Habitat Availability and Benthic Invertebrate Population Changes Following Alum Treatment and Hypolimnetic Oxygenation in Newman Lake, Washington

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Abstract
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. Following alum treatment and prior to hypolimnetic oxygenation, chaoborid densities doubled. This was due, in part, to changes in trophic structure which provided an abundant food source for early instar chaoborids. In addition, Newman Lake was stocked with fewer trout in 1990 and 1991 resulting in reduced predation on the invertebrate community. Alum treatment had no effect on chironomid or oligochaete populations. Hypolimnetic oxygenation prevented the formation of the anoxic zone and greatly reduced the extent of the hypoxic zone (≤ 4 mg/l dissolved oxygen). During the first year of oxygenation, average available trout habitat expanded deeper into the hypolimnion to include 97% of the lake's total volume during summer stratification. We hypothesize that the expansion of hypolimnetic trout habitat coupled with an increase in the number of rainbow trout stocked in Newman Lake greatly increased fish predation on selected species of bottom organisms. Consequently, merobenthic chaoborids experienced a 90% decline in abundance following oxygenation. Benthic chironomids and oligochaetes benefited fiom higher dissolved oxygen concentrations, in spite of increased fish predation pressures, and reached their maximum densities 16 to 23 months after oxygenation began.