Increasing prevalence of warm monomictic lakes in France over six decades under climate change

This study utilized temperature simulations from the Ottosson-Kettle-Prats Lake Model and a modified Lewis classification to analyze temporal changes in mixing-regime dynamics, temperature, density gradients, and meteorological indicators in 170 French lakes between 1959 and 1988 and 1989-2019. In the initial period, 55%, 30% and 15% of lakes were classified as warm monomictic, polymictic and dimictic respectively, with 57% of lakes experiencing anomalous regimes. Notably, 6% of the lakes, all at low altitudes (< 800 m a.s.l.) and located in central and north-eastern France shifted from dimictic to warm monomictic between the two periods, representing 77% of all low-altitude dimictic lakes. In 1989-2019, these lakes experienced a warmer climate with annual air temperatures increasing by an average of 0.19 °C/dec across all lakes, with an annual epilimnion warming averaging 0.25 °C/dec compared to 0.2 °C/dec for non-shifting lakes. Additionally, they became more stable and had the greatest density gradient change, averaging 0.07 kg/m³ between 1989 and 2019 and 1959-1988. In contrast, high-altitude lakes remained dimictic, showing an annual epilimnion warming averaging 0.19 °C/dec and a particular winter solar radiation decrease averaging -6.96 W/m2/dec in 1989-2019. Further, they did not show significant changes in average density gradients between the two periods. Our findings provide new insights into the mixing-regime dynamics of French lakes over the past six decades. This research is crucial for understanding the ecological impacts of physical alterations and for guiding management strategies under climate change. In the future, we expect less mixing in dimictic lakes, especially those at low altitudes.