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Abstract
In stream ecosystems, primary producers (i.e., algae) are an important food resource to consumers due to their high dietary quality relative to detritus. In headwater (or lower stream order) streams draining the southern Appalachian Mountains, forest ecosystems exert control over algal communities by regulating light availability to stream reaches, stream-water nutrient concentrations, flows, and temperatures. However, southern Appalachian forests are changing due to the massive die-off of a foundation tree species, expansion of a native invasive riparian shrub, and low-density development on once-forested mountainsides. In this dissertation, we used historical data sets, reach-scale riparian manipulations, trophic dynamic experiments, and stable isotope analyses to assess how riparian and basin-wide changes to southern Appalachian forests have affected algal communities and higher trophic levels. First, we compared algal biomass and community composition before and after the die-off of a foundation riparian tree, Tsuga canadensis (eastern hemlock). Thirteen years following hemlock death, we found lower algal standing crops and little change in community composition; findings suggest that this is likely due to the expansion of the native riparian shrub, Rhododendron maximum (rhododendron). Next, using plot-scale crayfish exclusion experiments nested within reach-scale rhododendron removal manipulations, we examined how rhododendron removal as a forest management strategy might affect algal communities while accounting for stream trophic dynamics. Short-term (2 year) response to rhododendron removal included higher total algal cell biovolume and a shift in algal community composition. Findings indicate that rhododendron removal resulted in an increase in the relative abundance of filamentous green algal taxa, with food webs potentially becoming more autochthonous. Finally, we compared algal and fish communities between forested and suburbanizing watersheds at three points in time over a ten-year period (2000, 2005, 2010). For both algae and fishes, we found evidence of biotic homogenization and taxonomic change within suburbanizing reaches, but not to the level where endemic taxa were extirpated. Combined studies show how slight changes to near-stream riparian areas or their forested watersheds can significantly change stream algal standing stocks and community composition, with changes to primary producers cascading up to higher trophic levels.