HYPORHEIC ZONE HYDRODYNAMICS, BIOGEOCHEMISTRY, AND MICROBIAL COMMUNITY DISTRIBUTIONS FOR THE SAN SABA RIVER, TEXAS

The hyporheic zone (HZ) plays a crucial role in riparian ecosystems by providing essential habitat and contributing to biogeochemical cycling. Although microorganisms are known to control most of the biogeochemical processes occurring in the HZ, past investigations of microbial communities within the HZ have been limited in scope. This study examines changes in geochemical processes, associated microbial communities, and other environmental factors at large and small scales within the HZ of the San Saba River in Central Texas. Pool-riffle sequences spanned approximately 228.6 to 22.4 kilometers upstream from the confluence of the San Saba and Colorado Rivers. Up to five different Bou Rouch (BR) samplers were installed at each pool-riffle sequence to access the HZ. Aqueous physiochemistry and microbial community compositions were determined from river water and water extracted from the HZ. Discharge was determined at each location and hydraulic gradients and bedload permeability were measured. Six pool-and-riffle locations had gaining hydrological conditions, such that groundwater was discharging into the HZ, and five locations had losing conditions, meaning that river water was recharging the HZ. Geochemical indicators of microbial activity generally reflected gaining/losing conditions, and dissolved sulfide and methane concentrations were greater in gaining reaches. The HZ had higher microbial diversity compared to the river, but also more unclassified taxonomic groups, which suggested that the HZ may be a reservoir for novel taxa. Microbial diversity was also higher from gaining sites. Gammaproteobacteria, Actinobacteria, Bacteroidia, and Cyanobacteria were more prevalent under losing conditions, and Planctomycetes, Dehalococcoidia, Bdellovibrionia, Anaerolineae, and Omnitrophia were more prevalent from gaining conditions. Furthermore, post-hoc multivariate testing confirmed the effects of lithology, hydrologic conditions, and the location within the pool-riffle sequence and distance along the San Saba River on various geochemical parameters and microbial communities. This study enhances our understanding of the spatial and environmental drivers shaping microbial communities and geochemical processes in the HZ of the San Saba River. The findings emphasize the significance of both large and small-scale factors in shaping microbial diversity and community structure within this ecologically important aquatic zone.