Mid- to late-Holocene land-use change and lake development at Dallund Sø, Denmark: Trends in lake primary production as reflected by algal and macrophyte remains

Diatom, macrofossil, pollen, Pediastrum and biogenic silica analyses were carried out on an 11-m sediment sequence from the Danish lake Dallund Sø, demonstrating major changes in the aquatic ecosystem over the last 7000 years. A diatom-phosphorus calibration model was applied to the fossil diatom record to reconstruct in-lake total phosphorus (TP) concentrations over this period. Prior to the introduction of agriculture to the region, c. 6000 years ago, the lake was relatively deep and had low diatom-inferred TP concentrations (c. 20 μg TP/L), with limited macrophyte growth. Moderate nutrient enrichment of the lake was inferred during the Bronze Age (1700-500 BC) and Iron Age (500 BC-AD 1050) periods and evidence for water-level lowering was observed. Marked eutrophication of the lake (reconstructed TP levels consistently > 100 μg/L) was associated with major changes in agriculture during the Mediaeval period (AD 1050-1536) and continued to the present day. These data document the long-term anthropogenic impact on Dallund Sø, a lake in an area with a long history of human activity.