Filling the gaps in stream size spectra: using electroshocking to collect large macroinvertebrates

Studies of stream benthic macroinvertebrate size distributions describe assemblages from random small area samplers or cobbles. Large organisms are often underestimated in these samples because of their mobility or low densities. This study compared the performance of cobble collection and a Hess sampler with electrobugging (using a backpack electroshocker to collect macroinvertebrates) to determine whether electrobugging could improve estimates of the biomass of large organisms. We sampled 12 stream sites, collecting cobbles and Hess samples and conducting 3–4 consecutive electrobugging passes (repeating until attrition was observed, following with kick samples). Electrobugging collected more biomass of mobile taxa than cobble or Hess samples, and collected greater taxonomic richness, particularly at sites with low algal biomass. However, it was less effective at collecting attached and immobile taxa. Electrobugging extended the size distribution range by up to eight size classes by collecting larger macroinvertebrates than the cobble or Hess methods, and collected significantly more taxa per size class. Size distributions created from only cobble or Hess samples clearly do not represent the entire benthic community, but our results indicate that electrobugging provides a means to improve estimates of the biomass of large, mobile taxa, providing a more accurate estimate of body size distributions.     

Comparisons of benthic communities adjacent to riparian native eucalypt and introduced willow vegetation

1. Willows, Salix spp., have been widely introduced as a riparian species in temperate Australia and New Zealand. The present study was a broad-scale observational survey to document the differences between reaches of river lined with willows and native vegetation in the community structure of benthic invertebrates and the resources which these plants use.
2. Nine rivers in south-eastern Tasmania were examined on three occasions in autumn, spring and summer. Taxa were identified to family level, with the exception of Oligochaeta and Acarina, and benthic organic matter (CPOM and FPOM) and epilithic biomass were measured for each reach.
3. Taxon diversity and evenness were lower in willowed reaches in autumn, and total macroinvertebrate density and number of taxa were lowest in willowed reaches in summer. No differences in the fauna between willowed and native reaches were observed in spring. Measures of community similarity of the fauna in willowed and native reaches were significantly different in autumn and summer, but not in spring.
4. The taxa responsible for the significant differences seemed to be responding to differences in food availability and habitat quality in reaches of each vegetation type. Organic matter standing stock was higher in willowed reaches in autumn although the influence of these litter inputs on the fauna were not marked. Epilithon biomass was highest in autumn and spring in willowed reaches when shading in these reaches was least.
5.  The most marked differences between willowed and native reaches were during summer low flows, when the instream fauna appeared to be responding to changes to shading, water quality and the quality of the habitat.     

典型河床底质组成中底栖动物群落及多样性

底栖动物是河流生态系统中食物链的重要环节。通过对长江、黄河、东江和拒马河等河流野外调查和采样分析研究了河床底质组成对底栖动物群落结构的影响规律。研究结果发现,不同河床底质组成中的底栖动物结构差别很大,不同地理位置而相同底质条件和水力条件的河流底栖动物群落组成相似,说明河床底质是影响河流底栖动物群落结构的关键因素,受地理位置和大气候的影响不大;利用多项生物指标分析了不同河床底质组成中底栖动物群落的多样性,卵石河床且有水生植物生长的河流底栖动物物种组成最丰富,大河中沙质河床不稳定,未采集到底栖动物;不同底质类型河床中的优势种群亦不同。并分析了采样所得底栖动物物种数与采样面积之间的关系,符合前者随后者呈幂指数增加的规律,当实测采样面积为1～2m^2时物种数变化不大,建议一般情况下最小采样面积应为1m^2。     

Patch based predation in a southern Appalachian stream

Streams are characterized by high degrees of patchiness that could influence the role of predators in these systems. Here we assess the impact of predatory benthic fishes on benthic macroinvertebrate density, biomass, and community structure at the patch scale in a fourth order stream in the southern Appalachians. We tested the role of predation in two different patch types: patches inhabited by adult mottled sculpin ( Cottus bairdi ) and random patches. We placed 30 basket pairs (one open to fish predation, and one from which fish predators were excluded) in the streambed at each patch type. We also tested for potential basket effects by setting up a basket control area. Although there was some evidence of basket artifacts on macroinvertebrate density in sculpin patches, these artifacts were not consistent and we do not feel that they affected our results because predators did not affect macroinvertebrate density. In random patches, predation did not significantly affect macroinvertebrate density or biomass. Predators significantly reduced macroinvertebrate biomass in sculpin patches but did not affect prey density. When the data-set was size-limited to exclude macroinvertebrates too large for consumption by sculpin, macroinvertebrate biomass did not differ significantly between exclusion and open baskets. This suggests that sculpin can reduce macroinvertebrate biomass through a combination of consumption and by predator-induced emigration of large macroinvertebrates into areas that are protected from sculpin. In addition, invertebrate predator biomass was higher in predator exclusion baskets in sculpin patches indicating that predation pressure remained high in the exclusion baskets despite fish exclusion. These results illustrate the heterogeneity of streams and the effect of small-scale differences (e.g. location of predators' territories) on local processes. Experiments that utilize these differences can provide insights into these stream processes.     

Effects of longitudinal variations in stream habitat structure on fish abundance: an analysis based on subunit-scale habitat classification

SUMMARY 1. Stream reaches contain assortments of various habitat types that can be defined at different spatial scales, such as channel unit (e.g. pools, riffles) and subunit (patches within channel units). We described longitudinal (upstream–downstream) patterns of stream habitat structure by considering subunits as structural elements, and examined their effects on the abundance of masu salmon ( Oncorhynchus masou ) and rosyface dace ( Leuciscus ezoe ) in a third-order tributary of the Teshio River in northern Hokkaido, Japan.
2. Nine subunit types were determined on the basis of water depth, current velocity and substrate, using 0.5 × 0.5 m grids. Although both masu salmon and rosyface dace used pools as a major habitat, the former preferred a subunit type occurring at pool heads (PH subunit) while the latter preferred a slow-current edge type (SE-2 subunit).
3. Along the course of the stream, slow-edge subunits (SE-1, 2 and 3) increased in frequency downstream while fast-edge subunits (FE-1 and 2) decreased, suggesting a downstream development of slow-current edges. Regression analyses indicated that longitudinal variation in masu salmon abundance was explained by the area of PH, rather than pools. Masu salmon density increased with the area of PH. Rosyface dace abundance was explained by a combination of water depth and the area of SE-2, both effects being positive.
4. Longitudinal variations in the abundance of both species were related to the abundance of their preferred habitat at the subunit scale, rather than channel-unit scale. The results emphasise the importance of fine-scale patchiness when examining stream fish habitats.     

Influence of urban tributaries on freshwater mussel populations in a biologically diverse piedmont (USA) stream

We investigated the distribution patterns of senescent and green leaf litter patches on a streambed to evaluate the hypothesis that the different immersion times of senescent leaves in long-term benthic storage and newly retained green leaves affect streambed distribution patterns in summer (June, July, and August). We counted all the leaf litter patches in the streambed of the study reach, comparing the physical condition of patches and classifying the trapping obstacles associated with each patch type. The distribution patterns of senescent and green leaf litter patches differed. Green leaf litter patches were more numerous at every sampling date, with most trapped by cobbles, whereas senescent leaf litter patches were retained by twig obstacles, backwaters, and cobbles. In June and July, senescent leaf litter patches were located in significantly deeper stream areas than were green leaf litter patches. The distribution of senescent leaves would be primarily determined during spring snowmelt-driven floods. We speculate that senescent leaves were originally located at the edges of pools in the main flow pathway of the channel, which overflowed in the floods. We conducted flume experiments to clarify the transport characteristics of senescent and green painted maple and manchurian alder leaves in the water column. Our hypothesis was that the transport characteristics of each leaf type differ when they first enter the water, because of differing leaf properties. The flume experiments showed that duration of surface flotation differed for senescent and green leaves and for the two tree species. These differences in the duration of leaves on the streambed and in the floating time of green leaves of different trees ensure varied food resources for macroinvertebrates in various physical conditions. Handling editor: B. Oertli     

Assessing acid stress in Swedish boreal and alpine streams using benthic macroinvertebrates

Sixty streams in northern Sweden were sampled for benthic macroinvertebrates in spring and autumn of 2000 as part of the European Union project AQEM (the Development and Testing of an Integrated Assessment System for the Ecological Quality of Streams and Rivers throughout Europe using Benthic Macroinvertebrates). Samples were taken using a harmonised multi-habitat sampling procedure and a large number of parameters describing the streams and their catchments were recorded for all sampling sites. From the stream water chemistry characteristics unbiased measures of acid stress were derived, based on the Swedish Ecological Quality Criteria (EQC) for acidity status. These criteria do not, however, assess the degree of human influence to a stream. Therefore, in the following analysis of classification, the Swedish EQC status criteria are applied as if they were a measure of human influence. Three new multimetric acid indices using benthic macroinvertebrates were developed for the AQEM project. These, together with a large number of other benthic macroinvertebrate indices including a number of indices aimed at detecting acid stress, were evaluated for their ability to detect acid stress, using both spring/early summer and autumn sampling. Surprisingly, the acid index that is in general use in northern Sweden worked well in spring/early summer, but because of a (probable) re-colonisation of more acid sensitive taxa, the index values changed from classifying streams as affected in spring/early summer to classify streams as non-affected by acid stress in autumn. Another Swedish acid index (included in the Swedish Environmental Quality Criteria developed by the Swedish EPA) and developed for southern Sweden was strongly correlated between spring/early summer and autumn and could thus be sampled in the autumn and indicate acid stress from spring flood declines in pH. This is of great importance since the local County boards of northern Sweden generally argue that sampling has to be done in the spring to see the effects of acid stress, when sampling is difficult for logistic reasons. Whether or not the three indices developed for the AQEM project (and extensively based on the south Swedish acid index) were truly better than the south-Swedish acid index to assess acid stress in northern Sweden could not be clearly determined in the present study.     

Abundance, diversity and fidelity of macroinvertebrates sheltering beneath rocks during tidal emersion in an intertidal cobble field: Does the intermediate disturbance hypothesis hold for less exposed shores with smaller rocks?

The community structure of macroinvertebrates occurring beneath cobbles during tidal emersion was investigated in an intertidal cobble field on the east coast of the south island of New Zealand. In order to test whether the intermediate disturbance hypothesis (IDH) holds in less exposed shores comprised exclusively of smaller rocks (cobbles) we measured 1) the size frequency distribution of cobbles (mass and basal surface area), 2) the incidence of barnacle encrustation as an indirect measure of cobble disturbance, 3) the abundances of the dominant macroinvertebrates occurring beneath cobbles (the crab Hemigrapsus crenulatus, the chiton Sypharochiton pelliserpentis and the snail Diloma zealandica), and 4) macroinvertebrate fidelity beneath a range of cobble sizes. Only large cobbles were found to be encrusted with barnacles, indicative of their greater resistance to physical disturbance by waves and currents. Mark and relocate experiments indicated fidelity to a given cobble over two tidal cycles was 11-14% for crabs and did not vary with cobble size. In contrast, fidelity to medium and large cobbles was higher for chitons (26-45%) and snails (17-31%) and generally increased with increasing cobble size. In contrast to the predictions of the IDH, there was a positive correlation between increasing cobble size and increasing species diversity of organisms found under the cobbles. This pattern is not simply attributable to greater space available beneath large cobbles as there was a significantly greater density of macroinvertebrates beneath medium and large, compared to the small cobbles. Increasing species diversity with increasing cobble size appears to be independent of patterns of species recolonization associated with small cobbles being subject to high disturbance or interspecific competitive hierarchies associated with low disturbance beneath large cobbles. Instead, factors such as aggregative behaviors, and reducing vulnerability to predators and desiccation stress are likely to explain the relationship between cobbles and their associated macroinvertebrate communities.     

Temporal changes in benthic macroinvertebrates and their interactions with fish predators after restoration in the Cheonggyecheon,a downtown stream in Seoul,Korea

The Cheonggyecheon (Cheonggye stream), a downtown stream in Seoul, Korea, was restored by the historic Cheonggyecheon Restoration Project (2003–2005), and its discharge and other environmental conditions are artificially regulated throughout the year. The aim of this study was to assess temporal changes in benthic macroinvertebrates and their interactions with fish predators in the Cheonggyecheon. Sampling was conducted twice a year (spring and autumn) at three sections of the stream from 2006 to 2010 using a Surber sampler (50 × 50 cm) for benthic macroinvertebrates and a cast net (mesh size, 10 × 10 mm) and a kick net (mesh size, 5 × 5 mm) for fishes. Analyses of annual precipitation and most water quality parameters showed no significant differences across the study period (P > 0.05). The species richness and density of benthic macroinvertebrates and fishes varied across the sampling period. Analyses of functional feeding groups and trophic guilds showed distinct trends, such as an increase in the number of insectivorous fish in the absence of invertebrate predators. Correlation and regression analyses for five major groups of benthic macroinvertebrates and fishes demonstrated that the density of certain groups of benthic macroinvertebrates (e.g. Tubificidae, Erpobdellidae, Baetidae, and Chironomidae) and insectivorous fishes (e.g. Gobioninae, Leuciscinae, Danioninae, and Gobiidae) were negatively correlated. In conclusion, in an artificially regulated stream like the restored Cheonggyecheon, where physical environments are nearly unchanged, fish predation can be a major factor controlling benthic macroinvertebrate communities.     

Effects of snow cover on the benthic fauna in a glacier-fed stream

1. Alpine streams above the tree line are covered by snow for 6–9 months a year. However, winter dynamics in these streams are poorly known. The annual patterns of macroinvertebrate assemblages were studied in a glacial stream in the Austrian Alps, providing information on conditions under the snow.
2. Snow cover influenced water temperature, the content of benthic organic matter and insect development. Taxa richness and abundance of macroinvertebrates did not show a pronounced seasonal pattern. The duration of the autumn period with stable stream beds was important in determining the abundance and composition of the winter fauna.
3. There were significant differences in species composition between summer and winter. Two potential strategies in larval survival were evident: adaptation to the extreme abiotic conditions in summer (e.g. Diamesa spp.) or avoidance of these conditions and development during winter (e.g. Ephemeroptera and Plecoptera).
4. A comparison of a stream reach with continuous snow cover and a stream reach that remained open throughout winter showed that conditions under snow are suboptimal. At the open stream site, with higher water temperatures and greater food supply (benthic organic matter content), abundance and taxa richness was higher and larval growth was faster. Several taxa were found exclusively at this site.
5. Winter conditions did not provide an entirely homogeneous environment, abiotic conditions changed rapidly, especially at the onset of snowfall and at snowmelt. Continuous monitoring is necessary to recognize spatial and temporal heterogeneity in winter environments and the fauna of alpine streams.     

Organic matter availability structures microbial biomass and activity in a Mediterranean stream

1. We compared microbial biomass (bacteria, fungi, algae) and the activity of extracellular enzymes used in the decomposition of organic matter (OM) among different benthic substrata (leaves, coarse and fine substrata) over one hydrological year in a Mediterranean stream.
2. Microbial heterotrophic biomass (bacteria plus fungi) was generally higher than autotrophic biomass (algae), except during short periods of high light availability in the spring and winter. During these periods, sources of OM shifted towards autochthonous sources derived mainly from algae, which was demonstrated by high algal biomass and peptidase activity in benthic communities.
3. Heterotrophic activity peaked in the autumn. Bacterial and fungal biomass increased with the decomposition of cellulose and hemicellulose compounds from leaf material. Later, lignin decomposition was stimulated in fine (sand, gravel) and coarse (rocks, boulders and cobbles) substrata by the accumulation of fine detritus.
4. The Mediterranean summer drought provoked an earlier leaf fall. The resumption of the water flow caused the weathering of riparian soils and subsequently a large increase in dissolved organic carbon and nitrate, which led to growth of bacteria and fungi.     

Coexistence of Fish Species in a Large Lowland River: Food Niche Partitioning between Small-Sized Percids,Cyprinids and Sticklebacks in Submersed Macrophytes

In the spring and summer of each year, large patches of submersed aquatic macrophytes overgrow the bottom of the alluvial Warta River downstream of a large dam reservoir owing to water management practices. Environmental variables, macroinvertebrates (zoobenthos and epiphytic fauna, zooplankton) and fish abundance and biomass were assessed at this biologically productive habitat to learn intraseasonal dynamics of food types, and their occurrence in the gut contents of small-sized roach, dace, perch, ruffe and three-spined stickleback. Gut fullness coefficient, niche breadth and niche overlap indicated how the fishes coexist in the macrophytes. Chironomidae dominated in the diet of the percids. However, ruffe consumed mostly benthic chironomids, while perch epiphytic chironomids and zooplankton. The diet of dace resembled that in fast flowing water although this rheophilic species occurred at unusual density there. The generalist roach displayed the lowest gut fullness coefficient values and widest niche breadth; consequently, intraspecific rather than interspecific competition decided the fate of roach. Three-spined stickleback differed from the other fishes by consuming epiphytic simuliids and fish eggs. The diet overlap between fishes reaching higher gut fullness coefficient values was rather low when the food associated with the submersed aquatic macrophytes was most abundant; this is congruent with the niche overlap hypothesis that maximal tolerable niche overlap can be higher in less intensely competitive conditions.     

Habitat attributes associated with short‐term settlement of Ozark hellbender (Cryptobranchus alleganiensis bishopi) salamanders following translocation to the wild

1. Organisms associated with lotic systems rank among the most threatened because of global change. Although translocation is being increasingly applied as a conservation strategy, most studies have focused on survival and recruitment of individuals, and few have attempted to identify how habitat attributes influence short‐term settlement of animals during the critical post‐release period. 2. We demonstrate the application of resource selection modelling in an information theoretic framework to identify release‐site characteristics that will increase the likelihood of settlement for a fully aquatic benthic stream salamander, the Ozark hellbender (Cryptobranchus alleganiensis bishopi). We fit discrete choice models using data from 29 radio‐tagged hellbenders that were translocated to two sites in the North Fork of the White River (NFWR), Missouri (U.S.A.). We defined resource availability at two spatial scales (stream reach and home range) and quantified abiotic habitat attributes at 3181 salamander locations and 6329 random available locations collected between May 2008 and August 2009. 3. At both sites and spatial scales, a single model received substantially greater support (0.96–1.00 of total model weight) than all other models, and top‐ranked models were similar in form and predictive ability. At both spatial scales, selection was positively influenced by the presence of cobble‐boulder substratum relative to bedrock and finer substrata. We also noted a negative interactive effect between distance to the nearest substratum particle large enough to provide cover (i.e. at least one axis ≥15 cm in length) and an increase in either a direct or relative (i.e. pool, run, and riffle) measure of water velocity. 4. Collectively, salamanders released in our study selected resources indicative of long‐term benthic microhabitat stability. However, despite strong selection of cobble‐boulder substratum, 8% (282 of 3181) of captive‐reared hellbender locations occurred in bank crevices and root masses. Although several studies have reported the importance of near bed hydraulics in determining occurrence of stream macroinvertebrates, our findings are the first to indicate that spacing among cobble‐boulder substrata may be important for hellbenders. 5. To increase the likelihood of short‐term settlement of captive‐reared hellbenders in the wild, we recommend prioritising release sites where the average distance between cobble‐boulder particles within habitat patches is minimised. In general, average spacing among cobble and boulder substrata should be <1 m in habitat patches where mean benthic water velocity exceeds 0.1 m s^?1, and <0.5 m where water velocity approaches 0.30 m s^?1. Based on home range sizes of captive‐reared Ozark hellbenders, the collective extent of suitable cobble‐boulder habitat patches within release sites should approximate at least 10 m² per salamander released.     

Effects of habitat availability on dispersion of a stream cyprinid

Synopsis We analyzed temporal changes in the dispersion of the rosyside dace,Clinostomus funduloides, (family Cyprinidae) in a headwater stream, to assess the role of habitat availability in promoting fish aggregation. The dace foraged alone and in groups of up to about 25 individuals, and dispersion varied significantly among monthly censuses conducted from May through December. In two of three study pools, dace aggregated during July, October and/or December, but spread out during other months, especially during September when dispersion did not differ significantly from random. Dispersion was not significantly correlated with the total amount of suitable habitat available to foraging dace, but during summer, corresponded to the availability of depositional areas adjacent to rapid currents. Foragers aggregated in eddies or depositional areas during high stream discharge in July, and shifted out of depositional areas when current velocities declined from July to September. During late autumn, however, aggregations formed independently of changes in habitat conditions, and dace dispersion did not vary significantly among months in a third pool. The study suggests that dace dispersion cannot be predicted from the overall availability of suitable habitat as estimated from point measurements of depth and velocity; both the occurrence of a specific habitat feature (i.e., eddies adjacent to high velocity currents) and seasonal differences in behavior more strongly influenced the spatial distribution of foragers.     

山地河流潜流层大型无脊椎动物群落组成及分布

潜流层大型无脊椎动物是河流生态系统重要的组成部分.2013年8月(夏季)、12月(冬季)和2014年4月(春季),在黑水滩河上游河段,采用人工基质法调查潜流层大型无脊椎动物.结果表明: 3个季节共采集大型无脊椎动物27种,其中夏季22种、冬季和春季各16种,各季节水生昆虫种类所占比例均较高,分别为81.8%、75.0%和62.5%;夏季群落密度显著低于冬季和春季,春季最高;冬季群落生物量显著高于夏季和春季,夏季最低;3个季节群落的物种丰富度指数、Shannon多样性指数和Pielou均匀度指数均没有显著性差异.空间分布上,大型无脊椎动物的密度和丰富度均随潜流层深度增加而呈现降低的趋势,大多数个体均分布在0～20 cm深度.群落以滤食者和收集者组成的集食者为绝对优势功能群.动物的相互作用、生活史策略和潜流层的理化条件影响着潜流层大型无脊椎动物的群落结构和时空分布.     

雅鲁藏布江中下游底栖动物物种多样性及其影响因素

雅鲁藏布江流域维系着丰富而独特的生物资源, 是全球生物多样性研究的热点区域。然而, 该流域底栖动物多样性的调查却极不充分。本文于2015年10月和2016年3月对雅鲁藏布江干流(朗县至墨脱段)和主要支流的底栖动物进行了调查, 并采用单因素方差分析(one-way ANOVA)和典范对应分析(canonical correspondence analysis)等对群落多样性格局进行解析。共采集到底栖动物270种, 隶属于5门8纲20目92科, 包括昆虫纲246种, 寡毛纲14种, 腹足纲4种, 其他动物6种。春季和秋季分别采集到底栖动物184种和214种, 优势种均以喜清洁和冷水的水生昆虫为主, 包括四节蜉属一种(Baetis sp.)、花翅蜉属一种(Baetiella sp.)、蚋属一种(Simulium sp.)、小突摇蚊属一种(Micropsetra sp.)和短石蛾属一种(Brachycentrus sp.)等。全流域平均密度为939.1 ind./m²,sp.)等。平均生物量为5.44 g/m²。底栖动物的物种组成、密度和多样性在季节和区域之间存在一定差异, 支流的多样性显著高于干流。典范对应分析显示, 海拔、流速、河宽和底质类型等环境因子是影响雅鲁藏布江流域底栖动物群落结构的关键环境因素, 而大峡谷地区多变的气候类型和地理阻隔是造成群落变化的根本原因。本研究可为雅鲁藏布江流域底栖动物多样性评估和环境监测提供重要的基础和参考。     

Metabolic patch dynamics in small headwater streams: exploring spatial and temporal variability in benthic processes

1. To gain a better understanding of the heterotrophic nature of small headwater streams in forested landscapes we explored the spatial and temporal variability of in‐stream organic matter processes. Three methods were used to measure the benthic metabolism of different in‐stream habitats in seven streams throughout a calendar year. This allowed us to analyse the contribution of various metabolic habitats (i.e. sediment, leaf litter, cobbles) to in‐stream metabolism during a natural flow regime. Furthermore, it allowed us to define in‐stream patchiness based on functional rather than structural elements. 2. Bacterial growth, measured using a leucine assay, displayed a quadratic relationship over time with a peak in warmer months and consistently higher bacterial growth in fine depositional (3.00–710.64 mg C m^?2 day^?1) than coarse gravel (38.84–582.85 mg C m^?2 day^?1) sediments. 3. Community metabolism, measured using dissolved oxygen chambers, showed distinct diel patterns and consistently greater net daily metabolism in leaf packs (?261.76 to ?24.50 mg C m^?2 day^?1) than fine depositional sediments (?155.00 to ?15.56 mg C m^?2 day^?1). Coarse gravel sediments (?49.55 to ?16.88 mg C m^?2 day^?1) and cobble habitats (?151.98 to 55.38 mg C m^?2 day^?1) exhibited the lowest metabolic rates. Modelled whole‐stream metabolism was highly variable among streams and temporal patterns appeared driven by temperature and the relative contribution of patch configuration as a function of flow. 4. Cellulose decomposition potential showed higher rates of microbial activity in fine depositional compared to coarse gravel sediments (30.5 and 29.1 kg average cotton tensile strength loss respectively), though there were higher rates of thread loss indicative of macroinvertebrate activity in gravel compared to depositional sediment (21% and 13% average thread loss respectively), with a slight quadratic trend. The high variability among habitats, streams and over time in this integrative measure may be explained by variability in local microbial activity as well as the potential for macroinvertebrates to contribute across patches. 5. There were strong relationships among benthic processes and habitat structure, nutrient status, stream temperature and flow. Different habitats had distinct metabolic characteristics and these characteristics appear to influence stream food webs and biogeochemical cycling depending on the relative abundance of habitats. Generally, within habitat variability was less than among habitat variability and among stream variability was less than temporal variability. Hence, in terms of the spatial and temporal heterogeneity of benthic processes, these small headwater streams showed predictable metabolic patterns. However, there were few correlations between differing measures of benthic metabolism at the same patch and this suggests that caution should be taken when attempting to infer the rates of one level of metabolic activity (e.g. whole community metabolism) based on another (e.g. bacterial productivity).     

Seasonal influence of brook trout and mottled sculpin on lower trophic levels in an Appalachian stream

1. In some situations fish have strong top‐down effects in stream communities while in others they seem to be relatively unimportant. Differences in the impact of fish may depend on a variety of factors including the foraging mode of the fish, interactions among fish species and temporal variation in environmental conditions and species interactions. 2. We investigated the effect of brook trout (Salvelinus fontinalis) and mottled sculpin (Cottus bairdi) on lower trophic levels in Appalachian streams and whether or not interactions between these fish changed their influence. Mesocosms were placed in a headwater stream in a randomized complete block design. Within blocks, mesocosms were randomly assigned to one of the following treatments: (i) no fish; (ii) sculpin only; (iii) trout only and (iv) both sculpin and trout. Fish biomass was the same in all three fish treatments. Invertebrate density and algal biomass in mesocosms were determined after 3 weeks. We repeated the experiment in the autumn, spring and summer to test for seasonality of fish effects. 3. The effect of fish on invertebrate assemblages was seasonal and depended on prey identity. Sculpin strongly suppressed grazer abundance in spring while trout had little effect on grazers in any season. The influence of both fish on insect predators was similar and relatively constant across seasons. We found little evidence of an interaction between sculpin and trout that strongly influenced their effect on prey across seasons. 4. None of the fish treatments influenced algal biomass during any of the seasons. Algal growth was also seasonal, with a two‐ to four‐fold increase in algal biomass in spring compared to autumn and summer. 5. Our results indicate that benthic and drift feeding fish differ in their effects on some, but not all prey. Furthermore, fish effects on prey were strongly seasonal for some, but not all prey types. While the temporal context is not commonly considered, our results indicate seasonality can be an important component of predator–prey interactions in streams.     

Spatio-temporal variation in macroinvertebrate assemblages of glacial streams in the Swiss Alps

1. Changes in water chemistry, benthic organic matter (BOM), and macroinvertebrates were examined in four different glacial streams over an annual cycle. The streams experienced strong seasonal changes in water chemistry that reflected temporal changes in the influence from the source glacier, especially in water turbidity, particulate phosphorus and conductivity.
2. Nitrogen concentrations were high (nitrate-N values were 130–274 μg L^–1), especially during spring snowmelt runoff. Benthic organic matter attained >600 g m^–2 dry mass at certain times, peaks being associated with seasonal blooms of the alga Hydrurus foetidus .
3. Macroinvertebrate taxon richness was two to three times higher (also numbers and biomass) in winter than summer suggesting winter may be a more favourable period for these animals. Benthic densities averaged 1140–3820 ind. m^–2, although peaking as high as 9000 ind. m^–2. Average annual biomass ranged from 102 to 721 mg m^–2, and reached >2000 mg m^–2 at one site in autumn.
4. Taxa common to all sites included the dipterans Diamesa spp. and Rhypholophus sp., the plecopterans Leuctra spp. and Rhabdiopteryx alpina , and the ephemeropterans Baetis alpinus and Rhithrogena spp. Principal components analysis clearly separated winter assemblages from those found in summer.