Synchrony and seasonality in bacterioplankton communities of two temperate rivers

The bacterioplankton community composition (measured with denaturing gradient gel electrophoresis of 16S ribosomal DNA [rDNA]) of two nonintersecting temperate rivers was nearly identical and changed synchronously over 2.5 yr, suggesting that intrinsic controls on bacteria were similar in the two rivers and that seasonal changes were driven by extrinsic factors such as climate. Most potential controls on community composition also exhibited synchrony; these included bacterial production rate (leucine incorporation), water temperature, river flow rate, and a suite of chemical measurements. Temperature and river flow rate were the best predictors of temporal patterns in diversity. However, diversity patterns also correlated with bacterial production and concentrations of dissolved organic nitrogen and nitrate, suggesting that diversity is directly or indirectly influenced by complex seasonal shifts in environmental conditions. Winter and summer communities were somewhat predictable over 3 yr, although these communities were not identical. Two polymerase chain reaction (PCR)-amplified clone libraries of 16S rDNA, constructed with summer samples from each river, were not significantly different and contained typical freshwater bacterioplankton of the beta-Proteobacteria, Bacteroidetes, and Actinobacteria, including members of five new freshwater bacterioplankton clusters. However, libraries also included several phylotypes related to bacteria from soil and sediment, indicating the potential importance of allochthonous organisms in river diversity.