Metacommunity ecology studies the local and regional factors that influence community assembly. Assembly can proceed based on local interactions that select for some taxa over others (i.e., species sorting), but it can also be influenced by immigration (i.e., mass effects) and by dispersal limitation. Stream ecosystems are ideal for studying assembly processes because their dendritic network structure allows for field tests of the role of dispersal directionality, while their habitat heterogeneity (e.g., planktonic vs. benthic) allows for tests of whether different assembly processes predominate in different habitat types. We predicted that the increased dispersal ability of planktonic bacteria should favor mass effects, while the decreased mobility of sediment-attached bacteria should favor species sorting and dispersal limitation. Further, we predicted high taxonomic turnover among headwater streams due to high rates of terrestrial immigration or dispersal limitation. We tested these predictions by characterizing the planktonic and sediment-associated bacterial communities, as well as environmental variables, in a stream network in the H. J. Andrews Experimental Forest, Oregon. To further distinguish between species sorting and dispersal limitation in sediment-associated bacterial communities, we conducted a field experiment that allowed bacterial communities to assemble on empty habitat patches of different quality across the stream network.
Our findings are consistent with the hypothesis that dispersal plays a more important role in community assembly for planktonic than for sediment communities. Bacterioplankton communities were distinct from and more diverse than sediment communities. Nearly 25% of planktonic taxa were absent from sediments, whereas only 13% of sediment taxa were absent from planktonic samples. This suggests that sediment bacteria often occur in plankton, presumably via resuspension, but many planktonic bacteria fail to colonize sediments and thus may be terrestrial-derived transients. Bacterioplankton communities show no decrease in similarity with environmental dissimilarity, but sediment communities do. These findings suggest that species sorting may be more important in sediments, while plankton may be more susceptible to mass effects. However, sorting is only seen in downstream sediments, perhaps because headwater communities exhibit greater exchange with surface waters and increased terrestrial inputs. The colonization experiment suggests that higher dissimilarity in headwater streams is not due to dispersal limitation, but rather, may be a consequence of higher immigration rates from terrestrial environments relative to downstream reaches, where terrestrial influence has been reduced. Together, this work suggests that metacommunity dynamics, and especially planktonic–sediment interactions, play an important role in structuring bacterial diversity in stream networks.