Influence of stream flow regime and temperature on growth rate of the riparian tree, Platanus wrightii, in Arizona

1. The growth of riparian trees in semi‐arid regions is influenced by stream flow regime, but the relative importance of base flow and seasonal floods on growth has not been explored. I examined abiotic influences on the growth of Platanus wrightii in four stream reaches in Arizona. All reaches had a bimodal pattern of discharge, but only two had continuous flow throughout the growing season.
2. In two reaches of Sycamore Creek without perennial flow, a large percentage of the annual variation in radial growth rate of P. wrightii was explained by annual and growing season flow rate. Growth was related to these same variables in a perennial reach of Sycamore Creek, but trees maintained higher growth during drought years than they did in the temporary reaches. At Oak Creek, a larger perennial stream, P. wrightii growth showed a bell‐shaped relationship with flow. These data suggest that growth rate is frequently limited by water availability at Sycamore Creek, but not at Oak Creek.
3. At both rivers, much of the annual surface flow occurs as winter floods. Oak Creek, however, maintains a high summer base flow even during years with no floods. Platanus wrightii growth was significantly related to winter flood frequency only at Sycamore Creek. The positive relationship of growth with stream flow and winter flood frequency at Sycamore Creek presumably occurs because the P. wrightii trees are dependent on the winter flows to recharge the shallow alluvial aquifer and to raise the level of ground water within the root zone.
4. Frequent summer floods increased the growth of trees in perennial and non‐perennial reaches alike. At perennial Oak Creek, summer flood frequency was the only variable linearly related to growth of P. wrightii. Summer flood frequency was a significant, but secondary, component of multiple‐regression growth models for trees in the perennial and non‐perennial reaches of Sycamore Creek. Summer floods may stimulate growth, in part, by replenishing limiting nutrients.
5. High temperature was negatively associated with the growth of P. wrightii at Sycamore Creek. The combination of drought and high temperature resulted in very low growth rate.
6. These results have implications for the management of flood and base flow regimes on regulated, diverted and pumped rivers.     

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.     

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.     

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.     

Hierarchy,spatial configuration,and nutrient cycling in a desert stream

Abstract Recent studies of nutrient cycling in Sycamore Creek in Arizona, USA, suggest that a thorough understanding requires a spatially explicit, hierarchical approach. Physical configuration determines the path that water follows as it moves downstream. Water follows flowpaths through surface stream components, the hyporheic zone beneath the surface stream, and the parafluvial (sand bar) zone. Characteristic biogeochemical processes in these subsystems alter nitrogen (N) species in transport, in part as a function of available concentrations of N species. At several hierarchical levels, substrate materials are an important determinant of nitrogen dynamics in desert streams. Sand is present in bars of variable size and shape, each of which can be considered a unit, interacting with the surface stream. Groups of these stream-sandbar units form a higher level, the reach. At the next higher scale, sandy reaches (runs) alternate with riffles. Where flowpaths converge, rates of N transformation are high and, as a result, change in concentration is a non-linear function of flowpath length. Disturbance by flash floods alters sandbar configuration. Between floods, the interaction of subsurface and surface flowpaths shapes configuration in each, thus a self-organizing element of spatial structure exists. Sandy runs are dominated by subsurface processes and are likely to be net nitrifiers while riffles are dominated by surface flow and are nitrogen fixers. Whether a stream ecosystem retains nitrogen, or transports it to downstream recipient systems, or is a net emitter of gaseous forms of N, depends upon the dynamics of a spatial mosaic of interacting elements. An understanding of the net effect of this mosaic requires a spatially explicit, hierarchical, multi-scale approach.     

Methanogenesis in Arizona,USA dryland streams

Methanogenesis was studied in five streams of central and southern Arizona by examining the distribution of methane in interstitial water and evasion of methane in three subsystems (hyporheic, parafluvial and bank sediments). In Sycamore Creek, the primary study site (studied during summer and early autumn), methane content of interstitial water exhibited a distinct spatial pattern. In hyporheic (sediments beneath the wetted channel) and parfluvial zones (active channel sediments lateral to the wetted channel), which were well oxygenated due to high hydrologic exchange with the surface stream and had little particulate organic matter (POM), interstitial methane concentration averaged only 0.03 mgCH₄-C/L. Bank sediments (interface between the active channel and riparian zone), in contrast, which were typically vegetated, had high POM, low hydrologic exchange and concomitantly low dissolved oxygen levels, had interstitial concentration averaging 1.5 mgCH4-C/L. Methane emission from Sycamore Creek, similar to methane concentration, averaged only 3.7 mgCH₄-C·m⁻²·d⁻¹ from hyporheic and parafluvial zones as opposed to 170 mgCH₄-C·m⁻²·d⁻¹ from anoxic bank sediments. Methane in four additional streams sampled (one sampling date during late winter) was low and exhibited little spatial variation most likely due to cooler stream temperatures. Interstitial methane in parafluvial and bank sediments of all four streams ranged from only 0.005 to 0.1 mgCH₄-C/L. Similarly methane evasion was also low from these streams varying from 0 to 5.7 mgCH₄-C·m⁻²·d⁻¹. The effects of organic matter and temperature on methanogenesis were further examined by experimentally manipulating POM and temperature in stoppered flasks filled with hyporheic sediments and stream water. Methane production significantly increased with all independent variables. Methane production is greatest in bank sediments that are relatively isolated hydrologically and lowest in hyporheic and parafluvial sediments that are interactive with the surface stream.     

Responses of macroinvertebrate communities to long‐term flow variability in a Sonoran Desert stream

Current global models predict a hotter and drier climate in the southwestern United States with anticipated increases in drought frequency and severity coupled with changes in flash flood regimes. Such changes would likely have important ecological consequences, particularly for stream and riparian ecosystems already subject to frequent hydrologic disturbance. This study assessed the potential response of aquatic macroinvertebrates to interannual variation in hydrology in a spatially intermittent desert stream (Sycamore Creek, AZ). We compiled data on the recovery of macroinvertebrate communities following spring floods, with successional sequences captured 11 times over a 16‐year period (1983–1999). This period encompassed a transition from perennial to intermittent flow in this system, and included a record drought in 1989–1990. Results show that while the size of floods initiating sequences had little explanatory power, changes in macroinvertebrate community structure during postflood succession were closely associated with antecedent flooding and drought. Year‐to‐year differences in benthic communities integrated taxon‐specific responses to antecedent disturbance, including differential resistance to channel drying, use of hyporheic refugia, and variable rates of recovery once stream flow resumed. The long‐term consequences of drying on community structure were only evident during later stages of postflood succession, illustrating an interaction between flood and drought recovery processes in this system. Our observations highlight the potential for predicted climate changes in this region to have marked and long‐lasting consequences for benthic communities in desert streams.     

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.     

Metabolism of a desert stream

SUMMARY. Rates of photosynthesis and community respiration were determined for benthic assemblages in Sycamore Creek, a Sonoran Desert stream in Arizona. Benthos in this stream can be separated into (1) mats of Cladophora glomerata and associated epiphytes and (2) assemblages of epipelic diatoms and blue-green algae. Community respiration and net photosynthesis were measured for these assemblages using submerged light-dark chambers in situ . Multiple regression analysis was used to predict (1) gross photosynthesis as a function of photosynthetically active radiation, temperature and chlorophyll-α concentration; and (2) community respiration as a function of temperature and biomass.
Calculations suggest that Sycamore Creek is autotrophic during the summer ( P/R = 1.7) and that the rates of gross photosynthesis ( P =8.5 g O₂ m⁻² day⁻¹) and community respiration ( R = 5.1 g O₂ m⁻² day⁻¹) are high for a small stream. Considerable difference exists between the Cladophora mat assemblages, in which mean P is 12.5gO₂m⁻² day⁻¹and the P/R ratio is 2.3, and the epipelic assemblages in which mean P is 4.4 g O₂m⁻² day⁻¹ and P/R is 0.96. The high rate of gross photosynthesis, low litter inputs, high biomass of algae and the intermittent but severe floods that characterize Sycamore Creek indicate that this stream and other similar desert streams are net exporters of organic matter and are, thereby, truly autotrophic stream ecosystems.     

Carbon and nitrogen transfer from a desert stream to riparian predators

Adult aquatic insects emerging from streams may be a significant source of energy for terrestrial predators inhabiting riparian zones. In this study, we use natural abundance delta(13)C and delta(15)N values and an isotopic (15)N tracer addition to quantify the flow of carbon and nitrogen from aquatic to terrestrial food webs via emerging aquatic insects. We continuously dripped labeled (15)N-NH(4) for 6 weeks into Sycamore Creek, a Sonoran desert stream in the Tonto National Forest (central Arizona) and traced the flow of tracer (15)N from the stream into spiders living in the riparian zone. After correcting for natural abundance delta(15)N, we used isotopic mixing models to calculate the proportion of (15)N from emerging aquatic insects incorporated into spider biomass. Natural abundance delta(13)C values indicate that orb-web weaving spiders inhabiting riparian vegetation along the stream channel obtain almost 100% of their carbon from instream sources, whereas ground-dwelling hunting spiders obtain on average 68% of their carbon from instream sources. During the 6-week period of the (15)N tracer addition, orb-web weaving spiders obtained on average 39% of their nitrogen from emerging aquatic insects, whereas spider species hunting on the ground obtained on average 25% of their nitrogen from emerging aquatic insects. To determine if stream subsidies might be influencing the spatial distribution of terrestrial predators, we measured the biomass, abundance and diversity of spiders along a gradient from the active stream channel to a distance of 50 m into the upland using pitfall traps and timed sweep net samples. Spider abundance, biomass and richness were highest within the active stream channel but decreased more than three-fold 25 m from the wetted stream margin. Changes in structural complexity of vegetation, ground cover or terrestrial prey abundance could not account for patterns in spider distributions, however nutrient and energy subsidies from the stream could explain elevated spider numbers and richness within the active stream channel and riparian zone of Sycamore Creek.     

A long-term perspective of dissolved organic carbon transport in Sycamore Creek, Arizona, USA

Dissolved organic carbon (DOC) dynamics were examined over five years (1989–1993) in Sycamore Creek, a Sonoran Desert stream, specifically focusing on DOC concentration in surface and hyporheic waters, and rates of export. In 1989 and 1990, the years of lowest stream discharge (0.08 and 0.04 m³ s^–1 annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgC l^–1 (weighted annual means). In contrast, from 1991 through 1993, a period of increased flow (1.1, 1.2 and 4.3 m³ s^–1), concentration was significantly lower (P<0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgC l^–1. Concentration exhibited little spatial variation between two sampling stations located 6 km apart along the mainstem or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five-year period from a mean rate of only 24 kgC d^–1 in 1990 to 2100 kgC d^–1 in 1993. Ninety percent of DOC was exported by flows greater than 2.8 m³ s^–1, and 50% during flows greater than 27 m³ s^–1; flows of 2.8 and 24 m³ s^–1 occurred only 9 and 1% of the time. The export of organic matter in Sycamore Creek appears to be coupled to El Niño-Southern Oscillation phenomena. The years of highest export, 1991–1993, had El Niño conditions while 1989 and 1990 had medial conditions.     

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.     

溪流潜流层大型无脊椎动物生态学研究进展

溪流潜流层是溪流表层水和地下水相互作用的群落交错区,生物多样性丰富,是溪流生态系统的重要组成部分.大型无脊椎动物位于潜流层食物网的顶层,直接影响着潜流层物质和能量动态,是河流健康潜在的指示生物,调节着潜流层的环境净化和生态缓冲功能,对溪流生态系统发挥着至关重要的作用.潜流层大型无脊椎动物类群按生活史划分为偶入动物、非典型潜流层动物和典型潜流层动物.潜流层孔隙大小、孔隙水流速、溶解氧、温度、可利用的食物源、渗透系数和水力停留时间是影响大型无脊椎动物在潜流层分布的主要因素.对于潜流层这样一个特殊的生态界面,针对不同的研究目的应该选择合适的取样和调查方法.潜流层大型无脊椎动物的生活史和生活史对策,在溪流生态系统物质循环和能量流动中作用的定量化分析,基于潜流层大型无脊椎动物的河流健康评价体系,以及潜流层作为“庇护地”对于大型无脊椎动物分布和进化的生态学意义,都值得进一步关注和深入研究.     

Patterns of hydrological exchange and nutrient transformation in the hyporheic zone of a gravel-bottom stream: examining terrestrial—aquatic linkages

• 1 The terrestrial-aquatic interface beneath a riparian corridor was investigated as a region of hydrological and biological control of nutrient flux. Subsurface flow paths were defined from the channel toward the riparian zone and also from the riparian zone toward the channel using tracer-injection studies. Solute transport had a rapid channel component (m min^?1) and a slow hyporheic flow component (mh^?1, m day^?1). Subsurface flow beneath the riparian zone approximated a straight path entering at meanders but could also cross beneath the stream, possibly using relic channels.
• 2 Dissolved oxygen (DO) concentration in the hyporheic zone ranged from <1.0 to 9.5mgl^?1 due to permeability variations in bankside sediments. DO concentration was related to the proportion of stream water in the lateral hyporheic zone, indicating that the channel water was the DO source.
• 3 The magnitude and riming of lateral water exchange was linked to previously published studies of nitrification and denitrificarion. Both nitrification potential and channel exchange decreased with distance from the channel and were absent at sites lacking effective exchange, due to low DO. Field amendment of ammonium to an aerobic flow path indicated nitrification potential under natural hydrological conditions. Denitrification potential was inversely related to channel exchange and was insignificant in channel sediments. Field amendment of acetylene plus nitrate to a flow path with low DO and minimal channel exchange indicated denitrificarion of amended nitrate.
• 4 Comparison of hydraulic head to distribution of the biologically important solutes DO, ammonium, and nitrate was useful for interpreting previous findings and conceptualizing the riparian zone as a functioning ecotone between terrestrial and aquatic systems.

     

The role of micro-organisms in the ecological connectivity of running waters

1. Riparian zones hold a central place in the hydrological cycle, owing to the prevalence of surface and groundwater interactions. In riparian transition zones, the quality of exfiltrating water is heavily influenced by microbial activities within the bed sediments. This paper reviews the role of micro-organisms in biogeochemical cycling in the riparian-hyporheic ecotone. 2. The production of organic substances, such as cellulose and lignin, by riparian vegetation is an important factor influencing the pathways of microbial processing in the riparian zone. For example, anaerobic sediment patches, created by entrainment of allochthonous organic matter, are focal sites of microbial denitrification. 3. The biophysical structure of the riparian zone largely influences in-stream microbial transformations through the retention of organic matter. Particulate and dissolved organic matter (POM and DOM) is retained effectively in the hyporheic zone, which drives biofilm development and associated microbial activity. 4. The structure of the riparian zone, the mechanisms of POM retention, the hydrological linkages to the stream and the intensity of key biogeochemical processes vary greatly along the river continuum and in relation to the geomorphic setting. However, the present state of knowledge of organic matter metabolism in the hyporheic zone suggests that lateral ecological connectivity is a basic attribute of lotic ecosystems. 5. Due to their efficiency in transforming POM into heterotrophic microbial biomass, attached biofilms form an abundant food resource for an array of predators and grazers in the interstitial environments of rivers and streams. The interstitial microbial loop, and the intensity of microbial production within the bed sediments, may be a primary driver of the celebrated high productivity and biodiversity of the riparian zone. 6. New molecular methods based on the analysis of the low molecular weight RNA (LMW RNA) allow unprecedented insights into the community structure of natural bacterial assemblages and also allow identification and study of specific strains hitherto largely unknown. 7. Research is needed on the development and evaluation of sampling methods for interstitial micro-organisms, on the characterization of biofilm structure, on the analysis of the biodegradable matter in the riparian-hyporheic ecotone, on the regulation mechanisms exerted on microbiota by interstitial predators and grazers, and on measures of microbial respiration and other key activities that influence biogeochemical cycles in running waters. 8. Past experiences from large-scale alterations of riparian zones by humans, such as the River Rhine in central Europe, undeniably demonstrate the detrimental consequences of disconnecting rivers from their riparian zones. A river management approach that uses the natural services of micro-organisms within intact riparian zones could substantially reduce the costs of clean, sustainable water supplies for humans.     

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.     

Of spates and species: responses by interstitial water mites to simulated spates in a subtropical Australian river

The 'hyporheic refuge hypothesis' predicts that the hyporheic zone, the saturated sediments below and alongside rivers and streams, is a refuge from the scouring effects of spates for many aquatic invertebrates including water mites. We tested this hypothesis in two lateral gravel bars and two riffles in a subtropical Australian river by collecting water mites from the hyporheic zone at two depths (10 and 50 cm) at two 'pre-flood' sampling times before experimentally diverting water through the sites for 14 h to simulate a spate. Taxon richness of mites washigh (46 taxa) and dominated by the Prostigmata, with nearly half the species being new to science. Oribatids were also common at the four sites. Samples were collected twice during each 'spate', and again soon after flow was returned to normal. The experimental spate induced changes in the strength and even direction of subsurface-surface water exchange; however, these changes seldom persisted after the experiment, nor after a subsequent natural spate. The hyporheic refuge hypothesis was not supported by our water mite data. Neither during nor shortly after the experimental spates did we find more epigean (surface-dwelling) water mites in downwelling zones where surface streamwater enters the hyporheic zone, demonstrating that these mites were not using the hyporheic zone as a refuge at these locations. There was also no evidence for a 'wash out' effect, because hyporheic mitedensities did not significantly decline late in the spate. Our data indicate that floods of the low magnitude simulated in this study apparently do not pose a lasting disturbance for hypogean water mites. The fact that the same response was found at four sites indicates that the hyporheic refuge hypothesis may not always be an appropriate explanation for rapid post-flood recolonisation. Possibly, the use of the hyporheic zone as a refuge from floods may be dictated by the strength of the disturbance and substrate composition and stability.     

Response of invertebrates to lotic disturbance: a test of the hyporheic refuge hypothesis

Summary Recovery following hydrological disturbances is usually rapid for lotic invertebrates. Stream ecologists have assumed that recovery is facilitated by behavioral migrations during floods down into the hyporheic zone (the interstitial spaces of a streambed) to seek temporary refuge from possible erosion (the hyporheic refuge hypothesis). We provide the first explicit test of this hypothesis by evaluating three predictions of the hypothesis. We coupled field observations of the response of meiofaunal invertebrates to floods with field and flume experiments. The study site was a sandy-bottom stream in northern Virginia. Prediction 1, that loss of fauna from a streambed during floods should be minimal as long as the depth of scour in the streambed is less than the depth of the hyporheic zone, was not supported for any taxon. For two floods which varied considerably in magnitude, 50–90% of the fauna was lost from the bed despite the fact that the depth of scour (10–30 cm) was significantly less than the total depth of the hyporheic zone (50 cm). Prediction 2, that fauna should move deeper into the bed at higher flows, was supported by field observations during only one of two floods and then only for rotifers. In flume experiments that tested for finer scale behavioral movements, significant vertical migrations were found for copepods and chironomids which moved 1.5–3.5 cm downward as mean velocity (3 cm off bottom) was increased from 5–23 cm/s. Movements down by rotifers were not found in the flume experiments. Prediction 3, that the hyporheic zone is the most important source of colonists to defaunated areas, was supported in part by field experiments. The hyporheic route was not the primary route for any taxon but it was as important for the rotifers and copepods as water column or streambed surface routes. We conclude that, even though smallscale (cm's) migrations into the streambed in response to increased flow may be observed for some taxa and the hyporheic zone may serve as a partial source of colonists following disturbances, movements down are not adequate in preventing significant losses of meiofauna during floods.     

Of spates and species: responses by interstitial water mites to simulated spates in a subtropical Australian river

The ‘hyporheic refuge hypothesis’ predicts that the hyporheic zone, the saturated sediments below and alongside rivers and streams, is a refuge from the scouring effects of spates for many aquatic invertebrates including water mites. We tested this hypothesis in two lateral gravel bars and two riffles in a subtropical Australian river by collecting water mites from the hyporheic zone at two depths (10 and 50 cm) at two‘pre-flood’ sampling times before experimentally diverting water through the sites for 14 h to simulate a spate. Taxon richness of mites washigh (46 taxa) and dominated by the Prostigmata, with nearly half the species being new to science. Oribatids were also common at the four sites. Samples were collected twice during each ‘spate’, and again soon after flow was returned to normal. The experimental spate induced changes in the strength and even direction of subsurface-surface water exchange; however, these changes seldom persisted after the experiment, nor after a subsequent natural spate. The hyporheic refuge hypothesis was not supported by our water mite data. Neither during nor shortly after the experimental spates did we find more epigean (surface-dwelling) water mites in downwelling zones where surface streamwater enters the hyporheic zone, demonstrating that these mites were not using the hyporheic zone as a refuge at these locations. There was also no evidence for a ‘wash out’ effect, because hyporheic mitedensities did not significantly decline late in the spate. Our data indicate that floods of the low magnitude simulated in this study apparently do not pose a lasting disturbance for hypogean water mites. The fact that the same response was found at four sites indicates that the hyporheic refuge hypothesis may not always be an appropriate explanation for rapid post-flood recolonisation. Possibly, the use of the hyporheic zone as a refuge from floods may be dictated by the strength of the disturbance and substrate composition and stability.     

山地河流浅滩生境潜流层大型无脊椎动物群落拓殖

通过包埋人工基质法研究大型无脊椎动物在山地河流潜流层中的拓殖过程。结果表明:群落个体密度在7—29 d呈"J"型增长,在29 d后骤然降低,55 d后呈波动趋势;物种丰富度在1—29 d呈增加趋势,29 d后呈波动状态;群落的生物量总体呈增加趋势。群落的物种丰富度、密度和生物量在第29、71和83天时没有显著性差异(P0.05),Shannon-Wiener多样性指数和Pielou均匀度指数在第55、71、83天没有显著性差异(P0.05),综合不同拓殖时间段物种的主成分分析,表明潜流层大型无脊椎动物群落在55 d后趋于稳定。群落优势种为摇蚊(Camptochironomus sp.)、河蚬(Corbicula fluminea)、四节蜉(Baetis sp.)、动蜉(Cinygmina sp.)、纹石蛾(Hydropsyche sp.)和扁泥甲科的一种(Psephenidae)。滤食者和收集者在整个拓殖过程中均是优势功能摄食群。群落拓殖过程是一个群落自身恢复能力和外部环境影响相互作用的过程,拓殖初期潜流层的结构是影响着无脊椎动物迁入的主要因素,中期动物的生活史特征起主要作用,稳定期之后群落可能受到各因素的综合影响。