The rates of denitrification and oxygen consumption processes were measured in sediment of four reservoirs in south - eastern Poland using special incubation chambers in natural environment. Chemical analyses included overlying water, bottom sediment and evaluated gas which components were identified by chromatographic method. The denitrification rate was calculated from the total nitrogen flux off sediments after subtracting the rates of the released nitrogen flux because of the changes in nitrogen gas solubility. The oxygen consumption rate was calculated from differences in dissolved oxygen concentration in overlying water.

Changes in analysed indicators were observed during experiments. In most cases, decrease in pH values, dissolved oxygen and nitrate concentrations were observed. In the other hand increase in ammonia ions concentration was found. The evaluated gas contained mainly methane and in less proportion nitrogen. Oxygen and carbon dioxide represented a small quantity of evaluated gas mixture. Oxygen was observed mainly on the beginning of experiments and based on the correlation analysis of all evaluated gases there were found that oxygen was a consequence of water desorption but not biochemical processes in bottom sediment.

Denitrification rates for all the investigated lakes resembled the values reported for eutrophic reservoirs. The rate of oxygen consumption resembled the values reported for majority of freshwater environments. Chemical oxygen consumption, as measured using formalin-killed controls, dominated in the total oxygen demand.

The factors influencing the rates of denitrification and oxygen consumption in bottom sediment such as light, temperature, nitrate and dissolved oxygen concentration, pH in overlying water and available organic matter in sediment were investigated. Denitrification rate was enhanced under natural light conditions. Temporal variations in denitrification rates and oxygen consumption appeared to be dominated by temperature. The rates of both processes were greater in higher temperature measuring from spring to autumn. The more nitrate concentration in overlying water and organic matter in sediment the greater the rate of denitrification. Organic matter concentration did not influence the oxygen consumption rate. As a result of pH of overlying water, nitrous oxide as the intermediate product of denitrification was not observed. Mutual dependence between the processes was analysed and the correlation between denitrification rate and oxygen consumption rate was found. This suggests the coupled denitrification-nitrification had greater contribution to the total process than denitrification, owing to nitrate diffusing from overlying water. In addition nitrate concentrations in water were relatively high and their consumption was low and much low in comparison with gaseous nitrogen production as a denitrification final product. Also dissolved oxygen which was presented in overlying water despite its continuous consumption, created favourable conditions for nitrification process.