A method for measuring the rates of nitrogen fixation in samples of lake water using N1" as a tracer is described, and a dctailcd description of glassware rcquircd is provided. The nitrogen present in the water is removed by flushing with Hc-0 mixture and then replaced by the labclcd nitrogen.After incubation and trcatmcnt, the isotope ratio in the organic material is dctcrmincd on a mass spcctrometcr. Analysts of lake water and (NHe) SOC as isotope ratio blanks arc dcscribcd and a r&urn6 of statistical data from these analyses is presented.

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Incorporation of N(15) into the fixed nitrogen fraction of natural lake waters has been studied for the purpose of estimating rates of primary nitrogen fixation. Experiments in Pymatuning Reservoir, Pennsylvania, in Lake Mendota, Wisconsin, and in two lakes in Alaska indicate that fixation occurs at measurable, sometimes high, rates.