Prehistoric increases in the pH of acid-sensitive Swedish lakes caused by land-use changes

Abstract

IN the past few decades, there has been considerable debate about whether vegetational and soil changes associated with changing land-use can cause surface-water acidification in lakes^1,2. Although it is now widely accepted that the severe and extensive acidification that has occurred recently in southern Scandinavia, northern Britain and North America^3,4 has been chiefly caused by atmospheric acid deposition, the role of changing land-use for moderate acidification is still not fully understood^5,6. Here we report analyses of sediment records from Swedish lakes, which provide evidence that land-use changes can have an important influence on the pH of acid-sensitive lakes. Following the expansion of an agrarian economy during the Iron Age (from about 2,500 years ago), pH increased from about 5.5 to about 6.5. Our results suggest that this pH increase was caused by burning, agriculture, forest grazing and other culture-related practices that increased the base saturation and pH of the soils, and enhanced the transport of base cations and nutrients from the soils to the surface waters, thus indicating these lakes may be naturally more acidic than had been thought. Following this period of alkalization, pH in many lakes fell to about 4.5 during the present century. Although cessation of former land-use practices could account for some of this change, the unprecedentedly low pH in recent years must be due predominantly to acid deposition.