Long-term ecological trajectories of alpine lakes since 13 500 cal. BP in the Mediterranean Alps (Mercantour national park, France) : between climate forcing and plurimillenial human pressure

Diatom (SEM)

The thesis is devoted to the reconstruction of long-term changes in two lacustrine ecosystems in the Southern Alps. This study aims at assessing (1) the lateglacial-holocene variability of water physico-chemistry, lake levels and aquatic species dynamics ; and (2) the environmental responses of the lacustrine ecosystem to external forcing (watershed and climate changes), with the study of Lake Allos (2200 m a.s.l in the Southern Alps, Mercantour Massif, 42 meters deep) and the Lake Petit (2200 m a.s.l in the Southern Alps, Mercantour Massif, 7 meters deep). The two sedimentary cored profiles provided a detailed record of the past dynamics of the lacustrine ecosystems since 13.500 cal. BP.

An integrative approach was carried out including (1) the study of fossils of aquatic bioindicators species (diatoms and ostracods) informing about changes in lacustrine living conditions ; (2) the analysis of oxygen isotopes recorded by these aquatic fossils tracing a palaeohydrological signal ; (3) a multiproxy comparison linking the aquatic environment with changes in watershed dynamics.

These results integrated into a rich multidisciplinary framework evidenced the major role of erosion processes and changes in vegetation cover as factors triggering lake responses to disturbance, with concomitant changes in aquatic communities and trophic levels at several timescales. The different geological settings and lake morphologies have also played a significant role, modulating changes in benthic and planktonic aquatic communities over long time periods marked by erosion of carbonates, silicates or pH changes. Finally, oxygen isotopes records allow to trace for the first time palaeohydrological changes in the region : this innovative approach represents an original outcome for reconstructing a reference past climate for the Southern Alps.