Bacterial community composition and carbon metabolism in a subtropical riverscape

To better understand the interaction between the abundance and composition of bacterial communities, physicochemical conditions, and bacterial metabolic function, we studied the patterns of bacterial community composition and metabolic function in relation to nutrients, land use, and network position in a large subtropical river. Composition varied across the watershed, and with variation in nutrients in particular. Although these changes were partially driven by overall abundance, bacterial groups responded to nutrients differently across the watershed. Most groups were correlated with suspended particulate materials, yet, abundance of beta-proteobacteria was highest with elevated nitrate, and abundance of Actinobacteria was highest with elevated soluble reactive phosphate. Land use weakly influenced composition. Landscape position influenced composition, with downstream mainstem riverine sites differing from more upstream and tributary sites. Both production and respiration were influenced by temperature and organic carbon, but we found that production and growth efficiency were not directly related. Production peaked at intermediate levels of abundance, but no groups were correlated with production. Respiration by free-floating bacteria was positively correlated with proportional abundance of alpha-proteobacteria, gamma-proteobacteria, and Actinobacteria. Finally, we found that land use was a relatively weak influence on community composition, but that variation in nutrients was relatively important for both composition and function.