Hydroxylamine (NH₂OH) is an intermediate of the marine nitrogen cycle and in marine environments dissolved NH₂OH is short-lived. In order to investigate the distribution of NH₂OH under varying oxygen conditions, its seasonal variability was investigated on a monthly basis from July 2005 to May 2006 at the time series station Boknis Eck located in the Eckernförde Bay (southwestern Baltic Sea). NH₂OH concentrations were generally low and close to the detection limit. However, a pronounced increase was observed after the seasonal thermohaline stratification period with low oxygen/anoxic conditions in the deep layers was terminated in November 2005. The increase of NH₂OH was associated with the re-oxygenation of the water column. We conclude that NH₂OH was produced in-situ during nitrification. We suggest that the detection of significant amounts of NH₂OH can be used as an indicator for a “fresh” nitrifying system.