Département Eaux de surface

Over the past millennium, the tropical Pacific Islands of Remote Oceania have experienced significant transformations caused by different waves of human settlement and climatic variability. However, many sites remain to be explored for their archeological potential, and the complex climatic setting of the tropical Pacific further complicates our understanding of past environmental and societal changes. In this study, we applied a multi-proxy approach to sediment cores extracted from ponds on the west coast of Espiritu Santo, Vanuatu to investigate past human-climate-environment dynamics. Through the analysis of human-associated proxies including fecal markers, palmitone (a specific lipid biomarker for taro), crop pollen and sedimentary charcoal, we inferred changes in human presence and activities. We reconstructed past hydroclimate from leaf wax hydrogen isotopes (δ²H_LW) and past temperatures from branched glycerol dialkyl glycerol tetraethers (brGDGTs). Changes in pollen reveal major shifts in local and regional vegetation. In our record, the period from 1000 to 1300 CE was characterized by warm/wet conditions concomitant with demographic expansion inland. Around 1400 CE, pollen, leaf wax distributions, and δ²H_LW data indicated a drier period. The coincident decrease in palmitone, despite high charcoal and fecal marker concentrations, suggested that drier conditions might have rapidly restricted taro cultivation, but not the overall population, which declined more than a century later. We hypothesize that the establishment of one of the earliest European settlements in Oceania in 1606 CE further disrupted local demographics with the introduction of epidemic diseases. This study contributes to our understanding of the intricate relationship between human activities, climatic fluctuations, and landscape modifications in Remote Oceania over the past millennium.

Lake Cadagno is a meromictic alpine lake characterized by permanent stratification, which creates a permanent anoxic environment that supports the growth of anoxygenic phototrophic sulfur bacteria. The seasonality and interseasonality of these microorganisms were examined over a three-year period (2019–2021) through regular monitoring of the water column. A variety of physical–chemical parameters, including temperature, conductivity, light, oxygen and sulfide concentrations, and the community composition of anoxygenic phototrophic sulfur bacteria in the chemocline were recorded, to observe potential influence of external weather conditions. Our findings indicate that, despite the lake’s consistent physical and chemical stratification, the composition of the anoxygenic phototrophic sulfur bacteria community exhibited notable variations in response to external environmental factors, including changes in rainfall and light irradiance. Specifically, we observed different growth dynamics in the purple (PSB) and green (GSB) sulfur bacteria communities over the three years of monitoring. These variations underscore the complexity of biogeochemical cycles in meromictic lakes and the impact of external environmental factors on this ancestral microbial community dynamics. The results provide valuable insights into the stability of redox-stratified environments, offering a modern analog for ancient aquatic ecosystems. This research emphasizes the importance of long-term regular monitoring to capture interannual dynamics and assess the implications of climate change on such unique ecosystems.

Projected increases in drought frequency and strength in Central Europe in the next two decades due to anthropogenic climate change pose challenges for European temperate forests. Understanding the correlation between drought stress, local conditions, and forest responses is crucial for effective forest management and climate mitigation measures. We examine how local water dynamics determine the response of Swiss forests during the European drought in 2018. We particularly investigate how increased atmospheric water demand, reduced soil water availability, and increased exposure of forests to potentially harsh abiotic conditions at the edge of the forest affect forest health. We used Sentinel-2 data to calculate the normalized difference water index (NDWI) as proxy for forest health. Weather data, data from a process based hydrological model, a digital elevation model, and airborne LiDAR data were used to assess hydrological drivers. Our analysis revealed that forest exposure and water availability were more important than atmospheric water demand in explaining forest drought resistance. Regions with more limited water availability (47 % of Switzerland) had systematically higher proportions of forest areas that exhibited weak drought resistance (R² = 0.56 for moderate NDWI decrease and 0.55 for severe NDWI decrease). Forest exposure (i.e. the degree to which a forest patch stands out from the surroundings) could best explain weak drought resistance, with strong statistical relationships (R² = 0.69, R² = 0.50). Finally, atmospheric water demand had only a moderate effect on weak drought resistance (R² = 0.45 and 0.27). Our findings highlight the complex interplay of local water dynamics and forest responses to drought, while providing insights on how forest structure and exposure conditions at local scales affect responses and need to be considered when examining forest health under changing climatic conditions.