the deepest layers (>16m) become anoxic. During summer overturn, alkalinity (ALK) is lowest (-8 mu eq l(-1)) in the whole water column, whereas pH reaches its minimum (4.88) at the surface during snowmelt. A decrease of pH from 5.8 to 5.4 during winter is caused by CO2 oversaturation, but deep water ALK increases to up to 130 mu eq l(-1) due to in-lake ALK generation by reductive processes and base cation (BC) release. The seasonal pattern of ALK in SOS is driven by in-lake processes in winter, the snowmelting in spring and watershed processes and precipitation during summer. Since 1989 summer sulfate concentrations in SOS, originating mainly from the catchment show a tendency to increase presumably caused by enhanced weathering. In contrast, SO42-. concentrations in other high mountain lakes which are dominated by atmospheric depositions show a decreasing trend. SOS is a good example for the complexity of interactions between catchment and in-lake processes which act at different time scales and depend on climate changes and atmospheric inputs.