Trends of quantitative change of organic matter in the Nevėžis

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 analyzes change of organic matter in the Neve˛is, which is under influence of agricultural activity, in comparison with that in the Skroblus, which has natural background. In the period of 1986–2009 the established amount of organic matter in the Neve˛is exceeded that in the Skroblus by up to 2.8 times. Water of the Neve˛is was of good (11 years, i.e. 11 times), that of the Skroblus – of very good (13 times) ecological condition. Three periods of organic matter fluctuation were determined in the Neve˛is: 1986-1991, 1992-2002 and 2003-2009. Fluctuation of organic matter was more related with changes of production intensity than with air or water temperature. Analysis of research results for the period of 1986-2009 according to Mann-Kendall test has detected different tendencies of change. In the Neve˛is basin air and water temperature insignificantly increased, but organic matter decreased. In the Neve˛is the highest concentration of organic matters was in 1988 (BOD7 = 5.66 mg O2/L), later it was significantly decreasing until 1994, and increased again starting in 1995; however, content of these matter did not exceed the amounts established at the beginning of the research. In the Skroblus basin air temperature insignificantly increased, water temperature decreased, organic matter significantly decreasedVytauto Didžiojo universitetasŽemės ūkio akademij