Influences of Dams On Downstream Larval Fish and Aquatic Macroinvertebrate Community Structure and Associated Physicochemical Variables

In the context of the serial discontinuity concept, we investigated influences dams have on structuring physicochemical variables and biotic communities along the stream continuum.  Seven streams were sampled (one in each of MD, OH, and WV and four in PA) for larval fish and macroinvertebrates at predetermined distances below dammed sites. We tested the prediction that biotic richness and diversity (Shannon Index) would increase as distance from dams increased.  Furthermore, we hypothesized that dams would influence water quality and habitat variables, which would aid in explaining the structure of biotic communities along the stream continuum.  Temperature, pH, and conductivity showed significant relationships with distance from dams (p<0.05; r = 0.91, 0.62, and 0.70 respectively).  Species richness and total number of individuals captured also increased with distance from dams (r = 0.76 and 0.82 respectively).  Multivariate methods (PCA and NMDS) were used to summarize and interpret gradients in physicochemcial and biotic data respectively.  Using fourth-corner analysis we also investigated species trait and habitat associations as they relate to longitudinal changes in physicochemical variables.  We suggest that dams may impact physicochemical variables and thus biota in a longitudinal fashion that is congruent with the serial discontinuity concept.