The Effects of a Small Reservoir on the Seasonality and Stability of Physicochemical Parameters and Macrobenthic Community Structure in a Rocky Mountain Stream

Physicochemical and biotic characteristics of a headwater stream community above an impoundment on the Sante Fe River, New Mexico, were compared to those in its tailwaters. There was less seasonality in abiotic factors, particularly in discharge and temperature below the impoundment. The streambed below the reservoir was more heterogenous and more stable than that above. Macroinvertebrate density was almost five times greater below the reservoir as a result of enhanced temperature and flow constancy, greater substrate stability and the abundance of aquatic macrophytes at this site. Species richness was also greater below the impoundment and was directly related to its habitat heterogeneity. Reduced discharge and more constant temperatures in conjunction with extremely high macroinvertebrate densities resulted in reduced diversity below the impoundment. Evenness was also reduced. An analysis of seasonal evenness variances and a study of the seasonal abundance patterns of individual taxa indicated that the more constant physicochemical conditions below the reservoir probably decreased biotic interactions and reduced the community stability at this site.