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The paper evaluates changes in the water temperature of the Vistula River – one of the longest rivers in Europe. Mean monthly and annual water temperatures from the period 1971–2017 for 11 stations along the entire length of the river revealed the increasing trends. The mean increase in water temperature in the analysed multi-annual period was 0.31 °C dec–1. In the majority of analysed stations, the key factor determining changes in the water temperature of the river was air temperature. The observed water warming in the Vistula River should be considered an exceptionally unfavourable situation in the context of importance of water temperature for a number of processes and phenomena occurring in river ecosystems. Given the scale of changes, fast measures should be undertaken to slow down the warming.

Lake Śniardwy is the largest among more than 7000 Polish lakes. So far, it has not been a subject of detailed investigations concerning long-term changes in water temperature or ice regime. A considerable change in thermal and ice conditions has been observed in the period 1972–2019. Mean annual water temperature increased by 0.44°C dec−1 on average, and was higher than an increase in air temperature (0.33°C dec−1). In the monthly cycle, the most dynamic changes occurred in April (0.77°C dec−1). In the case of ice cover, it appeared increasingly later (5.3 days dec−1), and disappeared earlier (3.0 days dec−1). The thickness of ice cover also decreased (2.4 cm dec−1). Statistical analysis by means of a Pettitt test showed that the critical moment for the transformations of the thermal and ice regime was the end of the 1980’s. In addition to the obvious relations with air temperature for both characteristics, it was evidenced that the occurrence of ice cover depended on wind speed and snow cover. The recorded changes in the case of Lake Śniardwy are considered unfavourable, and their consequences will affect the course of physical, chemical, and biological processes in the largest lake in Poland.

The paper presents the dynamics of daily water temperature fluctuations in Lake Kierskie (West Poland) and determines the effect of air temperatures and wind speed on the variability of the parameter (water temperature). The analysis was based on hourly water temperature values from the period from May 2012 to January 2018. The data analysis permitted the description of the thermal regime of waters in the lake in the hourly cycle in reference to each month, and designation of months characterised by the highest and lowest stability of water temperatures. More than half of the analysed days was characterised by a change in water temperature not exceeding 1°C. The course of water temperature showed strongly positive correlations with air temperature, and weak correlations with wind speed. Water temperatures in the lake were correlated the strongest with air temperatures with a 24 h time step.

The article presents the analysis of water level fluctuations in Lake Powidzkie in the years 1961–2015. The study shows a considerable decrease in mean water levels in the aforementioned multiannual period, averaging 9 cmꞏdecade−1. Such a situation is caused by natural as well as anthropogenic factors, co-determining water relations in the study area. The natural factors include the amount and distribution of precipitation, increase in air temperature and evaporation size, unfavourable relations between the lake and catchment or hydrogeological conditions. Anthropogenic factors particularly include long-term transformations of the natural environment in the region, currently associated with meliorations accompanying the nearby opencast brown coal mines and exploitation of groundwaters for municipal purposes. Water shortages occurring during dry periods were shown not to be compensated in the study area in humid years. This is particularly related to the regional lowering of the aquifer remaining in close relations with Lake Powidzkie. Counteracting the unfavourable hydrological situation is done through hydrotechnical infrastructure which partially limits water outflow from the lake through damming.

The paper presents the analysis of the course of ice phenomena on four lakes (Lednica, Żnińskie Duże, Biskupińskie, Powidzkie) in central Poland in the period 1976–2015. The obtained results show changes in their ice regime that occurred over the last 40 years. An advance of the term of decline of ice cover was determined (by 2.5 day·dec-1on average for all lakes), as well as a reduction of duration of ice cover (by 4.4 day·dec-1 on average for all lakes), and a decrease in its maximum thickness (by 2 cm·dec-1 on average for all lakes). Such a situation should be associated with the observed warming, as confirmed by the course of air temperature for station Gniezno, where in the period of the winter half-year (November-April), an increase in temperature was recorded by 0.37°C·dec-1.

In the context of the observed climatic changes, unfavourable extreme situations occur increasingly frequently, manifested by, among others, the occurrence of droughts and floods. In the case of Poland, the central part of the country, in administrative terms covering the Wielkopolskie Province, is considered a region with a deficit of water resources. Such a situation was and is determined by a combination of natural and artificial factors. The former ones result from low precipitation and high air temperatures. The latter are the effect of intensive economic development, including large-scale meliorations. Measures mitigating this unfavourable phenomenon are related to the expansion of small retention. Its scale in Wielkopolska is the largest in the country – both in terms of the number of objects and volume of accumulated water. The key component of increasing retention is damming of lakes. According to the presented results, considerable reserves are still available in its scope. With the assumption of increasing water level in lakes by 0.2 m, it could cause an increase in water resources by 31 million m3, and assuming such an increase at a level of 0.5 m, by 78 million m3.

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The article presents thermal-oxygen conditions of Lake Białe Włodawskie in the warm half-year (May-October) in the years 2012-2015. Due to its morphometric parameters, summer stratification develops in the lake. In the period analysed in the paper, the highest temperature was recorded in July 2012 (27.5°C), and the lowest at the bottom in May 2013 (4.2°C). The highest difference in temperature throughout the water column was recorded during measurements in July 2012. It amounted to 22.7°C (27.5°C at the surface and 4.8°C at the bottom). The highest mean temperature throughout the water column (determined as the mean from all of the analysed months) occurred in 2015 (9.7°C), and the lowest in 2013 (7.8°C). The highest water temperature corresponded to the highest air temperature for station Włodawa, located several kilometres north of the lake. The mean concentration of oxygen dissolved in water (as the mean from all of the analysed months) suggests that the highest value 4.7 [mgO2 · dm-3] occurred in 2012, and the lowest 3.0 [mgO2 · dm-3] in 2013. In vertical distribution, the maximum oxygen concentration amounted to 13.5 [mgO2 · dm-3] in May 2013, and the minimum value decreased to zero multiple times. Complete lack of oxygen occurred each time in August, September, and October. It appeared the earliest in June 2013 (at a depth of 26 m), and the latest in August 2015 (at a depth of 28 m). The thickest water layer without oxygen was recorded in August 2013 already at a depth of 11 m. The studied lake is prone to strong anthropopressure. Due to favourable morphometric parameters and high resistance to degradation, however, it still maintains its high recreational values, although such a state may change in the near future in the context of increasing anthropopressure and observed climatic changes. [ABSTRACT FROM AUTHOR]