Flowing north from its headwaters in the Bighorn Mountains in Wyoming, the Little Bighorn River runs through the Crow Indian Reservation in southeastern Montana. Swim holes along the river’s stretch are often occupied by children in the summer and surface water is used in traditional practices, including ingestion in ceremonial practices. This small-scale, discharge driven river system was investigated for fecal indicator organism (coliform and E. coli) and microbial community composition over a three-year period (2015-2017) across varying river conditions (pre-runoff, peak-runoff, receding discharge, summer, fall and winter). Six sites, selected for their potential of human contact, importance, and spatial variability, were routinely sampled. The goals of this study were to monitor and assess the spatial and temporal variability of fecal indicator organisms, and characterize the microbial community composition of riverbed sediment, planktonic and particulate associated suspended microbial habitats. We were interested in observing potential shifts in both indicator organism concentrations and microbial community structure on temporal and spatial scales. Indicator organism data was quantified as a most probable number and the community structure assessed as 16S rRNA gene libraries sequenced via Illumina MiSeq technology. This research was made possible through invitation from the Crow Environmental Health and Steering Committee.
Results/Conclusions
Statistical analyses compared indicator organism concentrations and community composition variance across categorical variables, showing significant differences between river conditions and location. Both differences in community composition and indicator organism concentrations between sites were most distinct on a date specific level. Indicator organism concentrations were highest during peak-runoff when discharge values were largest. Minimum abundance was recorded during winter. Significant differences between upstream and downstream sites were observed for several dates, with upstream sites being elevated more frequently than those downstream. Spatial connectivity played a role in community composition, with upstream sites resulting in similar planktonic and particulate associated community structure, as well as those downstream, exemplified by ADONIS and Random-Forest Classification analyses. Alpha-diversity also increased from upstream to downstream sites. The riverbed sediment community was relatively stable between sites and conditions. Mantel tests revealed significant correlations between microbial community assemblage patterns for particulate associated, planktonic communities and measured environmental variables. These results are significant to the Crow Indian community, highlighting the distribution and patterns of fecal indicator organism concentrations in surface waters. Further, as the current literature emphasizes lakes and large-order rivers, this study contributes to freshwater microbial ecology concerning small-order rivers across constrained spatial scales.