Macroinvertebrate drift density in relation to abiotic factors in the Missouri River

Changes in flow management to restore ecosystem health have been proposed as part of many restoration projects for regulated rivers. However, uncertainty exists about how the biota will respond to flow management changes. The objectives of this study were to estimate the relative importance of key abiotic predictor variables to aquatic macroinvertebrate drift densities in the Missouri River and to compare these results among reaches of the river. A multi-year, multi-location database of spring macroinvertebrate drift net sampling was used to develop relations between drift density and variables representing discharge, temperature, and turbidity in the Missouri River from Fort Randall Dam, South Dakota to the mouth of the Little Nemaha River, Nebraska. Multimodel inference using generalized linear mixed models and an information theoretic approach were used to estimate the relative importance of the predictor variables and the parameters. The results varied by reach. Discharge-related factors were more important at the upstream end of the study area, and turbidity was more important at the downstream end of the study area. Water temperature or degree days were also important predictors in the upstream reaches. The results below Gavins Point Dam suggest that increased macroinvertebrate drift densities are a response to reduced habitat and food availability. The results identify important variables for drift density that could be used in future experimental studies of flow manipulation for the Missouri or other large, regulated rivers.