Do Differences in Reservoir Discharge Temperatures Influence Downstream Aquatic Macroinvertebrate Communities? They Dam Near Do!

To evaluate the influences of hypo- and hyperlimnetic reservoir discharge on aquatic macroinvertebrate community structure along the stream continuum we sampled below six dammed sites (three hypolimnetic and three hyperlimnetic) and one reference stream.  Dammed sites were systematically sampled at predetermined distances up to 5,100 m along the stream gradient.  Along the stream continuum we evaluated differences in biotic richness, diversity (Shannon Index), and functional feeding groups between hypo- and hyperlimnetic discharges.  Influences of dams on water quality and instream habitat are hypothesized to aid in explaining the changes in the biotic indices.  Changes in physiochemical and biotic indices are expected to be non-linear and regulated in a multivariate fashion; therefore we employed PCA and NMDS to interpret these gradients.  Dams with hypolimnetic discharges had appreciably lower biotic indices when compared to those with hyperlimnetic discharges.  Community structure between hypolimnetic and hyperlimnetic dammed sites became more similar as distance from dams increased and approached that of the reference stream.  Our results suggest physiochemical changes due to dams likely impact aquatic macroinvertebrate community structure in a fashion congruent with the serial discontinuity concept.