The palaeolimnology of Soppensee (Central Switzerland), as evidenced by diatom, pollen, and fossil-pigment analyses

The development of Soppensee (Central Switzerland, 596 m a.s.l.) has been r econstructed using algal remains such as diatoms, chlorophytes and fossil p igments, as well as the pollen and spores of macrophytes. Sediment accumula tion in Soppensee began at the end of the last glacial period, approximatel y 15,000 yrs ago. During the Oldest Dryas biozone (> 12,700 radiocarbon yrs B.P.) the lake had low primary productivity. After reforestation with birc h and later pine, around 12,700 B.P., phases of summer anoxia occurred in t he lake. These anoxic conditions were most probably caused by additional ca rbon input from the catchment, as well as longer phases of stratification d ue to reduced wind exposure caused by the sheltering effect of increased tr ee cover. From the Younger Dryas biozone(10,800 to 10,000 radiocarbon yrs B .P.) onwards, Soppensee became meromictic for several millennia. The fossil diatom assemblages are dominated by planktonic alkaliphilous to alkalibiontic species with mainly meso- to eutrophic preferences. Diatom-in ferred total phosphorus reconstructions suggest meso- to eutrophic conditio ns throughout the Holocene. Eutrophic conditions are also suggested by the presence of pigments of cyanobacteria, including Oscillatoria species. Firs t human activity in the catchment is evidenced ca. 5000 radiocarbon yrs B.P . by the occurrence of cereal pollen. Diatom-inferred total phosphorus conc entrations also increased slightly during the Neolithic period. According to the fossil pigment record, meromictic conditions ended during the Iron Age. Deep-water anoxia, however, persisted at least during the str atification period. During the Middle Ages massive deforestation in the cat chment and around the lake changed the limnological conditions drastically. The lack of forest increased the wind fetch and, therefore, also the mixin g of the lake, while soil erosion and retting of hemp supplied additional n utrients. Because of intensive agriculture in its catchment, Soppensee has become hypertrophic and diatom assemblages have consequently changed comple tely in the last 50 yrs.