# Trends of Changes in Minimum Lake Water Temperature in Poland
## 3 Results  
Minimum water temperatures in the analysed lakes show a course similar to that of average temperatures. The highest values of the minimum temperatures occur in June, and the lowest in January. In July, the average values and medians of minimum temperatures for the analysed lakes reach 17.9 \({}^{\circ}\)C. The range of changes in minimum temperatures in July is from 10.6 to 24.1 \({}^{\circ}\)C. In January, the average values and medians of minimum temperatures for the analysed lakes are 0.6 and 0.4 \({}^{\circ}\)C, respectively. Minimum temperatures in the analysed lakes in January varied in a range from 0 to 3.1 \({}^{\circ}\)C. The highest amplitudes of minimum water temperatures occur in the analysed lakes in May (approximately 9 \({}^{\circ}\)C), and the lowest in January--approximately 2.4 \({}^{\circ}\)C. The course of the minimum temperatures in the analysed lakes is presented in Figure 2. The annual amplitudes of water temperature fluctuations in lakes were at a level from 19.7 \({}^{\circ}\)C in Lake Gardno to 24.1 \({}^{\circ}\)C in Lake Studzieniczne. The highest amplitudes of minimum monthly water temperature fluctuations occurred in May (in Lakes Slawskie, Gardno, Bachotek, Jeziorak, Studzieniczne), June (Charzykowskie, Hancza), July (Sepoleriskie, Selmet Wielki), and October (Mikolajskie). The lowest amplitudes usually occurred in January, and in Lakes Mikolajskie, Selmet Wielki, and Hancza in February. The cluster analysis allowed for the designation of two clusters of months in terms of the course of minimum water temperatures: the first cluster includes months from November to April, and the second one from June to September (Figure 3a). Based on minimum monthly water temperatures, lakes can also be divided into two clusters. The first one includes 8 lakes. Two subclusters can be designated within this cluster, each covering four lakes. The first subcluster includes Lakes Bachotek, Slawskie, Sepoleriskie, and Jeziorak. They are lakes located in the central and western part of Poland. The second subcluster includes Lakes Studzieniczne, Selmet Wielki, Charzykowskie, and Mikolajskie located in the east and central part of Poland. The second cluster includes two Lakes: Gardno and Hancza (Figure 3b). The lakes, however, differ from each other in terms of minimum water temperatures. This results from their location, but also their type. Figure 2: Fluctuations of monthly minimum water temperature in lakes: Slawskie (**1**), Sepolarńska (**2**), Charzykowskiie (**3**), Gardno (**4**), Bachotek (**5**), Jeziorak (**6**), Mikolajskie (**7**), Selmet Wielki (**8**), Studzieniczne (**9**) and Harôza (**10**). The analysis of minimum water temperatures in the lakes in the period 1972-2021 showed an increasing trend in 5 of them. The greatest fluctuations of minimum water temperatures occurred in Lake Selmet Wielki (0.15 \({}^{\circ}\)C per decade) and in Lakes Slawskie and Charzykowskie (0.13 \({}^{\circ}\)C per decade). Moreover, considerable fluctuations also occurred in Lakes Bachotek and Hancza (Figure 4). In the remaining five lakes, the changes were not statistically significant. Particularly high temperatures in the background of the analysed multiannual period were recorded in 2020. This is associated with the lack of ice cover in the winter season. Data analysis by means of the Wild Binary Segmentation 2 method showed the occurrence of numerous change-points in the data set of minimum annual water temperatures in Lakes Charzykowskie, Gardno, Studzieniczne, and Hancza. This primarily results from the specifics of the determination of minimum annual values that usually occurred in January, but also in December and February. The analysis of values of minimum temperatures T90% read from the curve of durations of water temperatures showed an increasing trend, except for Lakes Gardno and Jeziorak. Increasing trends of minimum water temperatures were increasing at a significance level of 0.01, and for Lake Sepolenskie they were significant at a level of 0.05.