Long-term changes and periodicity of ice phenomena in the high mountain Lake Morskie Oko (Tatra Mountains, Western Carpathians)

High montain lakes are regarded as sensitive indicators of environmental changes in local and global scales. In recent years, climate change has considerablely influenced the timing of ice phenomena in these lakes. The objective of the paper is an investigation of long-term changes and periodicity of ice phenomena in high mountain Lake Morskie Oko (1392.8 m a.s.l.) located in the Tatra Mts. in the period 1971–2020. The study employed the Lomb-Scargle periodogram, linear regression modelling for temporal trends, correlation analysis, parameters of variability, and flashiness index. Periodicity of the duration of ice phenomena (∼5.4 y) and ice cover on the lake (∼13.5 y) was observed, although both are statistically insignificant. Ice cover duration has been interrupted several times by winter thaws after 1996 whereas no such events had been reported earlier. The trend towards a delayed appearance of ice phenomena reaches 4.0 d·decade−1 (p < 0.001). The observed trend towards an earlier ice phenomena disappearance reaches 5.1 d·decade−1 (p < 0.001). It results in a decline of the number of days with ice phenomena on Lake Morskie Oko at a rate of 9.0 d·decade−1 (p < 0.001). The duration of ice cover on Lake Morskie Oko has been decreasing at a rate of 10.4 d·decade−1. It is attributed to later freeze-up (3.9 d·decade−1, p < 0.001) and earlier ice break-up (5.6 d·decade−1, p < 0.001). An increasing trend of average annual air temperature (0.4°C·decade−1, p < 0.001) in the period 1971–2020 is observed. A statistically significant correlation is recorded between ice phenomena and ice cover (beginning, end, duration), average annual air temperature, average air temperature of three summer months (June, July, August), and average air temperature of the six months period from June to November. Higher air temperature induces growing heat resources accumulated in the lake water in the summer and autumn seasons, potentially causing later initiation of ice phenomena in the lake. There is no statistically significant relationship between maximum ice thickness and any air temperature parameters. Variability of maximum ice thickness appears to have been significantly increasing in the second part of the investigated period (1996–2020).