Short-term responses of phytoplankton to nutrient enrichment and planktivorous fish predation in a temperate South American mesotrophic reservoir

The benthic macroinvertebrate community is an important component of stream diversity, because its members are fundamental connectors among the different trophic levels of running waters. In this study, we assessed alpha and beta diversities of benthic macroinvertebrates in three stream sites and four microhabitats: (i) moss in the air-water interface; (ii) submerged roots of terrestrial plants; (iii) leaf litter deposited in pools; (iv) stones in riffles. We constructed rarefaction curves and compared species richness among microhabitats for each stream site. Additionally, we evaluated which factor, stream site, or microhabitat, was most important in determining variation in assemblage structure, i.e., beta diversity. There was no significant difference among microhabitats in terms of taxa richness evaluated by rarefaction curves. Using partial Constrained Correspondence Analysis (pCCA), we found that microhabitat was most important in determining community composition, accounting for 42.02% of the total variation. Stream sites accounted for 22.27%. In accordance with the pCCA, exploratory multivariate methods (ordination and classification) revealed four distinct groups, corresponding to the four microhabitats, independent of stream sites. Our results indicated that differences among environmental conditions are much more important in the determination of stream assemblage structure than are differences in spatial location. Accordingly, adjacent microhabitats in a single stream site harbor macroinvertebrate assemblages more dissimilar than those found in a single microhabitat at different stream sites. Handling editor: D. Dudgeon     

The influence of chemistry and habitat features on the microcrustacea of some upland Welsh streams

SUMMARY. 1. Microcrustaceans were sampled during September 1990 from three microhabitats (margins, stony riffles and gravel) at thirteen stream sites in mid-Wales.
2. Patterns in the distribution, abundance and community structure of microcrustacean taxa were related to stream chemistry, physiography, substratum composition and marginal habitat structure. Patterns of distribution between microhabitats were also assessed.
3. Chemical variables were the strongest correlates with microcrustacean parameters. Total numbers of microcrustaceans and harpacticoid copepods were reduced at sites with high aluminium concentrations, and ostracods were scarce or absent at the most acidic sites. In contrast, cyclopoid copepods were more abundant and species rich at low pH.
4. Microcrustacean communities were most species rich at sites with a high percentage cover of macrophytes on the substratum. Cladoceran species richness and abundance, and the abundance of several microcrustacean species, were also positively correlated with percentage macrophyte cover.
5. Three site groupings produced by TWINSPAN classification were primarily related to percentage macrophyte cover and marginal habitat structure, with chemistry and land use also of importance.
6. Total microcrustacean abundance was highest in margins, but of all the species, only the harpacticoid Bryocamptus cuspidatus showed a strong microhabitat preference.     

Spatial patterns of invertebrate species richness in a river: the relationship between riffles and microhabitats

This study describes the pattern of invertebrate species richness in a river reach with large differences in habitat complexity at two, hierarchically nested, spatial scales. The aim was to determine whether the mass effect was likely to be increasing invertebrate species richness in epilithic microhabitats in this river. The mass effect is the process by which the species richness of a patch is increased when it acts as a ‘sink’ for species generated by ‘source’ patches. Microhabitat patch types in Mountain River, Tasmania, were distinguished on the basis of physical structure and orientation on the river bed. They were nested within two types of riffle with contrasting structural complexity: bedrock and boulder-cobble riffles. It was hypothesized that microhabitats with high species richness would act as source patches, contributing species to other microhabitats (sinks) and thereby increasing their species richness. Microhabitat sampling was carried out in four consecutive seasons and rarefaction was used to estimate riffle-scale species richness. Analysis of variance ( ANOVA ) was used to compare the identical microhabitats present in the contrasting riffle types, to detect evidence of the mass effect in either riffle type. The more structurally complex boulder-cobble riffles had higher species richness than did bedrock riffles. Amongst the microhabitats, the spaces beneath the cobbles had the most species. Microhabitats accounted for a higher percentage of the variation in species richness than did differences between riffles of the same type. No evidence was found for the operation of the mass effect in either riffle type. The majority of species found only in boulder-cobble riffles were unique to the beneath-cobble microhabitat and appeared to be unable to colonize other microhabitats, even as transients. In Mountain River, small-scale habitat characteristics appeared to be more important than larger-scale effects in determining microhabitat species richness.     

The effects of forest conversion to oil palm on ground‐foraging ant communities depend on beta diversity and sampling grain

Beta diversity – the variation in species composition among spatially discrete communities – and sampling grain – the size of samples being compared – may alter our perspectives of diversity within and between landscapes before and after agricultural conversion. Such assumptions are usually based on point comparisons, which do not accurately capture actual differences in total diversity. Beta diversity is often not rigorously examined. We investigated the beta diversity of ground‐foraging ant communities in fragmented oil palm and forest landscapes in Sabah, Malaysia, using diversity metrics transformed from Hill number equivalents to remove dependences on alpha diversity. We compared the beta diversities of oil palm and forest, across three hierarchically nested sampling grains. We found that oil palm and forest communities had a greater percentage of total shared species when larger samples were compared. Across all grains and disregarding relative abundances, there was higher beta diversity of all species among forest communities. However, there were higher beta diversities of common and very abundant (dominant) species in oil palm as compared to forests. Differences in beta diversities between oil palm and forest were greatest at the largest sampling grain. Larger sampling grains in oil palm may generate bigger species pools, increasing the probability of shared species with forest samples. Greater beta diversity of all species in forest may be attributed to rare species. Oil palm communities may be more heterogeneous in common and dominant species because of variable community assembly events. Rare and also common species are better captured at larger grains, boosting differences in beta diversity between larger samples of forest and oil palm communities. Although agricultural landscapes support a lower total diversity than natural forests, diversity especially of abundant species is still important for maintaining ecosystem stability. Diversity in agricultural landscapes may be greater than expected when beta diversity is accounted for at large spatial scales.     

β-diversity decline of aquatic insects at the microhabitat scale associated with agricultural land use

Several studies report the decline of β diversity caused by agricultural impacts in river ecosystems. However, the susceptible scale of β diversity to agricultural impacts and the indirect effects on β diversity within hierarchically nested ecosystems are unclear. We first tested the hypothesis that β diversity between microhabitats is significantly influenced by agricultural land use. We also examined the indirect effects of agricultural land use on α and β diversities at the microhabitat scale. Twelve microhabitat samples (25 cm²) were collected at 27 reaches within Kitamihorobetsu River, Japan. All reaches were classified into three agricultural intensities based on pasture area (low, middle, and high), and their hierarchical diversities (microhabitat, reach, and catchment) were calculated using additive partitioning. Indirect effects were demonstrated by structural equation modelling using indirect and direct environmental gradients. The α and β diversities at the microhabitat scale decreased significantly with agricultural intensity. Increasing pasture cover within catchments showed a negative correlation with habitat heterogeneity and water quality (habitat homogenization and water degradation) and a positive correlation with sand cover on streambeds (sedimentation of fine particles). The α diversity decreased with sedimentation of fine particles, and the β diversity decreased with habitat homogenization and water degradation. Our findings suggest that species diversity of aquatic insects at the microhabitat scale would be susceptible to agricultural land use. Furthermore, we emphasize that the diversity index used to evaluate restoration projects should be carefully selected because influential abiotic factors were different between α and β diversities.     

Temporal consistency in the long-term spatial distribution of macroinvertebrate drift along a stream reach

Previous studies of the spatial pattern of stream invertebrate drift have focused on spatial variation at microhabitat scales or landscape scales, or temporal variation over diel or seasonal scales. None have examined consistency in spatial variation over longer time scales (>1 year). This study examined invertebrate drift density and composition at fixed locations (terminal ends of 10 riffles) each month at day and night along a 1 km reach of a 2nd order stream over a period of nearly 2 years. Consistent differences in the density of macroinvertebrate drift between riffles over 2 years were observed. The only habitat characteristic observed to be related to invertebrate drift density was the length and size of riffles above sampling sites, with larger and longer riffles producing the highest drift densities. Consistent differences in the supply of drifting macroinvertebrates along a stream reach may have implications for the supply of colonists to substrate patches and the profitability of feeding positions for drift-feeding fish and other predators. Handling editor: D. Dudgeon     

不同喀斯特小生境中土壤丛枝菌根真菌的遗传多样性

为探明西南喀斯特地区小生境土壤中丛枝菌根真菌(AMF)的遗传多样性特征, 利用巢式PCR和变性梯度凝胶电泳相结合的分子生物学方法, 对茂兰3种植被类型下的小生境(石缝、石沟、土面) AMF遗传多样性进行了研究。结果发现: 各类小生境都含有丰富的AMF遗传多样性, 灌木林土面的多样性指数和物种丰富度最高, 为4.06和68; 次生林石缝的最低, 为3.16和29, 所研究的9个小生境多样性指数和物种丰富度的平均值分别高达3.67和48, 高于同类研究在其他地区的结果, 这可能主要与喀斯特生态系统复杂的结构和较高的植物多样性有关; 聚类分析显示各类小生境间的AMF群落结构差异显著, 相似性指数最高仅为0.45, 说明小生境所带来的空间异质性对AMF的遗传多样性产生了显著影响; 基因测序显示球囊霉属(Glomus)极有可能是喀斯特地区AMF的优势菌属, 在以后筛选喀斯特地区的高效生态恢复菌种时可重点考虑球囊霉属的一些菌种。     

Habitat effects on invertebrate drift in a small trout stream: implications for prey availability to drift-feeding fish

In this study, we focused on the drivers of micro- and mesohabitat variation of drift in a small trout stream with the goal of understanding the factors that influence the abundance of prey for drift-feeding fish. We hypothesized that there would be a positive relationship between velocity and drift abundance (biomass concentration, mg/m³) across multiple spatial scales, and compared seasonal variation in abundance of drifting terrestrial and aquatic invertebrates in habitats that represent the fundamental constituents of stream channels (pools, glides, runs, and riffles). We also examined how drift abundance varied spatially within the water column. We found no relationship between drift concentration and velocity at the microhabitat scale within individual pools or riffles, suggesting that turbulence and short distances between high- and low-velocity microhabitats minimize changes in drift concentration through settlement in slower velocity microhabitats. There were also minimal differences in summer low-flow drift abundance at the mesohabitat scale, although drift concentration was highest in riffle habitats. Similarly, there was no differentiation of drifting invertebrate community structure among summer samples collected from pools, glides, runs, and riffles. Drift concentration was significantly higher in winter than in summer, and variation in drift within individual mesohabitat types (e.g., pools or riffles) was lower during winter high flows. As expected, summer surface samples also had a significantly higher proportion of terrestrial invertebrates and higher overall biomass than samples collected from within the water column. Our results suggest that turbulence and the short length of different habitat types in small streams tend to homogenize drift concentration, and that spatial variation in drift concentrations may be affected as much by fish predation as by entrainment rates from the benthos. Handling editor: Robert Bailey     

The spatial heterogeneity of diatoms in eight southeastern Ohio streams: how far does a single riffle reach?

Periphyton is a commonly used biomonitoring tool for streams. Often only one or few riffles are sampled and assumed to be representative of a stream reach. Current literature focuses on periphyton heterogeneity at small scales, on individual rocks within a riffle, and larger scales, within watersheds or ecoregions. The intermediate scales, within single riffles or among riffles, have not been adequately addressed. The purpose of this research was to determine how many riffles must be sampled in order to represent a reach and whether the number of necessary riffles varied with stream health. Since periphyton is sensitive to habitat change, it was hypothesized that heterogeneity would be primarily partitioned among riffles. Eight to ten consecutive riffles were sampled at eight individual stream reaches. Sampled reaches were categorized based on previously collected bioassesment data: three non-attaining, three partially-attaining, and two fully-attaining water quality standards as defined by the Ohio Environmental Protection Agency. Data were analyzed using the Bray-Curtis Similarity Index, Hill’s N₂ dominance diversity index, and the Acid Mine Drainage Diatom Index of Biotic Integrity. Diatoms appeared to be patchily distributed within a reach. This patchiness often led to varied relative abundance of common species and the introduction or loss of rare species among riffles. To account for this variation within a reach, at least two riffles should be sampled. However, a multimetric index may correctly classify a stream based on a one-riffle sample. Variation does not appear to correspond directly to stream health, but to species richness and diversity.     

Microhabitat distribution of protostelids in a Tropical Wet Forest in Costa Rica

A microhabitat study of protostelids was carried out in a Tropical Wet Forest at the La Selva Biological Station in Costa Rica. Nine species were recorded from sterile wheat straws placed out and then re-collected over a period of six weeks from two different litter microhabitats in an area of primary forest. All nine species were present on straws placed in the aerial litter microhabitat, but only six species were present on straws placed in the forest floor litter microhabitat. Total colonies, percent of straws colonized, and mean number of species per straw increased significantly over time. One species (Schizoplasmodiopsis pseudoendospora) typical of temperate litter was the overwhelming dominant on the forest floor litter, while Echinostelium bisporum, a species rare in temperate litter microhabitats, was the single most abundant species in the aerial litter microhabitat. Both of these species had significantly increased frequencies over time. Two species abundant in temperate aerial litter microhabitats and one species abundant in temperate forest floor litter were rare at La Selva. Our data conform to those obtained in an earlier study carried out in tropical forests in the mountains of Puerto Rico and provide additional support towards developing a model of microhabitat distribution of protostelids in terrestrial ecosystems.     

Seasonal Shifts in Day-Time Resource Use of 0+ Barbel, Barbus Barbus

Habitat preference and diet of 0+ barbel were studied both on a meso- and a microhabitat scale in the River Sieg (Germany) between May 1993 and January 1995. Changes in mesohabitat use were observed. Barbel moved from shallow bays (larvae and step 1 juveniles) to gravel banks, and subsequently, to riffles (step 2 juveniles). Size-dependent shifts in microhabitat-use were observed during the second juvenile step. These juveniles left the shoreline and preferred microhabitats with stronger current velocities. 0+ barbel in riffles fed on Chironomidae and Ephemeroptera. No remarkable shifts in diet were detected between larvae and juveniles. We suggest that the observed ontogenetic shifts to habitats with high food supply and low predation pressure might contribute to the high abundances of barbel in the River Sieg.     

Environmental and spatial drivers of spider diversity at contrasting microhabitats

The relative importance of environmental and spatial drivers of animal diversity varies across scales, but identifying these scales can be difficult if a sampling design does not match the scale of the target organisms' interaction with their habitat. In this study, we quantify and compare the effects of environmental variation and spatial proximity on ground‐dwelling spider assemblages sampled from three distinct microhabitat types (open grassland, logs, trees) that recur across structurally heterogeneous grassy woodlands. We used model selection and multivariate procedures to compare the effects of different environmental attributes and spatial proximity on spider assemblages at each microhabitat type. We found that species richness and assemblage composition differed among microhabitat types. Bare ground cover had a negative effect on spider richness under trees, but a positive effect on spider richness in open grassland. Turnover in spider assemblages from open grassland was correlated with environmental distance, but not geographic distance. By contrast, turnover in spiders at logs and trees was correlated with geographic distance, but not environmental distance. Our study suggests that spider assemblages from widespread and connected open grassland habitat were more affected by environmental than spatial gradients, whereas spiders at log and tree habitats were more affected by spatial distance among these discrete but recurring microhabitats. Deliberate selection and sampling of small‐scale habitat features can provide robust information about the drivers of arthropod diversity and turnover in landscapes.     

Identifying the scales of variability in stream macroinvertebrate abundance, functional composition and assemblage structure

1. Many natural ecosystems are heterogeneous at scales ranging from microhabitats to landscapes. Running waters are no exception in this regard, and their environmental heterogeneity is reflected in the distribution and abundance of stream organisms across multiple spatial scales. 2. We studied patchiness in benthic macroinvertebrate abundance and functional feeding group (FFG) composition at three spatial scales in a boreal river system. Our sampling design incorporated a set of fully nested scales, with three tributaries, two stream sections (orders) within each tributary, three riffles within each section and ten benthic samples in each riffle. 3. According to nested anova s, most of the variation in total macroinvertebrate abundance, abundances of FFGs, and number of taxa was accounted for by the among‐riffle and among‐sample scales. Such small‐scale variability reflected similar patterns of variation in in‐stream variables (moss cover, particle size, current velocity and depth). Scraper abundance, however, varied most at the scale of stream sections, probably mirroring variation in canopy cover. 4. Tributaries and stream sections within tributaries differed significantly in the structure and FFG composition of the macroinvertebrate assemblages. Furthermore, riffles in headwater (second order) sections were more variable than those in higher order (third order) sections. 5. Stream biomonitoring programs should consider this kind of scale‐dependent variability in assemblage characteristics because: (i) small‐scale variability in abundance suggests that a few replicate samples are not enough to capture macroinvertebrate assemblage variability present at a site, and (ii) riffles from the same stream may support widely differing benthic assemblages.     

Spatial and Temporal Variation of Macroinvertebrate Drift in Two Neotropical Streams1

The detection of spatial variation in macroinvertebrate drift depends on the spatial scale of investigation in streams of the La Selva Biological Station, Costa Rica. Drift samples were taken in a spatially nested design, with two streams, two reaches per stream, two riffles per reach, and four replicate samples per riffle. Drift showed little variation among streams, but varied significantly at the scales of reach and riffle, with variation among samples also high. In addition, sampling took place at two temporal scales: diel and at two different periods that differed in rainfall conditions. Drift diel periodicity was a clear pattern, while only density of individuals varied among sampling periods. This is the first study of macroinvertebrate drift at multiple spatial scales, despite the recognition that multi‐scale studies are essential for a more complete understanding of community patterns and processes.     

Comparisons of benthic invertebrates between riffles and pools

Benthic species assemblages in upstream and downstream ends of riffles and in pools were investigated seasonally in the first five orders of an alluvial gravel stream with distinct pool and riffle channel form. Riffles comprised < 10% of stream area and were separated by pools with extensive bedrock substrate (ca. 15–85% of total surface area) which was scoured during periodic high flow. Virtually all taxa were more abundant in riffles than in pools, except chironomids which were more equally distributed. Inconsistent results were obtained for upstream-downstream comparisons within riffles. Intermittent headwater reaches (orders 1 & 2) which supported half as many taxa retained this pattern during periods of flow, although riffles at these sites were dry from mid-June to mid-November. Pools which contained more gravel, indicating less disturbance during high flow, had a richer assemblage of benthic species than other pools. Many invertebrates in pools may have been there as a result of drift from their preferred riffle habitats, but the presence of gravel in the pools indicates less intense flow disturbance during floods, provides protection from the mild scouring that does occur during floods, and provides refugia from predators.     

Spatial Variability of Plant Litter Decomposition in Stream Networks: from Litter Bags to Watersheds

The decomposition of plant litter plays a fundamental role in the cycling of carbon and nutrients and is driven by complex interactions of biological and physical controls, yet little is known about its variability and controls across spatial scales. Here we address the indirect effects of riparian canopy cover on litter decomposition and decomposers and their variability within a set of hierarchical scales (watershed, stream segments and reaches) controlling for confounding factors that could co-vary with canopy cover (for example, temperature and nutrients), in high-altitude subtropical streams. Total, microbial and invertebrate-driven decomposition rates were approximately 1.4–6.6 times higher in closed-canopy than in open-canopy watersheds. Riparian canopy cover accounted for 62–69% of total variability of decomposition rates and indirectly (via light availability and litter inputs) promoted fungal facilitation of shredders through leaf litter conditioning. In contrast to what we expected, much of the spatial variability in the decomposition occurred at smaller scale (4–20% of total variability among reaches versus <1% among watersheds) and coincided with the greatest variability in shredder abundance and fungal biomass (70 and 17% among reaches, respectively). We conclude that riparian canopy cover may be an important control of natural variability of litter decomposition at the watershed scale through its effects on fungal decomposers and shredder consumption. We also provide evidence of higher reach and minor watershed variability of litter decomposition in stream networks. Our results point to the importance of identifying the sources of natural variability of decomposition and how they interact within and among spatial scales.     

Microhabitat selection by a stream-dwelling amphipod: a muitivariate analysis approach

SUMMARY. 1. Colonization of microhabitat implants by the amphipod Gammarus pseudolimmnaeus in a small southern Ontario stream was studied in order to analyses the factors controlling habitat selection. The variables substrate particle size, current speed, presence of food and light were used in an analysis of covariance, with percentage weight of organic matter of silt and percentage interstitial space occluded by silt as the covariates.
2. Greatest numbers of amphipods settled on microhabitats featuring large substrate particles, no current and presence of food. There was also a positive relationship between total numbers and the volume of silt deposited on the microhabitats by the stream; small quantities of silt had a beneficial effect on colonization but larger quantities became detrimental.
3. The change from a positive effect occurred at approximately 25% occlusion of the interstitial space in large gravel ( diameter=3.2 cm) and at approximately 55% occlusion in small gravel ( diameter = 0.34 cm).
4. Large animals (6–16.0 mm long) were found predominantly in microhabitats featuring food and large substrate. Medium-sized animals (3–6.0 mm) were most commonly associated with no current and presence of food, and were positively affected by the amount of silt but, at the same time, were negatively affected by increasing occlusion of interstitial spaces by silt. Numbers of small Gammarus (<3.0 mm) were affected only by silt and in a similar manner to medium-sized animals.
5. Amphipod biomass was greatest in microhabitats featuring food and no current. Previous data on the behaviour of this species in laboratory stream-tanks are compared with the microhabitat selections seen.     

Determinants of stream bryophyte community structure: bringing ecology into conservation

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Inter and intraspecific variation in fish body size constrains microhabitat use in a subtropical drainage

Microhabitat characteristics are expected to influence the distribution of stream fish species at fine spatial scales (e.g., within riffle segments). Body size is probably the most important trait that constrains microhabitat occupation by fish, but the effect of intraspecific variation has been understudied. We investigated how physical microhabitat characteristics affect species and body size distribution of fish within a stream riffle segment in a coastal subtropical drainage of Brazil. Fishes were sampled by electrofishing 56 riffle plots along a 730-m long stream segment. Species composition was significantly related to four microhabitat characteristics: substrate size, flow velocity, distance to margin and depth. In addition, mean body size increased with increasing substrate size and depth of microhabitat sampling plots. However, when including species identity in linear mixed-effects models (LMM), we observed a different relationship between body size and microhabitat characteristics, but most of the variation was explained by species identity. Thus, we fitted LMMs separately for each species and found species-specific relations between intraspecific variation in body size and microhabitat characteristics. The low variation explained in the models suggests that other fine scale factors, such as biotic interactions and dispersal from adjacent habitat patches, should be incorporated in modeling microhabitat use by stream fish. Our findings suggest that body size is important by itself, but intraspecific variation in body size also constrains microhabitat use differently for each species, which may depend on other species-specific traits, such as morphology, behavior and life history.     

Litter decomposition across multiple spatial scales in stream networks

Spatial scale is a critical consideration for understanding ecological patterns and controls of ecological processes, yet very little is known about how rates of fundamental ecosystem processes vary across spatial scales. We assessed litter decomposition in stream networks whose inherent hierarchical nature makes them a suitable model system to evaluate variation in decay rates across multiple spatial scales. Our hypotheses were (1) that increasing spatial extent adds significant variability at each hierarchical level, and (2) that stream size is an important source of variability among streams. To test these hypotheses we let litter decompose in four riffles in each of twelve 3rd-order streams evenly distributed across four 4th-order watersheds, and in a second experiment determined variation in decomposition rate along a stream-size gradient ranging from orders 1 to 4. Differences in decay rates between coarse-mesh and fine-mesh litter bags accounted for much of the overall variability in the data sets, and were remarkably consistent across spatial scales and stream sizes. In particular, variation across watersheds was minor. Differences among streams and among riffles were statistically significant, though relatively small, leaving most of the total variance (51%) statistically unexplained. This result suggests that variability was generated mainly within riffles, decreasing successively with increasing scale. A broad range of physical and chemical attributes measured at the study sites explained little of the variance in decomposition rate. This, together with the strong mesh-size effect and greater variability among coarse-mesh bags, suggests that detritivores account, at least partly, for the unexplained variance. These findings contrast with the widespread perception that variability of ecosystem characteristics, including process rates, invariably increases (1) with spatial extent and (2), in stream networks, when analyses encompass headwaters of various size. An important practical implication is that natural variability need not compromise litter decomposition assays as a means of assessing functional ecosystem integrity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.