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The benthic biolayer is a shallow zone of reactive streambed sediments, widely believed to contribute disproportionately to whole‐stream reactions such as aerobic respiration and contaminant transformation. Quantifying the relative contribution of the biolayer to whole‐stream reactions remains challenging because it requires that hyporheic zone solute transport and reaction heterogeneity are explicitly captured within a single modeling framework. Here, we use field experiments and modeling to quantify the biolayer's aerobic reactivity relative to other stream compartments. We co‐injected and monitored several fluorescent tracers, including the reactive tracer resazurin, into a controlled experimental stream. We characterized reactive transport in the water column and at multiple depths in the hyporheic zone by fitting all data to a new mobile‐immobile model, using resazurin‐to‐resorufin conversion as an indicator of aerobic bioreactivity. Results show that the biolayer converted 8 times more resazurin to resorufin than all other stream compartments, and 80% of this conversion occurred within 2 reach advection times. This hotspot and hot moment behavior is attributed to the biolayer's ability to rapidly acquire, transiently retain, and rapidly degrade stream‐borne solutes. The model analysis shows that the majority of raz‐to‐rru conversion occurs in the biolayer across streams with a wide range of biolayer structural properties, including streams with a biolayer that is less reactive than deeper regions of the hyporheic zone. Together, our results show that the biolayer is a common feature of streams and rivers that should be considered in network‐scale models of aerobic reactivity.
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Unifying Advective and Diffusive Descriptions of Bedform Pumping in the Benthic Biolayer of Streams
Abstract Many water quality and ecosystem functions performed by streams occur in the benthic biolayer, the biologically active upper (~5 cm) layer of the streambed. Solute transport through the benthic biolayer is facilitated by bedform pumping, a physical process in which dynamic and static pressure variations over the surface of stationary bedforms (e.g., ripples and dunes) drive flow across the sediment‐water interface. In this paper we derive two predictive modeling frameworks, one advective and the other diffusive, for solute transport through the benthic biolayer by bedform pumping. Both frameworks closely reproduce patterns and rates of bedform pumping previously measured in the laboratory, provided that the diffusion model's dispersion coefficient declines exponentially with depth. They are also functionally equivalent, such that parameter sets inferred from the 2D advective model can be applied to the 1D diffusive model, and vice versa. The functional equivalence and complementary strengths of these two models expand the range of questions that can be answered, for example, by adopting the 2D advective model to study the effects of geomorphic processes (such as bedform adjustments to land use change) on flow‐dependent processes and the 1D diffusive model to study problems where multiple transport mechanisms combine (such as bedform pumping and turbulent diffusion). By unifying 2D advective and 1D diffusive descriptions of bedform pumping, our analytical results provide a straightforward and computationally efficient approach for predicting, and better understanding, solute transport in the benthic biolayer of streams and coastal sediments.
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- PAR ID:
- 10452366
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 11
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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