Impacts of warm winters and extreme rainstorms on the base consumption in a limed lake, southern Norway

The chemical composition of a limed lake, the two main inlets and the outlet was monitored during a period of 3 years. The winters of 1991–1992 and 1992–1993 were unusually warm while the winter of 1993–1994 was more normal. The lake surface water was wind exposed in the warm winters and as a consequence of frequent turnovers the acid input from the catchment mixed with the whole lake water body. In the winter of 1993–1994, the lake was ice-covered for approximately 4 months. During this period the drainage water from the catchment flowed to the outlet of the lake in the upper 2–3 m of the water column and only some of the acid input was neutralised. This is compared to a complete neutralisation in the winter of 1992–1993. The in-lake loss of alkalinity during this warm winter was approximately 29 μeq/l (November–June) compared to approximately 7 μeq/l lakewater in 1993–1994. Acid drainage from the catchment induced by an extraordinary rainstorm with heavy sea-salt deposition contributed to the in-lake alkalinity consumption in spring 1993. As winter temperatures above 0 °C and more frequent rainstorms may be common due to expected global warming, future increased lime consumption in-lakes may be projected in acidified areas as southern Norway.