Nitrifying activity in the surface water of the Włocławek Dam Reservoir (WDR) was determined by use of a nitrification selective inhibitor (ATU). The highest values of nitrification were observed in the stations nearer the dam, with lower flow velocity, where the activity amounted to up to 120 µg N/dm³/day. Nitrification process [%] involvement in biochemical oxygen demand increased with passing from lotic to lenitic conditions and reached maximum values up to 70% or even 100% BOD₅. Chemoautotrophic nitrifiers participation increase in oxygen consumption in the reservoir was probably caused by sedimentation of allochtonous, easily decomposable organic matter and a significant reduction in carbonaceous oxygen demand (C–BOD). Ammonium ion release from sediments was the decisive factor of shallow flooding nitrification.