Shifts in Streamwater Microbial Diversity Track Storm Hydrograph Dynamics

A thorough understanding of watershed response to precipitation events is critical as our climate shifts to produce increasingly extreme precipitation and thus hydrologic events. Hydrogeochemical tools, such as stable isotope analysis, are a common approach for tracking precipitation and identifying the source of surface water in catchments, however they sometimes lack the dimensionality necessary to capture the multitude of complex processes involved in streamflow generation and water storage. In contrast, aquatic microbial communities in streams comprise thousands of 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, where tracing streamwater sources with stable isotopes is confounded by the underlying geology. We collected daily DNA samples from the Marys River before, during, and after a large, isolated precipitation event. Stable water isotopes (δ18O and δ2H) were also analyzed. Though isotopes signatures exhibited relatively little variation, prior work in the catchment suggests that distinct pre-event, early-event, and late-event water sources are visible. DNA samples were translated into the relative abundance of different distinct taxa (~1000 in total) using 16S amplicon sequencing. Cluster analysis of the microbial composition similarly reveals coherent pre-event, early-event, and late-event microbial communities. Shifts in microbial diversity reflect changes in discharge over the course of the storm, and abundance-discharge relationships (analogous to a concentration-discharge geochemical analysis) reveal that some taxa are mobilized and others diluted over the course of the event. 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 demonstrates that microbial DNA is useful not only as an indicator of biodiversity but also as an innovative hydrologic tracer.