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1. Variability and drivers of CO 2 , CH 4 , and N 2 O concentrations in streams across the United States

   https://doi.org/10.1002/lno.12281  

   DelVecchia, Amanda G. ; Rhea, Spencer ; Aho, Kelly S. ; Stanley, Emily H. ; Hotchkiss, Erin R. ; Carter, Alice ; Bernhardt, Emily S. ( December 2022 , Limnology and Oceanography)

   Streams and rivers are major sources of greenhouse gases (GHGs) to the atmosphere, as carbon and nitrogen are converted and outgassed during transport. Although our understanding of drivers of individual GHG fluxes has improved with numerous site‐specific studies and global‐scale compilations, our ability to parse out interrelated physical and biogeochemical drivers of gas concentrations is limited by a lack of consistently collected, temporally continuous samples of GHGs and their associated drivers. We present a first analysis of such a dataset collected by the National Ecological Observatory Network across 27 streams and rivers across ecoclimatic domains of the United States. Average concentrations of CO₂ranged from 36.9 ± 0.88 to 404 ± 33 μmol L⁻¹, CH₄from 0.003 ± 0.0003 to 4.99 ± 0.72 μmol L⁻¹, and N₂O from 0.015 to 0.04 μmol L⁻¹and spanned ranges of previous global compilations. Both CO₂and CH₄were strongly affected by physical drivers including mean air temperature and stream slope, as well as by dissolved oxygen and total nitrogen concentrations. N₂O was exclusively correlated with total nitrogen concentrations. Results suggested that potential for gas exchange dominated patterns in gas concentrations at the site level, but contributions of in‐stream aerobic and anaerobic metabolism, and groundwater also likely varied across sites. The highest gas concentrations as well as highest variability occurred in low‐gradient, warmer, and nonperennial systems. These results are a first step in providing unprecedented, continuous estimates of GHG flux constrained by temporally variable physical and biogeochemical drivers of GHG production.

    

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2. Impacts of Carbonate Buffering on Atmospheric Equilibration of CO 2 , δ 13 C DIC , and Δ 14 C DIC in Rivers and Streams

   https://doi.org/10.1029/2023GB007860  

   Winnick, Matthew J. ; Saccardi, Brian ( February 2024 , Global Biogeochemical Cycles)

   Rivers and streams play an important role within the global carbon cycle, in part through emissions of carbon dioxide (CO₂) to the atmosphere. However, the sources of this CO₂and their spatiotemporal variability are difficult to constrain. Recent work has highlighted the role of carbonate buffering reactions that may serve as a source of CO₂in high alkalinity systems. In this study, we seek to develop a quantitative framework for the role of carbonate buffering in the fluxes and spatiotemporal patterns of CO₂and the stable and radio‐ isotope composition of dissolved inorganic carbon (DIC). We incorporate DIC speciation calculations of carbon isotopologues into a stream network CO₂model and perform a series of simulations, ranging from the degassing of a groundwater seep to a hydrologically‐coupled 5th‐order stream network. We find that carbonate buffering reactions contribute >60% of emissions in high‐alkalinity, moderate groundwater‐CO₂environments. However, atmosphere equilibration timescales of CO₂are minimally affected, which contradicts hypotheses that carbonate buffering maintains high CO₂across Strahler orders in high alkalinity systems. In contrast, alkalinity dramatically increases isotope equilibration timescales, which acts to decouple CO₂and DIC variations from the isotopic composition even under low alkalinity. This significantly complicates a common method for carbon source identification. Based on similar impacts on atmospheric equilibration for stable and radio‐ carbon isotopologues, we develop a quantitative method for partitioning groundwater and stream corridor carbon sources in carbonate‐dominated watersheds. Together, these results provide a framework to guide fieldwork and interpretations of stream network CO₂patterns across variable alkalinities.

    

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3. Spatiotemporal variability of gas transfer velocity in a tropical high‐elevation stream using two independent methods

   https://doi.org/10.1002/ecs2.3647  

   Whitmore, Keridwen M. ; Stewart, Nehemiah ; Encalada, Andrea C. ; Suárez, Esteban ; Riveros‐Iregui, Diego A. ( July 2021 , Ecosphere)

   Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO₂) to the atmosphere by transforming terrestrial carbon to gaseous CO₂. Studies of these environments are scarce, and estimates of CO₂fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO₂evasion and other biogeochemical processes. The first method, kinematick₆₀₀(k_600‐K), is derived from an empirical relationship between temperature‐adjustedk(k₆₀₀) and the physical characteristics of the stream. The second method, measuredk₆₀₀(k_600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO₂(pCO₂) and CO₂evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability ink₆₀₀at small spatial scales. Values ofk_600‐Kranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values ofk_600‐Mranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability ink₆₀₀was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates ofk₆₀₀values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams.

    

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4. River Corridor Sources Dominate CO 2 Emissions From a Lowland River Network

   https://doi.org/10.1029/2022JG006954  

   Kirk, Lily ; Cohen, Matthew J. ( January 2023 , Journal of Geophysical Research: Biogeosciences)

   Rivers and streams are control points for CO₂emission to the air (fCO₂), with emission rates often exceeding internal metabolism (net ecosystem production, NEP). The difference is usually attributed to CO₂‐supersaturated groundwater inputs from upland soil respiration and rock weathering, but this implies a terrestrial‐to‐aquatic C transfer greater than estimated by terrestrial mass balance. One explanation is that riparian zones—rich in organic and inorganic C but mostly neglected in terrestrial mass balances—contribute disproportionately tofCO₂. To test this hypothesis, we measuredfCO₂, NEP, and the lateral CO₂contributions from both terrestrial uplands (TER) and riparian wetlands (RIP) for seven reaches in a lowland river network in Florida, USA. NEP contributed about half offCO₂, but the remaining CO₂emission was generally much larger than measured TER. The relative importance of RIP versus TER varied markedly between contrasting hydrogeologic settings: RIP contributed 49% offCO₂where geologic confinement forced lateral drainage through riparian soils, but only 12% where unconfined karst allowed deeper groundwater flowpaths that bypassed riparian zones. On a land area basis, the narrow riparian corridor yielded far more CO₂than the terrestrial uplands (33.1 vs. 1.4 g‐C m⁻² yr⁻¹), resulting in river corridors (i.e., stream channel plus adjacent wetlands, NEP + RIP) sourcing 87% offCO₂to streams. Our findings imply that true terrestrial CO₂subsidies to streams may be smaller than previously estimated by aquatic mass balance and highlight the importance of explicitly integrating riparian zones into the conceptual model for terrestrial‐to‐aquatic C transfer.

    

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5. Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal

   https://doi.org/10.1002/eap.2830  

   Dodds, Walter K. ; Ratajczak, Zak ; Keen, Rachel M. ; Nippert, Jesse B. ; Grudzinski, Bartosz ; Veach, Allison ; Taylor, Jeffery H. ; Kuhl, Amanda ( March 2023 , Ecological Applications)

   Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure from local (land use/land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here we report on a decade‐long watershed‐scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before–after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, associated with a decline in streamflow, loss of grassy plant species, and other ecosystem‐scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for 3 years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when reseeded with grassland species. Rapid expansion of shrubs (Cornus drummondii,Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every 2 years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands, resulting in inexorable movement toward a new ecosystem state. Human pressures, such as climate change, atmospheric CO₂increases, and elevated atmospheric nitrogen deposition, could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well‐studied sites.

    

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