Raised water temperature lowers diversity of hyporheic aquatic hyphomycetes

1. The hyporheic zone of a permanent first‐order stream was divided into a treatment and a control section using a 1 m deep sheet‐metal barrier. During a 4‐month pre‐treatment period, water temperatures in two transects of the two sections were not different. Upon heating, the water temperature in the treatment transect increased by an average of 4.3 °C over values in the control transect. 2. Eleven bimonthly core samples were taken from a treatment and a control transect, and recovered CPOM was classified as twigs, wood, grass, roots, cedar and deciduous leaves. 3. In both transects, twigs were the most common and deciduous leaves the least common substrates. The number of leaf fragments declined significantly in the heat‐treated transect. 4. Diversity and frequencies of occurrence of aquatic hyphomycetes were highest on leaves and lowest on grass and wood. On leaves, their frequency of occurrence was higher in control than in treatment samples. 5. Preliminary results with amplified and cloned 18S DNA sequences revealed many fungal taxa with high affinities to Basidiomycota, particularly to Limnoperdon incarnatum. 6. By itself, higher water temperature due to global warming is likely to lower the availability of substrates for, and therefore the occurrence of, aquatic hyphomycetes.     

Seasonal and substrate preferences of fungi colonizing leaves in streams: traditional versus molecular evidence

Aquatic hyphomycetes are the main fungal decomposers of plant litter in streams. We compared the importance of substrate (three leaf species, wood) and season on fungal colonization. Substrates were exposed for 12 4-week periods. After recovery, mass loss, fungal biomass and release of conidia by aquatic hyphomycetes were measured. Fungal communities were characterized by counting and identifying released conidia and by extracting and amplifying fungal DNA (ITS2), which was subdivided into phylotypes by denaturing gradient gel electrophoresis (DGGE) and terminal-restriction fragment length polymorphism (T-RFLP). Mass loss, fungal biomass and reproduction were positively correlated with stream temperature. Conidial diversity was highest between May and September. Numbers of different phylotypes were more stable. Principal coordinate analyses (PCO) and canonical analyses of principal coordinates (CAP) of presence/absence data (DGGE bands, T-RFLP peaks and conidial species) showed a clear seasonal trend (Por=0.88). Season was also a significant factor when proportional similarities of conidial communities or relative intensities of DGGE bands were evaluated (P相似文献   

Taxon-specific fungal primers reveal unexpectedly high diversity during leaf decomposition in a stream

Traditional techniques for studying the fungal community composition in streams favour the detection and identification of aquatic hyphomycetes. Our objective was to use molecular techniques to determine the presence and contributions of other fungal groups. We designed primers specific for the ITS regions in Ascomycota, Basidiomycota, Chytridiomycota, Zygomycota and Oomycota. The primers were used to amplify DNA from linden, maple, and beech leaves, and birch wood submerged in a stream for 4 weeks in summer, autumn, winter and spring. The amplification products were separated by denaturing gradient gel electrophoresis. Ascomycota were present in large phylotype numbers (up to 21) on all substrates and all dates and represented ≥ 75 % of the fungal biomass. Basidiomycota were the second most abundant group in summer and autumn (up to 13 % on wood) and were absent only on linden and maple in spring. There were consistently large numbers of phylotypes from Chytridiomycota and their relative contribution to the microbial community peaked in winter on all substrates. Oomycota were present in summer and abundant only on wood. Zygomycota were present in low numbers and their estimated contribution to fungal biomass was ≤ 1%. Using primers to target individual groups facilitates a more balanced approach to studying fungal diversity in freshwater ecosystems.     

Aquatic Hyphomycete Species Are Screened by the Hyporheic Zone of Woodland Streams

Aquatic hyphomycetes strongly contribute to organic matter dynamics in streams, but their abilities to colonize leaf litter buried in streambed sediments remain unexplored. Here, we conducted field and laboratory experiments (slow-filtration columns and stream-simulating microcosms) to test the following hypotheses: (i) that the hyporheic habitat acting as a physical sieve for spores filters out unsuccessful strategists from a potential species pool, (ii) that decreased pore size in sediments reduces species dispersal efficiency in the interstitial water, and (iii) that the physicochemical conditions prevailing in the hyporheic habitat will influence fungal community structure. Our field study showed that spore abundance and species diversity were consistently reduced in the interstitial water compared with surface water within three differing streams. Significant differences occurred among aquatic hyphomycetes, with dispersal efficiency of filiform-spore species being much higher than those with compact or branched/tetraradiate spores. This pattern was remarkably consistent with those found in laboratory experiments that tested the influence of sediment pore size on spore dispersal in microcosms. Furthermore, leaves inoculated in a stream and incubated in slow-filtration columns exhibited a fungal assemblage dominated by only two species, while five species were codominant on leaves from the stream-simulating microcosms. Results of this study highlight that the hyporheic zone exerts two types of selection pressure on the aquatic hyphomycete community, a physiological stress and a physical screening of the benthic spore pool, both leading to drastic changes in the structure of fungal community.     

Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams

1. Leaf litter constitutes the major source of organic matter and energy in woodland stream ecosystems. A substantial part of leaf litter entering running waters may be buried in the streambed as a consequence of flooding and sediment movement. While decomposition of leaf litter in surface waters is relatively well understood, its fate when incorporated into river sediments, as well as the involvement of invertebrate and fungal decomposers in such conditions, remain poorly documented. 2. We tested experimentally the hypotheses that the small interstices of the sediment restrict the access of the largest shredders to buried organic matter without compromising that of aquatic hyphomycetes and that fungal decomposers in the hyporheic zone, at least partly, compensate for the role of invertebrate detritivores in the benthic zone. 3. Alder leaves were introduced in a stream either buried in the sediment (hyporheic), buried after 2 weeks of exposure at the sediment surface (benthic‐hyporheic), or exposed at the sediment surface for the entire experiment (benthic). Leaf decomposition was markedly faster on the streambed surface than in the two other treatments (2.1‐ and 2.8‐fold faster than in the benthic‐hyporheic and hyporheic treatments, respectively). 4. Fungal assemblages were generally less diverse in the hyporheic habitat with a few species tending to be relatively favoured by such conditions. Both fungal biomass and sporulation rates were reduced in the hyporheic treatment, with the leaves subject to the benthic‐hyporheic treatment exhibiting an intermediate pattern. The initial 2‐week stage in the benthic habitat shaped the fungal assemblages, even for leaves later subjected to the hyporheic conditions. 5. The abundance and biomass of shredders drastically decreased with burial, except for Leuctra spp., which increased and was by far the most common leaf‐associated taxon in the hyporheic zone. Leuctra spp. was one of the rare shredder taxa displaying morphological characteristics that increased performance within the limited space of sediment interstices. 6. The carbon budgets indicated that the relative contributions of the two main decomposers, shredders and fungi, varied considerably depending on the location within the streambed. While the shredder biomass represented almost 50% of the initial carbon transformed after 80 days in the benthic treatment, its contribution was <0.3% in the hyporheic one and 2.0% in the combined benthic‐hyporheic treatment. In contrast, mycelial and conidial production in the permanently hyporheic environment accounted for 12% of leaf mass loss, i.e. 2–3 times more than in the two other conditions. These results suggest that the role of fungi is particularly important in the hyporheic zone. 7. Our findings indicate that burial within the substratum reduces the litter breakdown rate by limiting the access of both invertebrate and fungal decomposers to leaves. As a consequence, the hyporheic zone may be an important region of organic matter storage in woodland streams and serve as a fungal inoculum reservoir contributing to further dispersal. Through the temporary retention of litter by burial, the hyporheic zone must play a significant role in the carbon metabolism and overall functioning of headwater stream ecosystems.     

Sources and seasonal patterns of dissolved organic matter (DOM) in the hyporheic zone

The hyporheic zone is a region underneath streambeds that integrates surface and groundwater. Although its location is central to biogeochemical linkages between the riparian zone, dissolved nutrients, and benthic biota, the seasonal quality and likely sources of dissolved organic matter (DOM) in the hyporheic zone are not well understood. To investigate DOM characteristics in the hyporheic zone, water from the surface and subsurface (at depths 20, 60, and 100 cm below the streambed) was sampled every 4 weeks from 2007 to 2008 in a third-order stream in southern Ontario. Using UV spectroscopy, measures of spectral slopes, aromaticity, and A ₂₅₄/A ₃₆₅ ratios (molecular weight) were obtained. Temporal changes in these measures were consistent with watershed processes such as shedding of leaf litter in the fall, and photochemical and biofilm influence in the spring and summer. The fluorescence index (a measure of relative DOM source) suggested that at the surface and in the downwelling zone, DOM microbial sources increased with depth in the sediment, regardless of the season. Excitation–emission matrices (EEMs) showed seasonally distinct, protein-like DOM components of bacterial origin that were stronger in the fall. Leachates from specific allochthonous DOM sources—leaf litter from Betula papyrifera (white birch) and Thuja occidentalis (white cedar)—and an autochthonous source, biofilm, were isolated and incubated with unfiltered surface water. EEMs from these leachates indicated that these sources could indeed help explain observed patterns of DOM in surface and subsurface waters. These results suggest that although DOM sources were relatively constant, biogeochemical processing within the hyporheic zone resulted a DOM pool that was temporally dynamic and altered the nature of organic matter transported downstream into lakes and rivers.     

Fungal Propagules and DNA in Feces of Two Detritus-Feeding Amphipods

Aquatic shredders (leaf-eating invertebrates) preferentially ingest and digest leaves colonized by aquatic hyphomycetes (fungi). This activity destroys leaf-associated fungal biomass and detritial resources in streams. Fungal counter-adaptations may include the ability to survive passage through the invertebrate's digestive tract. When fecal pellets of Gammarus tigrinus and Hyalella azteca were incubated with sterile leaves, spores of nine (G. tigrinus) and seven (H. azteca) aquatic hyphomycete species were subsequently released from the leaves, indicating the presence of viable fungal structures in the feces. Extraction, amplification, and sequencing of DNA from feces revealed numerous fungal phylotypes, two of which could be assigned unequivocally to an aquatic hyphomycete. The estimated contributions of major fungal groups varied depending on whether 18S or ITS sequences were amplified and cloned. We conclude that a variable proportion of fungal DNA in the feces of detritivores may originate from aquatic hyphomycetes. Amplified DNA may be associated with metabolically active, dormant, or dead fungal cells.     

Lignocellulolytic Enzyme Production by Aquatic Hyphomycetes Species Isolated from the Nile's Delta Region

Twenty-six species of aquatic hyphomycetes were isolated from woody sources (unidentified wood segments, leaf skeletons and neck of leaves and bark) in the North River Nile (Delta region). Alatospora acuminata, Anguillospora crassa, Flagellaspora penicillioides, Lunulospra curvula, Tetracladium marchalianum and Triscelophorus monosporus were the most common species. Temperature was the highest physico-chemical parameter affecting the aquatic hyphomycetes occurrence. Twelve species of hyphomycetes, isolated from woody substrates, were screened for their ability to produce extracellular lignocellulolytic enzymes on solid media. The enzymes tested included: endoglucanase, endoxylanase, beta-glucosidase, laccase, peroxidase, polyphenoloxidase, tyrosinase and beta-xylosidase. Three species, A. acuminata, F. penicillioides, T. monosporus, were positive for all tested enzymes. Also, A. longissima was positive for all enzymes except lignin-peroxidase. The ability to produce cellulase was 100% for all species while only, four species were positive for lignin-peroxidase. The ability of the species to produce other lignocellulotic enzyme ranged from 50% to 83%. Freshwater hyphomycetes have been shown to produce a rich array of enzymes able to degrade the polysaccharides of plant debris.     

An application of the plaster dissolution method for quantifying water velocity in the shallow hyporheic zone of an Appalachian stream system

1. A method for quantifying interstitial water velocity based on the dissolution rate of plaster of Paris standards was developed as part of a study of vertical, longitudinal (1–4 order sites) and seasonal variation in the biotic and physical characteristics of the shallow hyporheic zone (0–30 cm) of a headwater stream system in West Virginia, U.S.A.
2. A calibration model was developed using a water velocity simulation tank to relate mass loss of plaster standards to water velocity and temperature. The model was then used to calculate water velocity through artificial substrata embedded in the shallow hyporheic zone of four stream reaches based on in situ mass loss of plaster standards.
3. Water velocity in the hyporheic zone increased with stream order, was highest in early spring and winter during high stream base flows, and decreased with depth into the substratum. There was a strong interaction between depth and season: during periods of high stream discharge, water velocity through the upper level of the shallow hyporheic zone (0–10 cm into the substrate) increased disproportionately more than velocity at greater depths. Mean interstitial velocity in March ranged from 0 cm s^–1 in the lowest level (20–30 cm) to 3.5 cm s^–1 at the upper level (0–10 cm) at the first‐order site, and from 2.5 cm s^–1 (20–30 cm) to 9.5 cm s^–1 (0–10 cm s^–1) at the fourth‐order site. Gradients in stream discharge and sediment permeability accounted for treatment effects.
4. Use of calibrated data improved the ability to resolve among‐season differences in interstitial water movement over the use of uncalibrated mass loss data. For some applications of the plaster standard method, empirical calibration may not be necessary.     

Effects of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM) on the macroinvertebrates within the hyporheic region of a small stream

The effect of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM), on the benthic macroinvertebrates within the hyporheic region of Dam Creek, Ontario was investigated. Organisms were regularly found to a depth of 70 cm in the substrate throughout the study period. Of the eight major taxa found at both the treated and untreated sites only Tubificoidea exhibited a decrease in abundance attributable to TFM.One day after treatment, TFM concentrations were greatest at a depth of 55 cm, the greatest depth to which water samples were taken. The movement of TFM into the hyporheic region during the present study may be due to the large convective forces created by the rapid decrease in surface water temperature. These convective forces are usually greatest in the late fall and winter when TFM is not applied. The greater part of TFM stream treatments are carried out during the summer when surface water temperatures remain fairly constant. It is suggested that the hyporheic region may act as a refuge zone for benthic macroinvertebrates against the lampricide during most of the TFM application season.     

Exotic riparian vegetation lowers fungal diversity but not leaf decomposition in Portuguese streams

1. We characterised the fungal communities of eight streams in Portugal, four bordered by native deciduous forest and four bordered by pure stands of Eucalyptus globulus .
2. Aquatic hyphomycete species richness and evenness, but not numbers of water-borne conidia, of aquatic hyphomycetes were significantly lower in eucalypt bordered streams.
3. Multivariate analyses subdivided the fungal communities into two distinct groups corresponding to riparian vegetation.
4. Despite these differences in the dominant decomposer community, decay rates of eucalypt leaves (accounting for ≥98% of naturally occurring leaves in eucalypt bordered streams, absent in native forest) and chestnut leaves (occurring naturally in native forests) did not differ between the two groups of streams.     

Importance of transient storage zones for ammonium and phosphate retention in a sandy-bottom Mediterranean stream

1. The ability of hyporheic sediments to exchange water and retain ammonium and phosphate in the Riera Major stream ,North-East Spain, under different discharge conditions was measured by conducting short-term nutrient and chloride additions. 2. The mean exchange coefficients from free-flowing water to the storage zone (k₁) and vice versa (k₂) were 0.82 × 10–4 s^??1 and 7 × 10^??3 s^??1, respectively. The ratio of storage zone cross-sectional area to stream cross-sectional area (A_S/A) averaged 2.8 × 10–2 and was negatively correlated with discharge (r = –0.85, d.f. = 13, P < 0.001). 3. The percentage of hyporheic zone water which came from surface water varied as a function of discharge and hyporheic depth, ranging between 33% and 95% at 25 cm depth, and between 78% and 100% at 10 cm depth. 4. The nutrient retention efficiency in the hyporheic zone at 10 cm depth measured as uptake length (S_wh) was less than 3.3 cm for ammonium and 37 cm for phosphate. Higher nutrient retentions were measured in the sediments at 10 cm depth than at 25 cm, indicating that near-surface sediments were involved more actively in phosphate retention than the deeper hyporheic sediments. The lack of ammonium at any depth of the hyporheic zone showed that ammonium was very rapidly taken up in the surfacial sediments.     

Fungal communities on decaying leaves in streams: a comparison of two leaf species

Decomposition of leaf litter is a microbial mediated process that helps to transfer energy and nutrients from leaves to higher trophic levels in woodland streams. Generally, aquatic hyphomycetes are viewed as the major fungal group responsible for leaf litter decomposition. In this study, traditional microscopic examination (based on identification of released conidia) and phylogenetic analysis of 18S rRNA genes from cultivated fungi were used to compare fungal community composition on decomposing leaves of two species (sugar maple and white oak) from a NE Ohio stream. No significant differences were found in sporulation rates between maple and oak leaves and both had similar species diversity. From the 18S rRNA gene sequence data, identification was achieved for 12 isolates and taxonomic affiliation of 12 of the remaining 14 isolates could be obtained. A neighbor-joining tree (with bootstrap values) was constructed to examine the taxonomic distribution of the isolates relative to sequences of known operational taxonomic units (OTUs). Surprisingly, only 2 of the isolates obtained were aquatic hyphomycetes based on phylogenetic analysis. Overall, there were no differences between the two leaf types and a higher diversity was observed via culturing and subsequent 18S rRNA gene sequencing than by conidia staining. These differences resulted from the fact that traditional microscopy provides estimates of aquatic hyphomycete diversity while the other approach revealed the presence of both aquatic hyphomycete and non-aquatic hyphomycete taxa. The presence of this broad array of taxa suggests that the role of aquatic hyphomycetes relative to other fungi be re-evaluated. Even though the functional role of these non-aquatic hyphomycetes taxa is unknown, their presence and diversity demonstrates the need to delve further into fungal community structure on decomposing leaves.     

Reproductive ecology of the freshwater red alga Batrachospermum boryanum Sirodot in a temperature headwater stream

The surface and hyporheic fauna of a second-order reach of a northern California Coast Range stream (Big Canyon Creek, Lake Co., CA, USA) was examined using substrate colonization samplers (i.e. hyporheic pots) during both the wet and dry seasons in the prevailing Mediterranean climate of the region. In terms of total number of macroinvertebrate taxa, the surface (0 cm to −5.0 cm within the stream substrate) level had higher richness than any level within the hyporheic (−5.1 cm to −15.0 cm, −15.1 cm to −25.0 cm, or −25.1 cm to −35.0 cm) from the beginning of the dry season (May) to the beginning of the wet season (October); during this period the surface was not subject to disturbance from wet-season storms. During the wet season, richness at the surface was similar to that observed at any of the three hyporheic levels examined. Macroinvertebrate density at the surface was substantially reduced during the wet season; however, mean surface densities always exceeded those found within the hyporheic zone (from 25% to 78% of total macroinvertebrate numbers were found at the surface). Seasonal fluctuations in abundance of total macroinvertebrates and density of many constituent populations were less within the hyporheic zone than at the surface. Apparently, early instars of abundant surface taxa do not penetrate the interstices, and substrate disturbance due to spates is less in deeper levels. Compared with many hyporheic faunas described from other temperate-zone sites, this hyporheic community shows reduced numerical dominance by the Chironomidae.     

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.     

The ecology of aero-aquatic hyphomycetes

We observed 35 species of aero-aquatic hyphomycetes belonging to 21 genera and 4 unidentified taxa of the Clavariopsis, Dactilella, Helicoon, and Tetracladium genera. Substrate preferences were detected in some species. Species such as Anguillospora longissima, Tetracladium marchalianum, and Fusarium aqueductum were found on different types of substrata. Only 11 species are developed on pine needles, while 20 species were observed on leaves. Eight species of aquatic hyphomycetes (Alatospora acuminata, Angullospora aquatica, Lemonniera aquatica, Tetracladium setigerum, Tricladium angulatum, Tripospermum campelopardus, Septonema secedens, and Spermospora sp.) were revealed on leaves taken from water habitats, whereas 15 species were found in litter. The jointly grown species did not demonstrate any inhibiting effects on each other except for Tripospermum campelopardus, which inhibited the growth of other species (Table 3). The microcyclic development of a Dactilella sp. was detected for the first time while being incubated on leaves in the laboratory. The frequency of appearance changes, and the diversity of the species of aquatic hyphomycetes appears to be higher on intact leaves than on skeletonized ones. The seasonal dynamics of aquatic hyphomycetes with two peaks of mass conidia development (vernal and sharper autumnal) were described. Some species were observed throughout the entire vegetation season. The conidial development in the leaf samples incubated in the laboratory lasted for 30–40 days. Therefore, the method of incubation for 7–10 days recommended in the literature did not allow for the complete investigation of species diversity in aquatic hyphomycetes.     

Identification of Methanogenic archaea in the Hyporheic Sediment of Sitka Stream

Methanogenic archaea produce methane as a metabolic product under anoxic conditions and they play a crucial role in the global methane cycle. In this study molecular diversity of methanogenic archaea in the hyporheic sediment of the lowland stream Sitka (Olomouc, Czech Republic) was analyzed by PCR amplification, cloning and sequencing analysis of the methyl coenzyme M reductase alpha subunit (mcrA) gene. Sequencing analysis of 60 clones revealed 24 different mcrA phylotypes from hyporheic sedimentary layers to a depth of 50 cm. Phylotypes were affiliated with Methanomicrobiales, Methanosarcinales and Methanobacteriales orders. Only one phylotype remains unclassified. The majority of the phylotypes showed higher affiliation with uncultured methanogens than with known methanogenic species. The presence of relatively rich assemblage of methanogenic archaea confirmed that methanogens may be an important component of hyporheic microbial communities and may affect CH₄ cycling in rivers.     

Whole-stream nitrate addition affects litter decomposition and associated fungi but not invertebrates

We assessed the effect of whole-stream nitrate enrichment on decomposition of three substrates differing in nutrient quality (alder and oak leaves and balsa veneers) and associated fungi and invertebrates. During the 3-month nitrate enrichment of a headwater stream in central Portugal, litter was incubated in the reference site (mean NO₃-N 82 μg l⁻¹) and four enriched sites along the nitrate gradient (214–983 μg NO₃-N l⁻¹). A similar decomposition experiment was also carried out in the same sites at ambient nutrient conditions the following year (33–104 μg NO₃-N l⁻¹). Decomposition rates and sporulation of aquatic hyphomycetes associated with litter were determined in both experiments, whereas N and P content of litter, associated fungal biomass and invertebrates were followed only during the nitrate addition experiment. Nitrate enrichment stimulated decomposition of oak leaves and balsa veneers, fungal biomass accrual on alder leaves and balsa veneers and sporulation of aquatic hyphomycetes on all substrates. Nitrate concentration in stream water showed a strong asymptotic relationship (Michaelis–Menten-type saturation model) with temperature-adjusted decomposition rates and percentage initial litter mass converted into aquatic hyphomycete conidia for all substrates. Fungal communities did not differ significantly among sites but some species showed substrate preferences. Nevertheless, certain species were sensitive to nitrogen concentration in water by increasing or decreasing their sporulation rate accordingly. N and P content of litter and abundances or richness of litter-associated invertebrates were not affected by nitrate addition. It appears that microbial nitrogen demands can be met at relatively low levels of dissolved nitrate, suggesting that even minor increases in nitrogen in streams due to, e.g., anthropogenic eutrophication may lead to significant shifts in microbial dynamics and ecosystem functioning. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

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.     

Small-sized invertebrates in a gravel stream: community structure and variability of benthic rotifers

1. The Rotifera assemblage inhabiting the streambed surface and the hyporheic zone of a gravel stream was investigated between October 1991 and October 1992. Forty-two species of Monogononta and 27 of Bdelloidea were identified. Within these two classes, dominant species differed between the surface and the hyporheic zone. At the streambed surface, the abundance of monogonont rotifers showed a seasonal pattern with significantly higher densities in pools, whereas bdelloids showed no clear temporal trend and did not differ significantly among sites. In the hyporheic zone, the depth distribution differed among the two rotifer groups, bdelloids occurred in highest densities between 0 and 30 cm sediment depth, while monogononts were most abundant at greater depths.
2. Species composition differed greatly between successive sampling dates (min. 5 to max. 26 days) at both the streambed surface and the hyporheos. At the streambed surface and in the shallow hyporheos a significantly higher percentage of species was replaced in riffles than in pools.
3. Few measured hydrophysical variables were associated with the Rotifera assemblage structure. At the streambed surface, species richness was negatively correlated with water temperature and substratum heterogeneity, and Monogononta rotifer densities declined with water depth and substratum roughness.
4. Permutation tests carried out on temporal serial correlations showed that, at riffle sites at the streambed surface, bdelloid rotifer densities, rotifer species richness and diversity did not differ significantly from a temporal, near-random pattern. The hyporheic rotifer assemblage followed similar near-random patterns.