**5. Conductance of water in the reservoir**

**Figure 5.** Oxygen saturation (in %) of the Turawa reservoir in the hydrological year 2004—vertical profile:

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

50°44′10.28″N, 18°05′30.22″E.

384 Water Quality

Surface waters show large differences in water-specific conductance: from as low as 1μS cm−1 to 3000 μS cm−1. Surface water conductance is strongly influenced by discharged wastewater, mainly industrial wastewater whose specific conductance can be as high as 10,000 μS cm−1.

The concentration of mineral substances dissolved in water, which are responsible for water conductance, changes with time. The changes are caused by primary production, which diminishes the content of salts in water and also by the transfer of biogenic substances from bottom sediments to water which increases conductance. In the period 2004–2006, the monthly average specific conductance of water in the Turawa reservoir changed from 285 to 425 μS cm−1 (**Figure 6**). In the whole period studied, the lowest measured specific conductance was 272 while the highest was 495 μS cm−1 (average 339 μS cm−1).

**Figure 6.** Changes in the average monthly specific conductance of the Turawa reservoir in the hydrological years 2004– 2006.

In the season of snow melting (January and February), the conductance was the highest due to transfer to surface waters of inorganic compounds formed in the process of decay of organic matter present at the bottom and also due to runoff from spatial sources. The lowest specific conductance was measured in spring-summer season—most probably this resulted from maximum water level in the reservoir at that time. Statistically significant dependence (*R*<sup>2</sup> = 0.37) was found between the reservoir water level and conductance—the higher water level the lower the specific conductance. According to the regulations in force at that time, water in the Turawa reservoir was classified to purity class I with respect to that parameter in all measuring periods, in the hydrological years 2004–2006.
