During two periods of high pH and high ammonia concentrations, disappearance rates of ammonia of 20–30 μg N∙L−1∙h−1 were observed in Lake 885, a hypertrophic prairie lake. At these times, rates of ammonia uptake by phytoplankton were 2.6–4.3 μg N∙L−1∙h−1 as measured by a 15N method. The high excess rates of ammonia disappearance are best explained by volatilization of ammonia from the lake surface. The loss of ammonia induced nitrogen fixation; however, much larger quantities of ammonia were lost via ammonia volatilization after algal bloom collapsed than the algae could fix in one season.Key words: ammonia volatilization, prairie lakes, nitrogen limitation, nitrogen cycle