Abstract
Dissolved oxygen in water (DO) is one of the primary parameters determining its quality. It remains in close relation to water temperature. The article presents results of DO and water temperature measurements performed for five stratified lakes in Poland in the years 2007-2014. The measurements were performed with monthly frequency in the summer half-year (May-October) in the deepest place of each of the lakes. The compared data show the complex character of the course of both of the parameters. The analysis of depth profiles revealed the prevalence of a decreasing tendency, or no tendency was recorded - both in the case of DO and water temperature. Due to the incoherent character of the water in the case of stratified lakes, a more detailed comparison was performed, referring to changes occurring in zones developed during summer stratification. It was determined that the temperature of the epilimnion was directly dependent on air temperature, and in all cases it showed an increasing tendency. Simultaneously, along with an increase in temperature, in the majority of cases, DO concentration in the zone decreased. In reference to the waters of the hypolimnion, increasing, decreasing, and no tendencies were recorded. The above distribution is determined by individual factors of the lakes - morphometry, trophic status, groundwater alimentation, etc. Several years of measurements of DO and water temperature in depth profiles of several lakes in Poland constitute valuable information on the current transformations of the environment of this part of Europe. In the future, the information can provide the basis for long-term analyses over the next decades, enriching global knowledge on those parameters of key importance for water ecosystems.