Influence of geomorphology and urban land cover on stream fish assemblages in the Etowah River basin, georgia
Walters, David M
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The Etowah River basin supports a diverse and imperiled fish assemblage increasingly threatened by urbanization. I quantified relationships between fishes, geomorphology, and urbanization in 32 wadeable Piedmont streams in basins of 11-126 km2. I developed an Index of Biotic Integrity (IBI) using six fish metrics that tracked changes in habitat quality. Low quality sites had more tolerant fishes and higher centrarchid richness, but fewer darter, sculpin, and insectivorous cyprinid species and lower richness and density. IBI scores were positively correlated with basin forested land cover, indicating that biotic integrity was highest in streams draining least-disturbed catchments. A separate multivariate statistical analysis identified strong patterns in species composition that were highly correlated with stream geomorphology. Species composition in these streams shifted from centrarchids and other pool species to darters, cyprinids, and redhorse suckers that are riffle-run, benthic species. Richness and density were correlated with stream size, but species composition was correlated with reach-level variation in stream slope and related benthic habitat variables that were independent of stream size. These findings contrast with a prediction of the River Continuum Concept that species composition varies predictably along stream size gradients. My results support the Process Domain Concept that local geomorphic processes determine stream habitats and disturbance regimes that influence assemblage structure. Urbanization altered the relationships between geomorphology and fishes. After accounting for the effects of stream size and slope, richness, density, darters and sculpin, cyprinids, and endemic species declined with urban land cover, whereas centrarchids persisted and became the dominant species group. Residual analysis indicated that effects were greatest for streams with > 15% urbanization. Most of the development occurred since 1987, suggesting that fishes respond rapidly to urbanization. Urbanization appeared to homogenize fish assemblages by altering stream sediment regimes. Homogenization, quantified as the ratios of Endemic to Cosmopolitan species richness (Er:Cr) and abundance (Ea:Ca), was associated with high baseflow turbidity and finer beds. Urban land cover positively correlated with baseflow turbidity, and urbanized sites had finer beds and riffles than predicted by stream slope. Baseflow turbidity was the best indicator of urban impacts because it was statistically independent from slope.