Water temperature and dissolved oxygen content in a small lake in Karelia over an annual cycle against the background of regional climate variability

The article analyzes the results of water temperature and dissolved oxygen content measurements taken at a mooring in a small lake in southern Karelia from October 2022 to October 2024 and compares them with records from the previous 15-year period (2007-2022). The measurements were carried out using RBR Ltd. temperature and dissolved oxygen sensors at 14 depths with 1 min resolution over an annual cycle. The study aims to identify changes in the thermal, oxygen, and the mixing regimes of a small lake in southern Karelia under regional climatic variability. The fall and winter seasons of 2022/23 and 2023/24 were characterized by contrasting weather conditions – the former was warmer than the baseline and the latter was colder and snowier (precipitation in some months exceeded the baseline by 2-3 times). The open water period of 2024 was noticeably warmer than the baseline, and the precipitation amounts in individual months were 2-4 times lower than the baseline. During both cold seasons, anoxic conditions in the bottom layers of the lake lasted for about four months (from mid-January until ice breakup in early May), and during the open water periods of 2023 and 2024 their duration was about three weeks and 1.5 months, respectively. Stratification (the period with a more than 1 °C temperature difference in the water column) in 2024 lasted 119 days, which is 20-60 days more than in previous years of measurements. Due to the calm and warm weather in the summer and early fall, the lake mixing regime in 2024 was dimictic, while in previous years the polymictic regime prevailed (excluding 2022). The biological summer (the period with water temperature above 10 °C) lasted more than 140 days in 2023 and 2024. Based on the 2008-2024 data, we determined the relationship between the duration of the biological summer and the mean monthly air temperatures in May and September, with September temperatures making a greater contribution. The identified changes in the thermal, oxygen and mixing regimes may have an adverse effect on the lake ecosystem, necessitating continuing monitoring.