River-lake systems most often behave as hydrographic units, which undergo complex interactions, especially in the contact zone. One such interaction pertains to the role of a river in the dispersal of trace elements carried into and out of a lake. In this study, we aimed to assess the impact of rivers on the accumulation of heavy metals in bottom sediments of natural lakes comprised in postglacial river-lake systems. The results showed that a river flowing through a lake is a key factor responsible for the input of the majority of available fraction of heavy metals (Zn, Mn, Cd and Ni) into the water body and for their accumulation along the flow of river water in the lake. The origin of other accumulated elements were the linear and point sources in catchments. In turn, the Pb content was associated with the location of roads in the direct catchment, while the sediment structure (especially size of fraction and density) could have affected the accumulation of Cr and Zn, which indicated correlations between these metals and fine fraction. Our results suggest that lakes act as filters and contribute to the self-purification of water that flows through them. As a result, the content of most metals in lake sediments showed a decrease by approx. 75% between the upstream (inflow) and downstream (outflow) sections. The increased content of two metals only, such as chromium and cadmium (higher by 2.0 and 2.5 times, respectively, after passing through the lake), was due to the correlation of the metals with fine sand. Both the content and distribution pattern of heavy metals in lake sediments are indicative of the natural response of aquatic ecosystems to environmental stressors, such as pollutant import with river water or climate change. The complex elements creating the water ecosystem of each lake can counteract stress by temporarily removing pollutants such as toxic metals form circulation and depositing them mostly around the delta.

Pond management requires that a specific fish culture is conducted while taking into account both production possibilities and profitability, as well as the impact it may have on the natural environment. This study aimed to evaluate the effect of three water management systems used in rainbow trout culture on water quality in fish ponds. It was conducted at six trout farms and differing in water management strategy. After water had flown through the fishing ponds, its quality was significantly less impaired at farms operating in the flow and cascade systems. In turn, waters discharged from farms using the recirculation system were characterized by the poorest quality and lowest values on the Water Quality Index (WQI). It was found that the flow and cascade systems can be used to maintain the water quality and give less fish mortality for trout. It has been shown that the use of a water recirculation system in rainbow trout cultures significantly affects the quality of water in fishponds and can potentially lead to suppression of fish resistance and in extreme cases, to fish death. This study will help fish farmers in choosing the optimal variant of water management, taking into account both the best fish health with the least negative impact of fish farms on the environment.

The study analyzed the structure of water shortages in plant crops and the available groundwater resources that can be used to satisfy these needs. The research was carried out in Braniewo poviat, which can be considered representative of the conditions of Central and Eastern Europe. A clear upward trend in the temperature value was observed, which influenced the changes in the duration of thermal seasons and agricultural periods. It also increases the intensity of the evapotranspiration process, which results in the reduction of water resources. The presence of significant water shortages, especially in the cultivation of root crops, such as, for example, late potato or sugar beet, justifies the need to irrigate these plants. Due to unevenly distributed surface water resources, groundwater is used as a source of irrigation. It was found that in the case of many crops, the areas with the greatest water shortages were those with average or high abundance in available groundwater. When indicating the possibility of abstracting large amounts of groundwater for use in plant production in Braniewo poviat, one should consider the fact that, in the long term, their exploitation may cause negative environmental effects.