The Harsit Stream flowing into the Black Sea is an important watershed in the northeastern part of Turkey. The monthly averages of dissolved oxygen, carbonaceous biochemical oxygen demand, temperature, pH, total suspended solids, total phosphorus and total nitrogen were obtained for the water samples taken from the Tekke, Akcakale and Haciemin sites of the Harsit Stream. Water quality parameters were analyzed and evaluated according to the WHO standards and the Turkish Water Pollution Control Regulation. Initially, the variations in the dissolved oxygen profile were determined from dissolved oxygen, temperature and carbonaceous biochemical oxygen demand measurements by using the point source Streeter-Phelps equation. Critical dissolved oxygen deficit values were calculated for the monthly average from January -December 2005, which are below the limit value (4 mg L -1 ) excluding April (7.26 mg L -1 ) and May (5.98 mg L -1 ) in the Harsit Stream, resulting in it having second class water quality in terms of dissolved oxygen concentration, temperature, pH and biochemical oxygen demand. It is also seen that the minimum dissolved oxygen limit value (8 mg L -1 ), which is a desired quantity for drinking water, is under the limit value for all months in the case of the water mixture. In particular, during the summer months, the consumption speed of oxygen is greater than the production speed of oxygen with increasing temperature and decreasing discharge, and negative values (anoxia) of dissolved oxygen sag are seen. These situations call for significant precautions and treatment criteria.

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