Groundwater–surface water mixing shifts ecological assembly processes and stimulates organic carbon turnover

Abstract

Environmental transitions often result in resource mixtures that overcome limitations to microbial metabolism, resulting in biogeochemical hotspots and moments. Riverine systems, where groundwater mixes with surface water (the hyporheic zone), are spatially complex and temporally dynamic, making development of predictive models challenging. Spatial and temporal variations in hyporheic zone microbial communities are a key, but understudied, component of riverine biogeochemical function. Here, to investigate the coupling among groundwater–surface water mixing, microbial communities and biogeochemistry, we apply ecological theory, aqueous biogeochemistry, DNA sequencing and ultra-high-resolution organic carbon profiling to field samples collected across times and locations representing a broad range of mixing conditions. Our results indicate that groundwater–surface water mixing in the hyporheic zone stimulates heterotrophic respiration, alters organic carbon composition, causes ecological processes to shift from stochastic to deterministic and is associated with elevated abundances of microbial taxa that may degrade a broad suite of organic compounds.

Dispersal-Based Microbial Community Assembly Decreases Biogeochemical Function

“…We therefore pose that variation in βNTI may be an effective tool for predicting biogeochemical function when biotic and abiotic conditions lead to a mixture of stochastic and deterministic assembly processes. Natural systems have repeatedly shown such a mixture, and previous field observations have revealed connections between βNTI and biogeochemical function [2,14,60]. These outcomes support our model-based inference that βNTI-as a proxy for assembly processes-offers a practical means to inform models that represent the effects of ecological processes on biogeochemical function.…”

Colonization Habitat Controls Biomass, Composition, and Metabolic Activity of Attached Microbial Communities in the Columbia River Hyporheic Corridor

“…Based on this work and recent microbial community assembly studies in the Hanford 300 Area HC (15)(16)(17), deterministic selection imposed by the various DOC sources/ amounts in the three different habitats was expected to lead to distinct microbial assemblages in the colonized sand materials. Stegen et al (15) showed (using highresolution Fourier transform-ion cyclotron resonance mass spectrometry) significant differences in the properties of DOC among groundwater, river water, and hyporheic zone fluids, which were correlated with both microbial respiration and community composition.…”

Biogeochemical cycling at the aquatic–terrestrial interface is linked to parafluvial hyporheic zone inundation history

“…The hyporheic zone, the subsurface region where river water and groundwater (GW) mix, is characterized by heightened biogeochemical cycling and microbial interactions (McClain et al, 2003;Boulton et al, 2010;Mulholland and Webster, 2010;Stegen et al, 2016). Hyporheic sediment is exposed to different aquatic chemistries due to variation in hydraulic head that drives flow into and out of the hyporheic zone (Arntzen et al, 2006;Hucks Sawyer et al, 2009).…”

“…Previous studies of biogeochemical cycling in the parafluvial zone have sampled gravel bars to investigate hydrologic flow paths (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011) and have sampled during saturated conditions utilizing well/piezometer transects (Baker et al, 1999;Deforet et al, 2009;Briody et al, 2016;Graham et al, 2016;Stegen et al, 2016). Such studies have indicated that the parafluvial hyporheic zone is an active region of diverse biogeochemical transformations (Zarnetske et al, 2011;Briody et al, 2016) and microbial community dynamics (Baker et al, 1999;Graham et al, 2016Graham et al, , 2017Stegen et al, 2016), often influenced by residence times (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011).…”

“…Such studies have indicated that the parafluvial hyporheic zone is an active region of diverse biogeochemical transformations (Zarnetske et al, 2011;Briody et al, 2016) and microbial community dynamics (Baker et al, 1999;Graham et al, 2016Graham et al, , 2017Stegen et al, 2016), often influenced by residence times (Claret and Boulton, 2008;Deforet et al, 2009;Zarnetske et al, 2011). However, studies that have compared biogeochemical cycling in the parafluvial zone to other floodplain environments (e.g., forest, ponds, islands, wetlands) have found that stream and nearby gravel habitats have had lower respiration, bacterial abundance, enzyme activities, and sediment nitrogen nutrient leaching than adjacent pasture/grassland, vegetated islands, and/or mature forest habitats, but short flood pulses (24 h) have increased bacterial extracellular enzymes in sediment from both flooded riverbanks and wetlands (Burns and Ryder, 2001;Doering et al, 2011;Ostojić et al, 2013;Bodmer et al, 2016).…”