NITROGEN METABOLISM IN LAKES I. MEASUREMENT OF NITROGEN FIXATION WITH N super(15) 1

A method for measuring the rates of nitrogen fixation in samples of lake water using N super(15) as a tracer is described, and a detailed description of glassware required is provided. The nitrogen present in the water is removed by flushing with Hc-O mixture and then replaced by the labeled nitrogen. After incubation and treatment, the isotope ratio in the organic material is determined on a mass spectrometer. Analyses of lake water and (NH sub(4)) SO sub(4) as isotope ratio blanks are described and a resume of statistical data from these analyses is presented.