Effect of Climate Change on Water Temperature and Stratification of a Small, Temperate, Karstic Lake (Lake Kozjak, Croatia)

Abstract As closed systems, lakes are extremely vulnerable to climate change. Understanding the response to climate change is crucial for effective management and conservation of the lakes and their associated ecosystems. This study focuses on Lake Kozjak, Croatia, a small lake belonging to the Plitvice Lakes system. This system represents a unique hydrogeological karstic phenomenon, closely dependent on a delicate biochemical balance necessary for tufa formation. We apply a simple one-dimensional model, SIMO v.1.0, to predict future water temperature in Lake Kozjak under three scenarios (RCP2.6, RCP4.5 and RCP8.5) from 2006 to 2100. The model was calibrated using measured water temperature profiles and meteorological data from a nearby station. In addition to analyzing the average temperatures of the epilimnion, hypolimnion and the whole lake, we also studied the surface and bottom layer temperatures and their relation to specific forcing parameters. The Schmidt stability index was used as a quantitative indicator to assess lake stability. The simulation results indicate average lake water temperature increase of 0.51, 1.41 and 4.51 °C (100 y) −1 for RCP2.6, RCP4.5 and RCP8.5, respectively. This increase in the water temperature is not accompanied by a substantial strengthening of stratification under RCP2.6 and RCP4.5 scenarios due to the temperature raise being present both in the epilimnion and hypolimnion. However, significant lengthening of the stratification period is observed even for the most stringent scenario, 16, 28 and 47 d (100 y) −1 for RCP2.6, RCP4.5 and RCP8.5, respectively. The predicted water temperature increase and prolonged stratification period may carry serious ecological and environmental implications. Highlights • Mean lake water temperature is projected to increase by 0.51 to 4.51 °C (100 y) −1 . • Baseline scenario surface temperature increase of 5.2 °C (100 y) −1 is predicted. • Stratification period is predicted to lengthen by 16 (RCP2.6) to 47 days (RCP8.5). • Substantial stratification strengthening is expected only under RCP8.5.