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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. Aerobic methane synthesis and dynamics in a river water environment

   https://doi.org/10.1002/lno.12383  

   Alowaifeer, Abdullah M. ; Wang, Qian ; Bothner, Brian ; Sibert, Ryan J. ; Joye, Samantha B. ; McDermott, Timothy R. ( June 2023 , Limnology and Oceanography)

   Reports of aerobic biogenic methane (CH₄) have generated new views about CH₄sources in nature. We examine this phenomenon in the free‐flowing Yellowstone river wherein CH₄concentrations were tracked as a function of environmental conditions, phototrophic microorganisms (using chlorophylla, Chla, as proxy), as well as targeted methylated amines known to be associated with this process. CH₄was positively correlated with temperature and Chla, although diurnal measurements showed CH₄concentrations were greatest during the night and lowest during maximal solar irradiation. CH₄efflux from the river surface was greater in quiescent edge waters (71–94 μmol m⁻² d) than from open flowing current (~ 57 μmol m⁻² d). Attempts to increase flux by disturbing the benthic environment in the quiescent water directly below (~ 1.0 m deep) or at varying distances (0–5 m) upstream of the flux chamber failed to increase surface flux. Glycine betaine (GB), dimethylamine and methylamine (MMA) were observed throughout the summer‐long study, increasing during a period coinciding with a marked decline in Chla, suggesting a lytic event led to their release; however, this did not correspond to increased CH₄concentrations. Spiking river water with GB or MMA yielded significantly greater CH₄than nonspiked controls, illustrating the metabolic potential of the river microbiome. In summary, this study provides evidence that: (1) phototrophic microorganisms are involved in CH₄synthesis in a river environment; (2) the river microbiome possesses the metabolic potential to convert methylated amines to CH₄; and (3) river CH₄concentrations are dynamic diurnally as well as during the summer active months.

    

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3. Heterotrophic nitrogen fixation in response to nitrate loading and sediment organic matter in an emerging coastal deltaic floodplain within the Mississippi River Delta plain

   https://doi.org/10.1002/lno.11737  

   Li, Song ; Twilley, Robert R. ; Hou, Aixin ( April 2021 , Limnology and Oceanography)

   Increasing nitrate (NO₃⁻) loading in rivers due to agricultural fertilization alters benthic nitrogen (N) cycling and shifts coastal wetlands from being a net source to net sink of reactive N. Heterotrophic N₂fixation that converts N₂to reactive N is often assumed negligible in eutrophic ecosystems and excluded in coastal N budget evaluations. We investigated N₂fixation and denitrification in response to increasing NO₃⁻loading (0, 10, and 100 μM) and sediment organic matter (OM_sediment) concentrations in the emerging Wax Lake Delta. Continuous flow‐through incubations with³⁰N₂addition was applied to measure N₂fixation. The variation of N₂fixation rates from 0 to 437 μmol N m⁻²h⁻¹among different NO₃⁻and OM_sedimentconcentrations were comparable to the estimated denitrification rates of 141–377 μmol N m⁻²h⁻¹. Increasing overlying NO₃⁻concentrations reduced N₂fixation rates and facilitated denitrification rates at each OM_sedimentconcentration. However, 100 μM of overlying NO₃⁻did not thoroughly inhibit N₂fixation rates in sites with intermediate and higher OM_sedimentconcentrations (189 and 99 μmol N m⁻²h⁻¹, respectively). Both N₂fixation and denitrification increased with increasing OM_sedimentconcentrations, but the relative importance of these processes was impacted mostly by overlying NO₃⁻concentration as increasing NO₃⁻switched the dominance of N₂fixation to denitrification in benthic N cycling. This study highlights the importance of heterotrophic N₂fixation in coastal deltaic floodplains and emphasizes the necessity of including N₂fixation quantification in coastal N budget evaluation, not only in oligotrophic environment but also in eutrophic environment.

    

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4. Nutrient uptake in a simplified stream channel: Experimental manipulation of hydraulic residence time and transient storage

   https://doi.org/10.1002/eco.2012  

   Cunha, Davi Gasparini Fernandes ; Finkler, Nícolas Reinaldo ; Calijuri, Maria do Carmo ; Covino, Timothy P. ; Tromboni, Flavia ; Dodds, Walter K. ( August 2018 , Ecohydrology)

   Stream restoration efforts have aimed at increasing hydraulic residence time (HRT) and transient storage (TS) to enhance nutrient uptake, but there have been few controlled studies quantifying HRT and TS influences on nutrient uptake dynamics. We assessed the effects of HRT and TS on ammonium (NH₄⁺) and phosphate (PO₄³⁻) uptake through controlled experiments in an artificial channel draining a pristine tropical stream. We experimentally dammed the channel with artificial weirs, to progressively increase HRT, and performed NH₄⁺and PO₄³⁻additions to estimate uptake each time a weir was added. We also ran consecutive additions of NH₄⁺and PO₄³⁻with no weirs, to evaluate short‐term changes in uptake metrics. Also, NH₄⁺was injected alone to assess potential nitrification. We observed that NH₄⁺and PO₄³⁻uptake rates were much greater in the very first addition, probably due to luxury uptake. The weirs increased mean HRT (from 8.5 to 12 min) and depth (from 6.5 to 8.9 cm) and decreased mean water velocity (0.40–0.28 m s⁻¹). Surprisingly, damming decreased the relative size of transient storage zone (storage zone area/channel area,A_s/Afrom 0.72 to 0.55), indicating that greater depth increasedA, but notA_s. Greater HRT increased uptake rates and velocities of both nutrients (p < 0.05). The NH₄⁺conversion to NO₃⁻was estimated at 18% of NH₄⁺consumption, indicating that joint additions to measure NH₄⁺and NO₃⁻uptake would not be feasible in this system. Our results suggest that increases in HRT can lead to a greater short‐term retention of nutrients, with implications for stream management and restoration initiatives.

    

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5. Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

   https://doi.org/10.1002/lno.12003  

   Vizza, Carmella ; Jones, Stuart E. ; Hart, Julia A. ; West, William E. ; Lamberti, Gary A. ( December 2021 , Limnology and Oceanography)

   Ponds play a larger role in the global freshwater methane (CH₄) budget than predicted from surface area alone. To improve our understanding of pond CH₄dynamics, we measured summer CH₄production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH₄production (0.64, 0.086–1.3 mmol m⁻²d⁻¹; median, interquartile range), as assessed with slurry incubations, was positively related to water‐column temperature and chlorophylla(Chla), negatively influenced by oxygen levels, and varied with microbial community structure. Average water‐column CH₄concentrations (0.39, 0.21–0.87 μmol L⁻¹) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024–0.20 mmol m⁻²d⁻¹) assessed with flux chambers. Based on a mass balance approach, 39–99% of CH₄produced in ponds was oxidized. Pond ebullition (3.7, 0.60–24 mmol m⁻²d⁻¹) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH₄emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chla (n = 28). Our study sheds light on the factors that influence pond CH₄dynamics and demonstrates that pond ebullition is a significant CH₄source worthy of continued study.

    

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