Microbial communities reveal sources of streamflow in response to early-season storm event
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Persistent gaps in our understanding of watershed processes, including streamflow generation, attest to the limitations of current hydrologic tools and the urgent need to explore new approaches. Common hydrogeochemical tracing tools, such as stable isotope analysis, offer incomplete information, in part because the many complex processes involved in streamflow generation and water storage are integrated into a one- or two-dimensional datapoint. In contrast, aquatic microbial communities in streams are composed of thousands of unique taxa, originating from a variety of sources, including groundwater, sediment, stable upstream communities, and the upslope terrestrial environment. In this study, we explore the dynamics of the streamwater microbial community response to a precipitation event on the Marys River in Oregon, USA. We collected daily samples for DNA and stable water isotopes (δ18O and δ2H) from the Marys River before, during, and after a large, isolated precipitation event. Although variation in isotope ratios suggested distinct pre-event, early-event, and late-event water sources, microbial communities were much more sensitive and responded more dynamically to the stream event response. Microbial diversity metrics closely tracked streamflow volume, and cluster analysis of microbial community composition revealed coherent pre-event, early-event, and late-event communities. Furthermore, abundance of distinct taxonomies of microbes were diluted with increasing streamflow volume, including groups commonly associated with freshwater. In contrast, groups mobilized with increasing streamflow included taxa that are associated with terrestrial environments, suggesting a growing contribution of water from the hillslope. Thus, whereas traditional geochemical tracers offer aid in inferring the age of water in the stream channel, microbial communities, through their diversity and biogeochemical interaction with the environment, offer additional information on the sources and pathways of water to the stream channel. This study provides an approach for integrating information from DNA suspended in the water column into an investigation of a hydrologic response that incorporates tools from both hydrology and microbiology and contributes to growing evidence that microbial DNA is useful not only as an indicator of biodiversity but also as an innovative hydrologic tracer.