Résumé
Alpine lakes in the Mediterranean region are experiencing rapid warming, posing a threat to aquatic ecosystem diversity and functioning. Documenting how high-altitude lake ecosystems have responded to past warming and cooling events is of interest to scale modern changes in a long-term context, and to inform management strategies. This paleoenvironmental study focuses on Lake Petit (2200 m a.s.l., Southern Alps) and combines the analysis of subfossil diatoms and inorganic and organic sediment composition over the last 14,000 years. The results show a major ecosystem shift at the Late Glacial-Holocene transition, ca. 12,000 years ago. In addition, the alternating dominance of diatoms from the Fragilariaceae (e.g., Pseudostaurosira pseudoconstruens, Staurosira venter) and Gomphonemaceae (e.g., Gomphonema elegantissimum), along with high-amplitude fluctuations in biogenic silica concentrations, highlight rapid ecological shifts at four distinct periods: 10,100–9800, 9200–9000, 8100–8000, and 7000–6900 cal yr BP. These shifts reveal the occurrence of four centennial-scale cooling events at the beginning of the Holocene. During the warmer Middle-Holocene period, diatom assemblages are dominated by Staurosirella neopinnata, ending with a shift coinciding with the 4.2 ka climate event, described in a previous study. A progressive return of S. neopinnata is observed nowadays. Our findings suggest that during the Early Holocene, the lake ecosystem was capable of buffering centennial-scale cooling events, consistently returning to previous conditions. However, the two phases of major long-lasting ecological shift that occurred 7000 and 4200 years ago, likely driven by catchment-related factors such as pedogenesis and vegetation dynamics, led to stepwise changes in ecosystem functioning baselines. This paper raises once again questions about the capacity of modern aquatic high-altitude lake ecosytems to recover from ongoing climate change when local baselines are no longer in equilibrium with natural variability.