**9. Eutrophication processes**

of organic matter which can be oxidized by strong oxidizer as potassium permanganate

The basic water quality indicators mentioned above were monitored for the Turawa reservoir. In hydrological years 2004–2006, the monthly average BOD5 for the whole studied period was 5.2 mg O2 dm−3; it was changing in a wide range from 0.6 to 16.7 mg O2 dm−3 (**Figure 14**). The annual average BOD5 increased from 3.6 mg O2 dm−3 in 2004 to 8.6 mg O2 dm−3 in 2006. Occasionally, very high concentrations were found. In the period from June to September 2006,

**Figure 14.** Changes in the average monthly BOD5 concentration in the waters of the Turawa reservoir in the hydrologi-

In the above-measuring period, the monthly average CODMn parameter was 9.2 mg O2 dm−3 and changed from 5.2 to 9.2 mg O2 dm−3 (**Figure 15**). Occasionally, very high values of CODMn were measured, for example, from September to October 2006 the maximum CODMn values

Both BOD5 and CODMn values were season dependent; in winter were the lowest while in summer the highest (**Figures 14** and **15**) due to intensive eutrophication and resulting growth of algae and green algae during hot season. These organisms emit considerable amounts of

If water contains neither toxic substances nor organic matter resistant to biodegradation, the good correlation between BOD5 and CODMn can be expected. For the water of the Turawa

In October 2006, a series of other pollution parameters were determined. They included total organic carbon (TOC), and indicators of industrial pollution such as free cyanides, phenolic index, anionic surfactants, mineral oils and polycyclic aromatic hydrocarbons. Fortunately, the

organic compounds in metabolic processes and during decaying.

(CODMn) or potassium chromate (CODCr), respectively.

the maximum BOD5 values exceeded 40 mg O2 dm−3.

cal years 2004–2006.

392 Water Quality

exceeded 25 mg O2 dm−3.

reservoir, correlation coefficient was 0.79.

Eutrophication is a system response to the addition of nutrients, mainly phosphates, resulting in the 'bloom' or great increase of phytoplankton in a water body. Oversupply of nutrients, inducing explosive growth of phytoplankton and algae, results in the consumption of oxygen when these species die. The oxygen depletion level may lead to fish kills and a number of other effects reducing biodiversity. Generally, low oxygen content has many negative consequences [11, 12].

The eutrophication of the Turawa reservoir was mainly of anthropogenic origin. Nutrients were delivered by inflowing rivers, mostly the Mała Panew river; they come mainly from municipal wastewater, from arable land and forests surrounding the reservoir, and from tourist resorts. Considerable participation in eutrophication had nutrients released from bottom sediments gathering in the reservoir for tens of years. In 2004–2006, intensive release of phosphates from bottom sediments was observed during spring periods caused by water mixing. Phosphates in the form of compounds with iron or with aluminium were also released during summertime due to anaerobic conditions in the benthic layer. The intensive eutrophication in summer is favoured by hydrological and meteorological conditions such as small stream flow of inflowing rivers, low water level and high water temperature.
