AN ECOLOGICAL STUDY OF A REGULATED PRAIRIE STREAM IN WESTERN MONTANA

Abstract

Longitudinal and seasonal changes in biophysical variables were examined in this seminal study of an intermountain (Palouse) prairie stream. Hypolimnial releases from an irrigation reservoir controlled discharge, temperature and other habitat variables, thereby influencing distribution and abundance of zoobenthos. Dramatic changes in turbidity and nutrient concentrations occurred during the spring runoff and summer irrigation periods and following wind-driven sediment resuspension in the reservoir. Seasonal changes in the relative dominance of certain macroinvertebrate taxa were correlated with physical and chemical dynamics. Based on detrended correspondence analysis (DCA), specific conductance, temperature, dissolved oxygen and nitrates + nitrites were the variables most associated with seasonal macroinvertebrate biomass. Changes in taxa composition downstream from the dam were mainly related to species additions from site to site. Changes in pH and substratum were associated with longitudinal changes in biomass. For instance, the observed bimodal distribution of Cheumatopsyche sp. was correlated with percentage boulder substratum among the sites. On the other hand, DCA performed on macroinvertebrate densities emphasized pH, total ammonia, discharge and substratum. Species composition throughout the prairie segment of the stream was very different from that in the mountain canyon segment. The effects of regulation ameliorated significantly in a downstream direction from the dam. Presence of warm water species (e.g. Helicopsyche sp.) upstream and downstream of the reservoir were indicative of the native stream fauna of the intermountain prairies. Faunal responses to the ‘recovery’ gradient downstream from the dam provided a basis for the development of remedial management actions.