Research on the nitrogen content of water in a midfield pond, carried out during 1994–2003 in the area of the Experimental Station “Mokronosy” (Gniezno Lakeland), included measurements of the concentrations of nitrates, nitrites and ammonium ions in water. Considering the values of those parameters, the pond water belonged to the first purity class for most of the study period. During ten years of research, the concentrations of nitrates and nitrites showed a weak decreasing trend and those of ammonium ions remained steady. Greater concentrations of NO₃^- and NO₂^- occurred only at the time of drainage water inflows to the pond. The parameters studied exhibited seasonal variation. The concentration of nitrates and nitrites was higher in winter and spring months, whereas that of ammonium ions – during summer periods. The results show that the quality of water is not threatened with the analysed forms of nitrogen.

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