Monitoring of northern lakes in recent decades has revealed strong increases in water color and organic carbon concentrations, with important implications for the structure and function of freshwater systems. It has been argued that this browning of lake waters is a result of declining acid deposition, and that lakes were unnaturally clear during the period of high atmospheric sulfur deposition, since enhanced acidity in soils reduces organic matter mobility. However, this study presents historical water color data in 50 lakes for the last 80 years and shows that recovery from acidification is not sufficient to explain trends in lake water color for a majority of the lakes, and that browning may be more related to changes in land-use, from agriculture to modern forestry. The analysis is based on comparing long-term data of water color, acid deposition, water flow, spruce coverage and growing degree days.
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