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The pollution of the river water with organic matter deteriorates the chemical, biological and microbiological quality of the water and has a negative effect upon the biological variety of the water environment. The paper analyses the study period (1996-2000) seasonal change of organic matter in the upper reaches of the River Nevezis (study sites upstream and downstream Panevezys). The highest amount of organic matter (according to chemical oxygen demand and biochemical oxygen demand) got into the river under the influence of concentrated pollution, the fewer amount - under the influence of agricultural pollution and in the course of natural processes. The concentration of organic substances in the river water had a seasonal character: rivers with a better ecological state (the Juosta) were more polluted with organic matter in spring, when water temperature was below 10 deg C, but rivers with a worse ecological state (the Nevezis upstream and downstream Panevezys) – in summer (water temperature was higher than 10 deg C). This may be related with fresh pollution; in the former case organic matter might have come from the environment together with the runoff, in the latter case - due to decomposition of water vegetation and fauna residues in the river itself. In all rivers water temperature was by 0.1-4.4 deg C higher in 1996-2000 in comparison to that in 1986-1989, a significant temperature increases were observed in winter and autumn. The water temperature arose mostly below the town.

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