Changes in the chemical composition of water in the small Lake Vendyurskoe (Southern Karelia, Russia) over 50 years due to climate change and antropogenic impact

Studies of changes in the chemical composition of lake water due to climate variability and human impact are highly relevant for Karelia, which has more than 60,000 lakes. Lake Vendyurskoe, a typical small lake in Karelia, has been used for growing rainbow trout since 2009. In the 1970–90s, drainage and logging were practiced in the catchment of this lake. A change in the composition of major ions in the lake’s water and an increase in the labile component of organic matter were observed between 1971 and 2023. In 2021–2023, the concentration of sulfate ions decreased fourfold compared to 1971–1985, which may be due to both the cessation of drainage operations and a reduction in the concentration of sulfates in precipitation. Chloride ion concentrations were reduced by half. The 50-year decrease in bicarbonate concentrations was likely associated with increased precipitation. Total phosphorus concentrations in the lake water showed an upward trend over the 50 years (in both surface and bottom layers), indicating increased anthropogenic impact on the lake. During the summer stratification period, hypoxia develops in the deep-water parts of the lake and near trout cages, and phosphate concentrations in the bottom layers are quite high (up to 36 μg/L). The mobilization of phosphorus from bottom sediments under anoxic conditions creates a secondary phosphorus load, which may be the reason for the increased bioproductivity. A direct correlation was found between the duration of near-bottom hypoxia in summer and the average air temperature over the three summer months (R 2 = 0.51), suggesting deteriorating oxygen conditions in the lake during the open-water period as regional climate warming continues. The greatest oxygen depletion in the lake occurs in winters with early freeze-up and late ice-breaking. The inverse correlation of the duration of winter hypoxia with the November air temperature of the previous year (R 2 = 0.39) and the May air temperature of the current year (R 2 = 0.41), which influence the timing of ice formation and breakup, suggests the oxygen conditions in the lake during the winter have improved, with a shorter freeze-up period due to warming in the fall and spring. The observed changes in the water chemical composition provide reasoning for reducing the trout farming volumes.