Biological,chemical and physical characteristics of downwelling and upwelling zones in the hyporheic zone of a north-temperate stream

Along a single stream riffle, there is a typical flow pattern in which surface water enters the hyporheic zone in a downwelling zone at the head of the riffle and hyporheic water returns to the stream surface in an upwelling zone at the tail of the riffle. Distinct patterns of physical and chemical conditions in the hyporheic zone are likely to determine patterns of microbial activity and occurrence of hyporheic fauna. Interstitial water and core samples were taken at three depths in the downwelling and upwelling zones of a single riffle in the Speed River, Southern Ontario, Canada. Physical and chemical characteristics of the hyporheic water, bacterial density, protein content, detritus content and faunal composition of the hyporheic sediment were analysed. The downwelling and upwelling zones differed significantly in temperature, pH, redox potential, dissolved oxygen and nitrate with significant positive correlations occurring among the latter three. There were no differences in bacterial density or detritus content between the two zones nor between depths in either zone, but protein content, considered to be a measure of biofilm biomass, was significantly higher in the downwelling zone. Total density of hyporheic fauna and the number of taxa decreased with increasing depth in both upwelling and downwelling zones, and were positively correlated with surface water characteristics (oxygen, temperature and nitrate), sediment protein content and detritus; however, only a weak correlation was found with zone. The composition of taxa differed between the two zones, and faunal distribution was correlated with dissolved oxygen, detritus, protein content and depth.     

Nitrogen processing in the hyporheic zone of a pastoral stream

The distribution of nitrogen-transforming processes, and factors controlling their rates, were determined within the hyporheic zone of a lowland stream draining agricultural land. In the field, physicochemical parameters were measured along a 10m-long hyporheic flow line between downwelling and upwelling zones. Sediment cores were retrieved from the stream bed surface, and from 20, 40 and 60cm deep in each zone, and in the laboratory, water from the corresponding depth was percolated through each core at the natural flow rate. Concentrations of nitrogen species and oxygen were measured before and after flow through each core. Denitrification was measured using a ¹⁵N-nitrate tracer. Shallow and downwelling zone samples were clearly distinct from deeper and upwelling zone samples in terms of physicochemical conditions, microbial processes and factors controlling nitrogen processing. Denitrification was highest in surface and downwelling zone cores, despite high oxygen levels, probably due to high pore-water nitrate concentrations in these cores and isolation of the denitrifying bacteria from oxygen in the bulk water by the hyporheic biofilms. Denitrification was limited by oxygen inhibition in the downwelling group, and by nitrate availability in the upwelling group. Strong evidence indicated that dissimilatory nitrate reduction to ammonium, occurred in almost all cores, and outcompeted denitrification for nitrate. In contrast, nitrification was undetectable in all but two cores, probably because of intense competition for oxygen. Field patterns and lab experiments indicated that the hyporheic zone at this moderately N-rich site is a strong sink for nitrate, fitting current theories that predict where hyporheic zones are nitrate sinks or nitrate sources.     

Hyporheic zone hydrology and nitrogen dynamics in relation to the streambed topography of a N-rich stream

The influence of riffle-pool units on hyporheic zone hydrology and nitrogen dynamics was investigated in Brougham Creek, a N-rich agricultural stream in Ontario, Canada. Subsurface hydraulic gradients, differences in background stream and groundwater concentrations of conservative ions, and the movement of a bromide tracer indicated the downwelling of stream water at the head of riffles and upwelling in riffle-pool transitions under base flow conditions. Channel water also flowed laterally into the floodplain at the upstream end of riffles and followed a subsurface concentric flow path for distances of up to 20 m before returning to the stream at the transition from riffles to pools. Differences in observed vs predicted concentrations based on background chloride patterns indicated that the hyporheic zone was a sink for nitrate and a source for ammonium. The removal of nitrate in the streambed was confirmed by the loss of nitrate in relation to co-injected bromide in areas of downwelling stream water in two riffles. Average stream water nitrate-N concentrations of 1.0 mg/L were often depleted to <0.005 mg/L near the sediment-water interface. Consequently, an extensive volume of the hyporheic zone in the streambed and floodplain had a large unused potential for nitrate removal. Conceptual models based mainly on studies of streams with low nutrient concentrations have emphasized the extent of surface-subsurface exchanges and water residence times in the hyporheic zone as important controls on stream nutrient retention. In contrast, we suggest that nitrate retention in N-rich streams is influenced more by the size of surface water storage zones which increase the residence time of channel water in contact with the major sites of rapid nitrate depletion adjacent to the sediment-water interface.     

Spatial responses of hyporheic invertebrates to seasonal changes in environmental parameters

1. Spatial relationships between hyporheic invertebrates and subsurface water flow patterns, sediment characteristics, water physicochemical parameters and several possible food sources were compared over three seasons at one site beneath a riffle. Measures of food sources included particulate organic matter (POM), bacterial activity (aerobic respiration, nitrate respiration and mineralisation of organic nitrogen) and microbial abundance. 2. Patterns of water flow changed significantly over the 9‐month study period, from predominantly upwelling beneath the entire riffle in spring, to distinct differentiation between downwelling and upwelling zones in summer and autumn. Water physicochemical parameters changed accordingly, showing gradually weaker correlations with depth and stronger correlations with zone between spring and autumn. 3. Despite these changes, depth remained the strongest predictor of invertebrate richness, density and taxon composition throughout the study period. However, invertebrate distributions were secondarily correlated with water physicochemical parameters, and a minor gradient in invertebrate distributions between downwelling and upwelling zones became stronger from spring to summer. 4. The correlations between invertebrates and physicochemical parameters changed in both magnitude and direction with season. In spring, invertebrates showed a negative correlation with surface water infiltration, whereas in summer and autumn, the correlation was positive. Correlations were strongest in summer, when interstitial dissolved oxygen concentrations were lowest. 5. No relationships were found between hyporheic invertebrates and POM, microbial abundance or activity. This suggests that at this site, proximity to the streambed surface and physicochemical variables are more important than the abundance of food in controlling invertebrate distributions.     

Effects of buried leaf litter and vertical hydrologic exchange on hyporheic water chemistry and fauna in a gravel-bed river in northern New South Wales, Australia

1. Large amounts of coarse particulate organic matter (CPOM) are buried in the sand and gravel beds of many rivers during spates. The effects of these patchily distributed resources on hyporheic invertebrates and water chemistry are poorly understood. Buried CPOM may provide local ‘hot-spots’ of food for hyporheic detritivores and their predators, alter nutrient supply to nearby sediment biofilms, and generate habitat for some invertebrates. 2. To examine potential short-term effects on hyporheic water chemistry, nutrient concentrations and invertebrate assemblage composition, leaf packs were buried in downwelling (surface water infiltrating the hyporheic zone) and upwelling (hyporheic water emerging to the surface) zones at two sites along a gravel-bed river in northern New South Wales. At one site, pits were excavated to simulate leaf burial (procedural control) and plastic ‘leaves’ were buried to test whether invertebrates might respond to leaves as refuges rather than food. Hyporheic CPOM, sediment size fractions, and interstitial silt content were also quantified at these sites. 3. Dry weights of naturally buried CPOM (leaf litter and wood fragments) varied substantially (0.6–71.7 g L^–1 sediment). Amounts of CPOM did not differ between up- vs. downwelling zones or between sites. Hyporheic dissolved oxygen saturation was generally high (> 75%), and was lower in upwelling zones. The hyporheos was dominated taxonomically by water mites (≈ 20 species), whereas small oligochaetes were most abundant (40% of total abundance). Tiny instars of elmid beetle larvae and leptophlebiid mayfly nymphs were also common. Before experimental manipulation, faunal composition differed between up- and downwelling zones. In upwelling zones, bathynellaceans and blind peracarids were found, whereas small individuals of the surface benthos were common in samples from downwelling zones. This validated stratification of the experiment across zones of hydrologic exchange. 4. Twenty days after leaf burial, there was no effect of the treatments at either site on changes in most variables, including mean numbers of taxa and individuals per sample. Similarly, changes in faunal composition of the hyporheos in the treatments paralleled those in the controls except for a weak response in the buried leaves treatment in the upwelling zone at site 1. Artificially buried leaf litter does not seem to influence hyporheic water chemistry or fauna at these two sites. It is probable that naturally buried leaf litter is swiftly processed soon after entrainment and that repeating this experiment immediately after a flood may yield different results.     

Hyporheic community composition in a gravel-bed stream: influence of vertical hydrological exchange, sediment structure and physicochemistry

Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini‐piezometers, each installed in a different upwelling or downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates. 3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L^?1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than downwelling areas. 4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter. 5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations. 6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.     

The role of water exchange between a stream channel and its hyporheic zone in nitrogen cycling at the terrestrial-aquatic interface

The subsurface riparian zone was examined as an ecotone with two interfaces. Inland is a terrestrial boundary, where transport of water and dissolved solutes is toward the channel and controlled by watershed hydrology. Streamside is an aquatic boundary, where exchange of surface water and dissolved solutes is bi-directional and flux is strongly influenced by channel hydraulics. Streamside, bi-directional exchange of water was qualitatively defined using biologically conservative tracers in a third order stream. In several experiments, penetration of surface water extended 18 m inland. Travel time of water from the channel to bankside sediments was highly variable. Subsurface chemical gradients were indirectly related to the travel time. Sites with long travel times tended to be low in nitrate and DO (dissolved oxygen) but high in ammonium and DOC (dissolved organic carbon). Sites with short travel times tended to be high in nitrate and DO but low in ammonium and DOC. Ammonium concentration of interstitial water also was influenced by sorption-desorption processes that involved clay minerals in hyporheic sediments. Denitrification potential in subsurface sediments increased with distance from the channel, and was limited by nitrate at inland sites and by DO in the channel sediments. Conversely, nitrification potential decreased with distance from the channel, and was limited by DO at inland sites and by ammonium at channel locations. Advection of water and dissolved oxygen away from the channel resulted in an oxidized subsurface habitat equivalent to that previously defined as the hyporheic zone. The hyporheic zone is viewed as stream habitat because of its high proportion of surface water and the occurrence of channel organisms. Beyond the channel's hydrologic exchange zone, interstitial water is often chemically reduced. Interstitial water that has not previously entered the channel, groundwater, is viewed as a terrestrial component of the riparian ecotone. Thus, surface water habitats may extend under riparian vegetation, and terrestrial groundwater habitats may be found beneath the stream channel.     

Benthic algal response to hyporheic-surface water exchange in an alluvial river

We studied the response of benthic algae to points of hyporheic-surface water exchange in the main channel of the Middle Fork Flathead River within the Nyack Flood Plain, Montana. We examined hyporheic exchange at 120 sites using piezometers and measuring vertical hydraulic gradient (VHG), hydraulic conductivity, and vertical discharge. We removed benthic algae from a single cobble at each site, and we used VHG to group sampling sites for statistical analysis. Algal cell density and chlorophyll a concentration were significantly higher at sites with hyporheic discharge (+VHG, upwelling) compared to both sites with hyporheic recharge (−VHG, downwelling) and sites with no hyporheic-surface water exchange (=VHG, neutral) (ANOVA, P < 0.05). The assemblages of algae at upwelling sites were also significantly different from downwelling and neutral exchange sites (ANOSIM, P < 0.05). Filamentous green algae Stigeoclonium sp. and Zygnema sp. and a chrysophyte, Hydrurus foetidus (Villars) Trevisan were abundant at upwelling sites, whereas an assemblage of diatoms Achnanthidium minutissimum (Kützing) Czarnecki, Cymbella excisa Kützing, Diatoma moniliformis Kützing, and Gomphonema olivaceoides Hustedt, were the most abundant taxa at downwelling and neutral exchange sites, occurring attached to, or in close association with the stalks of Didymosphenia geminata (Lyngbye) Schmidt. These data show that benthic algal communities are structured differently depending on the direction of hyporheic flux in the main channel of a large alluvial river, suggesting that hyporheic-surface exchange may influence the spatial distribution of main-channel benthic algae in rivers with hyporheic-surface water connectivity. Handling editor: J. Padisak     

Responses of Hyporheic Meiofauna to Habitat Manipulation

Interactions between interstitial meiofauna and physicochemical parameters of the hyporheic zone were examined via an in situ experiment on the Speed River, Ontario. The manipulation comprised reversing upwelling and downwelling zones at the riffle scale, and was maintained for 1 month. Significant differences in physicochemical parameters were detected between zones and between treatments (control vs. manipulated). Depth-related variables, such as sediment particle size, were most important in structuring the hyporheic community during pre- and post-manipulation phases. Flow reversal was largely successful, with more significant changes occurring in the original downwelling zone. For example, change from downwelling to upwelling resulted in decreased larval chironomid dominance but an increase in the numbers of oligochaetes, nematodes, mites, and copepods. However, under field conditions, it was difficult to keep other variables, such as water temperature, constant and some of these may have contributed to the changes seen in the meiofauna.     

Habitat Heterogeneity and Associated Microbial Community Structure in a Small-Scale Floodplain Hyporheic Flow Path

The Nyack floodplain is located on the Middle Fork of the Flathead River, an unregulated, pristine, fifth-order stream in Montana, USA, bordering Glacier National Park. The hyporheic zone is a nutritionally heterogeneous floodplain component harboring a diverse array of microbial assemblages essential in fluvial biogeochemical cycling, riverine ecosystem productivity, and trophic interactions. Despite these functions, microbial community structure in pristine hyporheic systems is not well characterized. The current study was designed to assess whether physical habitat heterogeneity within the hyporheic zone of the Nyack floodplain was sufficient to drive bacterial β diversity between three different hyporheic flow path locations. Habitat heterogeneity was assessed by measuring soluble reactive phosphorous, nitrate, dissolved organic carbon, dissolved oxygen, and soluble total nitrogen levels seasonally at surface water infiltration, advection, and exfiltration zones. Significant spatial differences were detected in dissolved oxygen and nitrate levels, and seasonal differences were detected in dissolved oxygen, nitrate, and dissolved organic carbon levels. Denaturing gradient gel electrophoresis (DGGE) and cell counts indicated that bacterial diversity increased with abundance, and DGGE fingerprints covaried with nitrate levels where water infiltrated the hyporheic zone. The ribosomal gene phylogeny revealed that hyporheic habitat heterogeneity was sufficient to drive β diversity between bacterial assemblages. Phylogenetic (P) tests detected sequence disparity between the flow path locations. Small distinct lineages of Firmicutes, Actinomycetes, Planctomycetes, and Acidobacteria defined the infiltration zone and α- and β-proteobacterial lineages delineated the exfiltration and advection zone communities. These data suggest that spatial habitat heterogeneity drives hyporheic microbial community development and that attempts to understand functional differences between bacteria inhabiting nutritionally heterogeneous hyporheic environments might begin by focusing on the biology of these taxa.     

A mixing model analysis of stream solute dynamics and the contribution of a hyporheic zone to ecosystem function*

1. We monitored streamwater and streambed sediment porewaters from White Clay Creek (WCC), SE Pennsylvania, for dissolved organic carbon (DOC), dissolved oxygen (DO) and conductivity to investigate organic matter processing within the hyporheic zone. Dissolved organic carbon and DO concentrations were higher in the streamwater than in the porewaters and, in many cases, concentrations continued to diminish with increasing depth into the streambed. 2. Hydrological exchange data demonstrated that the permeability of the stream bed declines with depth and constrains downwelling, effectively isolating porewaters >30 cm from streamwater. 3. End‐member mixing analysis (EMMA) based on conductivity documented a DOC source and DO sink in the hyporheic zone. We calculated hyporheic streambed DOC fluxes and respiration from the EMMA results and estimates of water flux. Based upon our calculations of biodegradable DOC entering the hyporheic zone, we estimate that DOC supports 39% of the hyporheic zone respiration, with the remaining 61% presumably being supported by entrained particulate organic carbon. Hyporheic respiration averaged 0.38 g C m^?2 d^?1, accounted for 41% of whole ecosystem respiration, and increased baseflow ecosystem efficiency from 46 to 59%. 4. Advective transport of labile organic molecules into the streambed concentrates microbial activity in near‐surface regions of the hyporheic zone. Steep gradients in biogeochemical activity could explain how a shallow and hydrologically constrained hyporheic zone can dramatically influence organic matter processing at the ecosystem scale.     

Responses of hyporheic invertebrate assemblages to large-scale variation in flow permanence and surface–subsurface exchange

1. Flow permanence (the proportion of time that flowing water is present) strongly influences benthic invertebrate assemblages in ephemeral and intermittent river reaches. Effects of varying flow permanence on hyporheic invertebrate assemblages are not well understood, and have not previously been studied at large spatial scales. 2. We used a 52‐km long flow‐permanence gradient in the alluvial Selwyn River, New Zealand to assess hyporheic assemblage responses to variation in flow permanence and surface–subsurface exchange. The Selwyn mainstem consists of perennial and temporary reaches embedded in longer downwelling (losing) and upwelling (gaining) sections. 3. We predicted that hyporheic invertebrate diversity, density and assemblage stability would increase with increasing flow permanence. We further predicted that assemblage structure would be influenced by the relative contribution of downwelling and upwelling water at the reach‐scale. 4. Hyporheic invertebrates were collected at 15 river cross‐sections over a 13‐month period. As predicted, hyporheic taxon richness, density and assemblage stability varied directly with flow permanence. The distribution of taxa along the flow permanence gradient appeared to be related to desiccation resistance. However, it is possible that proximity to colonist sources also contributed to distribution patterns. 5. Taxon richness was significantly higher at sites in the gaining section compared with the losing section. Sites with high flow permanence in the gaining and losing sections supported distinct hyporheic assemblages, characterised by amphipods and isopods in the gaining section, and ostracods, Hydra sp. and the mayfly Deleatidium spp. in the losing section. 6. Results of the study suggest an expansion of the scope of the Hyporheic Corridor Concept to include large hyporheic flowpaths associated with unbounded alluvial plains rivers. Hyporheic assemblages in alluvial rivers are strongly influenced by large‐scale flow permanence gradients, large‐scale surface water–groundwater exchange, and their interactions.     

Nitrogen dynamics in the hyporheic zone of a forested stream during a small storm, Hokkaido, Japan

Water and dissolved nitrogen flows through the hyporheic zone of a 3rd-order mountain stream in Hokkaido, northern Japan were measured during a small storm in August 1997. A network of wells was established to measure water table elevations and to collect water samples to analyze dissolved nitrogen concentrations. Hydraulic conductivity and the depth to bedrock were surveyed. We parameterized the groundwater flow model, MODFLOW, to quantify subsurface flows of both stream water and soil water through the hyporheic zone. MODFLOW simulations suggest that soil water inflow from the adjacent hill slope increased by 1.7-fold during a small storm. Dissolved organic nitrogen (DON) and ammonium (NH ₄ ⁺ ) in soil water from the hill slope were the dominant nitrogen inputs to the riparian zone. DON was consumed via mineralization to NH ₄ ⁺ in the hyporheic zone. NH ₄ ⁺ was the dominant nitrogen species in the subsurface, and showed a net release during both base and storm flow. Nitrate appeared to be lost to denitrification or immobilized by microorganisms and/or vegetation in the riparian zone. Our results indicated that the riparian and hyporheic system was a net source of NH ₄ ⁺ to the stream.     

Surface–subsurface water exchange rates along alluvial river reaches control the thermal patterns in an Alpine river network

1. Water temperature is a key characteristic of stream ecosystems that is gaining scientific and managerial relevance as maximum temperatures in aquatic ecosystems increase worldwide.
2. To assess the effect of surface–subsurface water exchange on stream water temperature patterns, four alluvial reaches in the Tagliamento River basin (NE Italy), constrained by geomorphic knickpoints at the upper and lower end, and two to four hyporheic flowpaths within each reach, were continuously studied during summer 2007 and winter 2007–08. Water temperature was continuously monitored at the upstream and downstream knickpoints of the floodplains, as well as at discrete upwelling areas within each reach. Discharge and vertical hydraulic gradient were measured along the alluvial reaches, and the residence time and chemistry of upwelling water were assessed four times during the study.
3. Discharge variation along the study reaches revealed that massive hyporheic exchange occurred in all sites, ranging from 21% in reach 2–52% in reach 1. End member mixing analysis showed little influence of ground water, as almost all upwelling water was freshly infiltrated hyporheic water. Importantly, hyporheic exchange flows shaped surface temperature at the upwelling locations in all study reaches, providing potential thermal refugia for aquatic biota. At sites with highest hyporheic flow rates, net temperature change was also reflected at the floodplain scale.
4. The magnitude of the thermal change along a hyporheic flowpath was not related to the flowpath length but to the estimated ²²²Rn water age. Reduction in the diel thermal amplitude by hyporheic flows rather than net temperature change, reduced temperature extremes. Therefore, restoration activities to create thermal refugia should consider the role of hyporheic flows and enhance the exchange between surface and hyporheic waters.     

Alternative tactics in spawning site selection by brook trout (Salvelinus fontinalis) related to incubation microhabitats in a harsh winter environment

1. Species with distributions that span a broad range of latitudes may have populations that exhibit distinct life history traits associated with environmental gradients. The majority of previous studies have indicated a strong association between spawning site selection by brook trout (Salvelinus fontinalis) and the presence of upwelling groundwater, but does this generalisation extend to the thermal regimes experienced at northern sites? 2. We investigated the role of hyporheic flow in redd site selection by brook trout in a relatively high‐latitude boreal system. Hyporheic flows through streambed substratums can be dominated by groundwater or surface water and may be influenced by the presence of morphological features. For autumn spawners such as brook trout, embryos situated in microhabitats where hyporheic flow in the shallow substratum is groundwater dominant (i.e. warmer in winter) are likely to experience accelerated development rates relative to embryos in redds where there is downwelling surface water (i.e. colder in winter). 3. We measured vertical hydraulic gradients (VHG) at the microhabitat scale and spatial and temporal variation in upwelling/downwelling flow and thermal regimes in brook trout spawning/incubation habitats. Additionally, we noted the proximity of redd sites to stream morphological features (e.g. riffle crests). 4. Our results indicate that upwelling flow was not a decisive cue in redd site selection at the microhabitat scale (10⁰ m) as an approximately equal number of redds were situated in microhabitats with upward flow as compared to microhabitats with downward flow through the redd. Redds situated on bedforms with convex longitudinal profiles (e.g. riffle crests, log steps) were associated with downward flow, whereas redds not immediately adjacent to these bedform features were associated with upward flow. Winter streambed temperatures confirmed that both sites with steady upwelling (i.e. warm incubation regime) and downwelling (i.e. cold incubation regime) were indeed selected by spawners. 5. Our observations that spawners utilised both cold‐regime and warm‐regime sites suggests the existence of distinct reproductive tactics related to hyporheic flow patterns in this boreal system. As temperature is the dominant factor controlling rates of embryonic development, the use of spawning microhabitats with distinct thermal regimes implies substantial differences in the timing of hatching and the phenology of emergence.     

Faunal response to benthic and hyporheic sedimentation varies with direction of vertical hydrological exchange

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Interpreting spatial patterns in redox and coupled water–nitrogen fluxes in the streambed of a gaining river reach

Water pathways through permeable riverbeds are multi-dimensional, including lateral hyporheic exchange flows as well as vertical (upwelling and downwelling) fluxes. The influence of different pathways of water on solute patterns and the supply of nitrate and other redox-sensitive chemical species in the riverbed is poorly understood but could be environmentally significant. For example, nitrate-rich upwelling water in the gaining reaches of groundwater-fed rivers has the potential to supply significant quantities of nitrate through the riverbed to surface waters, constraining opportunities to deliver the goals of the EU Water Framework Directive to achieve ‘good ecological status’. We show that patterns in porewater chemistry in the armoured river bed of a gaining reach (River Leith, Cumbria) reflect the spatial variability in different sources of water; oxic conditions being associated with preferential discharge from groundwater and reducing conditions with longitudinal and lateral fluxes of water due to water movement from riparian zones and/or hyporheic exchange flows. Our findings demonstrate the important control of both vertical and lateral water fluxes on patterns of redox-sensitive chemical species in the river bed. Furthermore, under stable, baseflow conditions (<Q₉₀) a zone of preferential discharge, comprising 20 % of the reach by area contributes 4–9 % of the total nitrate being transported through the reach in surface water, highlighting the need to understand the spatial distribution of such preferential discharge locations at the catchment scale to establish their importance for nitrate delivery to the stream channel.     

Biotic and abiotic interactions between surface and interstitial systems in rivers

Water chemistry and community assemblages of surface and interstitial invertebrates were studied at seven sues on the French Rivers Rhône and Am at surface and at 50 cm depth into the bed sediments Chemical factors allowed differentiation of surface water from groundwater and detection of water exchanges defining clear downwelling and upwelling zones At some sites, interstitial water showed both surface and phreatic conditions, characterizing the underflows of the Rhone or of the Am In the interstitial area, most taxa showed no significant correlations with water chemistry Some epigean and hypogean fauna showed correlations with certain factors Clear relationships appeared between the water exchanges and the distribution of surface and interstitial faunas In interstitial samples, epigean and some hypogean species characterized the downwelling zones while others seemed to be strictly linked to the upwelling zones In surface samples, the presence of hypogean species was associated with regions of groundwater upwelling     

Denitrification in a nitrogen-limited stream ecosystem

Denitrification was measured in hyporheic, parafluvial, and bank sediments of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. We used three variations of the acetylene block technique to estimate denitrification rates, and compared these estimates to rates of nitrate production through nitrification. Subsurface sediments of Sycamore Creek are typically well-oxygenated, relatively low in nitrate, and low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our conservative estimates (unamended, oxic incubations and field chamber nitrous oxide accumulation), denitrification consumed 5–40% of nitrate produced by nitrification. We expected that denitrification would increase along hyporheic and parafluvial flowpaths as dissolved oxygen declined and nitrate increased. To the contrary, we found that denitrification was generally highest at the upstream ends of subsurface flowpaths where surface water had just entered the subsurface zone. This suggests that denitrifiers may be dependent on the import of surface-derived organic matter, resulting in highest denitrification rate at locations of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature dependent. Overall, the quantity of nitrate removed from the Sycamore Creek ecosystem via denitrification is significant given the nitrogen-limited status of this stream.     

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among thehighest rates of atmospheric nitrogen deposition in the United States. Somestreams draining Catskill catchments have shown dramatic increases in nitrateconcentrations while others have maintained low nitrate concentrations. Streamsin which exchange occurs between surface and subsurface (i.e. hyporheic) watersare thought to be conducive to nitrate removal via microbial assimilationand/ordenitrification. Hyporheic exchange was documented in the Neversink River inthesouthern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate(NO₃ ^–) losses along hyporheic flowpaths werenegligible. In this study, Neversink River water was amended with natural,bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series ofexperimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamicswere examined before and throughout a three week BDOC amendment. In addition,bacterial production, dissolved oxygen demand, denitrification, and sixextracellular enzyme activities were measured to arrive at a mechanisticunderstanding of potential DOC and NO₃ ^– removalalong hyporheic flowpaths. There were marked declines in DOC and completeremoval of nitrate in the BDOC amended mesocosms. Independent approaches wereused to partition NO₃ ^– loss into two fractions:denitrification and assimilation. Microbial assimilation appears to be thepredominant process explaining N loss. These results suggest that variabilityinBDOC may contribute to temporal differences in NO₃ ^–export from streams in the Catskill Mountains.