Benthic and hyporheic invertebrate assemblages along a gradient of increasing streambed colmation by fine sediment

Streambed colmation by fine sediment, e.g. the deposition, accumulation and storage of fines in the substrate, is a major environmental concern throughout the world. Nevertheless, the ecological effects of streambed colmation on both benthic and hyporheic invertebrate assemblages have rarely been considered simultaneously. We studied a continuum of a naturally increasing percentage of fine sediment in three temperate rivers and hypothesized that the increasing percentage of fine sediment would decrease both benthic and hyporheic invertebrate densities and diversities, and reduce the similarities between them. To test these hypotheses, we first compared heavily, moderately and lightly clogged reaches located in downwelling areas and sampled invertebrates in the benthic zone and at 3 different depths (10, 30 and 50 cm) in the hyporheic zone. Secondly, we modified the sediment grain size distribution experimentally by increasing the percentage of fine sediment and using artificial substrates. The increasing colmation halved the hyporheic taxonomic richness and reduced benthic and hyporheic densities to a third. Some taxa were found in both zones, mainly in high colmation (e.g. Baetidae) or low colmation contexts (e.g. Orthocladiinae, Cyclopoida and Harpacticoida). The dissimilarity between benthic and hyporheic fauna (only at ?50 cm) was significantly higher in heavily clogged reaches than in moderately and lightly clogged ones, suggesting reduced vertical exchange of invertebrates or differential impacts between zones. The total abundance, taxonomic richness, percentage of EPT taxa and densities of most organisms observed using the artificial substrates decreased linearly with the increasing percentage of fine sediment in the experiment. Only the Ephemeroptera Caenis spp. and Heptageniidae disappeared above 30 and 50 % of fine sediment, respectively, suggesting that the response to increasing colmation are strongly taxon-specific. High amount of fine sediments within the substrate significantly decreased habitat quality for benthic and hyporheic invertebrates and thus limit the production of streams and their capacity to recover after disturbance. Moreover, the use of hyporheic invertebrates seems more relevant than benthic invertebrates to assess the effect of colmation and thus could be tested in future research as indicators.     

Patterns in stream longitudinal profiles and implications for hyporheic exchange flow at the H.J. Andrews Experimental Forest,Oregon, USA

There is a need to identify measurable characteristics of stream channel morphology that vary predictably throughout stream networks and that influence patterns of hyporheic exchange flow in mountain streams. In this paper we characterize stream longitudinal profiles according to channel unit spacing and the concavity of the water surface profile. We demonstrate that: (1) the spacing between zones of upwelling and downwelling in the beds of mountain streams is closely related to channel unit spacing; (2) the magnitude of the vertical hydraulic gradients (VHGs) driving hyporheic exchange flow increase with increasing water surface concavity, measured at specific points along the longitudinal profile; (3) channel unit spacing and water surface concavity are useful metrics for predicting how patterns in hyporheic exchange vary amongst headwater and mid‐order streams. We use regression models to describe changes in channel unit spacing and concavity in longitudinal profiles for 12 randomly selected stream reaches spanning 62 km² in the H.J. Andrews Experimental Forest in Oregon. Channel unit spacing increased significantly, whereas average water surface concavity (AWSC) decreased significantly with increasing basin area. Piezometer transects installed longitudinally in a subset of stream reaches were used to measure VHG in the hyporheic zone, and to determine the location of upwelling and downwelling zones. Predictions for median pool length and median distance between steps in piezometer reaches bracketed the median distance separating zones of upwelling in the stream bed. VHG in individual piezometers increased with increasing water surface concavity at individual points in the longitudinal profile along piezometer transects. Absolute values of VHG, averaged throughout piezometer transects, increased with increasing AWSC, indicating increased potential for hyporheic exchange flow. These findings suggest that average hyporheic flow path lengths increase—and the potential for hyporheic exchange flow in stream reaches decreases—along the continuum from headwater to mid‐order mountain streams. Copyright © 2005 John Wiley & Sons, Ltd.     

Influence of macroinvertebrates on physico‐chemical and microbial processes in hyporheic sediments

The objective of this study was to measure the effects of invertebrates on the physical characteristics and microbial processes in hyporheic sediments. We investigated the impacts of an assemblage of three taxa (asellids, chironomid larvae, and tubificid worms) on sediment distribution, water fluxes, sediment organic carbon, biofilm (attached bacteria) characteristics, and O₂, dissolved organic carbon NO₃^?, NO₂^?, and NH₄⁺ concentrations in slow filtration sand–gravel columns. The results showed that invertebrates clearly modified the distribution of particles in the sediment column, probably because of the structures (tubes, macropores, and faecal pellets) produced by the three taxa in the sediment. Our assessment of water fluxes indicated that invertebrate activities led to an increase in the porosity of the sediment columns. In addition, aerobic (O₂ consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) microbial processes occurring in the sediment were stimulated in the presence of invertebrates. Finally, the present study demonstrates that invertebrates can act as ecosystem engineers in heterogeneous sediments that are under the influence of an advective flux of water. The solute residence time increased in columns containing the faunal assemblage. Micro‐organisms used more dissolved organic matter and nutrients in the presence of invertebrates because invertebrate activities increased the contact between the biofilm and water. We conclude that engineering by invertebrates in natural conditions modifies characteristics of the hyporheic zone and thereby enhances both the porosity of the sediment and the solute transport across the benthic interface. Copyright © 2002 John Wiley & Sons, Ltd.     

Vertical hydraulic exchange and the contribution of hyporheic community respiration to whole ecosystem respiration in the River Lahn (Germany)

To quantify the contribution of hyporheic community respiration to whole running-water ecosystem respiration in a cultural landscape setting, we studied the vertical hydraulic exchange in riffle–pool sequences of the River Lahn (Germany). We used flow through curves from four tracer experiments to estimate flow velocities in the surface and subsurface water. Generally, vertical exchange velocities were higher in riffle sections and a high temporal variability was observed (range of values 0.11–1.08 m day⁻¹). We then used (1) the exchange velocities and (2) time series of dissolved oxygen concentration in surface and subsurface water to calculate hyporheic respiration. Hyporheic respiration was estimated in a range of 10–50 mg O₂ m⁻³ day⁻¹ for the upper sediment layer (first 20 cm). It was much lower in the deeper sediment layer (20–40 cm), ranging from 0 to 10 mg O₂ m⁻³ day⁻¹ (volumes are volumes of interstitial water; the average porosity was 20%). We determined primary production and respiration of the biofilm growing on the sediment by modelling dissolved oxygen concentration time series for a 2,450 m long stream reach (dissolved oxygen concentrations with diurnal variations from 8 to 16 mg L⁻¹). Modelled respiration rates ranged from 2 to 21 g O₂ m² day⁻¹. All information was integrated in a system analysis with numerical simulations of respiration with and without sediments. Results indicated that hyporheic respiration accounted for 6 to 14% of whole ecosystem respiration. These values are much lower than in other whole system respiration studies on more oligotrophic river systems.     

Invertebrate colonization of leaves and roots within sediments of intermittent Coastal Plain streams across hydrologic phases

Ecological flows between habitats are vital for predicting and understanding structure and function of recipient systems. Ecological flows across riparian areas and headwater intermittent streams are likely to be especially important in many river networks because of the shear extent of these interfaces, their high edge-to-width ratio, and the alternation of wet and dry conditions in intermittent channels. While there has been substantial research supporting the importance of riparian-stream linkages above-ground, comparatively less research has investigated below-ground linkages. We tested the hypothesis that riparian roots are colonized by invertebrates as a food source within stream beds of intermittent headwater streams. We compared benthic invertebrate assemblages colonizing three types of buried substrates (leaves, roots, and plastic roots) among three intermittent Coastal Plain streams, each with a different riparian management treatment (clearcut, thinned, and reference), over a 1-year period. Invertebrate density was significantly lower in root litterbags than in plastic roots litterbags, but neither differed from densities in leaf litterbags. Total invertebrate abundances, however, were significantly higher in leaf and root litterbags compared to abundances in plastic root litterbags. Invertebrate biomass and richness did not vary among substrates, but invertebrate density, abundance, and richness all declined from the wet phase (September–December) through the dry phase (June–August). Meiofauna and aquatic dipterans were the primary colonizing invertebrates during the wet phase. Relative abundance of terrestrial taxa increased during the dry phase, but their absolute abundance remained lower than aquatic taxa during the wet phase. Invertebrate composition did not differ among substrate types, but was significantly different among streams and time periods. Cumulative number of dry days, degree days, and redox depth all strongly correlated with assemblage structure as indicated by ordination scores. Our results suggest that subsurface invertebrates respond to leaves and roots as food sources, but assemblage composition is not substrate specific. Colonization of leaves and roots within stream beds by aquatic and terrestrial taxa supports the idea that headwater intermittent streams are important interfaces for the reciprocal exchange of energy and materials between terrestrial and aquatic ecosystems.     

Control of river stage on the reactive chemistry of the hyporheic zone

We examined the influence of river stage on subsurface hydrology and pore water chemistry within the hyporheic zone of a groundwater‐fed river during the summer baseflow period of 2011. We found river stage and geomorphologic environment to control chemical patterns in the hyporheic zone. At a high river stage, the flux of upwelling water in the shallow sediments (>20 cm) decreased at sample sites in the upper section of our study reach and increased substantially at sites in the lower section. This differential response is attributed to the contrasting geomorphology of these subreaches that affects the rate of the rise and fall of a river stage relative to the subsurface head. At sites where streamward vertical flux decreased, concentration profiles of a conservative environmental tracer suggest surface water infiltration into the riverbed below depths recorded at a low river stage. An increase in vertical flux at sites in the lower subreach is attributed to the movement of lateral subsurface waters originating from the adjacent floodplain. This lateral‐moving water preserved or decreased the vertical extent of the hyporheic mixing zone observed at a low river stage. Downwelling surface water appeared to be responsible for elevated dissolved organic carbon (DOC) and manganese (Mn) concentrations in shallow sediments (0–20 cm); however, lateral subsurface flows were probably important for elevated concentrations of these solutes at deeper levels. Results suggest that DOC delivered to hyporheic sediments during a high river stage from surface water and lateral subsurface sources could enhance heterotrophic microbial activities. Copyright © 2013 John Wiley & Sons, Ltd.     

Invertebrate and microbial responses to inundation in an ephemeral river reach in New Zealand: effects of preceding dry periods

Inundation marks the shift from a terrestrial ecosystem to an aquatic ecosystem in ephemeral rivers. The forms and rates of responses by aquatic invertebrates and sediment microbes to inundation depend on desiccation resistance during preceding dry periods. We assessed invertebrate and microbial responses to inundation over a range of preceding dry periods in an ephemeral reach of the Selwyn River, New Zealand. Microbial response variables were dissolved oxygen consumption and non-specific esterase activity. Sampling sites along the reach had been continuously dry for 1–592 d prior to sample collection. The onset of flow simulated by an experimental inundation led to the appearance of aquatic invertebrates in all samples, but the assemblages varied with the length of the preceding dry period. Taxon richness decreased linearly with dry period length while density decreased exponentially. These patterns indicate that a large number of individuals from desiccation-sensitive taxa were eliminated soon after flow ceased, and a low-density assemblage composed of a small number of desiccation-resistant taxa persisted during prolonged dry periods. As with invertebrate density, sediment respiration and nonspecific esterase activity decreased with length of dry period, and were characterized by exponential decay functions. The results of the inundation experiments suggest that a temporal ecotone exists for about one week after the disappearance of flowing water, and before the terrestrial system stabilizes.     

Spatial variability in the hyporheic zone refugium of temporary streams

A key ecological role hypothesized for the hyporheic zone is as a refugium that promotes survival of benthic invertebrates during adverse conditions in the surface stream. Many studies have investigated use of the hyporheic refugium during hydrological extremes (spates and streambed drying), and recent research has linked an increase in the abundance of benthic invertebrates within hyporheic sediments to increasing biotic interactions during flow recession in a temporary stream. This study examined spatial variability in the refugial capacity of the hyporheic zone in two groundwater-dominated streams in which flow permanence varied over small areas. Two non-insect taxa, Gammarus pulex and Polycelis spp. were common to both streams and were investigated in detail. Hydrological conditions in both streams comprised a four-month period of flow recession and low flows, accompanied by reductions in water depth and wetted width. Consequent declines in submerged benthic habitat availability were associated with increases in population densities of mobile benthic taxa, in particular G. pulex. The reduction in the spatial extent of the hyporheic zone was minimal, and this habitat was therefore a potential refugium from increasing biotic interactions in the benthic sediments. Concurrent increases in the hyporheic abundance and hyporheic proportion of a taxon’s total (benthic + hyporheic) population were considered as evidence of active refugium use. Such evidence was species-specific and site-specific, with refugium use being observed only for G. pulex and at sites dominated by downwelling water. A conceptual model of spatial variability in the refugial capacity of the hyporheic zone during habitat contraction is presented, which highlights the potential importance of the direction of hydrologic exchange.     

Benthic invertebrate assemblages and functional feeding groups in the Paraná River floodplain (Argentina)

The benthic invertebrate assemblages and functional feeding groups in different mesohabitats of the Middle Paraná River–floodplain system were analyzed. Benthic invertebrates and bottom sediments were sampled in a secondary channel (center and bank mesohabitats), a temporal marginal fluvial wetland adjacent to the river, an isolated lake and a connected lake during low water level. Cluster analysis of average invertebrate densities based on the Bray Curtis dissimilarity index yielded a group composed by the mesohabitats with higher species richness, the floodplain lakes, banks mesohabitats and the wetland. The center mesohabitat of the main channel characterized by sandy sediments with low organic matter content and the lowest invertebrate densities and species richness was classified separately. Alpha diversity increased from the center mesohabitat (6 taxa) to the adjacent wetland (71 taxa), and were similar between the floodplain lakes (24 and 22 taxa) and the river bank mesohabitat (24 taxa). Gamma and beta diversities (Whittaker index) were 92 and 2.19, respectively. The highest turnover of taxa was between the river and the other mesohabitats and the lowest between floodplain lakes. Detrended correspondence analysis (DCA) showed a clear separation of wetland and banks from other mesohabitats (axis 1 and 2 explained 52.25% variance) explained by shredders and collector-filterers. The other mesohabitats were arranged in a gradient from the main channel mostly related to collector-gatherers to the connected lake and the isolated lake that were mostly characterized by predators and scrapers. The invertebrate assemblage complexity and functional feeding groups composition increased in the lateral dimension, from the center of the main channel to the temporal marginal fluvial wetland due to the influences of the spatial heterogeneity caused by different sources of organic matter inputs.     

Thermal profiles within river beds

This pilot study uses a chemical technique (KEtX) to establish vertical profiles of average monthly water temperature within river beds: the hyporheic zone at the interface between surface water and groundwater. Data are presented for two gravel-bed sites and one sand-bed site on the River Wissey, Norfolk, UK. From February to October 1992, average monthly temperatures were determined at 10cm intervals down 1 m profiles. A strong seasonal pattern was defined at all sites with hyporheic temperatures being relatively warm in winter and cool in summer. Isothermal periods occurred in March-April and September-October. Temperature ranges within the hyporheic were decreased (4.6–7.7°C) compared with those of surface waters (10 and 10.9°C). Temperature profiles were similar at all sites during winter, but the sand-bed site had relatively low temperatures at a standard depth within the hyporheic during summer and the temperature gradient became isothermal later in the autumn at this site. It is suggested that the influence of flows and substratum characteristics on temperature patterns, especially in regulated rivers, may have significant ecological implications, for example for determining the timing of salmonid fry and invertebrate emergence.     

Impacts of cascade run-of-river dams on benthic diatoms in the Xiangxi River, China

The ecological effects of small run-of-river dams on aquatic ecosystems are poorly understood, especially on downstream benthic algal communities. We examined impacts of such dams on the benthic diatom community at a regional scale in the Xiangxi River, China. A total of 90 sites were visited, which were divided into five habitats (H1–H5) according to impact extent of each dam. Using partial least squares (PLS) modeling, we developed two predictive models (diatom species richness and total diatom density) based on environmental variables of an unregulated habitat (H1). These models were then used to predict species richness and total densities at impacted habitats (H2–H5) and residuals, i.e. the differences between observed and predicted values, were used to evaluate impact strength of flow regulation. Significant impacts of flow regulation on diatom species richness were detected at three impacted habitats (H3–H5), where observed species richness were significantly higher—70.6, 63.9 and 46.6%, respectively—than predicted values. Then, possible mechanisms for observed impacts were discussed. Further research is necessary to address the potential negative impacts of cascade run-of-river dams on other aquatic organisms in different seasons, and to explore more appropriate mechanisms for such impacts, which may lead to sustainable management strategies and help to determine the optimal ecological water requirement for the Xiangxi River.     

Influence of the hyporheic zone on the phosphorus dynamics of a large gravel‐bed river,Garonne River,France

Phosphorus (P) concentrations in sediments and in surface and interstitial water from three gravel bars in a large river (Garonne River, southern France) were measured daily, downstream of a wastewater treatment plant for a city of 740 000 inhabitants (Toulouse). Measurements were made of vertical hydraulic gradient (VHG), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP) and total phosphorus (TP) in water and of three extractable forms of phosphorus (water extractable, NaOH extractable and H₂SO₄ extractable) in hyporheic sediments from the gravel bars. Dissolved phosphorus was the major contributor to TP (74–79%) in both interstitial and surface waters on all sampling dates, and in most cases surface water P concentrations were significantly higher than interstitial concentrations. Hyporheic sediment TP concentrations ranged between 269 and 465 µg g^?1 and were highest in fine sediment fractions. Acid‐extractable P, a non‐bioavailable form, represented at least 95% of sediment TP. A positive relationship was observed between VHG and TP in two of the gravel bars, with wells that were strongly downwelling having lower TP concentrations. These results suggest that in downwelling zones, hyporheic sediments can trap surface‐derived dissolved P, and that much of this P becomes stored in refractory particulate forms. Bioavailable P is mainly present in dissolved form and only occupies a small fraction of total P, with particulate P comprising the majority of total P. Copyright © 2009 John Wiley & Sons, Ltd.     

Interstitial flow through preferential flow paths in the hyporheic zone of the Oberer Seebach,Austria

We investigated interstitial flow velocities in the Oberer Seebach, Austria, with NaCl tracer injections at a sediment depth of 30 cm to estimate the hydraulic conditions experienced by invertebrates inhabiting the hyporheic zone. Flow velocity measured with tracers is taken as travel time of the water along a straight line between injection and sampling points, although the water flows around sediment particles, and thus travels a somewhat longer distance. From sections of stream sediment in which the interstitial spaces were replaced by concrete, we estimated that this difference amounts, on average, to 27% and used this factor to correct the results of our velocity measurements. Corrected interstitial water velocities ranged from 0.01 to 1.32 cm s^-1 and were independent of surface discharge. We also studied spatial flow patterns in the bed sediments with long-term tracer injections. The three-dimensional distribution of tracer concentrations 24 hours after the start of the injection indicated that interstitial water preferentially flows in a complex network of areas of high hydraulic connectivity. Reynolds numbers for flow in the hyporheic pore space ranged from 0.1 to 489, implying that the flow environment varies from laminar up to the zone of transition to turbulent flow. Therefore, invertebrates may have a size-related active choice of areas where either friction drag or pressure drag predominates. The consequence of flow patterns, such as those observed in our study, is that small-scale variability of hydraulic conditions may be an important determinant of the patchy invertebrate distribution in bed sediments.     

Characteristics of beaver ponds on deltas in a mountain environment

Many beaver ponds in the Rocky Mountains, that have been described in the literature, are in‐channel ponds that are relatively small and short‐lived. This paper describes floodplain beaver ponds on low‐gradient deltas in glacial finger lakes in Glacier National Park, Montana. These ponds are distinctly larger, probably fed by hyporheic flow, and stable and long‐lived. Ponds examined were, with one exception, 44 years old. Glacial discharge is present in each valley where beaver ponds occupy low‐gradient deltas, and this discharge likely sustains pond water level over the course of the summer. As glaciers recede and disappear, deltaic beaver ponds dependent on hyporheic flow may be negatively affected. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of biotope on invertebrate assemblages in lentic environments: A study of two perennial alkaline wetlands in the Western Cape,South Africa

As a step towards the biological assessment of wetlands in South Africa, this study investigates the influence of biotope characteristics on the spatial distribution of aquatic invertebrates. The aim was to assess whether different wetland biotopes support significantly different invertebrate assemblages in terms of the composition and abundance of microcrustaceans and macroinvertebrate taxa. During October 2006, three different biotopes were sampled within Verlorenvlei and Wave's Edge wetlands (Western Cape, South Africa) using a long-handled sweep net. Composition and abundance of invertebrate assemblages were compared between and within sites for each of the wetlands. Assemblage composition generally differed among biotopes within each wetland, as revealed by cluster analysis and MDS plots. At Verlorenvlei, biotopes formed distinctive clusters with low site-specific variability. Assemblage composition at Wave's Edge revealed coarser groupings with clusters distinguishing between vegetated and non-vegetated biotopes only. Biotopes within each wetland differed significantly in terms of taxon richness, Shannon diversity and mean total invertebrate biomass (g m⁻³), whilst mean total density (ind m⁻³) differed only between biotopes in Verlorenvlei. Considerable shifts in invertebrate assemblage structure corresponded to differences in electrical conductivity among sites at Verlorenvlei. For large physico-chemically heterogeneous wetlands such as Verlorenvlei, it is suggested that smaller physico-chemically homogenous zones should be identified a priori and within these areas vegetated biotopes should be sampled over the broadest possible spatial scale, whilst open-water biotopes can be sampled more narrowly. For small, reasonably homogenous wetlands such as Wave's Edge, we suggest a less broad spatial representation of biotopes and instead one should concentrate on increasing the number of sample repetitions per site.     

Abiotic features and macroinvertebrate colonization of the hyporheic zones of two tributaries of the river Elbe (Germany)

Whole-ecosystem experiments may reach statistical pertinence when reference and treatment sites are compared. Therefore at least two similar systems must be found. To this aim we tried to answer the question if the hyporheic zones of both streams have similar features. We postulated that both streams would be unsuitable for a comparison in an ecosystem experiment if the structure of their hyporheic invertebrate communities differs significantly. Although pH, electrical conductivity and nitrate concentration differed in the surface water, variability of all chemical criteria studied in the hyporheic zones was much lower between the two streams compared to the variability within each stream. Thus the structure of both hyporheic invertebrate communities did not differ significantly. Because of this uniformity both streams are regarded to be suitable for a comparison in a whole-ecosystem experiment.     

Geomorphology,hydrology, and aquatic vegetation drive seasonal hyporheic flow patterns across a gravel‐dominated floodplain

Across 1·7 km² of the Umatilla River floodplain (Oregon, USA), we investigated the influences of an ephemeral tributary and perennial ‘spring channel’ (fed only by upwelling groundwater) on hyporheic hydrology. We derived maps of winter and summer water‐table elevations from data collected at 46 monitoring wells and 19 stage gauges and used resulting maps to infer groundwater flow direction. Groundwater flow direction varied seasonally across the floodplain and was influenced by main channel stage, flooding, the tributary creek, and the location and direction of hyporheic exchange in the spring channel. Hyporheic exchange in the spring channel was evaluated with a geochemical mixing model, which confirmed patterns of floodplain groundwater movement inferred from water‐table maps and showed that the spring channel was fed predominantly by hyporheic water from the floodplain aquifer (87% during winter, 80% during summer), with its remaining flow supplied by upslope groundwater from the adjacent catchment aquifer. Summertime growth of aquatic macrophytes in the spring channel also influenced patterns of hyporheic exchange and groundwater flow direction in the alluvial aquifer by increasing flow resistance in the spring channel, locally raising surface water stage and adjacent water‐table elevation, and thereby altering the slope of the water‐table in the hyporheic zone. The Umatilla River floodplain is larger than most sites where hyporheic hydrology has been investigated in detail. Yet, our results corroborate other research that has identified off‐channel geomorphic features as important drivers of hyporheic hydrology, including previously published modeling efforts from a similar river and field observations from smaller streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 from InSAR and GPS observations

In this study, the spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 is monitored using interferometric synthetic aperture radar (InSAR) data acquired by the European Remote-Sensing Satellites (ERS-1 and ERS-2) and the Environmental Satellite (ENVISAT). These data are combined with continuous global positioning system (GPS) measurements to identify four discrete episodes of caldera subsidence and uplift, these episodes are: 1992–1995 (subsidence of 2.7 cm/year), 1996–2000 (subsidence of 0.5 cm/year, with local uplift of 1.7 cm/year at Norris), 2000–2004 (subsidence of 0.7 cm/year, with local uplift of 0.6 cm/year at Norris), and 2004–2009 (uplift of 3–8 cm/year, with local subsidence of 1–4 cm/year at Norris). We construct the full three-dimensional velocity field of Yellowstone deformation for 2005–2006 from ascending and descending ENVISAT orbits. The InSAR three-dimensional velocity field and three-component GPS measurements indicate that the majority of the observed deformation (3–8 cm/year) across the Yellowstone caldera and near Norris Geyser Basin (NGB) occurred in the vertical direction between the summers of 2005 and 2006. During this time, significant lateral displacements of 3–7 cm/year also occurred in the east–west direction at the southeastern and western rims of the Yellowstone caldera and in the area between Hebgen Lake and NGB. Minor north–south displacements of about 0.2 cm/year also occurred, however, in the southwestern section of the caldera and near Yellowstone Lake during the same period. The calculated three-dimensional velocity field for 2005–2006 implies the existence of two pressure-point sources, beneath the two structural resurgent domes in the Yellowstone caldera, connected by a planar conduit, rather than a single, large sill as proposed in previous studies. Furthermore, no measurable displacements occurred along any fault zone across the caldera during the entire period of observation (1992–2009). Therefore, we infer that magmatic and hydrothermal processes beneath the Yellowstone caldera and NGB were the main sources of deformation.     

Successional changes in soil and hyporheic nitrogen fertility on an alluvial flood plain: implications for riparian vegetation

In floodplain primary succession, vegetation colonizes nitrogen-poor alluvial deposits and fertility improves as soil nitrogen accumulates over time. It is generally assumed that vegetation assimilates the vast majority of its nitrogen from the soil; however, recent studies have suggested that the hyporheic zone also may be an important nitrogen source. We investigated the potential relative importance of hyporheic nitrogen by comparing fertility indices, specifically total (TN), dissolved inorganic (DIN), potentially mineralizable (PMN) and ion exchange resin nitrogen (IERN) in both soils and the hyporheic zone at early, mid and late succession stands on an expansive river flood plain. We also constructed mesocosms to assess growth of cottonwood cuttings with access to soil and/or hyporheic water. We found TN and PMN increased from early to mid succession in both the soil (to 10 cm) and hyporheic zone (in a 10 cm layer). While TN, DIN and PMN were an order of magnitude higher in the soil than in the hyporheic zone, IERN was higher in the hyporheic zone, indicating that subsurface flow through the flood plain may be important in delivering nitrogen to the root zone. However, even when flux was added to the hyporheic PMN pool, nitrogen availability in the hyporheic zone (in a 10 cm layer) was vastly lower than soil PMN (to 10 cm). Further, the instantaneous standing stock of DIN in the surface soil alone was about equal to the sum of the DIN pool, the mean subsurface flux and the PMN pool in a 10 cm layer of hyporheic zone. In the mesocosm experiment, cottonwood cuttings with access to both soil and hyporheic water grew fastest; however, they also had the lowest foliar nitrogen concentrations, indicating that this was not due to greater nitrogen availability. In the field, nitrogen content of cottonwood foliage increased along with soil (but potentially hyporheic as well) nitrogen accumulation during succession, suggesting the vegetation responded to increasing nitrogen fertility. We conclude that at least on a per unit-volume basis, the hyporheic zone probably provides little nitrogen relative to the surface soil, except on new alluvial bars that characteristically are nitrogen poor. Therefore, the hyporheic zone is probably a much smaller nitrogen source for mature forests relative to the surface soil unless the vegetation exploits a much larger volume of the hyporheic zone than surface soil.