Increasing water temperature of the largest freshwater lake on the Mediterranean islands as an indicator of global warming

Global warming has been monitored for many years. The increase in air temperature and changes in the distribution and frequency of high temperatures are recorded continually. Lakes are one of the important water resources for aquatic ecosystems and water supply, which are significantly affected by global warming. The increase in lake water temperature increases the evaporation from the free lake surface, lowering the lake level, and changes the water quality. In the last few decades, analysis of changes in lake water temperature has been increasing. In situ measurements of water temperature in Vrana Lake on Cres island (Croatia), the largest freshwater lake on the Mediterranean islands, were analysed over 43 years. The results showed that the mean annual lake surface water temperature (LSWT) increased by 0.47 °C decade-1 (p < 0.0001). The increase in the mean annual lake summer surface water temperature (July–September) was 0.44 °C decade-1 (p < 0.0001), and the maximum annual LSWT was 0.56 °C decade-1 (p < 0.0001). All these amounts are in accordance with the published data on the increase in water temperature in the investigated European lakes. The number of days with LSWT higher than 25 °C increased by almost 9 d decade-1. An increase in the minimum LSWT (0.17 °C decade-1) corresponding to isothermal conditions was also determined but was not statistically significant. The minimum mean monthly LSWT increased by 0.36 °C decade-1 (p < 0.0001). Because the increase in water temperature can negatively affect the lake's ecosystem, and become a threat to safe water supply; LSWT, thermal stratification and evaporation should be continuously monitored. The impacts of climate warming on the lake stratification and aquatic ecosystems need to be further investigated.