Aim of the study:
This study aimed to determine selected physicochemical indices of water in Rotcze lake (Łęczna-Włodawa Lake District) at particular times of the day and to determine the degree of eutrophication.

Material and methods:
Water samples for analysis were collected in two places - near the platform located on the bathing beach (first point) and in the vicinity of the platform situated among the abundant rush vegetation, away from the bathing beach (second point). Water samples were taken from both points five times, every six hours. In each case, samples were taken from a depth of about 60 cm using an open container. Immediately after their collection, the water temperature was measured, while the remaining parameters were tested within about an hour, i.e. pH, concentration of ammonia, nitrites, nitrates, phosphates, total hardness and carbonate. Additionally, the air temperature was measured.

Results and conclusions:
1. The trophic level in Rotcze lake is low, therefore the lake can be classified as an oligotrophic type. 2. The lake is not endangered by eutrophication, as the concentration of nutrients is negligible (below the quantification threshold), and it does not overgrow (during the growing season, rush vegetation accounts for up to 8% of the entire lake area). 3. Daily variation of water temperature is small (± 2 ºC), but it correlates with air temperature. 4. The pH value increased after dark (10:00 pm), which was caused by the photosynthesis process that took place during the day, leading to a decrease in CO2 concentration and alkalinization of the water.