Thermal, hydrological and isotopic modelling of lake systems : sensitivity to past and futur climate changes and improvement of palaeoclimatic reconstructions

Paleoclimatic archives preserved in lake sediments are commonly used in mid-latitudes to reconstruct past climate changes. Moreover, fresh-water lakes play an important role in regional economy because they offer tourism activities and fresh water and food resources.<br />In this thesis, we wanted to develop a simple conceptual model, including few lake-specific parameters. This model was calibrated and validated for two european lakes (Lac d'Annecy, France, and Ammersee, Germany) representative for the majority of deep and cold lakes in mid-latitudes. The model includes the representation of thermal processes (temperature of the water column and mixing behaviour) at a daily time step, the representation of the hydrologic and isotopic (δ18O) budget of the lake and of its watershed. During the thesis, recent hydrologic and isotopic observations have also been done for the calibration. The main processes are correctly reproduced at the seasonal time scale (steady-state simulation) and inter-annual time scale (transient simulation during the 20th century). <br />Using the model, sensitivity studies have been done to quantify uncertainties in reconstruction of paleoclimate, based on calcitic valves of benthic ostracods (Candona sp.) Due to (i) thermal behaviour of lakes, (ii) change in precipitation seasonality and (iii) watershed deforestation.<br />The model was also forced with the predictions of a climate model run under IPCC SERS-B2 scenario for the 21th century. These transient simulations predict a dramatic impact on the temperature and the circulation for both lakes. The deep water of Lac d'Annecy will probably be 2°c warmer during the 21th century which will deeply affect the cold-water fauna. Ammersee will be affected by an irreversible lack of ventilation after 2060.