The study investigates the influence of various ecosystems (agricultural vs. urban) surrounding a watercourse on the chosen indicators of water quality. In the years 1996–2001, water was sampled once a month from eleven cross-sections of the watercourse Kadaň and examined for the concentrations of dissolved oxygen, biodecomposable organic matter (BOD5), and organic substances (CODMn). The mean values of the respective indicators in the research period were: O2 – 7.26 mg ⋅ dm^-3, BOD5 – 2.12 mg ⋅ dm^-3, CODMn – 3.32 mg ⋅ dm^-3. On average, the O2 concentration was lowest in summer and autumn, most likely because of the intensive biological decomposition of organic matter, and had higher values downstream of agricultural areas than downstream of villages. BOD5 was lowest in the months of January to March, whose thermal conditions were unsuitable for the decomposition of organic matter by microorganisms living in the stream, and highest in May. The indicator reached higher values downstream of villages and tended to fall on the segments surrounded with agricultural areas, suggesting that the self-purification of water took place there. CODMn was lowest in December to March, and highest in June to August. This indicator had higher values downstream of urbanised areas, which may be due to the discharge of raw sewage into the watercourse. As follows from the results, various ecosystems differently influence the indicators of surface water quality.

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