A landscape perspective of surface–subsurface hydrological exchanges in river corridors

1. River corridors can be visualised as a three‐dimensional mosaic of surface–subsurface exchange patches over multiple spatial scales. Along major flow paths, surface water downwells into the sediment, travels for some distance beneath or along the stream, eventually mixes with ground water, and then returns to the stream. 2. Spatial variations in bed topography and sediment permeability result in a mosaic of patch types (e.g. gravel versus sandy patches) that differ in their hydrological exchange rate with the surface stream. Biogeochemical processes and invertebrate assemblages vary among patch types as a function of the flux of advected channel water that determines the supply of organic matter and terminal electron acceptors. 3. The overall effect of surface–subsurface hydrological exchanges on nutrient cycling and biodiversity in streams not only depends on the proportion of the different patch types, but also on the frequency distribution of patch size and shape. 4. Because nutrients are essentially produced or depleted at the downwelling end of hyporheic flow paths, reach‐scale processing rates of nutrients should be greater in stretches with many small patches (e.g. short compact gravel bars) than in stretches with only a few large patches (e.g. large gravel bars). 5. Based on data from the Rhône River, we predict that a reach with many small bars should offer more hyporheic refugia for epigean fauna than a reach containing only a few large gravel bars because benthic organisms accumulate preferentially in sediments located at the upstream and downwelling edge of bars during floods. However, large bars are more stable and may provide the only refugia during severe flood events. 6. In river floodplain systems exhibiting pronounced expansion/contraction cycles, hyporheic assemblages within newly created patches not only depend on the intrinsic characteristics of these patches but also on their life span, hydrological connection with neighbouring patches, and movement patterns of organisms. 7. Empirical and theoretical evidence illustrate how the spatial arrangement of surface–subsurface exchange patches affects heterogeneity in stream nutrient concentration, surface water temperature, and colonisation of dry reaches by invertebrates. 8. Interactions between fluvial action and geomorphic features, resulting from seasonal and episodic flow pulses, alter surface–subsurface exchange pathways and repeatedly modify the configuration of the mosaic, thereby altering the contribution of the hyporheic zone to nutrient transformation and biodiversity in river corridors.     

Diel variation in surface and subsurface microbial activity along a gradient of drying in an Australian sand-bed stream

1. Microbes play key roles in nutrient transformation and organic matter mineralisation in the hyporheic zone but their short‐term responses to diel variations in discharge and temperature are unknown. Rates of microbial esterase activity were hypothesised to vary vertically and along a gradient of moisture in a drying sand‐bed stream where discharge fluctuated daily in response to evapotranspiration. 2. At ‘fully saturated’, ‘moist’ and ‘dry’ locations in three sites along a drying Australian sand‐bed stream, microbial activity at three depths (surface, 10 and 30 cm) was assessed using fluorescein diacetate hydrolysis. Samples were collected in mid‐summer in the late afternoon and again at dawn to assess diel variation in hydrolytic activity at each site and depth. Data loggers tracked diel variations in temperature at each depth. 3. Hydrolytic activity was up to 10‐fold greater in the surface sediments in late afternoon than at dawn in all habitats, and was correlated with surface sediment temperature. Diel differences in activity were not detected at 10 cm, although daily thermal cycles were evident at this depth. Unexpectedly, activity was marginally higher at dawn at 30 cm in all habitats, perhaps reflecting lags in temperature at that depth. 4. Overall, microbial activity declined with depth, strongly correlated with vertical trends in total organic matter and concentrations of dissolved phosphorus. Particulate organic matter, probably buried during a flood 35 days earlier, appeared largely responsible for these vertical trends. On the other hand, there was little evidence for hydrological exchange between much of the hyporheic zone and the surface stream, implying that processes in the subsurface zone of this stream are effectively isolated during baseflow in mid‐summer. 5. Diel cycles of wetting and drying in the moist habitats did not enhance esterase activity relative to the dry or fully saturated habitats. Sediment moisture was not correlated with microbial activity, and mats of senescent algae appeared to inhibit water loss from surface sediments in the moist habitat. In this sand‐bed stream, local diel fluctuations in water level appear to have less influence on microbial activity and mineralisation of organic matter in the sediments than occasional floods that bury leaf litter and renew many hyporheic zone functions. Subreach‐scale processes seem to be the major driving force of microbial processes and nutrient cycling in this sand‐bed river.     

Influence of streambed sediment clogging on microbial processes in the hyporheic zone

1. The hyporheic zone plays a key role in hydrological exchange and biogeochemical processes in streambed sediments. The clogging of sediments caused by the deposition of particles in the bed of streams and rivers can decrease sediment permeability and hence greatly affect hyporheic microbial processes. 2. The main objective of this study was to determine the influence of sediment clogging on hyporheic microbial processes in three French rivers (the Usses, Drôme and Isère). In each river, microbial abundance and activity were studied at three depths (10, 30 and 50 cm) in the sediment at one unclogged (high porosity) and one clogged site (low porosity). 3. The results showed that the sediment clogging had inconsistent effects on microbial processes in the three rivers. Increases (Usses) or decreases (Drôme and Isère) in both aerobic and anaerobic processes were detected at the clogged sites compared to unclogged sites. These results suggest that microbial changes because of the sediment clogging are mainly mediated by the residence time of water within the hyporheic sediments. 4. A single model predicting the effect of clogging on hyporheic microbial processes cannot be applied generally to all rivers because the degree of clogging creates heterogeneous effects on flow rates between surface and interstitial waters. As a consequence, the influence of heterogeneous clogging on surface water–hyporheic exchanges needs to be evaluated by water tracing and hydraulic modelling to determine the links between microbial processes and hydraulic heterogeneity induced by clogging in hyporheic sediments.     

The effects of increased flow and fine sediment on hyporheic invertebrates and nutrients in stream mesocosms

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

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Factors controlling hyporheic respiration in a desert stream

1. Experimental manipulations were performed to determine the biological, chemical and physical attributes that govern sediment respiration in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream. 2. Hyporheic respiration per unit volume of sediment was inversely related to diameter of sediment particles, indicating that respiration is affected by availability of substrate for microbial colonization (i.e. sediment surfaces). Respiration rate per unit surface area on sediments was positively correlated with particle diameter, indicating greater metabolic activity of microbes on larger sediments. 3. Hyporheic respiration was more than twice as high in water collected from the surface flow than from subsurface flow. Further, hyporheic respiration was highest immediately following exposure of sediments to surface water and declined over time, presumably due to exhaustion of labile organic matter. 4. Microbial activity was stimulated by addition of algal leachate; however, amendments of leaf leachate had little effect. Respiration was also elevated with dextrose and leucine amendments, but not with inorganic nitrogen additions, indicating hyporheic respiration is carbon limited. 5. Water from the stream surface is probably enriched in labile organic matter derived from algae and stimulates respiration at points of hydrologic downwelling where surface water enters hyporheic sediments. The physical structure of sediments further affects metabolism by affecting the area available for microbial attachment.     

Net dissolved inorganic nitrogen production in hyporheic mesocosms with contrasting sediment size distributions

The interstitial spaces within streambeds are recognized as an important location of dissolved inorganic nitrogen (DIN) transformations in streams. However, it remains uncertain how physical characteristics of streambeds affect the magnitude and net outcome of subsurface nitrogen transformations. We tested whether the size distribution of streambed sediments, in isolation from the influence of streambed topography and groundwater upwelling, could affect net DIN uptake or production along interstitial flow paths. Mesocosms constructed from PVC pipe (15 cm diameter × 1 m long) were filled with either coarse gravel/cobble or gravel/cobble mixed with finer sediments (5 mesocosms per sediment treatment). Mesocosms were submerged in a stream and oriented, so that surface water flowed through the sediments. After 2 months incubation, we measured DIN in interstitial water at 20 cm intervals and dissolved oxygen at 10 cm intervals along mesocosm flow paths. In both sediment types, DIN concentrations increased longitudinally along mesocosm flow paths in the direction of interstitial flow, indicating net DIN production. Although DIN increased to higher concentrations in mesocosms with fine sediments, greater exchange flow through coarse sediments resulted in similar rates of net DIN production and delivery to surface water. Production of DIN in both sediment types was concentrated within the first 10 cm of interstitial flow paths, with no significant production further along the flow paths. Coarse sediments had higher rates of oxygen consumption per unit sediment volume than the coarse–fine sediment mix, suggesting interstitial water velocity may be an important factor affecting hyporheic microbial metabolism.     

Water-sediment exchanges control microbial processes associated with leaf litter degradation in the hyporheic zone: a microcosm study

The present study aimed to experimentally quantify the influence of a reduction of surface sediment permeability on microbial characteristics and ecological processes (respiration and leaf litter decomposition) occurring in the hyporheic zone (i.e. the sedimentary interface between surface water and groundwater). The physical structure of the water–sediment interface was manipulated by adding a 2-cm layer of coarse sand (unclogged systems) or fine sand (clogged systems) at the sediment surface of slow filtration columns filled with a heterogeneous gravel/sand sedimentary matrix. The influence of clogging was quantified through measurements of hydraulic conductivity, water chemistry, microbial abundances and activities and associated processes (decomposition of alder leaf litter inserted at a depth of 9 cm in sediments, oxygen and nitrate consumption by microorganisms). Fine sand deposits drastically reduced hydraulic conductivity (by around 8-fold in comparison with unclogged systems topped by coarse sand) and associated water flow, leading to a sharp decrease in oxygen (reaching less than 1 mg L⁻¹ at 3 cm depth) and nitrate concentrations with depth in sediments. The shift from aerobic to anaerobic conditions in clogged systems favoured the establishment of denitrifying bacteria living on sediments. Analyses performed on buried leaf litter showed a reduction by 30% of organic matter decomposition in clogged systems in comparison with unclogged systems. This reduction was linked to a negative influence of clogging on the activities and abundances of leaf-associated microorganisms. Finally, our study clearly demonstrated that microbial processes involved in organic matter decomposition were dependent on hydraulic conductivity and oxygen availability in the hyporheic zone.     

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.     

The effect of in-stream gravel extraction in a pre-alpine gravel-bed river on hyporheic invertebrate community

We investigated the effect of in-stream gravel extraction in a pre-alpine gravel-bed river on hyporheic invertebrate community, together with changes in the hyporheic geomorphology, physico-chemistry and biofilm activity. Hyporheic invertebrates were collected, together with environmental data, on seven sampling occasions from June 2004 to May 2005, at two river reaches—at the site of in-stream gravel extraction and at a site 2.5 km upstream. The hyporheic samples were taken from the river bed and from the gravel bars extending laterally from the stream channel. The invertebrate community was dominated by insect larvae (occasional hyporheos), followed by meiofauna (permanent hyporheos). Stygobionts were present at low species richness and in low densities. Gravel extraction from the stream channel led to changes in the patterns of water exchange between surface and subsurface and changes in the sediment composition at the site. Immediate reductions in density and taxonomic richness of invertebrates were observed, together with changes in their community composition. The hyporheic invertebrate community in the river recovered relatively fast (in 2.5 months) by means of density and taxonomic richness, while by means of community composition invertebrates needed 5–7 months to recover. The impact of fine sediments (<0.1 mm) on biofilm activity measured through ETS activity and hyporheic invertebrate density and taxonomic richness was strongly confirmed in this study.     

Abiotic aspects of channels and floodplains in riparian ecology

1. The ecology of riparian zones is enormously influenced by the heterogeneous sedimentary structures and associated complex hydrologic flow paths that mediate surface- and groundwater exchanges. Sedimentary structures form a three-dimensional, dynamic framework that controls subsurface flow and the vertical and horizontal exchange of water between channels and floodplains in gravel bed rivers. The modern structure of the bed sediments reflects the legacy of cut and fill alluviation for a particular river basin. 2. Highly permeable sedimentary textures, particularly open framework gravels, allow rapid exchange between surface and groundwaters. 3. Ground penetrating radar provides high resolution information on the nature and three-dimensional distribution of the sediments within the shallow subsurface (4–25 m) of gravel bed rivers. Bed sediments can be mapped at the decimeter scale. 4. Exchange and mixing of ground and channel water occurs along losing, gaining and flow-through reaches as determined by the hydraulic gradient and transmissivity of the bed sediments. 5. Spatial and temporal patterns of surface- and groundwater interactions can be quantified by mass flux measurements and by assessing geochemical contrasts. Natural tracers, such as temperature or radon, are well suited for mapping exchange sites and quantifying interactions. Artificial signals produced by injecting anions, like chloride, bromide and organic dyes are also useful. 6. The study of riparian ecosystems requires an understanding of the geomorphic structures and processes that build and maintain bed sediments and flow pathways through them.     

Consequences of a biogeomorphic regime shift for the hyporheic zone of a Sonoran Desert stream

1. Feedbacks between vegetation and geomorphic processes can generate alternative stable states and other nonlinear behaviours in ecological systems, but the consequences of these biogeomorphic interactions for other ecosystem processes are poorly understood. In this study, we describe the changes in the hydrological, geomorphic and biogeochemical characteristics of the hyporheic zone of a Sonoran desert stream (Sycamore Creek, Arizona, U.S.A.) in response to a transition from an unvegetated gravel‐bed state to densely vegetated wetlands (ciénegas). 2. A survey of the entire length of Sycamore Creek indicated that ciénegas occupied c. 18% of the stream, and were disproportionately represented in constrained canyons rather than wide, unconstrained valleys. 3. Vegetated patches were characterized by low concentrations of dissolved oxygen (DO) and nitrate and high concentrations of carbon dioxide and methane in the hyporheic zone. In contrast to unvegetated areas, hyporheic DO in ciénegas exhibited no relationship with vertical hydraulic gradients. 4. Increases in hyporheic DO following removal of vegetation by floods supports the hypothesis that these reduced conditions were the result of biogeochemical and geomorphic changes associated with vegetation establishment. In locations where vegetation persisted, hyporheic DO exhibited no response to flooding; in sections where vegetation was removed hyporheic DO closely tracked post‐flood increases in surface stream DO. 5. Shallow sediments in vegetated patches were finer and more organic‐rich than in unvegetated patches, due to increased deposition during floods. Conservative tracer additions indicated that hydrological exchange between the surface stream and hyporheic zone was much lower in ciénegas than in gravel‐bed reaches. 6. Vegetation establishment in desert streams not only alters the physical and chemical characteristics of the hyporheic zone, but also the nature of interactions between surface and hyporheic subsystems.     

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.     

Intensity of mineralization in the hyporheic zone of the prealpine river Bača (West Slovenia)

Respiratory electron transport system (ETS) activity and oxygen consumption in the interstitial water, and in the fine (i.e. silt) and coarse (sand) sediment fractions from the hyporheic zone of the prealpine river Bača (W Slovenia) have been measured in order to estimate the intensity of potential and actual carbon mineralization through microbial communities. Hyporheic samples from the river bed (RB) and gravel bars (GB) were compared. ETS activity and oxygen consumption of all fractions from the RB did not differ significantly from those from the GB. ETS activity and oxygen consumption of biofilm attached to 1 g of the silt were higher than of that attached to the same mass of the sand. A significant correlation between ETS activity and oxygen consumption indicated that the former should be a good indicator of intensity of bioactivity in hyporheic sediments. The ratio of ETS activity to oxygen consumption (ETS/R ratio) revealed that the oxygen consumption of microorganisms is responsible for approximately 60% of the metabolic potential in the hyporheic sediments. The contributions of different fractions of sediment to the total ETS activity differed between RB and GB. The contribution of microorganisms in the interstitial water and silt was higher in GB than in the RB, but the sand fraction contributed less to potential carbon loss in GB than in the RB. Average total respiratory carbon loss per volume through the hyporheic zone was higher in the RB than in GB. The main reasons suggested are the different intensity of exchange of surface water with the hyporheic zone, and the rate of consolidation of sediments, which is primarily a function of river hydrology and geomorphology. Handling editor: J. Padisak     

Relevance of large litter bag burial for the study of leaf breakdown in the hyporheic zone

Particulate organic matter is the major source of energy for most low-order streams, but a large part of this litter is buried within bed sediment during floods and thus become poorly available for benthic food webs. The fate of this buried litter is little studied. In most cases, measures of breakdown rates consist of burying a known mass of litter within the stream sediment and following its breakdown over time. We tested this method using large litter bags (15 × 15 cm) and two field experiments. First, we used litter large bags filled with Alnus glutinosa leaves (buried at 20 cm depth with a shovel) in six stations within different land-use contexts and with different sediment grain sizes. Breakdown rates were surprisingly high (0.0011–0.0188 day⁻¹) and neither correlate with most of the physico-chemical characteristics measured in the interstitial habitats nor with the land-use around the stream. In contrast, the rates were negatively correlated with a decrease in oxygen concentrations between surface and buried bags and positively correlated with both the percentage of coarse particles (20–40 mm) in the sediment and benthic macro-invertebrate richness. These results suggest that the vertical exchanges with surface water in the hyporheic zone play a crucial role in litter breakdown. Second, an experimental modification of local sediment (removing fine particles with a shovel to increase vertical exchanges) highlighted the influence of grain size on water and oxygen exchanges, but had no effect on hyporheic breakdown rates. Burying large litter bags within sediments may thus not be a relevant method, especially in clogged conditions, due to changes induced through the burial process in the vertical connectivity between surface and interstitial habitats that modify organic matter processing.     

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.     

Effects of augmentation of coarse particulate organic matter on metabolism and nutrient retention in hyporheic sediments

SUMMARY 1. Metabolic and biogeochemical processes in hyporheic zones may depend on inputs of coarse particulate organic matter. Our research focused on how differing quantity and quality of organic matter affects metabolism and nutrient retention in the hyporheic zone of a first-order Appalachian stream.
2. Sixteen plots were established on a tributary of Hugh White Creek, NC, U.S.A. Sediment was extracted and treated with leaves, wood, plastic strips or remained unamended. Following treatment, sediment was returned to the stream and, approximately 3 months later, samples were removed from each plot.
3. Aerobic and anaerobic metabolism were measured as the change in O₂ and CO₂ in recirculating microcosms. At the same time, we monitored other possible terminal electron accepting processes and changes in nutrient concentrations. Aerobic metabolism was low in all treatments and respiratory quotients calculated for all treatments indicated that metabolism was dominated by anaerobic processes.
4. Rates of anaerobic respiration and total (combined aerobic and anaerobic) respiration were significantly greater ( P  < 0.05) in plots treated with leaf organic matter compared to controls.
5. Addition of leaves, which had a low C:N ratio, stimulated respiration in hyporheic sediments. Anaerobic processes dominated metabolism in both control and amended sediments. Enhanced metabolic rates increased retention of many solutes, indicating that energy flow and nutrient dynamics in the subsurface of streams may depend upon the quantity and quality of imported carbon.     

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.     

The ecological significance of exchange processes between rivers and groundwater

1. This review focuses on the connectivity between river and groundwater ecosystems, viewing them as linked components of a hydrological continuum. Ecological processes that maintain the integrity of both systems and those that are mediated by their ecotones are evaluated. 2. The hyporheic zone, as the connecting ecotone, shows diverse gradients. Thus it can be characterized by hydrological, chemical, zoological and metabolic criteria. However, the characteristics of the hyporheic zone tend to vary widely in space and time as well as from system to system. The exact limits are difficult to designate and the construction of static concepts is inadequate for the representation of ecological processes. The hyporheic interstices are functionally a part of both the fluvial and groundwater ecosystems. 3. The permeability of the ecotone depends on the hydraulic conductivity of the sediment layers which, because of their heterogeneity, form many flowpath connections between the stream and the catchment, from the small scale of a single microhabitat to the large scale of an entire alluvial aquifer. Local up- and downwellings are determined by geomorphologic features such as streambed topography, whereas large-scale exchange processes are determined mainly by the geological properties of the catchment. Colmation—clogging of the top layer of the channel sediments—includes all processes leading to a reduction of pore volume, consolidation of the sediment matrix, and decreased permeability of the stream bed. Consequently, colmation can hinder exchange processes between surface water and groundwater. 4. Physicochemical gradients in the interstices result from several processes: (i) hyporheic flow pattern and the different properties of surface and groundwaters; (ii) retention, caused by the filtering effect of pore size and lithologic sorption as well as the transient storage of solutes caused by diminished water velocities; (iii) biogeochemical transformations in conjunction with local residence time. Each physicochemical parameter may develop its own vertical dynamics laterally from the active channel into the banks as well as longitudinally because of geomorphologic changes. 5. The river–groundwater interface can act as a source or sink for dissolved organic matter, depending on the volume and direction of flow, dissolved organic carbon concentrations and biotic activity. Interstitial storage of particulate organic matter is influenced mainly by grain size distribution and by spates involving bedload movement that may import or release matter, depending on the season. After initial transient and abiotic storage, hyporheic organic matter is mobilized and transformed by the biota. Micro-organisms account for over 90% of the community respiration. In subterranean waters most bacteria are attached to surfaces and remain in a biofilm. 6. Hyporheic interstices are functionally significant for phreatic and riverine metazoans because they act as a refuge against adverse conditions. The net flow direction exerts a dominant influence on interstitial colonization, but many other factors also seem to be important in structuring the hyporheos. 7. The hyporheic corridor concept emphasizes connectivity and interactions between subterranean and surface flow on an ecosystem level for floodplain rivers. It is a complementary concept to others which focus on surficial processes in the lateral and longitudinal dimensions. 8. The ecological integrity of groundwater and fluvial systems is often threatened by human activities: (i) by reducing connectivity; (ii) by altering exchange processes; and (iii) by toxic or organic contamination.     

Changes in microphytobenthos fluorescence over a tidal cycle: implications for sampling designs

The breakdown rate of leaf litter buried inside river sediments (i.e., in the hyporheic zone) remains poorly known. The burial of large bags (15 × 15 cm) used in the benthic layer generates disturbances of the vertical connectivity with surface water, and thus affects the breakdown rate. We performed field and laboratory experiments to test the suitability of two leaf litter containers: small litter bags (5 × 4 cm) and perforated stainless steel cylinders (6 cm long, 1.6 cm in diameter), both introduced inside sediments using mobile mini-piezometers (1 m long, 1.7 cm or 3 cm in diameter). We compared the two containers for (i) the hydrological exchanges toward leaf litter, (ii) the impact of a benthic shredder on leaf breakdown, and (iii) the response of leaf litter breakdown to changes in sediment characteristics. The two methods give similar patterns of breakdown rates with stream sediment characteristics, but the use of perforated stainless steel cylinders provides an artificial empty volume in which invertebrates can over degrade leaf litter. Small plastic bags do not alter physico-chemical conditions around leaf litter and give lower values of breakdown rates in the hyporheic zone.