Parafluvial Zone Research Articles

Seven decades of hydrogeomorphological changes in a near‐natural (Sense River) and a hydropower‐regulated (Sarine River) pre‐Alpine river floodplain in Western Switzerland

ABSTRACTHydropower alteration of the natural flow and sediment regime can severely degrade hydromorphology, thereby threatening biodiversity and overall ecosystem processes of rivers and their floodplains. Using sequences of aerial images, we quantified seven decades (1938/1942–2013) of spatiotemporal changes in channel and floodplain morphology, as well as changes in the physical habitats, of three floodplain river reaches of the Swiss pre‐Alps, two hydropower‐regulated and one near‐natural. In the Sarine River floodplain, within the first decades of hydropower impairment, the magnitude and frequency of flood events (Q2, Q10, Q30) decreased substantially. As a result, the area of pioneer floodplain habitats that depend on flood activity and sediment dynamic, such as bare sediments, decreased dramatically by approximately 95%. However, by 2013 vegetated areas had generally increased in comparison to the pre‐regulation period in 1943, indicating general vegetative colonization. Between 1943 and 2013, the active channel underwent essential narrowing (up to 62% width reduction in the residual flow reach) and habitat turnover rates were very low (5% of the total floodplain area changed habitat type five to six times). In contrast, from the 1950s onwards, the near‐natural floodplain of the Sense River experienced recurrent narrowing and widening, and frequent changes between bare and vegetated areas, reflecting the shifting habitat mosaic concept typical for natural floodplains. In the three reaches investigated, we found that the active floodplain width and erosion of vegetated areas were primarily controlled by medium to large floods (Q10, Q30), which combined with reduced time intervals between ordinary floods ≥ Q2 most likely mobilized streambed sediments and limited the ability of vegetation to establish itself on bare gravel bars within the parafluvial zone. These findings can contribute to restoration action plans such as controlled flooding and sediment replenishments in the Sarine and other floodplain rivers of the Alps. © 2020 John Wiley & Sons, Ltd.

River-floodplain restoration and hydrological effects on GHG emissions: Biogeochemical dynamics in the parafluvial zone.

The parafluvial zone is frequently exposed to drying-rewetting cycles with critical consequences for the biogeochemistry of soil and sediment in river-floodplain landscapes. Upon restoration of the hydrological connectivity, substantial changes in biogeochemical processes are expected. The effects of water fluctuation on the magnitude of GHG emissions were investigated in the parafluvial zone of a restored river floodplain in Austria. Sediment composition, DOM quality and N2O, CO2, CH4 fluxes were quantified during distinct hydrological periods (intermittent, desiccation and post flood) and along a hydrological gradient. The hydrological gradient ranged from non-flooded plots in the floodplain soil (used as reference plots after restoration), to rarely-flooded and frequently flooded sediment plots in the parafluvial zone. Enhanced biogeochemical turnover rates were identified during the intermittent period, when N2O and CO2 emissions peaked. In particular, the frequently flooded plots showed significantly higher CO2 and CH4 emissions compared to non-flooded and rarely-flooded plots. This indicates a strong effect of water level fluctuation on GHG emissions, with higher emissions occurring during transitional stages of drying and rewetting. Strong positive relationships were found between individual GHG fluxes, suggesting a tight link between C and N cycles. Both the C and N cycles are dependent on similar substrate characteristics that are governed by the quality of the DOM pool. Interestingly, drier sediments in the rarely-flooded plots were also active areas for emissions. This highlights the importance to include dry phases and sites in the overall C and N emission estimates of riverine landscapes. From the restoration point of view, N2O emissions in the parafluvial zone did not differ significantly from the emissions in the reference plots, whereas CO2 and CH4 fluxes did. When making management decisions to restore connectivity, one needs to carefully consider the interplay between nutrient removal from water versus GHG emissions, to reach maximum environmental benefits.

Characterizing the capacity of hyporheic sediments to attenuate groundwater nitrate loads by adsorption

Nitrate has been recognized as a global threat to environmental health. In this regard, the hyporheic zone (saturated media beneath and adjacent to the stream bed) plays a crucial role in attenuating groundwater nitrate, prior to discharge into surface water. While different nitrate removal pathways have been investigated over recent decades, the adsorption capacity of hyporheic sediments under natural conditions has not yet been identified. In this study, the natural attenuation capacity of the hyporheic-sediments of the Ghezel-Ozan River, located in the north-west of Iran, was determined. The sampled sediments (from 1 m below the stream bed) were characterized via XRD, FT-IR, BET, SEM, BJH, and Zeta potential. Nitrate adsorption was evaluated using a batch experiment with hyporheic pore-water from each study site. The study was performed in the hyporheic sediments of two morphologically different zones, including Z1 located in the parafluvial zone having the clay sediment texture (57.8% clay) with smectite/Illite mixed layer clay type and Z2 located in the river confluence area containing silty clay sediment texture (47.6% clay) with smectite/kaolinite mixed layer clay type. Data obtained from the batch experiment were subjected to pseudo-first order, pseudo-second order, intra-particle diffusion, and Elovich mass transfer kinetic models to characterize the nitrate adsorption mechanism. Furthermore, to replicate nitrate removal efficiencies of the hyporheic sediments under natural conditions, the sampled hyporheic pore-waters were applied as initial solutions to run the batch experiment. The results of the artificial nitrate solution correlated well with pseudo-second order (R2>95%; in both Z1 and Z2) and maximum removal efficiencies of 85.3% and 71.2% (adsorbent dosage 90 g/L, pH = 5.5, initial adsorbate concentration of 90 mg/L) were achieved in Z1 and Z2, respectively. The results of the nitrate adsorption analysis revealed that the nitrate removal efficiencies varied from 17.24 ± 1.86% in Z1 during the wet season to 28.13 ± 0.89% in Z2 during the dry season. The results obtained by this study yielded strong evidence of the potential of hyporheic sediments to remove nitrate from an aqueous environment with great efficiency.

Using radon to understand parafluvial flows and the changing locations of groundwater inflows in the Avon River, southeast Australia

Abstract. Understanding the location and magnitude of groundwater inflows to rivers is important for the protection of riverine ecosystems and the management of connected groundwater and surface water systems. This study utilizes 222Rn activities and Cl concentrations in the Avon River, southeast Australia, to determine the distribution of groundwater inflows and to understand the importance of parafluvial flow on the 222Rn budget. The distribution of 222Rn activities and Cl concentrations implies that the Avon River contains alternating gaining and losing reaches. The location of groundwater inflows changed as a result of major floods in 2011–2013 that caused significant movement of the floodplain sediments. The floodplain of the Avon River comprises unconsolidated coarse-grained sediments with numerous point bars and sediment banks through which significant parafluvial flow is likely. The 222Rn activities in the Avon River, which are locally up to 3690 Bq m−3, result from a combination of groundwater inflows and the input of water from the parafluvial zone that has high 222Rn activities due to 222Rn emanation from the alluvial sediments. If the high 222Rn activities were ascribed solely to groundwater inflows, the calculated net groundwater inflows would exceed the measured increase in streamflow along the river by up to 490 % at low streamflows. Uncertainties in the 222Rn activities of groundwater, the gas transfer coefficient, and the degree of hyporheic exchange cannot explain a discrepancy of this magnitude. The proposed model of parafluvial flow envisages that water enters the alluvial sediments in reaches where the river is losing and subsequently re-enters the river in the gaining reaches with flow paths of tens to hundreds of metres. Parafluvial flow is likely to be important in rivers with coarse-grained alluvial sediments on their floodplains and failure to quantify the input of 222Rn from parafluvial flow will result in overestimating groundwater inflows to rivers.

Role of ephemeral vegetation of emerging river bottoms in modulating CO2 exchanges across a temperate large lowland river stretch

Rivers and marginal wetlands contribute significantly to the carbon (C) exchange rate per unit area compared to adjacent terrestrial ecosystems, due to high C inputs, metabolic activity and CO2 supersaturation. Within riverscapes, the contribution of emerging bottoms (i.e. parafluvial zones) and ephemeral vegetation (i.e. microphytobenthos or annual vascular plant communities) to the C metabolism is understudied. The aim of this study was to evaluate the CO2 exchange rates at the water- and emergent sand bar-atmosphere interfaces within a lowland large temperate river stretch. CO2 fluxes were measured seasonally, in summer (August 2007) and winter (March 2008) via static closed chambers, together with the primary producers’ biomass (microphytobenthos and vascular macrophytes). Our results showed that the river was a CO2 source (between 0.2 and 7.6 mmol CO2 m−2 day−1), whilst the vegetated parafluvial zones acted as a net sink, with assimilation rates peaking at 623.4 mmol CO2 m−2 day−1. Emerging bare sediments were on the contrary net emitting CO2 systems (2.7–60.1 mmol CO2 m−2 day−1). Within lotic environments, seasonally emergent vegetated sand bars may represent important C fixation hot spots, with summer primary production particularly contributing to offsetting CO2 emissions and counterbalancing the heterotrophic metabolism of the saturated zones. Based on these outcomes, we suggest that studies addressing the strictly aquatic fluvial environment, which is generally a CO2 emitter, should be linked to those from transitional areas in order to better integrate terrestrial and aquatic C budgets.

Groundwater flow, nutrient, and stable isotope dynamics in the parafluvial-hyporheic zone of the regulated Lower Colorado River (Texas, USA) over the course of a small flood

Periodic releases from an upstream dam cause rapid stage fluctuations in the Lower Colorado River near Austin, Texas, USA. These daily pulses modulate fluid exchange and residence times in the hyporheic zone where biogeochemical reactions are typically pronounced. The effects of a small flood pulse under low-flow conditions on surface-water/groundwater exchange and biogeochemical processes were studied by monitoring and sampling from two dense transects of wells perpendicular to the river. The first transect recorded water levels and the second transect was used for water sample collection at three depths. Samples were collected from 12 wells every 2 h over a 24-h period which had a 16-cm flood pulse. Analyses included nutrients, carbon, major ions, and stable isotopes of water. The relatively small flood pulse did not cause significant mixing in the parafluvial zone. Under these conditions, the river and groundwater were decoupled, showed potentially minimal mixing at the interface, and did not exhibit any discernible denitrification of river-borne nitrate. The chemical patterns observed in the parafluvial zone can be explained by evaporation of groundwater with little mixing with river water. Thus, large pulses may be necessary in order for substantial hyporheic mixing and exchange to occur. The large regulated river under a low-flow and small flood pulse regime functioned mainly as a gaining river with little hydrologic connectivity beyond a narrow hyporheic zone.

Understanding parafluvial exchange and degassing to better quantify groundwater inflows using 222Rn: The King River, southeast Australia

222Rn is an important tracer for quantifying groundwater inflows to rivers, especially where groundwater and surface water have similar major ion and stable isotope geochemistry. Uncertainties in the 222Rn mass balance arise, however, from not accurately estimating the degree of degassing of 222Rn to the atmosphere and the extent to which interaction within the parafluvial zone provides an additional source of 222Rn. This study estimates both 222Rn production in the parafluvial zone and degassing along a 75km stretch of the King River, Australia, in order to more precisely quantify groundwater inflows. The contribution of 222Rn from the parafluvial zone (Fp) was estimated using 222Rn emanation rates from near-river sediments and assessment of the residence time of water in the parafluvial zone from 222Rn activities and Cl concentrations in water the alluvial sediments. Values of Fp range from 40,400Bq/m/day in the upper King to 8500Bq/m/day in the lower King, corresponding to differences in the mineralogy and the volume of the parafluvial zone. The gas transfer coefficient (k) was estimated by matching groundwater inflows to observed increases in river discharge during a period of low discharge; k decreases from 25day-1 in the upper King to 3day-1 in the lower King. These k values are higher than those from most empirical formulations, probably due to the extensive pool and riffle sections that promote degassing. The King River is gaining in its upper and middle sections but several of the lower reaches are losing. Groundwater inflows on a reach scale are as high as 10m3/m/day and cumulative inflows along the 75km stretch are up to 19,000m3/day. Groundwater inflows increase proportional to total flow reflecting the response of both groundwater and surface water systems to rainfall. Uncertainties in the calculated groundwater inflows are reduced by independently estimating k from the discharge data; however, calculated inflows are up to 40% higher if parafluvial flow were not taken into account. •Predicted Rn activities from emanation measurements agree with those measured in groundwater.•Contribution of Rn from parafluvial zone is assessed.•Rn degassing is independently assessed by comparison with flow gauge data.•Rn degassing is higher than predicted from empirical formulations due to pool and riffle sections.•King River transitions from gaining in upper sections to losing in lower sections.

Landscape Influences on Ecosystem Function: Local and Routing Control of Oxygen Dynamics in a Floodplain Aquifer

Perspectives addressing either “local” or “routing” control over ecological form and function alternatively emphasize ambient conditions or recognize that form and processes at one location may be heavily influenced by other points in space. In their natural state, floodplains of montane gravel-bed rivers are organized laterally into parafluvial zones, where scour occurs annually, and orthofluvial environments, within which active (that is, early successional) and passive (that is, late successional) zones differ in regard to flooding influences. Vertically, these rivers interact with extensive alluvial aquifers throughout the length of their floodplains. We addressed how local and routing controls organize riverscape structure and function by investigating seasonal dynamics of interstitial dissolved oxygen (DO) vertically within the alluvial aquifer, longitudinally along flowpaths, and laterally across active and passive orthofluvial (PO) zones in the Nyack floodplain, Middle Fork Flathead River, MT. Groundwater DO was positively correlated to pH and negatively correlated electrical conductivity (EC) across all seasons. In some wells (7 of 19), DO was significantly lower and EC greater at shallow depths, and the magnitudes of vertical zonation were related to dissolved organic carbon concentrations. Longitudinal declines in DO were always and exclusively observed along flowpaths of the PO zone. At the same time, persistent hypoxic patches within the active orthofluvial environment suggest metabolic hotspots that contribute to local control of physicochemistry. In ecosystems heavily influenced by material transport, local and routing controls may be manifested simultaneously as distinct patterns of organization, which are expressed at different spatial and temporal scales.

Flow variability and longitudinal patterns in parafluvial water chemistry, aquatic invertebrates and microbial activity

Summary Parafluvial zones occur in emergent gravel bars and other river sediment structures where exchange with surface water is limited to horizontal inflow and outflow. Within parafluvial zones, interstitial water flows down longitudinal flowpaths in response to hydraulic-head gradients. Changes in physical and biogeochemical conditions along these flowpaths lead to the formation of longitudinal gradients in solute concentrations, biota and microbial activity. The influence of river-flow variability on interstitial residence times, flowpath sizes and solute influx suggests that flow variability also affects longitudinal gradients in parafluvial zones. We studied associations between longitudinal gradients and flow variation in the forms of flow fluctuations and variable water sources. We used data from parafluvial zones in a New Zealand river to test four predictions: (i) longitudinal gradients in solutes, invertebrates and microbial activity are steeper in runoff-dominated losing and neutral reaches compared with groundwater-dominated gaining reaches; (ii) moderate-to-large floods eliminate longitudinal gradients, which reform during recessions; (iii) between floods, gradients in parafluvial chemistry change in steepness and direction in response to changes in river-flow levels; and (iv) between floods, invertebrate abundance and diversity are positively related to river flow. In Study 1, we measured solute concentrations, invertebrate composition and microbial activity over a range of flow levels in vegetated gravel bars with contrasting water sources (alluvial groundwater in a gaining reach, and hillslope runoff in losing and neutral reaches). In Study 2, we made repeated measurements in unvegetated gravel bars before and after a one-in-2-year flood to assess its effects on longitudinal gradients in nutrients and invertebrate communities. In Study 1, we detected gradients along parafluvial flowpaths in , dissolved reactive phosphorus and major ion concentrations and in pH, electrical conductivity and invertebrate density. There was little congruence between bars in different reaches, or among variables; some gradients became steeper as flow increased, and others shifted from negative to positive or vice versa or approached zero. Parafluvial chemistry was strongly affected by water source; median concentrations of most solutes were 2–20 times higher in the groundwater-dominated parafluvial zone than in the runoff-dominated parafluvial zones. Prior to the flood during Study 2, the parafluvial zones were characterised by negative longitudinal gradients in dissolved organic carbon (DOC) and dissolved organic N (DON) and positive gradients in . These gradients were eliminated by the flood, but and DOC gradients redeveloped during the post-flood period. Differences between the steepness and direction of gradients in Study 1 and the pre-flood period in Study 2 may be related to differences in terrestrial vegetation, as well as differences in flow regimes. Associations between river-flow levels and parafluvial invertebrate communities differed in the two studies, which corresponded to different flow ranges. Invertebrate density and taxon richness were positively correlated with flow over a range of low flows in Study 1, but there was no detectable correlation over a range of high flows in Study 2. These results suggest that beneficial effects of flow fluctuations on invertebrate communities peak at moderate flow levels, beyond which invertebrate–flow relationships become unstable.

Analogous Modeling of Nutrient Transformation in Iskar River Sediments at Different Moisture Content: Microbiological and Enzymological Indicators

ABSTRACTThis study was focused on the R&D of the elements of the systems for early warning of risk events as well as on the selection of the mechanisms and indicators for their management. The aim was to derive the mathematical dependences, bioalgorithms, and indicative correlations for upgrading of the HSPF hydrological model. For this purpose analogous modeling of transformation of organics in the river sediments of the parafluvial zone as a function of moisture (in%) was performed. This upgrading can be applied for correct and effective prognosis of the rate of self-purification in the Iskar River in the section from Beli Iskar Reservoir to Iskar Reservoir.The results showed that the combination of total dehydrogenase, nitrate reductase and phosphatase activities could be considered an effective instrument for water quality management and for prediction of the rate of transformation processes. The derived linear mathematical dependences can be applied to calculate other microbiological and kinetic indicators important for monitoring, as well as to create special modules for the HSPF model with a well-functioning verification strategy.

Do parafluvial zones have an impact in regulating river pollution? Spatial and temporal dynamics of nutrients, carbon, and bacteria in a large gravel bar of the Doubs River (France)

The changes in both physical and chemical properties of interstitial water were investigated within a large gravel bar to determine if the parafluvial zone was a hotspot for nutrient transformations in a lowland eutrophic river, the Doubs (Eastern France). Interstitial water was sampled in 30 piezometers along five transects across the gravel bar, surface water was sampled in main and chute channels. Five campaigns of sampling were performed among summer and winter. In both interstitial and surface samples, water chemistry (electrical conductivity, pH, chloride, dissolved oxygen concentrations—DO) and nutrient concentrations (nitrate—NO3 −, ammonium—NH4 +, soluble reactive phosphorus—SRP, dissolved organic carbon—DOC) were measured. Moreover, temperature, water level, sediment grain size distribution and total bacterial abundance were assessed along a flowpath through a lateral gravel bar of the Doubs River. Measurements of water table elevation in the bar, main and chute channels indicated that the parafluvial flowpath was perpendicular to the main channel. Very low changes in chloride concentration and electrical conductivity showed minor groundwater input along the flowpath. The parafluvial zone was 0.9 m thick under the mean piezometric level, hydraulic gradients along the flowpath were 0.3%, and the discharge of interstitial water through the bar calculated at low flow was 40.6 m3 day−1. Most changes in interstitial nutrients occurred during the warm season, suggesting that biotic mechanisms occurred. Along the flowpath, DO, DOC, and bacterial abundance declined, while phosphate increased. Temporal trends of nitrate were less clear, exhibiting a slight increase during spring (organic matter biodegradation) and a decrease during summer (denitrification). Using the parafluvial discharge and nutrient concentrations of water infiltrating into and seeping from the gravel bar, the retention capacity of the bar was 171 gC day−1 for DOC and 48.3 gN day−1 for nitrate. Phosphate production ranged from 0.65 to 2.3 gP day−1. These values were low compared to the nutrient fluxes in the river Doubs, suggesting a minimal local impact of the parafluvial zone in regulating of river pollution.

Integrating hydraulic conductivity with biogeochemical gradients and microbial activity along river–groundwater exchange zones in a subtropical stream

The pervious lateral bars (parafluvial zone) and beds (hyporheic zone), where stream water and groundwater exchange, are dynamic sites of hydrological and biological retention. The significance of these biogeochemical ‘hotspots’ to stream and groundwater metabolism is largely controlled by filtration capacity, defined as the extent to which subsurface flowpaths and matrix hydraulic conductivity modify water characteristics. Where hydraulic conductivity is high, gradients in biogeochemistry and microbial activity along subsurface flowpaths were hypothesized to be less marked than where hydraulic conductivity is low. This hypothesis was tested in two riffles and gravel bars in an Australian subtropical stream. At one site, gradients in chemical and microbial variables along flowpaths were associated with reduced hydraulic conductivity, longer water residence time and reduced filtration capacity compared with the second site where filtration capacity was greater and longitudinal biogeochemical trends were dampened. These results imply that factors affecting the sediment matrix in this subtropical stream can alter filtration capacity, interstitial microbial activity and biogeochemical gradients along subsurface flowpaths. This hydroecological approach also indicates potential for a simple field technique to estimate filtration capacity and predict the prevailing hyporheic gradients in microbial activity and biogeochemical processing efficiency, with significant implications for stream ecosystem function.

River flow controls ecological processes and invertebrate assemblages in subsurface flowpaths of an ephemeral river reach

We present the first measurements of solutes, invertebrates, and microbial activity in the semi-perched hyporheic, parafluvial, and riparian flowpaths of an ephemeral river channel. Dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations decreased as water from an adjacent river mainstem moved through the flowpaths. DOC, DON, and DOP processing rates decreased with increasing mainstem flow and increased with parafluvial zone length. These patterns suggest that the surface water zones of perched river systems are organic nutrient sources to subsurface flowpaths and that parafluvial zones of these systems are strong sinks for organic matter. No longitudinal changes were detected in NO3–concentrations, and relationships between NO3–processing and hydrological variables were not significant. NO3–concentrations were uniformly high, and microbial activity and DOC and dissolved reactive phosphorus (DRP) concentrations were low, suggesting that biological NO3–removal was carbon- or phosphorus-limited. Invertebrate assemblages also varied between subsurface zones: density and taxon richness in the hyporheic and parafluvial zones were higher than in the riparian zone, and evenness was higher in the riparian zone than in the hyporheic or parafluvial zones. Distinct invertebrate assemblages in riparian zones may reflect greater hydrologic stability compared with hyporheic and parafluvial zones.

Lateral and longitudinal patterns within the stygoscape of an alluvial river corridor

In landscape ecology, landscapes are viewed as hierarchically-organized groups of interacting components or zones. This perspective is applied in studies of riverscapes with increasing frequency,particularlyin studies ofchannel habitats and channel-floodplain interactions. Stygoscapes, the subsurface portions of riverscapes, have received less attention. Stygoscapes are composed of three saturated zones beneath and lateral to river channels: the hyporheic, parafluvial and riparian zones. These zones are differentiated on the basis of hydrological, biological and chemical conditions, and the degree of interaction with the channel surface. In this paper, we describe lateral and longitudinal patterns in the shallow (30 cm - 10 m depth) stygoscape of the alluvial Selwyn River, New Zealand. We used abutting hyporheic, parafluvial and riparian zones in large-scale downwelling (losing) and upwelling (gaining) sections of the river mainstem to compare physico-chemical and biological conditions across a broad stygoscape. The stygoscape zones had the same arrangement in both sections: hyporheic zones within the wetted channel, riparian zones furthest from the channel, and parafluvial zones between the hyporheic and riparian zones. Nitrate and dissolved reactive phosphorus concentrations and electrical conductivity were significantly higher, and dissolved organic phosphorus concentrations and pH levels lower, in the gaining section than the losing section. In both sections, ammonium, dissolved reactive phosphorus, and dissolved organic carbon concentrations and pH were higher in the hyporheic zone than the riparian zone. Within the losing section, DON and DOP concentrations were higher in the hyporheic zone than the riparian zone. Physico-chemical conditions were intermediate in the parafluvial zones of both river sections. These among-zone differences suggest that hyporheic zones are sinks for dissolved nutrients, and that effects of hyporheic sinks are greatest in losing river sections with unidirectional flow from the surface to the hyporheic zone. There were large differences among stygoscape zones in invertebrate assemblage composition, and smaller differences between the losing and gaining sections. Invertebrate densities, taxon richness and % insect taxa were higher in hyporheic zones than riparian zones. Invertebrate densities were intermediate in the parafluvial zones, and taxon richness and % insect taxa were similar in parafluvial and riparian zones. The spatial patterns we observed in the Selwyn River suggest that hydrological connectivity between zones, and permeability within zones, are among the most important determinants of chemical and biological dynamics in stygoscapes. Differences in large-scale upwelling and downwelling, and differences in water source (runoff and groundwater) appeared to be of secondary importance.

The influence of size, hydrological characteristics and vegetation cover on nitrogen, phosphorus and organic carbon cycling in lowland river gravel bars (Doubs River, France)

The role of large gravel bars in nutrient retention may vary according to their physical characteristics and their vegetation. The goal of this study was to estimate nutrient fl uxes and retention capacity of four gravel bars of a fi fth-order river, the Doubs River (Eastern France), during low river discharge in summer and autumn. Patterns of water chemistry, nutrient concentrations, and hydrological characteristics were investigated at -0.3 m depth, using fi ve sampling points in the downwelling zone and fi ve in the upwelling zone of each bar. The dissolved oxygen (DO) concentrations strongly decreased inside the bars, reaching values below 1 mg L -1 in the upwelling zones in summer, but remained between 1.1 and 4.4 mg L -1 in autumn. Dissolved organic carbon (DOC) concentrations decreased by 21 % to 44 % inside the bars or remained stable (in two bars in autumn). In the upwelling zones they were signifi cantly positively correlated to shrub cover percentage at the bar surface, suggesting a DOC supply by leaf decomposition and root exudates. Nitrate concentrations decreased by 50 % and phosphate increased by 102 % on average along the interstitial fl ow path in summer, with similar values in the four upwelling zones. Vegetation cover and hydrological characteristics had no signifi cant effect on nitrogen retention nor on phosphorus production. In summer, retention of nitrate and DOC amounted to 16.5 and 25 g day -1 per 100 m of gravel bar respectively, with differences among bars linked to the intensity of interstitial discharges. Hence, net retention of DOC and NO3 - in the parafl uvial zone of the Doubs River was relatively high, while release of SRP was low and limited to the sum- mer period.

Subsystems, flowpaths, and the spatial variability of nitrogen in a fluvial ecosystem

Nutrient dynamics in rivers affect biogeochemical fluxes from land to oceans and the atmosphere. Fluvial ecosystems are thus important environments for understanding spatial variability in nutrient concentrations. At the San Pedro River in semi-arid Arizona, USA, we investigated how variability in dissolved inorganic nitrogen (DIN) was regulated by subsystem type and hydrological flowpaths. The three subsystems we compared were the riparian zone, parafluvial (gravel bar) zone, and surface stream. DIN concentration was greater in the riparian zone than in the surface stream, suggesting that the riparian zone retains DIN and is a source of N for the surface stream. Parafluvial zones were too variable to generalize how they regulate DIN. Our hypothesis that subsystem type regulates DIN oxidation was too simple. The riparian and parafluvial zones host a mosaic of oxidizing and reducing conditions, as they exhibited highly variable ammonium to nitrate (NH4 :NO3 ) ratios. Surface stream DIN was dominated by NO3 . Along a subsurface flowpath in the riparian zone, we did not observe spatial covariation among the N forms and transformations involved in miner- alization. We also compared spatial variability in solute concentrations between flowpaths and non- flowpath reference areas. Our mixed results suggest that spatial variability is regulated in part by flowpaths, but also by solute-specific processing length along a flowpath. To under- stand the distribution of N in fluvial ecosystems, subsystem type and flowpaths are readily discern- able guides, but they should be coupled with other mechanistic factors such as biota and soil type.

The Spatial Structure of Variability in a Semi-arid, Fluvial Ecosystem

The arrangement and composition of flowpath types within a given network are thought to govern its functioning. This concept assumes that different flowpath types are functionally distinct. We investigated this assumption in a fluvial ecosystem by comparing the riparian zone, parafluvial zone (in-channel gravel bars), and surface stream. We hypothesized that differences in advection, uptake, and sorption would render material cycles more (a) open and (b) mutable in the surface stream, whereas the converse would occur in the riparian zone, and an intermediate state would be seen in the intervening parafluvial zone. To test our first hypothesis, we predicted that spatial heterogeneity in solute concentrations would be least in the surface stream, greater in the parafluvial zone, and greatest in the riparian zone. Using a null model, we ascertained that this pattern was shown by all solute species we examined (nitrate, ammonium, total dissolved inorganic nitrogen [DIN], dissolved organic N, total dissolved N, soluble reactive phosphorus, dissolved organic carbon, and chloride). To test our second hypothesis, we predicted that temporal change in spatial heterogeneity would be greatest in the surface stream, less in the parafluvial zone, and least in the riparian zone. Nitrate, DIN, and chloride showed this pattern. In particular, surface stream inorganic N was less spatially variable following months of high rainfall. According to an extant hypothesis, these results suggest that inorganic N processing may be a stable function in this ecosystem. Other solute species did not support our second prediction, perhaps because their retention and release dynamics are influenced principally by geochemistry. Generally, our findings indicate that a geomorphic template can generate spatial patterns in ecosystem function, warranting an expansion of the spiraling framework to a variety of flowpath types.

The hydraulic characteristics and geochemistry of hyporheic and parafluvial zones in Arctic tundra streams, north slope, Alaska

Sodium bromide and Rhodamine WT were used as conservative tracers to examine the hydrologic characteristics of seven tundra streams in Arctic Alaska, during the summers of 1994–1996. Continuous tracer additions were conducted in seven rivers ranging from 1st to 5th order with samples collected from instream, hyporheic, and parafluvial locations. Tracer data was used as input for a computer model to estimate hydrologic characteristics of each study reach. While solute concentrations during the tracer additions indicated that steady-state or “plateau” conditions had been reached, interstitial samples indicated that there were additional hyporheic and parafluvial zones that had not been fully labeled at the time of apparent steady state in the stream channel (plateau). Exchange between channel and hyporheic water was a function of location within a pool–riffle sequence, with rapid downwelling at the head of riffles and delayed upwelling in riffle tails. The extent of exchange between channel and hyporheic water was positively correlated with apparent streambed hydraulic conductivity. Tracer additions indicated interstitial velocities ranging from 0.030 to 0.075 cm s −1 and hydraulic conductivities from 2.4 to 12.2 cm s −1. Hyporheic and in-channel samples were collected for N, P, DO, and CO 2 analyses in conjunction with conservative tracer additions in four of the stream reaches for which the interstitial velocities were also determined. Transformation rates based on these data indicated that there was rapid nitrification of mineralized organic N and production of ammonium, phosphate, and carbon dioxide in the interstitial zones of all four reaches. Dissolved oxygen did not appear to be limiting in the reaches studied. The hyporheic zone of all four reaches was a source of nitrate, carbon dioxide, and ammonium to the channel water based on the average concentration of upwelling waters. Increased contact time with hyporheic and parafluvial zones was related to decreased temperature and increased conductivity. Net nitrogen flux from the hyporheic zone was equivalent to 14–162% of benthic N uptake requirements for the Kuparuk River. These observations are important because we expected that the presence of continuous permafrost in this Arctic environment would limit the importance of hyporheic processes, either physically (i.e., through the presence of a restricting thaw bulb in the permafrost) or biogeochemically (i.e., through low temperatures). Instead, we found that biogeochemical processes in the hyporheic zone of these Arctic streams are at least as important as it is in similar temperate stream ecosystems.

Effects of Coarse Particulate Organic Matter on Fungal Biomass and Invertebrate Density in the Subsurface of a Headwater Stream

Links between groundwater invertebrates and their potential food resources were examined using biofilm development on fine wood. Little is known about biofilm development and organic matter content of lateral subsurface (i.e., parafluvial) environments and hyporheic habitats (upwelling and downwelling zones). Eighteen experimental baskets containing river rocks were paired by treatment in which 1 basket was supplemented with wood (six 7.2 cm × 12 cm strips of oak wood veneer). Nine pairs of baskets were buried 12 to 15 cm below the surface in Gallina Creek, a 1st-order mountain stream in northern New Mexico in late summer 1997. Three pairs were buried beneath the stream bank (i.e., parafluvial zone) and 6 pairs were buried in the hyporheic zone. Baskets were distributed along upwelling and downwelling reaches to assess the potential hydrologic influence of subsurface-surface exchange. Open baskets of wood veneer were placed on the streambed surface to compare fungal biomass on the surface with the subsurface. Wood in both hyporheic and parafluvial baskets was colonized by fungi, but fungal biomass was significantly greater on wood in surface water than in hyporheic and parafluvial zones. In addition, fungal biomass on hyporheic wood was significantly greater than on parafluvial wood. A similar pattern (i.e., surface > hyporheic > parafluvial) was observed for dissolved oxygen. In contrast, concentrations of retained particulate organic matter were significantly higher in the parafluvial than the hyporheic zone. Invertebrate densities were significantly greater in baskets supplemented with wood and were greater in the hyporheic zone than in the parafluvial zone. Our data suggest that wood and associated microbial biofilms represent an important food resource for interstitial invertebrate communities.

The impact of flash floods on microbial distribution and biogeochemistry in the parafluvial zone of a desert stream

Summary1. The impact of flash flooding on microbial distribution and biogeochemistry was investigated in the parafluvial zone (the part of the active channel lateral to the surface stream) of Sycamore Creek, a Sonoran Desert stream in central Arizona.2. It was hypothesized that subsurface bacteria were dependent on the import of algal‐derived organic matter from the surface stream, and it was therefore predicted that microbial numbers and rates of microbially mediated processes would be highest at locations of surface to subsurface hydrologic exchange and at times when algal biomass was high.3. Prior to a flash flood on 19 July 1994, chlorophyll a was high (≈ 400 mg m–2) in the surface stream and microbial numbers were highest at the stream–parafluvial interface and declined along parafluvial flowpaths, supporting the hypothesized algal–bacterial linkage. Immediately following the flash flood, chlorophyll a was low (≈ 7 mg m–2), and microbial numbers were reduced at the stream–parafluvial interface.4. Counter to expectations, parafluvial functioning (in terms of nitrate production and dissolved oxygen decline along flowpaths) re‐established immediately after the flood receded. Therefore, material other than algal exudates supported parafluvial metabolism immediately postflood, and terrestrially derived dissolved organic matter is the likely source.5. Algae in the surface stream recovered quickly following flooding, but recovery of parafluvial bacteria lagged somewhat behind. These results highlight the importance of surface–subsurface interaction to stream ecosystem functioning and show that the nature of these interactions changes substantially in successional time.