Watershed hydrology and salinity, but not nutrient chemistry, are associated with arid-land stream microbial diversity

Microbiota in streams drive many ecosystem functions, including whole-stream metabolism, nitrogen (N) cycling, and the production of basal resources that fuel stream food webs. Interactions between surface water and shallow, subsurface groundwater produce the oxygen and nutrient gradients that influence these microbially mediated biogeochemical functions. Microbial nutrient processing is often limited by nutrient availability, but we lack a clear understanding of the relationships between hydrology, water chemistry, microbial composition, and nutrient cycling. In this study we evaluated the prediction that the microbial (bacterial and archaeal) assemblage composition in surface and subsurface water (both within and among stream reaches) would be related to dissolved nutrient concentrations, surface-subsurface hydrologic connectivity, and reach-scale N cycling rates. To evaluate our predictions, we collected data on water chemistry, whole-stream hydrological connectivity, biogeochemical function, and surface and subsurface water microbial assemblage composition at 6 streams in the southwestern USA. We found no correlation between microbial assemblage composition and stream nutrient concentrations or cycling rates, but observed that subsurface waters in some streams had higher taxonomic richness than surface waters. Instead, differences in microbial assemblage composition among the study streams were correlated with both watershed size and stream water Br- concentrations. These results suggest that the longer, deeper groundwater flowpaths in larger watersheds, which promote solute accumulation, influence streamwater microbial assemblage composition. These contrasting streamwater microbial assemblages were associated with different reach-scale sinks for N immobilization (fine benthic organic matter versus filamentous algae). Overall, results suggest that the catchment-scale history of water movement, and salinity, may affect both microbial diversity and the fate of N. The mechanisms by which stream microbial diversity influences ecosystem function require more attention. In particular, we need a better understanding of how microbial biogeography is influenced by the spatiotemporal heterogeneity of stream ecosystems.