Bacteria inhabiting boreal freshwaters are part of metacommunities where local assemblages are often linked by the flow of water in the landscape, yet the resulting spatial structure and the boundaries of the network metacommunity have never been explored. Here, we aimed at reconstructing the spatial structure of the bacterial metacommunity in a complex boreal aquatic network in Quebec, Canada, by determining the changes in taxonomic composition across 220 bacterial assemblages inhabiting the entire terrestrial/aquatic continuum, ranging from the smallest headwater streams and their surrounding soils and soilwaters, to the largest rivers and lakes.
Results/Conclusions
We show that the network metacommunity has a directional spatial structure that changes sequentially from the terrestrial environment to the smallest streams, rivers and finally lakes. Interestingly, all bacterioplankton communities were numerically dominated by taxa derived from the terrestrial environment, which accounted for over 75% of all aquatic sequences. In the smallest headwater streams this dominance was due to the continuous immigration of bacteria washed from the soils, most of which gradually disappeared along the hydrologic continuum. However, a few of these terrestrial taxa were able to grow and become progressively abundant along a gradient of increasing water residence time, and largely dominated downstream ecosystems. Our results indicate that boreal freshwater bacterioplankton communities share a common terrestrial origin and that local community assembly is determined by hydrologically driven variations in the balance between mass effects and species sorting of terrestrial taxa along the aquatic continuum. This implies that soil bacterial assemblages are clearly part of the network metacommunity, and highlights that understanding the mechanisms that shape communities in these aquatic networks requires expanding the scale of study beyond the limits of the aquatic environment.