Changes in benthic macroinvertebrate population density and diversity were studied in a mesotrophic lake with seasonal hypoxia/anoxia prior to and following aluminum sulfate (alum) treatment and hypolimnetic oxygenation. Historically, low oxygen concentrations created a benthic zone with reduced fish predation pressures which allowed chaoborids to flourish. Population densities of chironomids and oligochaetes were suppressed by prolonged periods of summer anoxia Benthic macroinvertebrate community diversity values were indicative of poor water quality and/or habitat quality.