[en] Nitrogen fixation in Danish lake water is determined by to methods: by acetylene reduction and ¹⁵N determination. ¹⁵N determinations in gas and water were carried out by optical spectrometry using a commercial spectrometer. Measurements carried out in a Danish eutrophic lake demonstrate the applicability of the methods. Simultaneous determinations of acetylene reduction were performed. The results of the investigations substantiate the need for frequent calibration of the acetylene reduction method by the ¹⁵N uptake method. (author)