32 research outputs found
Dammed water quality—Longitudinal stream responses below beaver ponds in the Umpqua River Basin, Oregon
Beaver-related restoration (BRR) has gained popularity as a means of improving stream ecosystems, but the effects are not fully understood. Studies of dissolved oxygen (DO) and water temperature, key water quality metrics for salmonids, have demonstrated improved conditions in some cases, but warming and decreased DO have been more commonly reported in meta-analyses. These results point to the contingencies that can influence outcomes from BRR. We examined water quality related to beaver ponds in a diverse coastal watershed (Umpqua River Basin, OR, USA). We monitored water temperature 0–400m above and below beaver ponds and at pond surfaces and bottoms across seven study sites from June through September of 2019. DO was also recorded at two sites at pond surfaces and pond bottoms. Downstream monthly mean daily maximum temperatures were warmer than upstream reference locations by up to 1.9°C at beaver dam outlets but this heating signal attenuated with downstream distance. Downstream warming was greatest in June and July and best predicted by pond bottom temperatures. DO at pond surfaces and bottoms were hypoxic (≤5 mg/L) for more than half of the 32-day monitoring period. Water temperatures increased for short distances below monitored beaver ponds and observed oxygen conditions within ponds were largely unsuitable for salmonid fishes. These findings contrast with some commonly stated expectations of BRR, and we recommend that managers consider these expectations prior to implementation. In some cases, project goals may override water quality concerns but in streams where temperature or DO restoration are objectives, managers may consider using BRR techniques with caution
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Diurnal timing of warmer air under climate change affects magnitude, timing and duration of stream temperature change
Stream temperature will be subject to changes because of atmospheric warming in the future. We investigated the effects of the diurnal timing of air temperature changes – daytime warming versus nighttime warming – on stream temperature. Using the physically based model, Heat Source, we performed a sensitivity analysis of summer stream temperatures to three diurnal air temperature distributions of +4 °C mean air temperature: i) uniform increase over the whole day, ii) warmer daytime and iii) warmer nighttime. The stream temperature model was applied to a 37-km section of the Middle Fork John Day River in northeastern Oregon, USA. The three diurnal air temperature distributions generated 7-day average daily maximum stream temperatures increases of approximately +1.8 °C ± 0.1 °C at the downstream end of the study section. The three air temperature distributions, with the same daily mean, generated different ranges of stream temperatures, different 7-day average daily maximum temperatures, different durations of stream temperature changes and different average daily temperatures in most parts of the reach. The stream temperature changes were out of phase with air temperature changes, and therefore in many places, the greatest daytime increase in stream temperature was caused by nighttime warming of air temperatures. Stream temperature changes tended to be more extreme and of longer duration when driven by air temperatures concentrated in either daytime or nighttime instead of uniformly distributed across the diurnal cycle.Keywords: climate change, stream temperature, air temperature, nighttime warming, diurnal distribution, daytime warmin
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Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale
Excess NO₃⁻ in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO₃⁻. Hyporheic zones can have high denitrification potentials, but their role in reach- and network-scale NO₃⁻ removal is unknown because it is difficult to estimate. We used independent and complementary multiscale measurements of denitrification and total NO₃⁻ uptake to quantify the role of hyporheic NO₃⁻ removal in a 303-m reach of a 3rd-order agricultural stream in western Oregon, USA. We characterized the reach-scale NO₃⁻ dynamics with steady-state ¹⁵N-NO₃⁻ tracer-addition experiments and solute-transport modeling, and measured the hyporheic conditions via in-situ biogeochemical and groundwater modeling. We also developed a method to link these independent multiscale measurements. Hyporheic NO₃⁻ removal (rate coefficient λ[subscript HZ] = 0.007/h) accounted for 17% of the observed total reach NO₃⁻ uptake and 32% of the reach denitrification estimated from the ¹⁵N experiments. The primary limitations on hyporheic denitrification at the reach scale were availability of labile dissolved organic C and the restricted size of the hyporheic zone caused by anthropogenic channelization (sediment thickness ≤ 1.5 m). Linking multiscale methods made estimates possible for hyporheic influence on stream NO₃⁻ dynamics. However, it also demonstrated that the traditional reach-scale tracer experimental designs and subsequent transport modeling cannot be used alone to directly investigate the role of the hyporheic zone on reach-scale water and solute dynamics.Keywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interactionKeywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interactio
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Power-law residence time distribution in the hyporheic zone of a 2nd-order mountain stream
We measured the hyporheic residence time distribution in a 2nd-order mountain stream at the H. J. Andrews Experimental Forest, Oregon, and found it to be a power-law over at least 1.5 orders of magnitude in time (1.5 hr to 3.5 d). The residence time distribution has a very long tail which scales as t[superscript −1.28], and is poorly characterized by an exponential model. Because of the small power-law exponent, efforts to characterize the mean hyporheic residence time (t[subscript s]) in this system result in estimates that are scale invariant, increasing with the characteristic advection time within the stream channel (t[subscript ad]). The distribution implies the hyporheic zone has a very large range of exchange timescales, with significant quantities of water and solutes stored over time-scales very much longer than t[subscript ad]. The hyporheic zone in such streams may contribute to short-time fractal scaling in time series of solute concentrations observed in small-watershed studies.Keywords: Fractals and multifractals, Hydrology, Mathematical Geophysics, Surface water quality, Geomorphology, Groundwater transportKeywords: Fractals and multifractals, Hydrology, Mathematical Geophysics, Surface water quality, Geomorphology, Groundwater transpor
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Labile dissolved organic carbon supply limits hyporheic denitrification
We used an in situ steady state ¹⁵N-labeled nitrate (¹⁵NO₃⁻) and acetate (AcO⁻) well-to-wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as AcO⁻ influences microbial denitrification in the hyporheic zone of an upland (third-order) agricultural stream. The experimental wells receiving conservative (Cl⁻ and Br) and reactive (¹⁵NO₃⁻) solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (<1.5 mg O₂ L⁻¹). All receiving wells demonstrated ¹⁵N₂ production during ambient conditions, indicating that the hyporheic zone was an environment with active denitrification. The subsequent addition of AcO⁻ stimulated more denitrification as evidenced by significant δ¹⁵N₂ increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NO₃⁻ and DO in the two wells most hydrologically connected to the injection. The rate of nitrate removal in the hyporheic zone increased from 218 kg ha⁻¹ yr⁻¹ to 521 kg ha⁻¹ yr⁻¹ under elevated AcO⁻ conditions. In all receiving wells, increases of bromide and ¹⁵N₂ occurred without concurrent increases in AcO⁻, indicating that 100% of AcO⁻ was retained or lost in the hyporheic zone. These results support the hypothesis that denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply
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Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone
Biogeochemical reactions associated with stream nitrogen cycling, such as
nitrification and denitrification, can be strongly controlled by water and solute residence
times in the hyporheic zone (HZ). We used a whole‐stream steady state ¹⁵N‐labeled
nitrate (¹⁵NO₃⁻) and conservative tracer (Cl⁻) addition to investigate the spatial and
temporal physiochemical conditions controlling the denitrification dynamics in the HZ of
an upland agricultural stream. We measured solute concentrations (¹⁵NO₃⁻, ¹⁵N₂ (g), as
well as NO₃⁻, NH₃, DOC, DO, Cl⁻), and hydraulic transport parameters (head, flow rates,
flow paths, and residence time distributions) of the reach and along HZ flow paths of
an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in
all wells) with flow path lengths up to 4.2 m and corresponding median residence
times greater than 28.5 h. The HZ transitioned from a net nitrification environment at
its head (short residence times) to a net denitrification environment at its tail (long
residence times). NO₃⁻ increased at short residence times from 0.32 to 0.54 mg‐N L⁻¹ until
a threshold of 6.9 h and then consistently decreased from 0.54 to 0.03 mg‐N L⁻¹.
Along these same flow paths, declines were seen in DO (from 8.31 to 0.59 mg‐O₂ L⁻¹) and
DOC (from 3.0 to 1.7 mg‐C L⁻¹). The rates of the DO and DOC removal and net
nitrification were greatest during short residence times, while the rate of denitrification was
greatest at long residence times. ¹⁵NO₃⁻
tracing confirmed that a fraction of the NO₃⁻
removal was via denitrification as ¹⁵N₂ was produced across the entire gravel bar HZ.
Production of ¹⁵N₂ across all observed flow paths and residence times indicated that
denitrification microsites are present even where nitrification was the net outcome. These
findings demonstrate that the HZ is an active nitrogen sink in this system and that the
distinction between net nitrification and denitrification in the HZ is a function of residence
time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and
water residence time characterizations will improve mechanistic predictions of nitrogen
cycling in streams
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Changes in hyporheic exchange flow following experimental wood removal in a small, low-gradient stream
We investigated the response of hyporheic exchange flow (HEF) to wood removal in a small, low-gradient, gravel bed stream in southeast Alaska using a series of groundwater models built to simulate HEF for the initial conditions immediately after wood removal and 1 month, 2 years, 4 years, and 16 years following wood removal. The models were based on topographic surveys of the stream channel and surrounding floodplain, and surveyed water surface elevations (WSEs) were used to assign stream boundary conditions. Using the groundwater flow model, MODFLOW, and the particle tracking model, MODPATH, we calculated hyporheic exchange fluxes, their residence time distributions, and both longitudinal and plan view spatial patterns of downwelling and upwelling zones. In the first few years, streambed scour and sediment deposition smoothed the streambed and WSE profile, reducing HEF. Also, large contiguous patches of downwelling or upwelling were fragmented, nearly doubling the total number of patches present on the streambed. As the stream continued to adjust to the loss of wood, those trends began to reverse. Accretion of sediment onto alternating bars resulted in better developed pool-riffle morphology, enhanced HEF, and increased residence times and also resulted in downwelling and upwelling zones coalescing into elongated patches along bar margins. This study showed that the hyporheic zone is sensitive to changes in wood loading and that initial changes in HEF resulting from the direct effects of wood removal were contrary to longer-term channel adjustments to changes in wood loading
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Coupled transport and reaction kinetics control the nitrate source-sink function of hyporheic zones
The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O₂), ammonium (NH₄), nitrate (NO₃), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification-denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O₂ from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO₃ to the stream. We further show that whether the HZ is a net NO₃ source or net NO₃ sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O₂ (i.e., the Damköhler number, Da[subscript O2]), where HZs with Da[subscript O2] < 1 will be net nitrification environments and HZs with Da[subscript O2] ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO₃. Our model results suggest that only estimates of residence times and O₂ uptake rates are necessary to predict this nitrification-denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO₃
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Comprehensive multiyear carbon budget of a temperate headwater stream
Headwater streams comprise nearly 90% of the total length of perennial channels in global catchments. They mineralize organic carbon entering from terrestrial systems, evade terrestrial carbon dioxide (CO₂ ), and generate and remove carbon through in-stream primary production and respiration. Despite their importance, headwater streams are often neglected in global carbon budgets primarily because of a lack of available data. We measured these processes, in detail, over a 10 year period in a stream draining a 96 ha forested watershed in western Oregon, USA. This stream, which represents only 0.4% of the watershed area, exported 159 kg C ha⁻¹ yr⁻¹, similar to the global exports for large rivers. Stream export was dominated by downstream transport of dissolved inorganic carbon (63 kg C ha⁻¹ yr⁻¹) and by evasion of CO₂ to the atmosphere (42 kg C ha⁻¹ yr⁻¹), leaving the remainder of 51 kg C ha⁻¹ yr⁻¹ for downstream transport of organic carbon (17 kg C ha⁻¹ yr⁻¹ and 34 kg C ha⁻¹ yr⁻¹ in dissolved and particulate form, respectively)This is the publisher’s final pdf. The article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/%28ISSN%292169-8961/Keywords: carbon, evasion, carbon fluxes, stream, biogeochemistr