A TEMPORAL PERSPECTIVE OF LAKE ACIDIFICATION IN SWEDEN

pH trends from the last deglaciation to present day are assessed for a cid clear-water lakes in South Sweden in relation to natural developme nt, land use, and acid deposition. A general model for pH development is presented, based on diatom analysis of sediment cores from about 20 lakes, providing, in part, a new view of acidification history. Four periods are identified: (I) the natural long-term acidification period (12000 B.P.-2300 B.P. or later), with gradually decreasing pH values from about 7 to 5.5 resulting from soil acidification and declining fl uxes of base cations from catchment soils; (II) the anthropogenic alka lization period (2300B.P.-1900A.D.), when pH increased to above 6 due to land use; (III) the recent acidification period (about 1900 A.D.-pr esent), when pH decreased to about 4.5 due to acid deposition and poss ibly altered land use; and (IV) the liming period (1970s - present), w hen pH often increases to values above 7 following liming to counterac t acidification. The implications of past pH changes are discussed in the context of contemporary lake acidification and liming policy.