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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.

An analysis is presented of changes in the parameters of ice phenomena on selected Polish lakes over the years 1961-2010 against the background of variations in the intensity of the North Atlantic Oscillation. The following features were analysed: the start and end of ice phenomena and ice cover, their duration, and the stability of ice cover at observation sites of the Institute of Meteorology and Water Management on lakes located in various physical-geographic regions. Multi-year tendencies of change in the ice parameters under study were identified. The most statistically significant negative trends were observed for the ending dates of ice phenomena and their duration. Changes in the ice characteristics in the different NAODJFM stages were determined on the basis of their deviations: between years with high (> 1.7) and low (< -1.09) values of the winter NAO index, and from average values from the years 1961-2010. Changes in the intensity of the North Atlantic Oscillation affect all the parameters of ice phenomena except their starting dates. In a negative NAODJFM stage, ice phenomena and ice cover end later and last longer; also, the maximum thickness of ice cover is greater, its forms later and is more stable than in a positive stage and in average conditions

The evolution of lakes, which, in the final stage, leads to their disappearance, is mainly based on the analysis of surface area change. However, there is one more, frequently overlooked process, which determines lake disappearance - namely the lake shallowing. The present paper presents the direction and rate of such evolution, taking both these processes into account. It is a comparative analysis of 9 lakes in the Żnin region - which has the greatest water deficit in the whole of Poland. Based on bathymetric plans from two periods (early 20th century and early 60’s of the 20th century), the author was able not only to evaluate the scale of surface area change but also to determine the value of lake basins volume change. Both these values were negative and amounted 9.1% and 14.9% respectively. Assuming that the said processes are invariable, the prospective period of lake functioning is about 500 years when taking into consideration surface area change only, and is 150 years shorter when allowing for aquatic resources change. The presented approach to lake disappearance analysis, which treats this phenomenon as a decline in aquatic resources rather than a simple decrease in surface area, is more reliable and therefore, scientific works in this field should take the results of lake shallowing into consideration.

In the past, human intervention in water relations focused mainly on draining wetlands and lakes. In general, this was aimed at increasing the area of agricultural land or creating new settlement areas. In the case of lakes, the consequence of such actions was to reduce their level and in many cases to completely dry them. The maintenance of appropriate, assumed environmental conditions requires, in the case of drainage systems, maintaining the high efficiency of the drainage system, which is time- and cost-intensive. Neglecting such an approach, or intentional actions in this respect may lead to the restoration (to a different spatial extent) of drained lakes. The examples presented in the paper indicate that this process is quite common. In the context of the need to expand the water retention of the country, a detailed search of such reservoirs should be carried out and their renaturalization should be considered.

On the basis of hydrographic maps in the scale 1:50,000, an inventory of crenological objects in the lower part of the Vistula basin up to the Drwęca river basin in the south was made. The total number of theseobjects in the analyzed area is 308, with the highest number of solid sources – 161, followed by leakages – 125 and periodic sources – 17. The total efficiency of all outflows is very low, as it was estimated at 0.075 m3∙s–1. The most efficient source reaches only 1.0 dm3∙s–1, and only a few 0.7 dm3∙s–1.