AN ECOLOGICAL STUDY OF A REGULATED PRAIRIE STREAM IN WESTERN MONTANA

Longitudinal and seasonal changes in biophysical variables were examin ed in this seminal study of an intermountain (Palouse) prairie stream. Hypolimnial releases from an irrigation reservoir controlled discharg e, temperature and other habitat variables, thereby influencing distri bution and abundance of zoobenthos. Dramatic changes in turbidity and nutrient concentrations occurred during the spring runoff and summer i rrigation periods and following wind-driven sediment resuspension in t he reservoir. Seasonal changes in the relative dominance of certain ma croinvertebrate taxa were correlated with physical and chemical dynami cs. Based on detrended correspondence analysis (DCA), specific conduct ance, temperature, dissolved oxygen and nitrates + nitrites were the v ariables most associated with seasonal macroinvertebrate biomass. Chan ges in taxa composition downstream from the dam were mainly related to species additions from site to site. Changes in pH and substratum wer e associated with longitudinal changes in biomass. For instance, the o bserved 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 ca nyon 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 ind icative of the native stream fauna of the intermountain prairies. Faun al responses to the 'recovery' gradient downstream from the dam provid ed a basis for the development of remedial management actions.