Movement patterns of invertebrates in temporary and permanent streams

Summary Although it has been shown that invertebrates recolonize reflooded temporary streams from permanent refuges, e.g., the hyporheic zone, it has not been shown that they actively move into these refuges as streams dry. Substrate filled cages and drift nets were used to monitor invertebrate movement in two temporary streams and a permanent stream prior to and during drying to determine whether invertebrates leave drying riffles and enter flooded riffles. Invertebrate movement was essentially unidirectional in the permanent stream with downstream drift and with-in-substrate downstream movement dominating. In the temporary stream, movement vertically downward toward the hyporheic zone and upstream movement substantially contributed to a departure from a unidirectional pattern. In addition, prior to stream drying the relative colonization rate was higher and drift rate was lower in the temporary streams than in the permanent stream. During drying of the temporary stream, upstream movement continued to dominate but hyporheic movement was unimportant. Further, the upstream movement did not occur at the end of the riffle where it would lead to migration into non-drying riffles. Thus, even though movement patterns were different in permanent and temporary streams the pattern observed during stream drying would result in the concentration and subsequent death of invertebrates in drying riffles. This observation demonstrates that movement patterns of stream invertebrates do not necessarily result in behavioral avoidance of a dry period of temporary fiffles.     

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

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

Characterizing seasonal changes in physicochemistry and bacterial community composition in hyporheic sediments

The hyporheic zone of stream ecosystems is a critical habitat for microbial communities. However, the factors influencing hyporheic bacterial communities along spatial and seasonal gradients remain poorly understood. We sought to characterize patterns in bacterial community composition among the sediments of a small stream in southern Ontario, Canada. We used sampling cores to collect monthly hyporheic water and sediment microbial communities in 2006 and 2007. We described bacterial communities terminal-restriction fragment length polymorphism (TRFLP) and tested for spatial and seasonal relationships with physicochemical parameters using multivariate statistics. Overall, the hyporheic zone appears to be a DOC, oxygen, and nitrogen sink. Microbial communities were distinct from those at the streambed surface and from soil collected in the adjacent watershed. In the sediments, microbial communities were distinct between the fall, spring, and summer seasons, and bacterial communities were more diverse at streambed surface and near-surface sites compared with deeper sites. Moreover, bacterial communities were similar between consecutive fall seasons despite shifting throughout the year, suggesting recurring community assemblages associated with season and location in the hyporheic zone. Using canonical correspondence analysis, seasonal patterns in microbial community composition and environmental parameters were correlated in the following way: temperature was related to summer communities; DOC (likely from biofilm and allochthonous inputs) influenced most fall communities; and nitrogen associated strongly with winter and spring communities. Our results also suggest that labile DOC entering the hyporheic zone occurred in concert with shifts in the bacterial community. Generally, seasonal patterns in hyporheic physicochemistry and microbial biodiversity remain largely unexplored. Therefore, we highlight the importance of seasonal and spatial resolution when assessing surface- and groundwater interactions in stream ecosystems.     

Spatial and temporal variability of macroinvertebrates in spawning and non-spawning habitats during a salmon run in Southeast Alaska

Spawning salmon create patches of disturbance through redd digging which can reduce macroinvertebrate abundance and biomass in spawning habitat. We asked whether displaced invertebrates use non-spawning habitats as refugia in streams. Our study explored how the spatial and temporal distribution of macroinvertebrates changed during a pink salmon (Oncorhynchus gorbuscha) spawning run and compared macroinvertebrates in spawning (riffle) and non-spawning (refugia) habitats in an Alaskan stream. Potential refugia included: pools, stream margins and the hyporheic zone, and we also sampled invertebrate drift. We predicted that macroinvertebrates would decline in riffles and increase in drift and refugia habitats during salmon spawning. We observed a reduction in the density, biomass and taxonomic richness of macroinvertebrates in riffles during spawning. There was no change in pool and margin invertebrate communities, except insect biomass declined in pools during the spawning period. Macroinvertebrate density was greater in the hyporheic zone and macroinvertebrate density and richness increased in the drift during spawning. We observed significant invertebrate declines within spawning habitat; however in non-spawning habitat, there were less pronounced changes in invertebrate density and richness. The results observed may be due to spawning-related disturbances, insect phenology, or other variables. We propose that certain in-stream habitats could be important for the persistence of macroinvertebrates during salmon spawning in a Southeast Alaskan stream.     

Field and Microcosm Studies of Decomposition and Soil Biota in a Cold Desert Soil

In the extreme cold desert soil of the McMurdo Dry Valleys of Antarctica, we studied the effects of changing moisture and temperature on rates of decomposition and the activity and abundance of soil organisms. Our objective was to understand how moisture and temperature structure invertebrate communities and control important ecosystem processes and soil biotic activity in this extreme environment. First, in a field experiment, we manipulated soil moisture and temperature and compared cotton strip decomposition rates at two dry valley sites with different moisture regimes. At both sites, live nematode abundance and activity were unchanged by soil treatments over the 2-year study. In the same plots, the cotton strips did not decompose, despite soil warming and the addition of moisture. The results suggest that biological activity in the McMurdo Dry Valleys is severely limited and that soil organisms are not responsive to improving environmental conditions. Second, in microcosms, we manipulated dry valley soil moisture at a constant temperature of 10°C and measured the rates of key soil processes. Soil respiration, nitrification, and the decomposition of cotton strips were all greater in dry valley soils that were wetted to 10% moisture content, as compared to soils at 0.6%. These results indicate that the decomposition potential for dry valley soils is high when moisture and temperature limitations are removed. In the field, however, this process was extremely slow, and biota did not respond to improving environmental conditions. Soil processes appear to be limited primarily by the extreme desiccation of the dry valleys. Ecosystems processes are likely restricted to the brief periods following infrequent snowfall, melt, and soil wetting that permit the activity of soil microbes and biota. Received 23 May 2001; Accepted 7 September 2001.     

Bacterial community dynamics in the hyporheic zone of an intermittent stream

The dynamics of in situ bacterial communities in the hyporheic zone of an intermittent stream were described in high spatiotemporal detail. We assessed community dynamics in stream sediments and interstitial pore water over a two-year period using terminal-restriction fragment length polymorphism. Here, we show that sediments remained saturated despite months of drought and limited hydrologic connectivity. The intermittency of stream surface water affected interstitial pore water communities more than hyporheic sediment communities. Seasonal changes in bacterial community composition was significantly associated with water intermittency, phosphate concentrations, temperature, nitrate and dissolved organic carbon (DOC) concentrations. During periods of low- to no-surface water, communities changed from being rich in operational taxonomic units (OTUs) in isolated surface pools, to a few OTUs overall, including an overall decline in both common and rare taxa. Individual OTUs were compared between porewater and sediments. A total of 19% of identified OTUs existed in both porewater and sediment samples, suggesting that bacteria use hyporheic sediments as a type of refuge from dessication, transported through hydrologically connected pore spaces. Stream intermittency impacted bacterial diversity on rapid timescales (that is, within days), below-ground and in the hyporheic zone. Owing to the coupling of intermittent streams to the surrounding watershed, we stress the importance of understanding connectivity at the pore scale, consequences for below-ground and above-ground biodiversity and nutrient processing, and across both short- and long-time periods (that is, days to months to years).     

Invertebrate community patterns in Mediterranean temporary wetlands along hydroperiod and salinity gradients

1. Temporary aquatic habitats often are inhabited by a unique fauna and flora and contribute significantly to regional diversity. Temporary wetlands around the world are disappearing rapidly. The individual and interacting impacts of factors influencing community structure and dynamics in temporary wetlands are not always well known.
2. Camargue wetlands are mainly characterized by variable salinity and hydroperiod. The individual and combined impacts of these local factors, together with regional variables, on invertebrate communities remain unknown. We therefore characterized and sampled invertebrates in 30 temporary wetlands along salinity and hydroperiod gradients in the Camargue (Southern France) 3, 5 and 7 months after inundation.
3. Over the three sampling occasions, a total of 17 cladoceran species and 49 macroinvertebrate taxa were identified. Hydroperiod and salinity were the most important variables explaining variation in taxonomic composition and can be considered key factors shaping the invertebrate communities in Camargue wetlands. The impact on taxon richness was significantly positive for hydroperiod but significantly negative for salinity. Regional factors had no significant effect on the structure of the studied invertebrate communities, suggesting that dispersal was not limiting and that species sorting was the most important structuring process.
4. The results of this study suggest that the combined and interacting effects of salinization and hydrological modification of Mediterranean temporary wetlands (due to water management, climate change, etc.) can result in reduced diversity in large numbers of Mediterranean wetlands and induce a considerable decline in regional diversity of aquatic invertebrates.     

How do patch quality and spatial context affect invertebrate communities in a natural moss microlandscape?

Globally, moss associated invertebrates remain poorly studied and it is largely unknown to what extent their diversity is driven by local environmental conditions or the landscape context. Here, we investigated small scale drivers of invertebrate communities in a moss landscape in a temperate forest in Western Europe. By comparing replicate quadrats of 5 different moss species in a continuous moss landscape, we found that mosses differed in invertebrate density and community composition. Although, in general, richness was similar among moss species, some invertebrate taxa were significantly linked to certain moss species. Only moss biomass and not relative moisture content could explain differences in invertebrate densities among moss species. Second, we focused on invertebrate communities associated with the locally common moss species Kindbergia praelonga in isolated moss patches on dead tree trunks to look at effects of patch size, quality, heterogeneity and connectivity on invertebrate communities. Invertebrate richness was higher in patches under closed canopies than under more open canopies, presumably due to the higher input of leaf litter and/or lower evaporation. In addition, increased numbers of other moss species in the same patch seemed to promote invertebrate richness in K. praelonga, possibly due to mass effects. Since invertebrate richness was unaffected by patch size and isolation, dispersal was probably not limiting in this system with patches separated by tens of meters, or stochastic extinctions may be uncommon. Overall, we conclude that invertebrate composition in moss patches may not only depend on local patch conditions, in a particular moss species, but also on the presence of other moss species in the direct vicinity.     

Does carbon enrichment affect hyporheic invertebrates in a gravel stream?

For a period of one year we injected a solution of stream water enriched with glucose and inorganic nitrogen and phosphorus at two experimental sites into the hyporheic sediments of the Oberer Seebach, Austria. The biofilm reacted with a quantitative increase after two weeks. The hyporheic invertebrates were sampled with the Cage Pipe Trap method, where the number of trapped animals is determined by the spatial density and the activity of the invertebrates. Within two and six weeks, the hyporheic invertebrates exhibited a reaction indicating an utilization of the new food resources. Over a longer period of one year, three different reaction patterns appeared. The number of nematods and ostracods increased extensively, presumably caused by the modification of the spatial structure of the environment due to biofilm growth. The number of the small sized invertebrates decreased, reflecting the reduced feeding effort. And the number of the large insect larvae increased indicating that these group is mainly limited by space. The hyporheic zone is described as a ‘self-cleaning DOC filter’, an attribute that is particularly assigned to the ecotone between the riparian soil zone and the stream hyporheic zone.     

Perspectives and predictions on the microbial ecology of the hyporheic zone

1. Studies of hyporheic microbial ecology have suggested an important role for hyporheic microbial processes in stream ecosystem functioning. Using evidence from microbial communities in other aquatic habitats, some predictions are made concerning the diversity of microbial types and microbial processes likely to occur in the hyporheic zone, and the relative importance of these various types to the hyporheic ecosystem. 2. It is predicted that the biofilm growth form of interstitial micro-organisms will create a variety of microniches, allowing coexistence of a great diversity of microbial types, and promoting the activity of some otherwise poor competitors. It is further predicted that the confluence of reduced groundwaters and aerobic surface waters will favour chemolithotrophic processes in the hyporheic zone, but that these will contribute significantly to hyporheic production only if surface water is very low in dissolved organic carbon, or the groundwater is extremely reduced, such as by the influence of riparian wetlands. A variety of anaerobic respiratory pathways, such as nitrate, ferric ion, sulphate and even methanogenic respiration will be employed in the hyporheic zone, with biofilm dynamics permitting these to occur even in aerobic sediments. Anaerobic pathways may account for a significant proportion of total hyporheic organic matter mineralization. 3. The role of fungi in hyporheic dynamics is, as yet, almost completely unstudied. However, it is expected that they will be important in breaking down buried particulate organic matter (POM), which may account for a large proportion of total stream POM. 4. Physicochemical conditions in hyporheic sediments appear to be highly heterogeneous, and this heterogeneity may be very important in the cycling of certain nutrients, especially nitrogen, which involves a series of steps requiring different conditions. 5. Various new techniques are now available by which biofilm dynamics and in situ microbial processes may be measured. Studies are recommended of intact microbial communities both at the microscale of the biofilm and at the scale of the heterogeneities occurring in hyporheic sediments. Studies are needed that measure actual rates of microbial processes under in situ conditions.     

Benthic and hyporheic macroinvertebrate distribution within the heads and tails of riffles during baseflow conditions

The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. Riffle heads were found to contain a greater proportion of interstitial fine sediment than riffle tails. Significant differences in the composition of benthic communities were associated with the amount of fine sediment. Riffle tail habitats supported a greater abundance and diversity of invertebrates sensitive to fine sediment such as EPT taxa. Shredder feeding taxa were more abundant in riffle heads suggesting greater availability of organic matter. In contrast, no significant differences in the hyporheic community were recorded between riffle heads and tails. We hypothesise that clogging of hyporheic interstices with fine sediments may have resulted in the homogenisation of the invertebrate community by limiting faunal movement into the hyporheic zone at both the riffle heads and tails. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale.     

Hyporheic invertebrates as bioindicators of ecological health in temporary rivers: A meta-analysis

Worldwide, many rivers cease flow and dry either naturally or owing to human activities such as water extraction. However, even when surface water is absent, diverse assemblages of aquatic invertebrates inhabit the saturated sediments below the river bed (hyporheic zone). In the absence of surface water or flow, biota of this zone may be sampled as an alternative to surface water-based ecological assessments. The potential of hyporheic invertebrates as ecological indicators of river health, however, is largely unexplored. We analysed hyporheic taxa lists from the international literature on temporary rivers to assess compositional similarity among broad-scale regions and sampling conditions, including the presence or absence of surface waters and flow, and the regional effect of hydrological phase (dry channel, non-flowing waters, surface flow) on richness. We hypothesised that if consistent patterns were found, then effects of human disturbances in temporary rivers may be assessable using hyporheic bioindicators. Assemblages differed geographically and by climate, but hydrological phase did not have a strong effect at the global scale. However, hyporheic assemblage composition within regions varied along a gradient of higher richness during wetter phases. This indicates that within geographic regions, hyporheic responses to surface drying are predictable and, by extension, hyporheic invertebrates are potentially useful ecological indicators of temporary river health. With many rivers now experiencing, or predicted to experience, lower flows and longer dry phases owing to climate change, the development of ecological assessment methods specific to flow intermittency is a priority. We advocate expanded monitoring of hyporheic zones in temporary rivers and recommend hyporheic invertebrates as potential bioindicators to complement surface water assessments.     

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。     

Effects of eucalyptus afforestation on leaf litter dynamics and macroinvertebrate community structure of streams in Central Portugal

To test the hypothesis whether afforestation with Eucalyptus globulus affects litter dynamics in streams and the structure of macroinvertebrate aquatic communities, we compared streams flowing through eucalyptus and deciduous forests, paying attention to: (i) litterfall dynamics, (ii) accumulation of organic matter, (iii) processing rates of two dominant leaf species: eucalyptus and chestnut, and (iv) macroinvertebrate community structure. The amount of allochthonous inputs was similar in both vegetation types, but the seasonality of litter inputs differed between eucalyptus and natural deciduous forests. Eucalyptus forest streams accumulated more organic matter than deciduous forest streams. Decomposition of both eucalyptus and chestnut leaf litter was higher in streams flowing through deciduous forests. The eucalyptus forest soils were highly hydrophobic resulting in strong seasonal fluctuations in discharge. In autumn the communities of benthic macroinvertebrates of the two stream types were significantly different. Deciduous forest streams contained higher numbers of invertebrates and more taxa than eucalyptus forest streams. Mixed forest streams (streams flowing through eucalyptus forests but bordered by deciduous vegetation) were intermediate between the two other vegetation types in all studied characteristics (accumulation of benthic organic matter, density and diversity of aquatic invertebrates). These results suggest that monocultures of eucalyptus affect low order stream communities. However, the impact may be attenuated if riparian corridors of original vegetation are kept in plantation forestry.     

Structure and seasonal dynamics of hyporheic zone microbial communities in free-stone rivers of the estern United States

The hyporheic zone of a river is characterized by being nonphotic, exhibiting chemical/redox gradients, and having a heterotrophic food web based on the consumption of organic carbon entrained from surface waters. Hyporheic microbial communities constitute the base of food webs in these environments and are important for maintaining a functioning lotic ecosystem. While microbial communities of rivers dominated by fine-grained sediments are relatively well studied, little is known about the structure and seasonal dynamics of microbial communities inhabiting the predominantly gravel and cobble hyporheic zones of rivers of the western United States. Here, we present the first molecular analysis of hyporheic microbial communities of three different stream types (based on mean base discharge, substratum type, and drainage area), in Montana. Utilizing 16S rDNA phylogeny, DGGE pattern analysis, and qPCR, we have analyzed the prokaryotic communities living on the 1.7 to 2.36 mm grain-size fraction of hyporheic sediments from three separate riffles in each stream. DGGE analysis showed clear seasonal community patterns, indicated similar community composition between different riffles within a stream (95.6–96.6% similarity), and allowed differentiation between communities in different streams. Each river supported a unique complement of species; however, several phylogenetic groups were conserved between all three streams including Pseudomonads and members of the genera Aquabacterium, Rhodoferax, Hyphomicrobium, and Pirellula. Each group showed pronounced seasonal trends in abundance, with peaks during the Fall. The Hyphomicrobium group was numerically dominant throughout the year in all three streams. This work provides a framework for investigating the effects of various environmental factors and anthropogenic effects on microbial communities inhabiting the hyporheic zone.     

Soil nematodes and desiccation survival in the extreme arid environment of the antarctic dry valleys

Soil nematodes are capable of employing an anhydrobiotic survivalstrategy in response to adverse environmental conditions. TheMcMurdo Dry Valleys of Antarctica represent a unique environmentfor the study of anhydrobiosis because extremes of cold, salinity,and aridity combine to limit biological water availability.We studied nematode anhydrobiosis in Taylor Valley, Antarctica,using natural variation in soil properties. The coiled morphologyof nematodes extracted from dry valley soils suggests that theyemploy anhydrobiosis, and these coiled nematodes showed enhancedrevival when re-hydrated in water as compared to vermiform nematodes.Nematode coiling was correlated with soil moisture content,salinity, and water potential. In the driest soils studied (gravimetricwater content <2%), 20–80% of nematodes were coiled.Soil water potential measurements also showed a high degreeof variability. These measurements reflect microsite variationin soil properties that occurs at the scale of the nematode.We studied nematode anhydrobiosis during the austral summer,and found that the proportion of nematodes coiled can vary diurnally,with more nematodes vermiform and presumably active at the warmesttime of day. However, dry valley nematodes uncoiled rapidlyin response to soil wetting from snowmelt, and most nematodeactivity in the Dry Valleys may be confined to periods followingrare snowfall and melting events. Anhydrobiosis represents animportant temporal component of a dry valley nematode's lifespan. The ability to utilize anhydrobiosis plays a significantrole in the widespread distribution and success of these organismsin the Antarctic Dry Valleys and beyond.     

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.     

Naturally Saline Boreal Communities as Models for Reclamation of Saline Oil Sand Tailings

Reclaimed landscapes after oil sands mining have saline soils; yet, they are required to have similar biodiversity and productivity as the predisturbance nonsaline landscape. Given that many species in the boreal forest are not tolerant of salinity, we studied the effects of soil salinity on plant communities in natural saline landscapes to understand potential plant responses during the reclamation process. Vegetation–soil relationships were measured along transects from flooded wetlands to upland forest vegetation in strongly saline, slightly saline, nonsaline, and reclaimed boreal landscapes. In strongly saline landscapes, surface soil salinity was high (>10 dS/m) in flooded, wet‐meadow, and dry‐meadow vegetation zones as compared to slightly saline (<5 dS/m) and nonsaline (<2 dS/m) landscapes. Plant communities in these vegetation zones were quite different from nonsaline boreal landscapes and were dominated by halophytes common to saline habitats of the Great Plains. In the shrub and forest vegetation zones, surface soil salinity was similar between saline and nonsaline landscapes, resulting in similar plant communities. In strongly saline landscapes, soils remained saline at depth through the shrub and forest vegetation zones (>10 dS/m), suggesting that forest vegetation can establish over saline soils as long as the salts are below the rooting zone. The reclaimed landscape was intermediate between slightly saline and nonsaline landscapes in terms of soil salinity but more similar to nonsaline habitats with respect to species composition. Results from this study suggest it may be unrealistic to expect that plant communities similar to those found on the predisturbance landscape can be established on all reclaimed landscapes after oil sands mining.     

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.     

Diversity patterns of leaf-associated aquatic hyphomycetes along a broad latitudinal gradient

Information about the global distribution of aquatic hyphomycetes is scarce, despite the primary importance of these fungi in stream ecosystem functioning. In particular, the relationship between their diversity and latitude remains unclear, due to a lack of coordinated surveys across broad latitudinal ranges. This study is a first report on latitudinal patterns of aquatic hyphomycete diversity associated with native leaf-litter species in five streams located along a gradient extending from the subarctic to the tropics. Exposure of leaf litter in mesh bags of three different mesh sizes facilitated assessing the effects of including or excluding different size-classes of litter-consuming invertebrates. Aquatic hyphomycete evenness was notably constant across all sites, whereas species richness and diversity, expressed as the Hill number, reached a maximum at mid-latitudes (Mediterranean and temperate streams). These latitudinal patterns were consistent across litter species, despite a notable influence of litter identity on fungal communities at the local scale. As a result, the bell-shaped distribution of species richness and Hill diversity deviated markedly from the latitudinal patterns of most other groups of organisms. Differences in the body-size distribution of invertebrate communities colonizing the leaves had no effect on aquatic hyphomycete species richness, Hill diversity or evenness, but invertebrates could still influence fungal communities by depleting litter, an effect that was not captured by the design of our experiment.