Urbanization, sedimentation, and the homogenization of fish assemblages in the Etowah River Basin, USA

We tested the hypothesis that urbanization alters stream sediment regimes and homogenizes fish assemblages in 30 sub-basins of the Etowah River. Sediment variables included average particle size ( mean phi) of the stream bed, percent fines (< 2 mm) in riffles, and baseflow turbidity (NTU). Homogenization was quantified as ratios of endemic to cosmopolitan species richness (E-r: C-r) and abundance (E-a: C-a). High NTU and fine stream beds were associated with homogenized assemblages (i.e., lower E: C ratios). Mean phi and NTU were significantly correlated with E: C ratios (r = -0.74 to -0.76) and, when combined using multiple regression, accounted for 73% of the variance in ratios. Stream slope strongly covaried with mean phi ( r = - 0.92) and percent fines in riffles ( r = - 0.79), but multiple regression models showed that urbanized sites had finer beds and riffles than predicted by slope alone. Urban land cover was the primary predictor of NTU (r(2) = 0.42) and, combined with slope in multiple regression, explained 51% of the variance in NTU. Our results indicate that stream slope is a background variable predicting particle size and E: C ratios in these streams. Urbanization disrupts these relationships by transforming clear streams with coarse beds into turbid streams with finer beds. These conditions favor cosmopolitan species, ultimately homogenizing fish assemblages. Bed texture was linked to urbanization; however, NTU was the best indicator of urban impacts because it was statistically independent from slope.