The degree to which small natural dams affect the native bacterial nitrogen cycling community was explored by molecular methods. The identities and relative abundances of ammonia oxidizing bacteria in the sediment surrounding an artificial dam both at the surface and in the hyporheic zone were characterized. Analyses were performed using tRFLP of the conserved amoA gene using a semi-nested degenerate PCR approach. Additionally, an amoA gene library was constructed to characterize the most dominant sediment genotypes. The results of the tRFLP analyses showed clear differences between the upstream and downstream communities at different depths in the sediment column. Non-metric multidimensional scaling ordination of the tRFLP data set produced a stable one-dimensional solution with significant correlations to oxygen, pH, nitrate, and dissolved organic nitrogen levels. The sample corresponding to the hyporheic zone downstream of the dam showed 28-50% higher amoA richness and higher diversity than the other samples. All gene fragments sequenced from the samples grouped with sequences of the Nitrosospira type. Ordination of 16S rDNA tRFLP data revealed a two dimensional data structure, one axis of which had similar chemical correlation characteristics as the amoA model axis. Taken together, the results from this study suggest that the presence of the dam creates physical and chemical heterogeneity that may foster genetic diversity and community changes amongst ammonia oxidizing bacteria. (C) 2013 Elsevier Ltd. All rights reserved.