Microbiota in stream waters drive ecosystem functions including respiration, nitrification and dentitrification, and their production fuels stream food webs. The streambed scale interaction between surface water and shallow subsurface groundwater sets up the oxygen and nutrient gradients that drive and maintain these microbially mediated biogeochemical functions at the reach and drainage scales. Yet, the extent to which differences in the surface-subsurface stream microbial composition and diversity are associated with differences in stream ecosystem function is not well understood. This study measured heterogeneity in surface and subsurface water bacterial community composition (BCC) within and among stream reaches with contrasting levels of surface-subsurface hydrologic connectivity, and evaluated the prediction that BCC heterogeneity was related to reach scale ecosystem N- and C- cycling rates. To do this, we collected data on water geochemistry, whole-stream function, and BCC and diversity (using Illumina MiSeq 16S rRNA amplicon sequencing) at six streams in the southwestern USA.
Results/Conclusions
Results showed a positive, but weak, relationship between stream reach scale surface-subsurface interaction and BCC beta-diversity, and between denitrification rate and BCC beta-diversity, providing weak support for our hypothesis. Furthermore, stream water salinity was the strongest correlate with BCC, in that higher salinity and larger watershed size was associated with lower diversity and distinct composition of stream bacteria. As is often observed in arid-land watersheds, stream salinity was associated with larger basin area; this suggests that the longer, deeper groundwater flowpaths in larger watersheds may impact stream water microbial community composition via selection and/or dispersal of certain types of bacteria. The mechanistic links between stream microbial diversity and ecosystem function deserve more attention, and given the highly spatiotemporally dynamic nature of stream ecosystems, should be considered within the template of factors driving microbial biogeography.