oa Isochrysidales Bloom Dynamics and Alkenone Production in Swiss Lakes

Past temperature reconstructions are essential for studying climate variability and evaluating the accuracy of climate models. Alkenones are long-chain ketones produced by haptophytes in the order Isochrysidales that are well preserved in sediments. Their unsaturation level records water temperature and can therefore be used as paleothermometer to reconstruct past climate. Isochrysidales are divided in three phylogenetic groups. Their application has been validated in oceans, where exclusively Group III are found, and more recently in high-latitude lakes, where Group I and II are found. However, few studies have calibrated the use of alkenones as paleothermometers in mid-latitude lakes. This study focuses on the production of Group I alkenones in two Swiss lakes (Greifensee, 436masl and lake St. Moritz, 1786masl). The bloom of the Isochrysidales algae, expected in end of February for Greifensee and middle of April in St. Moritz, will be followed with high-resolution sampling of the water column (every 5m) for three weeks. The final aim is to first, characterize alkenone production in detail in these two lakes (timing, depth, temperature, light intensity, nutrient composition of the water, impact of ice coverage and trophic state) and second, develop in-situ calibrations of the response of the unsaturation index (UK37) to temperature.