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Abstract Dissolved organic carbon (DOC) is a key variable impacting stream biogeochemical processes. The relationship between DOC concentration (C) and stream discharge (q) can elucidate spatial and temporal DOC source dynamics in watersheds. In the ephemeral glacial meltwater streams of the McMurdo Dry Valleys (MDV), Antarctica, the C‐qrelationship has been applied to dissolved inorganic nitrogen and weathering solutes including silica, which all exhibit chemostatic C‐qbehavior; but DOC‐qdynamics have not been studied. DOC concentrations here are low compared to temperate streams, in the range of 0.1–2 mg C l−1, and their chemical signal clearly indicates derivation from microbial biomass (benthic mats and hyporheic biofilm). To investigate whether the DOC generation rate from these autochthonous organic matter pools was sufficient to maintain chemostasis for DOC, despite these streams' large diel and interannual fluctuations in discharge, we fit the long‐term DOC‐qdata to a power law and an advection‐reaction model. Model outputs and coefficients of variation characterize the DOC‐qrelationship as chemostatic for several MDV streams. We propose a conceptual model in which hyporheic carbon storage, hyporheic exchange rates, and net DOC generation rates are key interacting components that enable chemostatic DOC‐qbehavior in MDV streams. This model clarifies the role of autochthonous carbon stores in maintaining DOC chemostasis and may be useful for examining these relationships in temperate systems, which typically have larger sources of bioavailable autochthonous organic carbon than MDV streams but where this autochthonous signal could be masked by a stronger allochthonous contribution.
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Modeling Benthic Versus Hyporheic Nutrient Uptake in Unshaded Streams With Varying Substrates
Abstract Assessments of riverine ecosystem health and water quality require knowledge of how headwater streams transport and transform nutrients. Estimates of nutrient demand at the watershed scale are commonly inferred from reach‐scale solute injections, which are typically reported as uptake velocities (vf). Multiple interacting processes controlvf, making it challenging to predict howvfresponds to physical changes in the stream. In this study, we linkvfto a continuous time random walk model to quantify howvfis controlled by in‐stream (velocity, dispersion, and benthic reaction) and hyporheic processes (exchange rate, residence times, and hyporheic reaction). We fit the model to conservative (NaCl) and nitrate (NO3−‐N) pulse tracer injections in unshaded replicate streams at the Notre Dame Linked Experimental Ecosystem Facility, which differed only in substrate size and distribution. Experiments were conducted over the first 25 days of biofilm colonization to examine how the interaction between substrate type and biofilm growth influenced modeled processes andvf. Model fits of benthic reaction rates were ∼8× greater than hyporheic reaction rates for all experiments and did not vary with substrate type or over time. High benthic reactivity was associated with filamentous green algae coverage on the streambed, which dominated total algal biomass. Finally,vfwas most sensitive to benthic reaction rate and stream velocity, and sensitivity varied with stream conditions due to its nonlinear dependence on all modeled processes. Together, these results demonstrate how reach‐scale nutrient demand reflects the relative contributions of biotic and abiotic processes in the benthic layer and the hyporheic zone.
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- Award ID(s):
- 1705770
- PAR ID:
- 10460167
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 124
- Issue:
- 2
- ISSN:
- 2169-8953
- Page Range / eLocation ID:
- p. 367-383
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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