Aquatic insects in Tahoe basin streams: The link between stream restoration and biological assessment

The purpose of this research was to quantify the effects of anthropogenic habitat degradation and restoration on stream insects in the Tahoe basin, located in the Sierra Nevada Mountains of California and Nevada. Two hypotheses were tested: (1) Restored streams have similar aquatic communities to reference streams; and (2) Degraded streams have lower biodiversity and species richness indices than reference or restored sites. Five streams were chosen in three categories: reference, impacted and degraded, and impacted and restored. Sampling was conducted during the streams' baseflow conditions of 1999 and 2000. Five riffles at each of two sites per stream, within and above a current or future restoration, were sampled. Four categories of environmental variables were measured: geographical, physical, chemical and biological. The surveys conducted in the Tahoe basin, revealed that: (1) There were a total of 93 genera present in the Ephemeroptera, Plecoptera, and Trichoptera (EPT) orders and 27 families identified for the remainder of the aquatic insect orders; (2) In 1999, the environmental variables that best categorized the streams were characterized by shaded riffles and larger sized stream bottom substrate, and high temperature/low dissolved oxygen gradient, (Principal Components #2 and #3, respectively); (3) In 2000, the Principal Components #2 and #4, comprising of the low temperature/high dissolved oxygen gradient (PCA #2) and the high temperature/high conductivity (PCA #4) gradient represented the most important abiotic characteristics of the streams; (4) Insect abundances were significantly different by treatment with the greatest numbers in the impacted sites, followed by the reference sites and the lowest abundances in the restored sites; (5) Differences in EPT taxa richness were not significant, although values were slightly lower in the impacted sites compared to the reference and restored sites; (6) EPT diversity (Shannon-Weiner Index) was significant by both treatment and year; and (7) the Biotic Index values ranged from 0.63 to 5.65, giving a water quality rating of excellent to fair. The practical and applied benefits of this research was to assist policy makers by providing direction to spending future monies for stream restoration projects in the Tahoe basin.