COUPLED BIOGEOCHEMICAL CYCLING AND HYDROGEOLOGIC PROCESSES IN THE HYPORHEIC ZONE OF THE SAN SABA RIVER, TEXAS

Although biogeochemical processes in the hyporheic zone (HZ) of riparian ecosystems are controlled by microbial communities, the spatial scale and the extent of these processes vary considerably. Because of this, accurately modeling river and HZ biogeochemical processes is challenging. The goal of this study was to examine HZ hydrogeology, geochemistry, and microbial communities at varying spatial scales (riffle to watershed) in the San Saba River of the Colorado River watershed in Central Texas. This river was selected because previous efforts have been made to understand how human activities, such as groundwater extraction, impact water resources, but effects of the HZ on river hydrology and biogeochemistry remain largely undetermined.

The main hypothesis tested was that groundwater contributions to the HZ regulate existing geochemistry and microbial communities. Specifically, if the river is hydrologically gaining (i.e., groundwater discharges into the river), less organic matter and higher reduced gases will be observed, compared to the HZ in segments of the river that are losing (i.e., river water recharges the groundwater). Water was collected from the HZ using Bou-Rouch samplers, and analyzed for major and trace ions, inorganic and organic carbon and nitrogen species, and dissolved gases. Environmental DNA was extracted from filters and used for sequencing 16S rRNA genes, from which taxonomic profiles were evaluated using multivariate statistical approaches to evaluate compositional changes.

Results indicated that microbial communities varied in individual riffle locations, among the HZ sites and overlying surface water, and from upstream to downstream along the whole river system. Differences in community composition correlated with hydrogeology, geochemistry, and spatial position at a pool-riffle sequence, such that gaining headwater sites had lower relative abundances of photosynthetic bacteria, like cyanobacteria, but higher hydrogen sulfide and methane concentrations that indicate anaerobic conditions in the HZ. Results from this study provide strong support for integrating microbial community diversity information into future ecological models to understand how riverine ecosystems respond to climate change and to improve river restoration outcomes.