Abstract. Long-term effects of climate change on lakes globally will include a substantial modification in the thermal regime and the oxygen solubility of lakes, resulting in the alteration of ecosystem processes, habitats, and concentrations of critical substances. Recent efforts have led to the development of long-term model projections of climate change effects on lake thermal regimes and oxygen solubility. However, such projections are hardly ever confronted with observations extending over multiple decades. Furthermore, global-scale forcing parameters in lake models present several limitations, such as the need of significant downscaling. In this study, the effects of climate change on thermal regime and oxygen solubility were analyzed in the four largest French peri-alpine lakes over 1850–2100. We tested several one-dimensional (1D) lake models' robustness for long-term variations based on up to 63 years of limnological data collected by the French Observatory of LAkes (OLA). Here, we evaluate the possibility of forcing mechanistic models by following the long-term evolution of shortwave radiation and air temperature while providing realistic seasonal trends for the other variables for which local-scale downscaling often lacks accuracy. Based on this approach, MyLake, forced by air temperatures and shortwave radiations, predicted accurately the variations in the lake thermal regime over the last 4 to 6 decades, with RMSE < 1.95 ∘C. Over the previous 3 decades, water temperatures have increased by 0.46 ∘C per decade (±0.02 ∘C) in the epilimnion and 0.33 ∘C per decade (±0.06 ∘C) in the hypolimnion. Concomitantly and due to thermal change, O2 solubility has decreased by −0.104 mg L−1 per decade (±0.005 mg L−1) and −0.096 mg L−1 per decade (±0.011 mg L−1) in the epilimnion and hypolimnion, respectively. Based on the shared socio-economic pathway SSP370 of the Intergovernmental Panel on Climate Change (IPCC), peri-alpine 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. Together, these results highlight a critical alteration in lake thermal and oxygen conditions in the coming decades, and a need for a better integration of long-term lake observatories data and lake models to anticipate climate effects on lake thermal regimes and habitats.

&lt;p&gt;Lake systems are facing long-term (&gt;150 years) changes around the world acting on multi-decadal to centennial scales. Historic temperature warming at global scales, projected to continue by the end of the century, acting concomitant with eutrophication has modified ecosystem functioning in complex ways. Process-based lake models have emerged as powerful tools to assess the effects of climate and human activities on ecosystems, as well as the responses under future scenarios since they take into account the processes in the boundaries lake-catchment and lake-atmosphere. Most of these models are constrained by short-term monitoring limnological records, traditionally ranging from days to a few decades, potentially limiting&amp;#160;the robustness of long-term reconstructions. The integration of lake modeling and paleolimnological records can overcome the short-term monitoring data temporal scale, thereby providing a long-term perspective on lake ecosystem dynamics related to climate variability and human pressures. The present study develops a methodological framework using paleolimnological records from well-dated lake sediment records to constrain, validate and model temporal changes in water quality over a period of 250 years (1850&amp;#8211;2100). Lake Geneva (France, Switzerland) was selected as a case study in face of its similarity with other peri-alpine lakes and its representativeness as it is one of the most studied and well-known lentic ecosystems in the world. The 1D hydrodynamic-biogeochemical GLM-AED2 model was applied to simulate dissolved oxygen, nutrients, and chlorophyll-a concentrations along the water column. Pluri-decadal series of limnological data monthly collected by the French Observatoire des LAcs&amp;#160;(OLA database) were used to calibrate and validate the model. In addition, model outputs were further validated with published paleolimnological records for the past 170 years. Preliminary results of the calibration procedure show that the GLM-AED2 model accurately predicts the magnitude and seasonal dynamics of the state variables with goodness-of-fit metrics under the literature range (&lt;em&gt;e.g.&lt;/em&gt; RMSE = 0.96 mg L&lt;sup&gt;&amp;#8211;1&lt;/sup&gt; and RRMSE = 25% for dissolved oxygen; RMSE = 6.53 ug L&lt;sup&gt;&amp;#8211;1&lt;/sup&gt; and RRMSE = 37% for chlorophyll-a, both in the epilimnion). The integration of a one-dimensional lake model, paleolimnological records, and in situ measurements supports a better understanding of the historical dynamics and provides more robust long-term hindcast/forecast simulations to elucidate the impacts of climate change and critical implications for lake management and planning.&lt;/p&gt;

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.