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

Stegen JC, Fredrickson JK, Wilkins MJ, Konopka AE, Nelson WC, Arntzen EV, Chrisler WB, Chu RK, Danczak RE, Fansler SJ, Kennedy DW, Resch CT, Tfaily M - Nat Commun (2016)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, Washington 99352, USA.

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.

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Solutes and associated mixing models.(a–c) Dissolved solutes as a function Cl− concentration with mixing model expectations indicated by solid lines. DIC, dissolved inorganic carbon; NPOC, non-purgable organic carbon. (d) Deviation from the upper/steeper DIC mixing model as a function of the deviation from the lower/steeper NPOC mixing model. To focus on positive DIC deviations, data points were restricted to Cl−<7 mg l−1. Linear model (dashed line) statistics are provided. In all panels, river stage at the time of sample collection in the hyporheic zone is indicated by a gradient from blue (low river stage) to red (high river stage); values corresponding to colours are provided in the colour bar.
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f3: Solutes and associated mixing models.(a–c) Dissolved solutes as a function Cl− concentration with mixing model expectations indicated by solid lines. DIC, dissolved inorganic carbon; NPOC, non-purgable organic carbon. (d) Deviation from the upper/steeper DIC mixing model as a function of the deviation from the lower/steeper NPOC mixing model. To focus on positive DIC deviations, data points were restricted to Cl−<7 mg l−1. Linear model (dashed line) statistics are provided. In all panels, river stage at the time of sample collection in the hyporheic zone is indicated by a gradient from blue (low river stage) to red (high river stage); values corresponding to colours are provided in the colour bar.

Mentions: A sampling scheme was employed adjacent to and within the Columbia River (Fig. 1), to capture a range of river-stage conditions across ∼1 week. Although large, long-term fluctuations in river stage (Fig. 2a) are principally driven by snowpack melt and runoff across the Columbia River watershed, short-term fluctuations (Fig. 2b) are caused by up-river flow regulation at the Priest Rapids dam. Sampling across different river-stage conditions and across different spatial locations (vertically and horizontally distributed; Fig. 1) translated into a range of groundwater–surface water mixing conditions in the hyporheic zone as shown by substantial variation in Cl− concentration within hyporheic zone samples (Fig. 3); Cl− concentration was used as a conservative tracer. The three non-conservative (that is, reactive) analytes showed remarkable differences between groundwater and surface water, which allowed for evaluations of mixing-model deviations (Fig. 3); reactive analytes included non-purgable organic carbon (NPOC), dissolved inorganic carbon (DIC) and nitrate. Specific conductance—an alternative tracer of groundwater–surface water mixing conditions38—was logged in river water and groundwater wells. The relatively high and stable specific conductance values in groundwater wells—combined with low and stable values in the river—during the sampling period (Supplementary Fig. 1) provide evidence that water sampled from groundwater wells and from the river represented distinct end members.


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

Stegen JC, Fredrickson JK, Wilkins MJ, Konopka AE, Nelson WC, Arntzen EV, Chrisler WB, Chu RK, Danczak RE, Fansler SJ, Kennedy DW, Resch CT, Tfaily M - Nat Commun (2016)

Solutes and associated mixing models.(a–c) Dissolved solutes as a function Cl− concentration with mixing model expectations indicated by solid lines. DIC, dissolved inorganic carbon; NPOC, non-purgable organic carbon. (d) Deviation from the upper/steeper DIC mixing model as a function of the deviation from the lower/steeper NPOC mixing model. To focus on positive DIC deviations, data points were restricted to Cl−<7 mg l−1. Linear model (dashed line) statistics are provided. In all panels, river stage at the time of sample collection in the hyporheic zone is indicated by a gradient from blue (low river stage) to red (high river stage); values corresponding to colours are provided in the colour bar.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4829693&req=5

f3: Solutes and associated mixing models.(a–c) Dissolved solutes as a function Cl− concentration with mixing model expectations indicated by solid lines. DIC, dissolved inorganic carbon; NPOC, non-purgable organic carbon. (d) Deviation from the upper/steeper DIC mixing model as a function of the deviation from the lower/steeper NPOC mixing model. To focus on positive DIC deviations, data points were restricted to Cl−<7 mg l−1. Linear model (dashed line) statistics are provided. In all panels, river stage at the time of sample collection in the hyporheic zone is indicated by a gradient from blue (low river stage) to red (high river stage); values corresponding to colours are provided in the colour bar.
Mentions: A sampling scheme was employed adjacent to and within the Columbia River (Fig. 1), to capture a range of river-stage conditions across ∼1 week. Although large, long-term fluctuations in river stage (Fig. 2a) are principally driven by snowpack melt and runoff across the Columbia River watershed, short-term fluctuations (Fig. 2b) are caused by up-river flow regulation at the Priest Rapids dam. Sampling across different river-stage conditions and across different spatial locations (vertically and horizontally distributed; Fig. 1) translated into a range of groundwater–surface water mixing conditions in the hyporheic zone as shown by substantial variation in Cl− concentration within hyporheic zone samples (Fig. 3); Cl− concentration was used as a conservative tracer. The three non-conservative (that is, reactive) analytes showed remarkable differences between groundwater and surface water, which allowed for evaluations of mixing-model deviations (Fig. 3); reactive analytes included non-purgable organic carbon (NPOC), dissolved inorganic carbon (DIC) and nitrate. Specific conductance—an alternative tracer of groundwater–surface water mixing conditions38—was logged in river water and groundwater wells. The relatively high and stable specific conductance values in groundwater wells—combined with low and stable values in the river—during the sampling period (Supplementary Fig. 1) provide evidence that water sampled from groundwater wells and from the river represented distinct end members.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, Washington 99352, USA.

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.

Show MeSH
Related in: MedlinePlus