COMPARATIVE ECOLOGY OF BENTHIC COMMUNITIES IN NATURAL AND REGULATED AREAS OF THE FLATHEAD AND KOOTENAI RIVERS, MONTANA

A comparative study was made of environmental variables and the density, biomass, diversity, and species composition of macroinvertebrates in areas downstream from a dam with a hypolimnetic release (Hungry Horse Dam on the Flathead River) and a dam with a selective withdrawal system (Libby Dam on the Kootenai River). A major objective of this study was to examine the response of macroinvertebrate communities to defined environmental gardients in temperature, flow, substrate, and food-related variables (periphyton, particulate organic carbon in the seston). In addition, the effects of experimental manipulations in discharge on macroinvertebrate drift and stranding were assessed, and the effects of temperature on the growth rates and emergence of five species of insects were measured.; Macroinvertebrate species diversity was severely reduced and composition was grossly altered at the station below the hypolimnetic release dam; species richness and Shannon diversity were intermediate at the stations downstream from the dam with the selective withdrawal system. Each of three ordination techniques used to assess similarities between macroinvertebrate communities produced a spatial gradient from the station most stressed by regulation to the reference station. The annual mean densities of macroinvertebrates were 1.5 to 2.5 times higher at the regulated than the reference stations; annual mean biomass was 1.5 times higher at the station nearest the dam on the Kootenai River but was approximately the same at regulated and reference stations on the Flathead River.; Correlation analyses between environmental variables and macroinvertebrate data produced significant correlations between rates of change in velocity, substrate heterogeneity, POC in the seston, AFDW and Chl a in the periphyton, benthic primary productivity, and macroinvertebrate diversity and composition.; Macroinvertebrate surface drift increased an order of magnitude during increasing discharges. High shoreline drift densities were measured after flow reductions when the prior discharge regime had been sustained at high levels.; Regulation produced subtle effects on the life histories of five species of insects.