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.
Understanding how water and solutes enter and propagate through freshwater landscapes in the Anthropocene is critical to protecting and restoring aquatic ecosystems and ensuring human water security. ...However, high hydrochemical variability in headwater streams, where most carbon and nutrients enter river networks, has hindered effective modelling and management. We developed an analytical framework informed by landscape ecology and catchment hydrology to quantify spatiotemporal variability across scales, which we tested in 56 headwater catchments, sampled periodically over 12 years in western France. Unexpectedly, temporal variability in dissolved carbon, nutrients and major ions was preserved moving downstream and spatial patterns of water chemistry were stable on annual to decadal timescales, partly because of synchronous variation in solute concentrations. These findings suggest that while concentration and flux cannot be extrapolated among subcatchments, periodic sampling of headwaters provides valuable information about solute sources and subcatchment resilience to disturbance.
Although the flux of dissolved organic carbon (DOC) through freshwaters is nearly equivalent to the net carbon uptake of all terrestrial ecosystems, uncertainty remains about how source processes ...(carbon production and location) and transport processes (hydrologic connectivity and routing) interact to determine DOC flux across flow conditions and ecoregions. This limits our ability to predict the fluvial carbon flux responses to changes in climate and land use. We used DOC concentration and discharge patterns with ensemble modeling techniques to quantify DOC flux behavior for 1,006 U.S. watersheds spanning diverse climate and land cover conditions. We found that DOC flux was transport‐limited (concentration increased with discharge) in 80% of watersheds and that this flux behavior spanned ecoregions and watershed sizes. The generality of transport limitation demonstrates how coupling discharge models with widely available watershed properties could allow DOC flux to be efficiently integrated into landscape and Earth system models.
Plain Language Summary
When water flows through ecosystems, it picks up dissolved organic carbon (DOC) from plants and soils, sometimes determining whether the ecosystem is a net carbon source or sink. DOC is also an important water quality parameter and understanding how it is produced and transported affects society's ability to provide water for industrial, agricultural, and domestic uses. Because DOC flux through rivers varies widely with flow and in different regions, DOC flux remains a major source of uncertainty in the global carbon cycle. Based on one of the largest and most geographically diverse analyses of river DOC dynamics to date, we found surprising similarities in DOC flux behavior. From southwestern deserts to northeastern forests, hydrologic flow, not DOC sources, determined DOC flux behavior in 80% of watersheds in the conterminous United States. In other words, DOC concentration systematically increased with river flow, even during large flow events, indicating that organic matter stocks provide ample DOC to maintain delivery to rivers. Additional analysis of this large data set identified several landscape and climate conditions that predict DOC flux behavior in watersheds. Together, these findings demonstrate that watershed DOC flux can be simulated across spatial scales using river flow and widely available watershed properties.
Key Points
Across ecoregions of the United States, 80% of watersheds express transport limitation in DOC flux behavior
Wetland abundance in watersheds is nonlinearly related to river DOC flux behavior
The generality of transport limitation in DOC flux presents an opportunity for improving ecosystem carbon balance models
It is important to understand how dissolved organic carbon (DOC) is processed and transported through stream networks because DOC is a master water quality variable in aquatic ecosystems. ...High-frequency sampling is necessary to capture important, rapid shifts in DOC source, concentration, and composition (i.e. quality) in streams. Until recently, this high-frequency sampling was logistically difficult or impossible. However, this type of sampling can now be conducted using in-situ optical measurements through long-term, field-deployable fluorometers and spectrophotometers. The optical data collected from these instruments can quantify both DOC concentration and composition properties (e.g., specific ultra-violet absorbance at 254nm, spectral slope ratio, and fluorescence index). Previously, the use of these sensors was limited to a small number of specialized users, mainly in Europe and North America, where they were used predominantly in marine DOC studies as well as water treatment and management infrastructure. However, recent field demonstrations across a wide range of river systems reveals a large potential for the use of these instruments in freshwater environments, heightening interest and demand across multiple environmental research and management disciplines. Hence, this review provides an up-to-date synthesis on 1) the use of spectroscopy as a diagnostic tool in stream DOC studies, 2) the instrumentation, its applications, potential limitations and future considerations, and 3) the new watershed DOC research directions made possible via these in-situ optical sensors.
Display omitted
•DOC is a master water quality parameter.•Mechanisms of DOC processing are inadequately quantified in riverine environments.•In-situ optical sensors can enhance DOC studies via high-resolution temporal data.•We discuss these new sensors, their uses, limitations, and future considerations.•We review and highlight new research directions revealed by these optical sensors.
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.
Key Points
The hyporheic zone can retain and remove dissolved organic carbon and nitrate
Conservative and stable isotope (15N) tracers show coupled C and N dynamics
Dissolved organic carbon quality and quantity limit hyporheic denitrification
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, DaO2), where HZs with DaO2 < 1 will be net nitrification environments and HZs with DaO2 ≪ 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 O2uptake rates are necessary to predict this nitrification‐denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO3.
Key Points
Hyporheic (HZ) N is controlled by coupled transport and reaction kinetics
Ratio of HZ residence time to O2 reaction time controls N source‐sink dynamics
We present a process‐based scaling relationship for hyporheic N transformations
As environmental change in the Arctic accelerates, there is a growing need to accurately quantify the response of Arctic ecosystems throughout the year. To assess the temporal coverage of ...observations of carbon and nutrient fluxes, we used literature synthesis, quantitative meta-analysis, and exploration of a novel biogeochemical dataset from one of the best-documented Arctic ecosystems: the headwaters of the Kuparuk River in Northern Alaska. The meta-analysis of 204 peer-reviewed studies revealed a strong temporal gap in observations of biogeochemistry and hydrology of the Kuparuk River, with substantially fewer observations from the early and late 'shoulders' of the thaw season (defined as the period before snowmelt or after plant senescence). To test and illustrate how much this bias might influence fundamental ecosystem level measurements, such as riverine carbon and nutrient fluxes, we used high-frequency, in-situ water chemistry sensors to estimate riverine export budgets across the thaw season for dissolved organic carbon (DOC) and nitrate (NO3−) in the Kuparuk headwaters. With this novel dataset, we found that a large proportion (∼30%) of the annual export of DOC and NO3− occurred during the shoulder seasons, which are not well characterized even for this well-documented Arctic system. These analyses raise the broader question: what ecological information are we missing by giving these seasons the 'cold shoulder'? As climate change alters seasonality, filling this major data gap in the shoulder seasons is crucial to understand the response of Arctic ecosystems.
Arctic river icings are surface ice accumulations that can be >10 km2 in area and >10 m thick. They commonly impact the hydrology, geomorphology, and ecology of Arctic river environments. Previous ...examination of icing dynamics in Arctic Alaska found no substantial changes in extent through 2005. However, here we use daily time series of satellite imagery for 2000–2015 to demonstrate that the temporal persistence and minimum summertime extent of large icings in part of Arctic Alaska and Canada have declined rapidly. We identified 122 large ephemeral icings, and 70 are disappearing significantly earlier in the summer, with a mean trend of −1.6 ± 0.9 day−1 for fully ephemeral features. Additionally, 14 of 25 icings that usually persist through the summer have significantly smaller minimum extents (−2.6 ± 1.6% yr−1). These declines are remarkably rapid and suggest that Arctic hydroclimatic systems generating icings, and their associated ecosystems, are changing rapidly.
Key Points
River icings are important hydrological and ecological components of many Arctic river systems
MODIS images from 2000 to 2015 show large changes in temporal persistence and minimum extent of icings in northern Alaska
Continued decline in river icings would likely result in alterations to hydrology and ecology of Arctic rivers
Plain Language Summary
Liquid water emerging from groundwater and flowing through Arctic rivers during the winter often freezes into large ice features, which are called river icings. These icings, which are found in nearly all parts of the Arctic, create wide, gravelly river channels that can be important habitat for animals. When icings melt during the summer, they help keep rivers flowing when other water sources are limited. Up until now, no study has systematically looked at whether these features are changing in response to warming temperatures. We use daily satellite imagery available over northern Alaska from 2000 to 2015 in order to test whether icings are becoming smaller or disappearing earlier in the summer. Of 147 features examined, we found that 84 are either becoming smaller (for those that persist throughout the summer) or are disappearing earlier (for those that fully melt each summer). None are becoming larger or disappearing later. These changes may be directly related to warming temperatures, but they may also be happening because climate change is altering how rivers and groundwater interact. If these trends continue, we may see changes in the form of many Arctic rivers and impacts on the habitat of animals like fish and caribou.
Recent observations reveal a paradox of anaerobic respiration occurring in seemingly oxic‐saturated sediments. Here we demonstrate a residence time‐based explanation for this paradox. Specifically, ...we show how microzones favorable to anaerobic respiration processes (e.g., denitrification, metal reduction, and methanogenesis) can develop in the embedded less mobile porosity of bulk‐oxic hyporheic zones. Anoxic microzones develop when transport time from the streambed to the pore center exceeds a characteristic uptake time of oxygen. A two‐dimensional pore‐network model was used to quantify how anoxic microzones develop across a range of hyporheic flow and oxygen uptake conditions. Two types of microzones develop: flow invariant and flow dependent. The former is stable across variable hydrologic conditions, whereas the formation and extent of the latter are sensitive to flow rate and orientation. Therefore, pore‐scale residence time heterogeneity, which can now be evaluated in situ, offers a simple explanation for anaerobic signals occurring in oxic pore waters.
Key Points
Denitrification occurs in anoxic microzones of bulk oxic hyporheic sediments
Microzones develop in less mobile porosity due to increased local residence time
Geophysical methods have potential to evaluate hyporheic less mobile porosity
PDF
