Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.

/pdf/plumbing-the-global-carbon-cycle-integrating-inland-waters-1enndt5kqx.pdf

Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget

The term ''urban stream syndrome'' describes the consistently observed ecological degra- dation of streams draining urban land. This paper reviews recent literature to describe symptoms of the syndrome, explores mechanisms driving the syndrome, and identifies appropriate goals and methods for ecological restoration of urban streams. Symptoms of the urban stream syndrome include a flashier hy- drograph, elevated concentrations of nutrients and contaminants, altered channel morphology, and reduced biotic richness, with increased dominance of tolerant species. More research is needed before generaliza- tions can be made about urban effects on stream ecosystem processes, but reduced nutrient uptake has been consistently reported. The mechanisms driving the syndrome are complex and interactive, but most impacts can be ascribed to a few major large-scale sources, primarily urban stormwater runoff delivered to streams by hydraulically efficient drainage systems. Other stressors, such as combined or sanitary sewer overflows, wastewater treatment plant effluents, and legacy pollutants (long-lived pollutants from earlier land uses) can obscure the effects of stormwater runoff. Most research on urban impacts to streams has concentrated on correlations between instream ecological metrics and total catchment imperviousness. Recent research shows that some of the variance in such relationships can be explained by the distance between the stream reach and urban land, or by the hydraulic efficiency of stormwater drainage. The mech- anisms behind such patterns require experimentation at the catchment scale to identify the best management approaches to conservation and restoration of streams in urban catchments. Remediation of stormwater impacts is most likely to be achieved through widespread application of innovative approaches to drainage design. Because humans dominate urban ecosystems, research on urban stream ecology will require a broadening of stream ecological research to integrate with social, behavioral, and economic research.

https://www.jstor.org/stable/10.1899/04-028.1

The urban stream syndrome: current knowledge and the search for a cure

Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that show disproportionately high reaction rates relative to the surrounding matrix, whereas hot moments are defined as short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the carbon (C) and nitrogen (N) cycles, we show that hot spots occur where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants. Hot moments occur when episodic hydrological flowpaths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flowpaths, we can forge a better mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary so that biogeochemical hot spots can be identified at broader spatiotemporal scales and factored into quantitative models. We specifically recommend that resource managers incorporate both natural and artificially created biogeochemical hot spots into their plans for water quality management. Finally, we emphasize the needs for further research to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements, improve our ability to predict their occurrence, assess their importance in landscape biogeochemistry, and evaluate their utility as tools for resource management.

/pdf/biogeochemical-hot-spots-and-hot-moments-at-the-interface-of-1vpqinkleu.pdf

Biogeochemical Hot Spots and Hot Moments at the Interface of Terrestrial and Aquatic Ecosystems

The interactions between groundwater and surface water are complex. To understand these interactions in relation to climate, landform, geology, and biotic factors, a sound hydrogeoecological framework is needed. All these aspects are synthesized and exemplified in this overview. In addition, the mechanisms of interactions between groundwater and surface water (GW–SW) as they affect recharge–discharge processes are comprehensively outlined, and the ecological significance and the human impacts of such interactions are emphasized. Surface-water and groundwater ecosystems are viewed as linked components of a hydrologic continuum leading to related sustainability issues. This overview concludes with a discussion of research needs and challenges facing this evolving field. The biogeochemical processes within the upper few centimeters of sediments beneath nearly all surface-water bodies (hyporheic zone) have a profound effect on the chemistry of the water interchange, and here is where most of the recent research has been focusing. However, to advance conceptual and other modeling of GW–SW systems, a broader perspective of such interactions across and between surface-water bodies is needed, including multidimensional analyses, interface hydraulic characterization and spatial variability, site-to-region regionalization approaches, as well as cross-disciplinary collaborations.

/pdf/interactions-between-groundwater-and-surface-water-the-state-521l9polte.pdf

Interactions between groundwater and surface water: the state of the science

Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg Nkm � 2 � yr � 1 ) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.

/pdf/denitrification-across-landscapes-and-waterscapes-a-ei9e1947rb.pdf

Denitrification across landscapes and waterscapes: a synthesis.

1 – Quantifying Hydrologic Interactions between Streams and Their Subsurface Hyporheic Zones

We determined the role of the hyporheic zone (the subsurface zone where stream water and shallow groundwater mix) in enhancing microbially mediated oxidation of dissolved manganese (to form manganese precipitates) in a drainage basin contaminated by copper mining. The fate of manganese is of overall importance to water quality in Pinal Creek Basin, Arizona, because manganese reactions affect the transport of trace metals. The basin-scale role of the hyporheic zone is difficult to quantify because stream-tracer studies do not always reliably characterize the cumulative effects of the hyporheic zone. This study determined cumulative effects of hyporheic reactions in Pinal Creek basin by characterizing manganese uptake at several spatial scales (stream-reach scale, hyporheic- flow-path scale, and sediment-grain scale). At the stream-reach scale a one-dimensional stream-transport model (including storage zones to represent hyporheic flow paths) was used to determine a reach-averaged time constant for manganese uptake in hyporheic zones, 1/ls, of 1.3 hours, which was somewhat faster but still similar to manganese uptake time constants that were measured directly in centimeter-scale hyporheic flow paths (1/lh 5 2.6 hours), and in laboratory batch experiments using streambed sediment (1/l 5 2.7 hours). The modeled depths of subsurface storage zones (ds 5 4-17 cm) and modeled residence times of water in storage zones (ts 5 3-12 min) were both consistent with direct measurements in hyporheic flow paths (dh 5 0-15 cm, and t h 5 1-25 min). There was also good agreement between reach-scale modeling and direct measurements of the percentage removal of dissolved manganese in hyporheic flow paths ( f s 5 8.9%, and fh 5 9.3%). Manganese uptake experiments in the laboratory using sediment from Pinal Creek demonstrated (through comparison of poisoned and unpoisoned treatments) that the manganese removal process was enhanced by microbially mediated oxidation. The cumulative effect of hyporheic exchange in Pinal Creek basin was to remove approximately 20% of the dissolved manganese flowing out of the drainage basin. Our results illustrate that the cumulative significance of reactive uptake in the hyporheic zone depends on the balance between chemical reaction rates, hyporheic porewater residence time, and turnover of streamflow through hyporheic flow paths. The similarity between the hyporheic reaction timescale (1/ls ' 1.3 hours), and the hyporheic porewater residence timescale (t s ' 8 min) ensured that there was adequate time for the reaction to progress. Furthermore, it was the similarity between the turnover length for stream water flow through hyporheic flow paths (Ls 5 stream velocity/storage-zone exchange coefficient ' 1.3 km) and the length of Pinal Creek (L ' 7 km), which ensured that all stream water passed through hyporheic flow paths several times. As a means to generalize our findings to other sites where similar types of hydrologic and chemical information are available, we suggest a cumulative significance index for hyporheic reactions, Rs 5 lst sL/Ls (dimensionless); higher values indicate a greater potential for hyporheic reactions to influence geochemical mass balance. Our experience in Pinal Creek basin suggests that values of R s . 0.2 characterize systems where hyporheic reactions are likely to influence geochemical mass balance at the drainage-basin scale.

/pdf/effect-of-enhanced-manganese-oxidation-in-the-hyporheic-zone-3rruqohyrk.pdf

Effect of enhanced manganese oxidation in the hyporheic zone on basin‐scale geochemical mass balance

[1] Stream denitrification is thought to be enhanced by hyporheic transport but there is little direct evidence from the field. To investigate at a field site, we injected 15NO3−, Br (conservative tracer), and SF6 (gas exchange tracer) and compared measured whole-stream denitrification with in situ hyporheic denitrification in shallow and deeper flow paths of contrasting geomorphic units. Hyporheic denitrification accounted for between 1 and 200% of whole-stream denitrification. The reaction rate constant was positively related to hyporheic exchange rate (greater substrate delivery), concentrations of substrates DOC and nitrate, microbial denitrifier abundance (nirS), and measures of granular surface area and presence of anoxic microzones. The dimensionless product of the reaction rate constant and hyporheic residence time, λhzτhz define a Damkohler number, Daden-hz that was optimal in the subset of hyporheic flow paths where Daden-hz ≈ 1. Optimal conditions exclude inefficient deep pathways where substrates are used up and also exclude inefficient shallow pathways that require repeated hyporheic entries and exits to complete the reaction. The whole-stream reaction significance, Rs (dimensionless), was quantified by multiplying Daden-hz by the proportion of stream discharge passing through the hyporheic zone. Together these two dimensionless metrics, one flow-path scale and the other reach-scale, quantify the whole-stream significance of hyporheic denitrification. One consequence is that the effective zone of significant denitrification often differs from the full depth of the hyporheic zone, which is one reason why whole-stream denitrification rates have not previously been explained based on total hyporheic-zone metrics such as hyporheic-zone size or residence time.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/wrcr.20492

Hyporheic zone denitrification: Controls on effective reaction depth and contribution to whole-stream mass balance

Previously regarded as the passive drains of watersheds, over the past 50 years, rivers have progressively been recognized as being actively connected with off-channel environments. These connections prolong physical storage and enhance reactive processing to alter water chemistry and downstream transport of materials and energy. Here we propose river corridor science as a concept that integrates downstream transport with lateral and vertical exchange across interfaces. Thus, the river corridor, rather than the wetted river channel itself, is an increasingly common unit of study. Main channel exchange with recirculating marginal waters, hyporheic exchange, bank storage, and overbank flow onto floodplains are all included under a broad continuum of interactions known as “hydrologic exchange flows.” Hydrologists, geomorphologists, geochemists, and aquatic and terrestrial ecologists are cooperating in studies that reveal the dynamic interactions among hydrologic exchange flows and consequences for water quality improvement, modulation of river metabolism, habitat provision for vegetation, fish, and wildlife, and other valued ecosystem services. The need for better integration of science and management is keenly felt, from testing effectiveness of stream restoration and riparian buffers all the way to reevaluating the definition of the waters of the United States to clarify the regulatory authority under the Clean Water Act. A major challenge for scientists is linking the small-scale physical drivers with their larger-scale fluvial and geomorphic context and ecological consequences. Although the fine scales of field and laboratory studies are best suited to identifying the fundamental physical and biological processes, that understanding must be successfully linked to cumulative effects at watershed to regional and continental scales.

/pdf/river-corridor-science-hydrologic-exchange-and-ecological-xomi7rai8g.pdf

River corridor science: Hydrologic exchange and ecological consequences from bedforms to basins

Tracer experiments are valuable tools for analyzing the transport characteristics of streams and their interactions with shallow groundwater. The focus of this work is the design of tracer studies in high-gradient stream systems subject to advection, dispersion, groundwater inflow, and exchange between the active channel and zones in surface or subsurface water where flow is stagnant or slow moving. We present a methodology for (1) evaluating and comparing alternative stream tracer experiment designs and (2) identifying those combinations of stream transport properties that pose limitations to parameter estimation and therefore a challenge to tracer test design. The methodology uses the concept of global parameter uncertainty analysis, which couples solute transport simulation with parameter uncertainty analysis in a Monte Carlo framework. Two general conclusions resulted from this work. First, the solute injection and sampling strategy has an important effect on the reliability of transport parameter estimates. We found that constant injection with sampling through concentration rise, plateau, and fall provided considerably more reliable parameter estimates than a pulse injection across the spectrum of transport scenarios likely encountered in high-gradient streams. Second, for a given tracer test design, the uncertainties in mass transfer and storage-zone parameter estimates are strongly dependent on the experimental Damkohler number, DaI, which is a dimensionless combination of the rates of exchange between the stream and storage zones, the stream-water velocity, and the stream reach length of the experiment. Parameter uncertainties are lowest at DaI values on the order of 1.0. When DaI values are much less than 1.0 (owing to high velocity, long exchange timescale, and/or short reach length), parameter uncertainties are high because only a small amount of tracer interacts with storage zones in the reach. For the opposite conditions (DaI .. 1.0), solute exchange rates are fast relative to stream-water velocity and all solute is exchanged with the storage zone over the experimental reach. As DaI increases, tracer dispersion caused by hyporheic exchange eventually reaches an equilibrium condition and storage-zone exchange parameters become essentially nonidentifiable.

/pdf/experimental-design-for-estimating-parameters-of-rate-cuu2fwo0ol.pdf

Judson W. Harvey

Papers

1 – Quantifying Hydrologic Interactions between Streams and Their Subsurface Hyporheic Zones

Effect of enhanced manganese oxidation in the hyporheic zone on basin‐scale geochemical mass balance

Hyporheic zone denitrification: Controls on effective reaction depth and contribution to whole-stream mass balance

River corridor science: Hydrologic exchange and ecological consequences from bedforms to basins

Experimental design for estimating parameters of rate-limited mass transfer: Analysis of stream tracer studies