Dispersal limitation, environmental filtering, mass effects, and species sorting are just a few of the processes that shape local community structure from the regional species pool. However, these processes remain understudied in lotic ecosystems. Stream networks are hierarchical, branching, and (largely) one-directional. Microorganisms, such as bacteria, may be particularly susceptible to the flow-through nature of stream ecosystems because their dispersal is highly dependent on stream flow. On the other hand, bacteria that inhabit streambed sediments may experience reduced washout risk, allowing them to establish more stable local populations in the sediments.
To test these predictions, we sampled a stream network at the H. J. Andrews Experimental Forest, Oregon, USA, in June of 2015. Across the 6400 ha catchment, we sampled surface water and sediment-associated microbial communities, and measured a suite of environmental variables. We extracted DNA from each microbial community and sequenced the V-4 region of the 16S rRNA gene on the Illumina MiSeq platform. We calculated local and regional biodiversity metrics, and used Principal Coordinates of Neighbor Matrices (PCNM) and partitioned the variance in community structure into environmental and spatial components.
Downstream surface water communities are more highly connected to upstream communities than downstream sediment-associated communities are to upstream sediments. Using elevation as a surrogate for along-stream distance, elevation-decay relationships suggest that downstream surface waters remain more closely related to upstream surface waters, while sediments experience a steeper decay. Dispersal limitation by elevation may affect sediment-associated communities more than surface water communities, while mass effects may occur more frequently in surface waters.
Furthermore, surface water communities are consistently more diverse and distinct from sediment-associated communities. Surface water communities from a subset of the larger catchment are more similar to one another than surface water communities from across the catchment. However, sediment-associated communities do not share this spatial pattern. We propose that strong environmental filters exclude most surface water bacteria from stream sediments, and that bacteria in the surface waters may be transient, and potentially influenced by the neighboring terrestrial ecosystem. Thus, sediment-associated communities may be colonized from the regional pool of propagules that disperse downstream via surface water flow.