A PRE-INDUSTRIAL ACIDIFICATION EVENT SHOWN BY DIATOM ANALYSIS OF A RECENTLY ACIDIFIED LAKE IN SOUTHERN NORWAY

Abstract

The pre-iridustrial pH history of a lake in an area of southern Norway which presently has heavy acid precipitation is elucidated on the basis of an 800-year record of its diatom assemblage. The pH history is interpreted in terms of the pH-tolerance of the recorded assemblages. An acidification event appears to have started around AD 1350, and to have lasted for a maximum of 300 years, as indicated by the marked increase in the relative abundance of Asterionella ralfsii W. Sm. This event coincided with the abandonment of farms in the catchment area and re-establishment of coniferous forest following the Black Death, as shown by pollen analysis. Even in the unlikely case that A. ralfsii be considered acidophilus, the pre-industrial event is evident. A. ralfsii disappeared about AD 1700 after a consistent decline from about AD 1600. This is consistent with the re-establishment of farms and deforestation of the area. The lake supported a rich fish population until about 1950, when the fish were eradicated by the recent acidification event; however, the name of the lake, Tveitå Fiskeløstjern, indicates that it was devoid of fish in the Middle Ages. The recent acidification event is indicated by Semiorbis hemicyclus (Ehrenb.) Patr. and Tabellaria binalis (Ehrenb.) Grun., possibly a combined effect from vegetational changes and atmospheric acid deposition.