During the last decade, an increasing number of studies have used lake sediment DNA to reconstruct past environmental changes and human activities. This emerging tool has great potential as it may allow the direct identification of all organisms present in the lake, the catchment and potentially the atmosphere. However, as other proxies used by “palaeo communities”, the potential of lake sediment DNA might be affected by biases related to both analytical and taphonomic (DNA source, transfer and deposit) processes. In order to ensure reliable interpretations of lake sediment DNA results, it is thus crucial that we understand these processes, which were a little studied until now.
Here we will present DNA analyses applied on several lakes in the French Alps and focused on plants and mammals. Our main results suggest that DNA (extracellular DNA) from the catchment area is more effectively archived in lakes with high detrital inputs, i.e. allochthonous sedimentation. The source of eroded material is also important. Our data suggest that the soil surface horizons are the main source of “catchment DNA” transferred to the lake bottom. However, lakes not dominated by allochthonous sedimentation provide interesting sedimentary context to study past dynamic of macrophytes.
For the Northern French Alps, we also used our data to trace past agro-ecosystems evolution. Especially, we show a first important development of pastoral activities during the late Iron Age and Roman Period, with sheep and cows at high altitude (around 2000-2500 m asl). A second important phase of pastoral activities is then recorded from 900 to 1800-1900 AD. On several lakes, we record a change of practices between the 12th and 14th century, with a specialisation toward herds mostly made of cows (i.e. without or with few sheep). In some cases, this change corresponds to enrichment in nutrient of terrestrial and aquatic ecosystems.