SEDIMENT-CHEMISTRY RESPONSE TO LAND-USE CHANGE AND POLLUTANT LOADING IN A HYPERTROPHIC LAKE, SOUTHERN SWEDEN

Responses to recent land-use changes and pollutant loading in the sedi ment of a hypertrophic lake in southern Sweden were studied by compari son of geochemical, pollen and magnetic records with historical land-u se data. A chronology was constructed for the last two centuries by co rrelating changes in the pollen diagram to major events in the land-us e history. Sediment accumulation was low (mean c. 0.2 g cm(-2) yr(-1)) prior to 1800 AD, when less than 25% of the catchment was arable land . Reorganization of the agrarian system during the 19th century increa sed the annually tilled area by 300%, which accelerated soil erosion a nd substantially increased the accumulation of allochtonous matter in the lake. Since the turn of the century 90% of the catchment has been ploughed every year. The deposition of elastic matter in the lake has, however, decreased due to a gradual rerouting of the drainage system, which has reduced the effective catchment area by c. 85%. Authigenic vivianite (Fe-3(PO4)(2).8H(2)O) is a major P phase in the preindustria l non-sulphidic sediments, which suggests that the sediments at that t ime served as a fairly efficient sink for P. The arable expansion, inc reased manuring and, eventually, the introduction of artificial fertil izers during the 19th century led to a massive influx of nutrients, wh ich elevated primary production in the lake. Subsequent development of bottom water anoxia around 1900, in combination with an additional po llutant burden of sulphate within the lake basin, led to major alterat ions of the biogeochemical cycles. The most critical change in the pos t-1900 sediments involved the cycling of Fe and P. The linkage between the lacustrine P and Fe cycles can explain that FeS formation was par alleled by a release of P from the sedimentary pool. This supply of P to the lake basin must have supplemented the nutrient supply by modern agriculture and contributed to recent hypertrophication. The bacteria l sulphate reduction also affected the generation of alkalinity which supported a significant calcite precipitation in the post-1900 sedimen ts. S is enriched 10-fold in the post-1900 sediments compared to prein dustrial values. Along with the rise in S, soot particles derived from fossil fuel combustion appear in the sediments for the first time. Th erefore, Bussjosjon is thought to be a good example of how a well-buff ered, highly productive lake may respond to the pollution by sulphur f rom acid rain.