**4. Oxygen saturation**

ature increase (from 0°C). Heavier water moves downwards while colder water from the bottom layer moves up resulting in water mixing and constant temperature in a vertical profile

The period of summer water stagnation is characterized by temperature drop with depth. This is typical water stratification whereby the upper layer of the highest temperature close to air temperature is epilimnion and the coldest bottom layer is hypolimnion. The water between these two layers is metalimnion. Due to the small depth of the reservoir (in summer rarely more than 10 m in the deepest point) and intensive wind mixing in summer stagnation period thermal stratification was not observed—vertical temperature difference did not exceed 4°C

**Figure 4.** Changes in water temperature in the Turawa reservoir in the hydrological year 2004—vertical profile:

At the beginning of September, the period of autumn circulation starts in the Turawa reservoir. Due to cooling of surface layer, wind and convective mixing, the difference between the water

[4].

382 Water Quality

(**Figure 4**).

50°44′10.28″N, 18°05′30.22″E.

The oxygen dissolved in surface water comes mainly from the atmosphere and photosynthesis processes; its content is one of the most important parameters characterizing the quality of water. Oxygen is necessary for the life of fishes and other water animals. The oxygen content depends on the physical (temperature) parameters and on biological processes. Water pollution generally results in drop of oxygen content. In the hydrological years 2004–2006, the Turawa reservoir water saturation with oxygen was typical (60–100%) from autumn to spring with the highest values in winter (**Figure 5**). In that period, the average oxygen saturation in vertical profile ranged from 58 to 102%.

**Figure 5.** Oxygen saturation (in %) of the Turawa reservoir in the hydrological year 2004—vertical profile: 50°44′10.28″N, 18°05′30.22″E.

Due to increase of nutrients in limnic waters in summer, the oxygen saturation profiles are highly modified. In the Turawa reservoir, epilimnion water was oversaturated with oxygen (from 104 to 164%) because of intensive growth of phytoplankton which produces oxygen in the process. In hypolimnion, phytoplankton growth is limited and oxygen is consumed by decaying organic matter which results in drop of oxygen content. The oxygen deficit (saturation smaller than 60%) occurred from June to September. In July and August, the bottom layer of up to 6–7-m thick was completely depleted of oxygen. In fact, every year the oxygen saturation conditions of water in the Turawa reservoir were getting worse in summer period due to intensification of eutrophication processes. This is very unwanted process since oxygen deficiency is detrimental for the river fauna. With respect to oxygen content, water in the reservoir was classified to purity class III [6].
