Seasonality of Group I alkenone production and in-situ UK37-Temperature calibrations for mid-latitude Swiss lakes

Climate models are based on our understanding of the Earth climate system. To improve their accuracy, we rely on regional paleotemperature reconstructions, that can also be used to validate model outputs. Many existing paleoclimate proxies reconstruct mean annual or summer temperatures. Spring, which is essential for biodiversity and agricultural activities, is underrepresented, resulting in an incomplete perception of past climate, especially regarding seasonal variations. Lacustrine alkenones are lipid biomarkers that appear as a promising paleothermometer to quantitatively record spring temperatures in freshwater lakes. They are long chain ketones (35 to 42 carbons) with a varying number of double bonds (2 to 4) produced exclusively by the haptophyte phytoplankton in the Order Isochrysidales. These algae were found to respond to water temperature changes by altering relative proportions of the produced alkenones. The UK37 index has been extensively used to reconstruct sea surface temperatures in the past. Alkenone producing algae are divided into three major phylogenetic groups shaped largely by salinity. Among these groups, Group I dominate freshwater lakes, making them potential powerful tools for reconstructing continental spring temperatures. Alkenone seasonality was resolved in several studies conducted in high-latitude lakes which found alkenones occurring at the ice-off (spring-summer). The question arises regarding which seasonal temperatures are recorded by the UK37 index in mid latitude lakes? When other proxies show great uncertainty, three robust in-situ calibrations with low uncertainties were developed for the correlation of UK37 values to temperatures in high-latitude lakes. However, so far, no monitoring studies have been conducted in mid-latitude lakes, and no in-situ calibration has been established. Therefore, we conducted high-frequency monitoring of alkenone production in two Swiss lakes with very distinct settings for comparison: Greifensee (453masl, lowland and not ice-covered), and Lake St. Moritz (1768masl, alpine and ice-covered). The monitoring consisted in taking water samples over a full year on Greifensee, and from spring to summer in Lake St. Moritz, and retrieving sediment traps regularly to study alkenone deposition into the sediments in parallel with production in the water. We also collected environmental data such as salinity, nutrient contents, chlorophyll concentrations, temperature, and light radiation. We describe the seasonality of alkenone production in these two lakes and draw the first calibrations between the UK37 and temperature. The timing of alkenone production will be compared with variations in the environmental parameters to estimate the bloom drivers. With this work, we aim to establish a reliable continental spring temperature proxy.