A Study of the Macroinvertebrate Drift in the Des Moines River

Abstract

The problem. To determine the effects of Red Rock Dam on one segment of the aquatic fauna, the "drifting" macroinvertebrates. Procedure. Drift samples were taken at two points on the Des Moines River. One sampling station was located above Red Rock Reservoir and one below. Samples were taken every other week at each station, on two consecutive nights, from August 7, 1973, through November 1, 1973, and April 3, 1974, through May 28, 1974. All sampling periods began at sunset, and ended one hour later. Samples were taken in a Wildco, 33 cm X 48 cm, 491 micron mesh drift net. Findings. Thirty-one taxa were sampled above the reservoir, while twenty-three were sampled below the reservoir. Total number of organisms per cubic meter were calculated. A decrease in the total number per cubic meter was observed below the dam. The control station had 16.7 times more organisms per cubic meter. The only taxa that were more numerous below the reservoir were the filter-feeding Trichoptera, Hydropsyche, and Cheumatopsyche. This is apparently a result of the washout of detritus and lentic zooplankton, such as Leptodora, from the reservoir. A positive correlation between numbers in the drift and volume was noted, except as a result of major variations caused by life cycle stages of an individual taxon. Conclusions. The total number of organisms and the number of different species of drifting macroinvertebrates in the Des Moines River was reduced downstream from Red Rock Dam. Recommendations. The following three topics are proposed extensions of this study: 1) A study of the diurnal variation of drift in the Des Moines River. 2) A study of the transverse variation of the drift across the Des Moines River. 3) A comparison of the phytoplankton and zooplankton above and below Red Rock Reservoir.