NITROGEN FIXATION IN TWO WISCONSIN LAKES1

Abstract

Rates of biological nitrogen fixation in two Wisconsin lakes arc given. In Lake Wingra, fixation continues at moderate rates throughout much of the ice‐free season. In Lake Mendota, the rate is normally zero throughout the ice‐free season, but occasional, sometimes high, rates are observed. Auxiliary experiments on the effects of incubation time, added glucose, added combined nitrogen, light intensity, and temperature on nitrogen fixation in lake water are discussed. The effect of incubation time on the rate of fixation varies from time to time. Glucose had no consistent effect on nitrogen fixation in the dark, although stimulation was observed in some experiments. In the light, glucose appeared to inhibit fixation. The effects of added ammonia and nitrate were inconsistent. At times, various added concentrations, particularly of ammonia, increased rates of nitrogen fixation. Rates of nitrogen fixation varied with light intensity. The rate increased to a maximum at 3,750 lux, then declined as the intensity increased further. Rates of nitrogen fixation increased at a constant rate with temperature in the range 16–30C.