Charophyte population dynamics during the Late Quaternary at Lake Bibersee, Switzerland

Oospores of 11 charophyte species were found in the Late Quaternary gyttja deposits of take Bibersee near the town of Zug. Except for the Boreal period rarely more than three different species were growing simultaneously during the Holocene. This compares well to typical Characeae lakes of the same size today, and shows that considerable changes in the hydrophyte diversity have taken place through time. During the first part of the Holocene the lake was oligotrophic and the species composition was mainly regulated by lake-level fluctuations due to climatic factors. During the younger periods of the Holocene the lake became mesotrophic, and pH values were for a longer period slightly alkaline. During the Early to Middle Bronze Age (1900-1400 BC) the considerable alterations in the hydrophyte composition and the simultaneous extensive prehistoric agriculture on the fertile shores of the lake point to human impact as the primary cause for changes in charophyte diversity.