Legacy of early anthropogenic effects on recent lake eutrophication (Lake Bénit, northern French Alps)

Résumé En

Mountain lakes are integrated sentinels of changes in the terrestrial environment, where these changes threaten the quality of the ecosystem services these lakes provide, including high biodiversity, economic and leisure activities. Few evidentiary records exist of the long-term relationships between human pressure and observed impacts. Multiproxy analyses of the Lake Bénit sediment sequence, including dating, grain-size, geochemistry, pollen, non-pollen palynomorphs and chironomid assemblage reconstructions, allowed reconstruction of past environmental evolution and lake trophic changes. Combined with soil analyses of the catchment, these data provide a record of the relationships between human activities and the lake-catchment ecosystem, and show the effect of inundation of the shore previously used as pasture. From 2100 to 1100 yrs cal. BP, the catchment was forested. During the Middle Ages, grazing deforested the catchment, triggering an increase in erosion and a change in sediment sources. The lake remained oligotrophic over most of the last millennia. The trophic state changed abruptly in the 20th century with intensification and multiplication of tourist activities in the catchment, i.e., fishing, hiking, while pastoral activities decreased. The sudden eutrophication coincides with an artificial increase of the lake water level in AD 1964 to improve fishing activities. A release of phosphorus (P) from the flooded soils was observed, which may be responsible for the current eutrophication. One thousand years of grazing practices would have led to the observed P concentrations in the soils of the lake shore, transferred by the cattle to this area. Our study highlights the combined effects of past and recent activities on the current eutrophication process, and the legacy of both soils and early anthropogenic activities.