PREHISTORIC INCREASES IN THE PH OF ACID-SENSITIVE SWEDISH LAKES CAUSED BY LAND-USE CHANGES

IN the past few decades, there has been considerable debate about whet her vegetational and soil changes associated with changing land-use ca n cause surface-water acidification in lakes1,2. Although it is now wi dely accepted that the severe and extensive acidification that has occ urred recently in southern Scandinavia, northern Britain and North Ame rica3,4 has been chiefly caused by atmospheric acid deposition, the ro le of changing land-use for moderate acidification is still not fully understood5,6. Here we report analyses of sediment records from Swedis h lakes, which provide evidence that land-use changes can have an impo rtant influence on the pH of acid-sensitive lakes. Following the expan sion of an agrarian economy during the Iron Age (from about 2,500 year s ago), pH increased from about 5.5 to about 6.5. Our results suggest that this pH increase was caused by burning, agriculture, forest grazi ng and other culture-related practices that increased the base saturat ion and pH of the soils, and enhanced the transport of base cations an d nutrients from the soils to the surface waters, thus indicating thes e 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 r ecent years must be due predominantly to acid deposition.