Diatoms and their preservation in the sediments of Lake Neuchâtel (Switzerland) as evidence of past hydrological changes

Abstract

Core LNO3 is 1136 cm long and includes uninterrupted sediments from the Oldest Dryas to historic time which have been submitted for geological, geochemical, palynological and diatom analysis. Thirty samples have been investigated for diatoms, focussing especially on their preservation, which changed during major hydrological events. In the lower part of the core (Oldest Dryas to beginning of Atlantic), with low carbonate, but high clastic silicate content, the diatoms are rare but are well preserved. In the upper well carbonated part of the core (Atlantic to historical periods), the diatoms are abundant but show traces of dissolution.

Between 1136 and 909 cm (Oldest Dryas) the flora is made up of benthic species principally of the genus Fragilaria. This flora is often the first to develop after the last glaciation. Between 879 and 569 cm (Bolling to beginning of Atlantic), littoral and terrestrial species adapted to eutrophic conditions are dominating. They suggest a strong fluvial impact on sedimentation, which corroborates the hypothesis of past inflows of the River Aar into the lake. Between 529 cm and 1 cm (Atlantic to historical periods) planktonic species of the genera Cyclotella, Aulacoseira or Stephanodiscus are dominating. This suggests that the upper well carbonated sediments were mainly formed by pelagic precipitation, without influence of the River Aar. The trophic level of the lake seems to have decreased, then increased again during prehistoric deforestation and development of agriculture in the watershed.