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

Abstract

The hyporheic zone, where surface water and groundwater mix, is an important microbial habitat where biogeochemical reactions influence water quality. We show that spatial variability in hyporheic flow in the East River near Crested Butte, CO, drives heterogeneity in streambed geochemical conditions and microbial community assemblages, but the diversity of microbial assemblages remains nearly constant throughout the reach. In July 2018, we collected approximately 100 pore water samples at 20-cm depth and analyzed them for anions, cations, dissolved organic carbon, dissolved organic matter (DOM) quality, and basic water quality parameters. Vertical hydraulic head gradients were also measured to assess the potential for upward or downward flow, and heat tracing was used to quantify vertical flux rates at a subset of locations. We found that regions of the streambed that are more groundwater-dominated contain less dissolved oxygen, higher concentrations of reduced metals, and more microbially processed, recalcitrant DOM, while more surface water-dominated locations contain higher dissolved oxygen concentrations and terrestrially derived, labile DOM. 16S rRNA gene sequencing of extracted DNA revealed that microbial community composition varies with geochemical gradients related to hyporheic flow. These findings provide a better understanding of hyporheic controls on streambed biogeochemistry during the baseflow season, which is expected to lengthen with climate change in alpine watersheds due to earlier snowmelt onset and reduced snowpack.Plain Language Summary Groundwater and surface water mixing in streambeds (hyporheic exchange) is important for nutrient and carbon cycling and influences the overall quality of surface water. In this study, we aimed to map relationships between hyporheic exchange, pore water chemistry, and microbial communities in the streambed of an alpine river during low flow conditions. We found that regions of the streambed with greater surface water influence had larger concentrations of dissolved oxygen and microbially available carbon compounds. The composition of streambed microbial communities also shifted with changes in pore water chemistry, though communities were all similarly diverse. As climate change progresses, earlier snowmelt onset and reduced snowpack should alter flow and biogeochemical processes in streambeds. This study provides a baseline for exploring future changes in alpine streambeds.

Geomorphic and Sedimentological Controls on Hyporheic Flow in an Alpine River

“…Sediment Collection. Forty-three sediment samples were collected during August 1 -August 13, 2018 at locations corresponding to pore water sampling locations described in Nelson et al (2019) (Figure 2). The sediment grab samples were collected using a standard shovel, retrieving the top 5-8 cm of sediment.…”

“…The meander has an average channel gradient of 0.01 m/m, an average channel width of 8 m, and flow depths ranging from 3 cm in the riffles to greater than 1 m in pools at baseflow. The reach is underlain by Cretaceous Mancos shale (Nelson et al, 2019).…”

“…This study was part of a broader project to understand the microbiology and biogeochemistry of hyporheic water within a meandering alpine river. Details on pore water chemistry and microbial community composition are available in Nelson et al (2019).…”

Stability of Floodplain Subsurface Microbial Communities Through Seasonal Hydrological and Geochemical Cycles

“…Particularly in floodplain soils that host TRZs and other organic-rich layers, microorganisms have been shown to produce compounds that control redox states, and determine solubility and mobility of contaminants, including metals such as Fe and U (Davis et al, 2006;Mouser et al, 2015). Previous work has indicated that these microorganisms may be distributed based on the geochemical environment in which they reside and its respective seasonal changes (Hug et al, 2015;Danczak et al, 2016a;Nelson et al, 2019), and also that microbial metabolisms in groundwater-influenced soils are interconnected and interdependent (Wrighton et al, 2014;Anantharaman et al, 2016).…”

“…Even in water-saturated conditions, the tortuosity and size variability of pores lead to uneven oxygen diffusion rates and, hence, variable oxygenation of pores within the same soil horizon (Fan et al, 2014;Neira et al, 2015;Hu et al, 2018). Finally, differences in seasonal dynamics and stability between microorganisms sampled from soil versus groundwater (Hug et al, 2015;Danczak et al, 2016b;Nelson et al, 2019) imply that a complete view of microbial controls on geochemistry may require both types of samples to be analyzed in floodplain soils, or further work is needed to determine which is more important.…”