Past and future climate change effects on thermal regime and oxygen solubility of four peri-alpine lakes

Abstract. Climate change modifies the thermal regime and the oxygen solubility of lakes globally, resulting in the alteration of ecosystem processes, lake habitats and concentrations of key parameters. The use of one-dimensional (1D) lake model for global scale studies has become the standard in lake research to evaluate the effects of climate change. However, such approach requires global scale forcing parameters which have several limitations that are barely discussed, such as the need of serious downscaling. Furthermore, projections of lakes' thermal regime are hardly ever confronted with long-term observations that extent for more than a few decades. These shortfalls limit the robustness of hindcast/ forecast simulations on decadal to centennial timescales. In this study, several 1D lake models' robustness was tested for long-term variations based on 63 years of limnological data collected by the French Observatory of LAkes (OLA). Here we evaluate the possibility to force mechanistic models by following the long-term evolution of shortwave radiation and air temperature while providing realistic seasonal trend for the other parameters for which local scale downscaling often lacks accuracy. Then, the effects of climate change on the thermal regime and oxygen solubility were analyzed in the four-largest French peri-Alpine lakes. Our results show that 1D lake models forced by air temperatures and short-wave radiations accurately predict variations in lake thermal regime over the last four to six decades, with RMSE <1.95 °C. During the last three decades, water temperatures have increased by 0.46 °C decade^–1 (±0.02 °C) in the epilimnion and 0.33 °C decade^–1 (±0.06 °C) in the hypolimnion. Concomitantly and due to thermal change, O₂ solubility has decreased by -0.104 mg L^–1 decade^–1 (±0.005 mg L^–1) and -0.096 mg L^–1 decade^–1 (±0.011 mg L^–1) in the epilimnion and hypolimnion, respectively. Based on the ssp370 socio-economic pathway of the IPCC, perialpine lakes could face an increase of 3.80 °C (±0.20 °C) in the next 70 years, accompanied by a decline of 1.0 mg L^–1 (±0.1 mg L^–1) of O₂ solubility. These results suggest important degradation in lake thermal and oxygen conditions and a loss of habitats for endemic species.