Exploring the River Continuum Concept for Bacterioplankton

Lotic ecosystems such as rivers and streams are unique in that they represent a continuum of both space and time during the transition from headwaters to the river mouth. As microbes have very different controls over their ecology, distribution and dispersion compared to macrobiota, we wished to explore biogeographical patterns within a river catchment and uncover the major drivers structuring bacterioplankton communities. Water column samples collected from the river channel across the River Thames Basin, UK, covering the transition from headwater tributaries to the lower reaches of the main river channel, were characterised using pyrosequencing. This approach revealed an ecological succession in the bacterial community composition along the river continuum, moving from Bacteroidetes dominated in the headwaters to an Actinobacteria dominated community downstream. Location of the sampling point in the river network (measured as the cumulative water channel distance upstream) was found to be the most predictive spatial feature above other measures of position such as Euclidian distance between sites and site catchment area; inferring that ecological processes pertaining to temporal community succession are of prime importance in driving the assemblages of riverine bacterioplankton communities. A decrease in bacterial activity rates and an increase in the abundance of low nucleic acid (LNA) bacteria relative to high nucleic acid (HNA) bacteria were found to correspond with these downstream changes in community structure, indicating that functional changes correspond with the taxonomic succession. Our findings suggest that bacterial communities across the Thames basin exhibit a marked ecological succession along the river continuum, and that this is primarily driven by water residence time rather than the physiochemical status of the river.