Historical modelling of changes in Lake Erken thermal conditions

Historical lake water temperature records are a valuable source of information to assess the influence of climate change on lake thermal structure. However, in most cases such records span a short period of time and/or are incomplete, providing a less credible assessment of change. In this study, the hydrodynamic GOTM (General Ocean Turbulence Model, a hydrodynamic model configured in lake mode) was used to reconstruct daily profiles of water temperature in Lake Erken (Sweden) over the period 1961-2017 using seven climatic parameters as forcing data: wind speed (WS), air temperature (Air T), atmospheric pressure (Air P), relative humidity (RH), cloud cover (CC), precipitation (DP), and shortwave radiation (SWR). The model was calibrated against observed water temperature data collected during the study interval, and the calibrated model revealed a good match between modelled and observed temperature (RMSE = 1.089 degrees C). From the long-term simulations of water temperature, this study focused on detecting possible trends in water temperature over the entire study interval 1961-2017 and in the sub-intervals 1961-1988 and 1989-2017, since an abrupt change in air temperature was detected in 1988. The analysis of the simulated temperature showed that epilimnetic temperature increased on average by 0.444 and 0.792 degrees C per decade in spring and autumn in the sub-interval 1989-2017 Summer epilimnetic temperature increased by 0.351 degrees C per decade over the entire interval 1961-2017. Hypolimnetic temperature increased significantly in spring over the entire interval 1961-2017, by 0.148 and by 0.816 degrees C per decade in autumn in the subinterval 1989-2016. Whole-lake temperature showed a significant increasing trend in the sub-interval 1989-2017 during spring (0.404 degrees C per decade) and autumn (0.789 degrees C per decade, interval 1989-2016), while a significant trend was detected in summer over the entire study interval 1961-2017 (0.239 degrees C per decade). Moreover, this study showed that that changes in the phenology of thermal stratification have occurred over the 57-year period of study. Since 1961, the stability of stratification (Schmidt stability) has increased by 5.365 J M-2 per decade. The duration of thermal stratification has increased by 7.297 d per decade, corresponding to an earlier onset of stratification of similar to 16 d and to a delay of stratification termination of similar to 26 d. The average thermocline depth during stratification became shallower by similar to 1.345 m, and surface-bottom temperature difference increased over time by 0.249 degrees C per decade. The creation of a daily time step water temperature dataset not only provided evidence of changes in Erken thermal structure over the last decades, but is also a valuable resource of information that can help in future research on the ecology of Lake Erken. The use of readily available meteorological data to reconstruct Lake Erken's past water temperature is shown to be a useful method to evaluate long-term changes in lake thermal structure, and it is a method that can be extended to other lakes