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.     

Macroconsumer effects on insect detritivores and detritus processing in a tropical stream

1. Few studies have assessed the effects of macroconsumers, such as fishes and shrimps, on detritus and detritivores.
2. We used an underwater electric field to prevent macroconsumers from feeding in and on leaf packs in a lowland stream in Costa Rica and thus to determine their effects on the density of insect detritivores and decay rates of leaves.
3. Exclusion of macroconsumers resulted in significantly higher densities of small invertebrates inhabiting leaf packs. Most of these were collector–gatherers, none were shredders.
4. Despite the increase in invertebrate density, decay rates of leaves were not statistically different. These findings contrast with results from temperate streams showing that increases in the density of invertebrates in leaf packs typically result in an increased rate of decay.
5. Leaf decay rates and invertebrate densities were also compared between leaf packs placed in electric exclusion treatments and those placed in coarse (2 cm) plastic net bags (as used in many previous studies). Our results suggest that using such netting in tropical streams may deter macroconsumers, which can affect insect density and, potentially, decay rates of organic matter.     

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.     

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.     

The breakdown of buried and surface-placed leaf litter in an upland stream

The breakdown of buried leaves (Eucalyptus viminalis) was investigated using surface-placed and buried leaf packs in a riffle of the Acheron River, Victoria. Leaf packs buried to a depth of 10 cm were rapidly colonized by invertebrates, with the total numbers of individuals and species exceeding those on surface leaf packs. A larger proportion of leaves in buried leaf packs was grazed in comparison with those on the surface, with the intensity of grazing also being higher for leaves in buried packs. Both surface and buried leaf packs broke down rapidly with no significant difference in weight loss with time. The high level of breakdown of buried E. viminalis leaf litter observed in this study suggests that the hyporheos of Australian headwater streams may significantly contribute to the decomposition of particulate organic matter.     

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.     

Invertebrate colonization and processing of maple leaf litter in a forested and an agricultural reach of a stream

Processing of maple leaf (Acer saccharum Marsh) packs, their colonization by invertebrates and nutrient dynamics in leaves were investigated in a forested reach and agricultural reach of Canagagigue Creek, Ontario. Shredders, Pycnopsyche, and collectors, Ephemerella subvaria, Stenonema vicarium and Baetis were significantly more numerous in packs at the forest site than in packs at the agricultural site, whereas filter feeders, especially blackflies, were significantly more numerous in packs at the agricultural site. Weight loss of litter packs was nearly equivalent at the two sites. However, there were major differences in the mechanism of processing between the sites. Physical abrasion and microbial activity governed weight loss of maple leaf packs at the agricultural site, whereas processing was governed mainly by microbial and invertebrate activity and, to a much lesser extent, by physical abrasion at the forest site. Both shredders and collector species played an important role in the processing of leaf material at the forest site. Greater uptake of N and P (P<0.05 in spring) and higher C concentrations were observed in leaf packs at the forest site than the agricultural site. Therefore, the results support the concept of retention of nutrients in forested areas and their export in deforested (agricultural) areas. Findings also indicated that the processing of leaf litter is not an efficient means of monitoring changes in stream ecosystems since leaf processing is affected by many factors, particularly physical abrasion.     

Role of transported particulate organic matter in the macroinvertebrate colonization of litter bags in streams

1. The exposure of mesh litter bags has been widely used to investigate the role of benthic macroinvertebrates in leaf processing in freshwaters. In this sense, several studies have related litter bag breakdown rates to the presence of colonizing invertebrates. A possible confounding factor in such experiments is that the litter bags trap suspended particulate organic matter that can itself attract invertebrate colonists, irrespective of the intended experimental treatment.
2. We attempted to quantify the accumulation of particulate organic matter (POM) within litter bags and to investigate its possible impact on macroinvertebrate density and richness. In seven headwater forested streams we exposed mesh bags filled either with beech leaves ( Fagus sylvatica ) or with plastic strips of an equal surface area.
3. Principal component analysis (PCA) showed that bag type and stream were the main explanatory variables for invertebrate colonization and POM accumulation within the bags. In contrast, there was little variation among sampling dates (6.4% of the total inertia).
4. The accumulated POM within the bags was substantial (up to 8.83 g ash-free dry mass (AFDM)) but highly variable among sites (mean from 0.32 to 4.58 g AFDM). At each of the seven sites, both richness and abundance of invertebrates were positively correlated with the mass of accumulated POM in bags. Macroinvertebrate colonization (notably taxon richness) was directly linked with the quantity of POM accumulated.
5. Our findings provide evidence of a potential pitfall in linking invertebrates to litter processing in mesh bags, particularly when large amounts of POM, entrained in stream flow, accumulate within the bags. An evaluation of the POM mass trapped in litter bags could account for the erratic patterns sometimes observed in their colonization by invertebrates.     

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.     

Effects of burial on leaf litter quality,microbial conditioning and palatability to three shredder taxa

Summary 1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf‐shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf‐associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second‐order stream where leaf‐associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C/P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels.     

Estimating the biomass of freshwater mussels (Bivalvia: Unionidae) from shell dimensions

Packs of autumn-shed maple leaves were placed at coal ash effluent-exposed and reference sites in streams on December 5, 1977 and removed after 27 and 96 days. Leaf surface area (cm²/leaf) and disc weight (ash-free dry wt/15 mm disc) were greater at the effluent-exposed site than at the reference site after 96 days (p < .001). ATP content of leaves from the reference stream quadrupled between 27 and 96 days while ATP content of effluent-exposed leaves remained low. Macroinvertebrates colonized the leaf packs in the reference site but were not found on or in effluent-exposed packs. We concluded that leaf processing beyond the leaching of soluble organics did not occur in the effluent-exposed packs owing to reduced colonization and decomposition by fungi. Since stream invertebrates prefer decomposed leaf material and animals grow faster on leaves colonized by microbes, the ash effuent appears to indirectly affect macroinvertebrates by interfering with leaf decomposition and thus reducing the quality of their food.     

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.     

Leaf processing in a woodland stream*

Detritus processing by a small woodland stream is analysed by following the loss of weight of 10 g, single species accumulations of riparian leaves. The daily loss rates are expressed as exponential coefficients after the data are fitted by least squares. Comparisons are made between two sites on a small hardwater trout stream during two seasons. Leaf processing rates form a continuum from a low of 0.5%/day to a high of 2.0%/day. Differences between species of leaf are observed, but significant differences between fall and winter processing and between the two sites are not. The response of the invertebrate community to differences in leaf species is investigated using controlled, artificial streams where significant differences in the effect of the invertebrates are related to the ability ofthe leaf to be processed. Evidence suggests that differential invertebrate colonization of leaf packs is a function of microbial colonization and conditioning. The data are used to develop a general scheme of leaf pack processing.     

Effects of forest clearcutting on leaf breakdown in a southern Appalachian stream

SUMMARY. Effects of forest clearcutting on rates of leaf breakdown were studied in Big Hurricane Branch, a second-order stream located at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains of North Carolina, USA. Breakdown rates of leaves of three tree species were measured in the stream before, during and after the catchment was clearcut. Changes in the stream attributable to logging and associated activities—principally road building—were increased stream flow, increased sediment transport, elevated water temperatures, increased nitrate concentrations and decreased allochthonous organic inputs. Breakdown rates of all three leaf species were slowed during clearcutting and accelerated later. Following logging the breakdown rate of dogwood leaves was equal to the pre-treatment rate, and white oak and rhododendron leaves broke down faster than prior to treatment. We attribute the slow breakdown during treatment to burial of the leaf packs in sediment. Subsequent acceleration may have been due to a lack of alternative food sources for invertebrate detritivores.     

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.     

Leaf breakdown patterns in a NW Italian stream: Effect of leaf type, environmental conditions and patch size

We studied the decomposition process and macroinvertebrate colonisation of leaf packs to determine to what extent leaf consumption and invertebrate abundance depend on the pollution level, season, leaf type and patch size. We exposed 400 leaf packs made of two leaf types, alder and chestnut, at two sites of the Erro River (NW Italy) with different environmental alteration levels. Leaf packs were set out as three patch sizes (alone, or in groups of 6 or 12). A first experiment was carried out in winter and a second in summer. Leaf packs were retrieved after 15, 30, 45 and 60 days of submersion to determine the leaf mass loss and to quantify the associated macroinvertebrates. Natural riverbed invertebrates were collected in the same areas. Patch size, season, leaf type and pollution level significantly affected mass loss. The breakdown process was faster for alder leaves, during summer, at the unpolluted site, and in smaller patches. Leaf type and patch size did not affect macroinvertebrate density and richness, but the highest taxon richness was found in winter and at the unpolluted site. There were more shredders and predators than in the natural riverbed. Our study supports two recent ideas regarding leaf processing in streams: that patch size influences the leaf breakdown rate and that the breakdown rate can be used to evaluate water quality and environmental health.     

The Influence of Pack Size and Position,Leaf Type,and Shredder Access on the Processing Rate of Atherosperma moschatum Leaves in an Australian Cool Temperate Rainforest Stream

Factors affecting the processing rate of packs of southern sassafras (Atherosperma moschatum) leaves were investigated in an Australian cool temperate rainforest stream pool. Processing rate was strongly influenced by pack weight fitting a linear inverse relationship. Processing was not significantly (p>0.05) affected by whether the packs were placed on bricks or free on the stream bed. Free packs, but not packs on bricks, were processed more rapidly near the bank than in midstream (p = 0.050). Packs placed in 300 μm mesh bags, with one side unsealed were processed at the same rate as packs attached to bricks or free on the stream bed, but packs in sealed 300 μm mesh bags were processed significantly more slowly (p<10⁻⁴). Abscissed leaves were processed significantly more slowly than fresh leaves (p<10⁻⁵) and at a rate not significantly different to leaves sealed in mesh bags (p = 0.197). There was no consistent pattern of difference between processing rates in summer and winter. The results indicate that the size of litter packs may be a more important factor in influencing processing rate estimates than the method of attachment of the packs. The slow rate of processing of abscissed leaves compared with fresh leaves indicates that they were at most, slightly influenced by stream shredders. The absence of a consistent pattern of difference between summer and winter processing rates is consistent with the results of previous litter processing studies from southeastern Australia although no clear causal factor can be identified.     

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

Nitrogen (N) retention in streams is an important ecosystem service that may be affected by the widespread burial of streams in stormwater pipes in urban watersheds. We predicted that stream burial suppresses the capacity of streams to retain nitrate (NO₃ ^?) by eliminating primary production, reducing respiration rates and organic matter availability, and increasing specific discharge. We tested these predictions by measuring whole-stream NO₃ ^? removal rates using ¹⁵NO₃ ^? isotope tracer releases in paired buried and open reaches in three streams in Cincinnati, Ohio (USA) during four seasons. Nitrate uptake lengths were 29 times greater in buried than open reaches, indicating that buried reaches were less effective at retaining NO₃ ^? than open reaches. Burial suppressed NO₃ ^? retention through a combination of hydrological and biological processes. The channel shape of two of the buried reaches increased specific discharge which enhanced NO₃ ^? transport from the channel, highlighting the relationship between urban infrastructure and ecosystem function. Uptake lengths in the buried reaches were further lengthened by low stream biological NO₃ ^? demand, as indicated by NO₃ ^? uptake velocities 17-fold lower than that of the open reaches. We also observed differences in the periphyton enzyme activity between reaches, indicating that the effects of burial cascade from the microbial to the ecosystem scale. Our results suggest that stream restoration practices involving “daylighting” buried streams have the potential to increase N retention. Further work is needed to elucidate the impacts of stream burial on ecosystem functions at the larger stream network scale.     

Effects of crayfish on leaf processing and invertebrate colonisation of leaves in a headwater stream: decoupling of a trophic cascade

I performed a field experiment to test the hypotheses that omnivorous crayfish both promote breakdown of leaves (basal resources) and decouple any potential trophic cascade by simultaneously affecting intermediate consumers as well as their basal resource. Leaf packs were placed inside in situ artificial channels, which excluded or allowed access to crayfish. During a 4-week period, crayfish greatly promoted leaf processing, with decomposition rates among the fastest ever recorded from temperate streams. Crayfish also affected invertebrate abundance in the leaf packs. As a result of resource consumption, predation and bioturbation, crayfish treatments contained significantly lower densities of invertebrates. In contrast, exclusion of crayfish did not promote leaf decay via increased colonisation by detritivores, primarily because of the conspicuous lack of shredder insects in New Zealand streams. The results support the hypothesis that omnivorous top consumers decouple cascading chains through simultaneous direct and indirect effects on intermediate consumers and basal resources. Decapod consumers, which have been largely ignored in leaf decomposition studies, can be key leaf processors in temperate streams where shredder insects are poorly represented. Received: 8 February 2000 / Accepted: 14 April 2000     

Comparisons of the Colonisation by Invertebrates of Three Species of Wood,Alder Leaves,and Plastic “Leaves” in a Temperate Stream

Small woody debris in streams is abundant, and may be a food source or may provide a substrate on which other food sources such as biofilm may develop, both of which may be significant to invertebrates in times of food scarcity. We examined patterns of invertebrate colonisation of small woody debris (veneers of red alder, Douglas‐fir, and western red cedar), red alder leaves, and plastic (as an inert substrate to mimic leaves). Invertebrate colonisation was high on alder leaves, but low on wood substrates and plastic, controlling for the available surface area. Detritivorous invertebrates had significantly higher colonisation rates of alder leaves versus the other four substrates, whereas predators and collectors did not (consistent with their use of these as substrates and not food). All wood decreased in mass by <15% and leaves by ∼50% over the 75 days of the experiment. For all taxa tested, there was no significant difference in their colonisation of the wood veneers versus the plastic sheets. These results suggest that wood was not directly used by these invertebrates as a food source, or that there could be similar biofilm development on the surfaces of these substrates. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)