Microbial community assembly and dynamics within a mixed-use temperate watershed

Streams and rivers play key roles in global biogeochemical cycles, acting as major transport pathways and gatekeepers governing the movement of carbon and nutrients between land and sea. We are interested in the assembly and composition of the pelagic microbial communities of streams. Pelagic stream environments are inhabited by unique microbial communities that are distinct from the prevailing microbial populations of surrounding soils and the stream benthos.  We are utilizing high-throughput 16S rRNA gene sequencing to comprehensively characterize microbial community composition and dynamics across a large stream network.           

We have collected a time series of stream microbial community samples distributed throughout a mixed-use, temperate watershed.  We have found that small, headwater streams are typified by high-diversity microbial communities with low representation of known aquatic microbial taxa.  As water moves through the stream network, overall species diversity decreases while the relative abundance of aquatic microbes increases.  This is consistent with previous studies of low-disturbance environments.  However, for two of the five watershed surveys that we conducted, this pattern was disrupted, with species diversity remaining high and the abundance of aquatic microbes low across all stream orders.  We hypothesize that this disruption in community assembly was caused by precipitation-driven alterations in discharge and associated changes in dispersal of microbes from terrestrial environments and in-stream residence time.  Together, these data suggest that microbial community assembly in streams is a highly dynamic process that is shaped by both intrinsic properties of the stream and external environmental factors.