Hyporheic Zone Microbiome Assembly Is Linked to Dynamic Water Mixing Patterns in Snowmelt‐Dominated Headwater Catchments

Abstract

Terrestrial and aquatic elemental cycles are tightly linked in upland fluvial networks. Biotic and abiotic mineral weathering, microbially mediated degradation of organic matter, and anthropogenic influences all result in the movement of solutes (e.g., carbon, metals, and nutrients) through these catchments, with implications for downstream water quality. Within the river channel, the region of hyporheic mixing represents a hot spot of microbial activity, exerting significant control over solute cycling. To investigate how snowmelt-driven seasonal changes in river discharge affect microbial community assembly and carbon biogeochemistry, depth-resolved pore water samples were recovered from multiple locations around a representative meander on the East River near Crested Butte, CO, USA. Vertical temperature sensor arrays were also installed in the streambed to enable seepage flux estimates. Snowmelt-driven high river discharge led to an expanding zone of vertical hyporheic mixing and introduced dissolved oxygen into the streambed that stimulated aerobic microbial respiration. These physicochemical processes contributed to microbial communities undergoing homogenizing selection, in contrast to other ecosystems where lower permeability may limit the extent of mixing. Conversely, lower river discharge conditions led to a greater influence of upwelling groundwater within the streambed and a decrease in microbial respiration rates. Associated with these processes, microbial communities throughout the streambed exhibited increasing dissimilarity between each other, suggesting that the earlier onset of snowmelt and longer periods of base flow may lead to changes in the composition (and associated function) of streambed microbiomes, with consequent implications for the processing and export of solutes from upland catchments. Plain Language SummarySeasonal changes in river discharge in high-altitude watersheds affect patterns of surface and groundwater mixing in the hyporheic zone (the region in the riverbed where these two water sources interact) that impacts how carbon compounds and dissolved metals are transported.One key constraint with respect to hyporheic mixing concerns the rate of water flow, a process driving the change in oxygen concentration in the riverbed. Oxygen concentration controls rates of carbon degradation and metal inputs from surrounding rocks, which, in turn, affect downstream water quality. In this study, we examined a stream in an alpine watershed, the East River, in the Upper Colorado River Basin, sampling a representative meander at discrete depths throughout the year for microbiological and geochemical analyses. We also installed data loggers to continuously collect temperature information to understand the extent of hyporheic mixing. We found that the movement of dissolved materials was strongly correlated with seasonal changes in flow. Under maximum flow, increased oxygen concentration stimulates microbial degradation of carbon compounds, and conversely, during minimum flow, decreased oxygen c...

Heterogeneity in Hyporheic Flow, Pore Water Chemistry, and Microbial Community Composition in an Alpine Streambed

“…Counter to our hypothesis, we demonstrate a lack of strong relationship between microbial diversity and groundwater‐related parameters such as conductivity and DO across a reach at the same streambed depth; we expected less diversity with greater groundwater influence (Danczak, Sawyer, et al, ). Interestingly, Saup et al () measured vertical DO and conductivity profiles at the same site and observed similar concentration ranges but a larger range in microbial diversity, compared to our lateral sampling scheme. They report DO and conductivity values of 1.92 to 8.9 ppm and 190 to 479 μS/cm, respectively (compared to 2.49 to 8.44 ppm and 263.7 to 544.7 μS/cm, this study).…”

“…The goal of this study was to understand relationships between heterogeneity in water flow, geochemistry, and microbial communities within the hyporheic zone of a meandering channel across lateral gradients. Previous data from our field site (East River, Colorado, USA) show seasonal variability in hyporheic pore water chemistry and microbial assemblages along vertical gradients at three locations (Saup et al, ). Microbial community diversity and aerobic respiration rates increase during the spring snowmelt season when oxic surface water infiltrates the hyporheic zone due to increased turbulence and bedform‐current interactions.…”

“…The study site lies within the U.S. Department of Energy‐supported Lawrence Berkeley National Laboratory's Watershed Science Focus Area and is immediately downstream of its Pumphouse intensive study site location (38.922245°N, −106.950650°W). The 200‐m reach encompasses one meander (Figure ) and contains three vertical profiles where microbial composition and pore water chemistry have been monitored across seasons (Saup et al, ). The reach has an average channel gradient of 0.01 and average channel width of 8 m. Flow depth ranges from approximately 3 cm in riffles to 1 m in pools during baseflow.…”

Ecological Assembly Processes Are Coordinated between Bacterial and Viral Communities in Fractured Shale Ecosystems

“…Such environmental filtering has been observed when divergent geochemical conditions affected community composition ( 24 ). If βNTI is less than −2, homogenous selection pushed the observed communities toward a common configuration, as can be observed when river water exerts a potentially common set of stressors in some riverbeds ( 25 , 26 ). When |βNTI| is less than 2, however, communities are as different as expected by random chance due to stochastic processes.…”

Groundwater–Stream Connectivity Mediates Metal(loid) Geochemistry in the Hyporheic Zone of Streams Impacted by Historic Mining and Acid Rock Drainage

“…Hydrological parameters such as permeability, transient storage zone area, fluid exchange rate, and residence time influence hyporheic zone characteristics (e.g., Miller et al, 2006;Boano et al, 2014). These parameters vary seasonally as a function of changing temperature (Weber et al, 2010), discharge (Wondzell, 2006), and microbial processes (Saup et al, 2019). Seasonal changes in stream discharge and subsurface saturation may, in turn, affect the kinetics of metal(loid) release from the hyporheic zone, given that abiotic and biotic process rates are hypothesized to be inherently different in areas characterized by variable fluid saturation and redox chemistry compared to permanently oxic or anoxic environments (Borch et al, 2010).…”