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.
Main aim of the study was to determine the temporal and spatial patterns of relations between monthly and annual average river flow (RF) and water temperature (WT) for 53 rivers in Poland. The research made use of monthly and annual WT and RF for 88 water gauges for the period 1971–2015. Correlations were established using the Spearman’s rank correlation coefficient and the similarity of RF–WT relations was determined using the Ward’s hierarchical grouping. It was demonstrated that correlations between average annual RF and WT were negative (for >85% of water gauges) and statistically significant (p<0.05) only for 30% of water gauges. It was confirmed that the studied RF–WT relations underwent seasonal changes. Positive correlations were clearly predominant in the winter months, while from April to September these relations were negative and statistically significant. The RF–WT relations were also characterized by spatial differences and this had been confirmed by separation of seven groups of water gauge profiles distinguished with the help of the Ward's hierarchical grouping method. The strongest RF–WT relations were apparent in the case of mountainous rivers, for which snow melt supply and summer rainfall supply were predominant, and lakeland rivers, which had a considerable share of groundwater supply. These were classified as cold rivers, as opposed to the cool rivers in the lowland belt, for which the RF–WT relations were the weakest. The results obtained may contribute to the elaboration of an appropriate management strategy for river ecosystems, which are assigned important economic and environmental functions.
The main purpose of the research was to determine the conditions affecting ice phenomena, including the three-phase cycle of ice: expansion, retention and decay of the ice cover on selected rivers of the Baltic coastal zone in the Northern Poland (Przymorze region). The analysis has been elaborated for the years 1951–2010 against the backdrop of currently occurring climatic changes, with particular emphasis on the development and phase variability of the NAO. The article presents the impact of the variability in atmospheric circulation which has manifested in an increase in air temperature, over the last 20 years, on thermal conditions during winter periods in the South Baltic Coastal Strip. The increase in air temperature has contributed to an increase in the temperature of river waters, thus leading to a shortening of the duration of ice phenomena on rivers in the Przymorze region. The article also brings to light an increased occurrence of winter seasons classified as cool, and a disruption in the occurrence of periods classified as normal over the last 30 observed years. The research has demonstrated a significant dependence between the seasonal change in air temperature and the variability of thermal conditions of water, which has a direct impact on the variability of the icing cycle of rivers in the Przymorze region. The authors also show that the variability in forms of ice phenomena for individual river sections is determined by the local factors, i.e. anthropogenic activity, impact of urbanized areas or inflow of pollutants.
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.