The study evaluated the effect of environmental conditions and morphometric parameters on lake water temperature changes. The analysis was carried out on the basis of 14 lakes located in northern Poland. The assessment was based on the daily water and air temperatures from 1972 to 2016. It took into account the location of lakes (latitude, longitude, altitude) morphometric parameters (surface area, maximum and mean depth, volume), hydrological processes (rate of water exchange, course of ice phenomena), and trophic status (water transparency) as factors that can modify lake water temperature changes. Direction and rate of air and water temperature changes were analysed by means of Mann–Kendall’s and Sen’s tests. Cluster analysis (CA) was applied to group lakes characterised by similar water temperature changes. The effect of climatic and non-climatic parameters on a lake’s water temperature was assessed on the basis of principal component analysis (PCA). Water temperatures in the lakes in the years 1972–2016 were characterised by a higher rate of increase of 0.43 °C·dec⁻¹ than the air temperature decrease of 0.34 °C·dec⁻¹. The analysis showed a faster rate of heating of waters in western Poland. This can be explained by shorter duration of ice cover. Moreover, the changes of water temperature were affected by other factors, including the location of the lakes, their morphometric parameters, wind speed, water transparency and water exchange time.

Processes of lake shrinkage as a result of the action of natural and anthropogenic factors in recent decades have been considerably accelerated. On the other hand, lake capacities may have increased as a consequence of damming. These changes are typically analyzed in relation to changes in lake area, while horizontal and vertical components are excluded. The aim of this study was to assess changes in morphometry in several lakes in central western Poland and to determine changes in water resources in selected lakes as a result of changes in their morphometry. These changes were analyzed based on archival data concerning the area and capacity of lake basins, collected from studies prepared by the Institute of Inland Fisheries in the early 1960s and current bathymetric studies. These investigations showed that within approx. 50 years a considerable decrease was recorded in the area of the analyzed group of lakes, amounting to 14.8% (i.e., 172.6 ha). As a result of changes in the area and shallowing of lakes, the water resources stored in lakes decreased by a total of 7.7%, which amounts to 3.6 million m³. Referring to the factors determining the above-mentioned transformations, it was found that both climatic and anthropogenic conditions were disadvantageous. While in the study period no trends were observed for changes in precipitation, a significant increase in temperature was found, thus contributing to increased evaporation. This situation was compounded by human activity connected with land reclamation operations or agricultural activity.

The paper discusses changes in the hydrological regime of high mountain Lake Morskie Oko located in the Tatra Mountains, in the Tatra Mountains National Park, a UNESCO biosphere reserve (MaB). According to the research conducted in the years 1971–2015, its water stages decreased by 3.5 cm·dec⁻¹, mean annual water temperature increased by 0.3°C·dec⁻¹ and the duration of ice phenomena and ice cover was reduced by 10 day·dec⁻¹. No considerable changes in maximum values of ice cover thickness were recorded. Such tendencies are primarily caused by long-term changes in climatic conditions – air temperature and atmospheric precipitation. The hydrological regime of the lake was also determined by changes in land use in the lake’s catchment and its location in high mountains.

The effect of water level reduction on nutrient concentrations and productivity of emergent macrophytes was analyzed in a shallow, polymictic lake, located in Central-Western Poland. The water level was regulated for irrigation purposes from 1974 to 2002, with an average amplitude of 0.85 m. In 2002, the water level was reduced by 0.60 m during winter time and by 0.40 m in summer time. Nutrient concentrations, nitrogen, phosphorus, and potassium standing stocks and aboveground biomass production were compared among four vegetation types along with a moisture gradient in the littoral zone. This study was conducted in the littoral zone of Lake Niepruszewskie during the growing seasons in 1999–2001 (1st period with high water level) and 2005–2008 (2nd period with low water level). The reduction of water level resulted in a negative effect on biomass production and nutrient concentrations. Stoichiometric indexes, like N:P and N:K ratios, indicate that the shift in nutrient limitation due to changes in hydrological conditions caused a reduction of the nutrient uptake by vegetation and affected productivity. The decrease in the aboveground biomass production was more marked at sites that were less affected by drainage, i.e., permanently flooded. However, in drawdown areas, plant growth decreased. Species including Phragmites australis and Carex acutiformis coped better with the reduction of water levels than Typha angustifolia and Glyceria maxima.