Effect of increased winter drawdown on benthic macroinvertebrates in Namakan Reservoir, Voyageurs National Park
Kraft KJ. 1988. Effect of increased winter drawdown on benthic macroinvertebrates in Namakan Reservoir, Voyageurs National Park. NPS Research/Resources Managment Report MWR - 12. National Park Service - Midwest Regional Office. Omaha, NE
The effect of increased winter drawdown on benthic macroinvertebrates in Namakan Reservoir, Voyageurs National Park, was studied by comparing invertebrate samples from Reservoir locations with those from Rainy Lake which had little drawdown. During the three years of the study, drawdown averaged 2.5 meters in the Reservoir and 1.1 meters in Rainy Lake. Macroinvertebrates collected at depths of 1, 2, 3, 4 and 5 meters at three Reservoir locations were compared with those collected at two locations in Rainy Lake. Dead, stranded invertebrates were commonly found in the Reservoir during winter drawdown. One taxon, the isopod genus Asellus, was common in Rainy Lake but absent from Reservoir samples. Five other taxa, Caenis, Chaoborus, Gastropoda, Hexagenia, and Sialis, showed marked reductions in density in the Reservoir locations compared to Rainy Lake locations. Invertebrate density showed more variation at the Reservoir locations than in Rainy Lake, especially at drawdown depths of one to three meters. Mean invertebrate diversity values were lower at the Reservoir locations, and the difference in diversity values between the Reservoir and Rainy Lake locations was greatest at depths of one and two meters. Mean equitability values were lower at Reservoir locations, especially at depths of one and two meters. The range of equability values was much greater at reservoir locations because of lower values found there. Invertebrate density at Reservoir locations was significantly higher in the year with the least drawdown during the study.